172 Applied Process Design for Chemical and Petrochemical Plants
The "single" mechanical seal is made of a rotating ele- used under various conditions 111 the wide variety of
ment fixed to the shaft (or shaft sleeve), and a stationary process fluids.
element fixed to the pump casing [16]. The average unbalanced external seal is good for pres-
The "double" seal is for severe sealing problems where sures of about 30 psig, while the balanced design will han-
out-leakage to the environment cannot be tolerated and dle 150 psig. Special designs will handle much higher
must be controlled. (See Figures 3-31C and 3-310.)
Depending upon the fluid's characteristics, the vent
Flush
between the double seals (Figures 3-31A and B) may be Gland connection
gas�et
purged with process liquid, or a different liquid or oil, or
it may be connected to a seal pot and vent collection to Drive pi,n Gland
prevent leakage to the air/ environment. There are tech- Stuffing-box $ring
housing ',,
niques for testing for leakage of the inner seal by measur- '
ing the vent space pressure through the seal liquid surge '
port. This should be essentially atmospheric (depending
on the vent system backpressure). This allows detection
before the leakage breaks through the outer seal.
Figure 3-18 illustrates a seal installed in a conventional
stuffing box with cooling liquid flow path. Figures 3-18, 3-
l9A, 3-198, 3-20 and 3-21 identify the fundamentals of
mechanical seals, even though there are many specific
designs and details. These various designs are attempts to
correct operational problems or seal weaknesses when
I I Primary seal '.
'I
Mechanical-seal , elements )
hardware
-. Secondary seals_,�
e,-� : -- . - =-_]
I Ir;;'. , ��� Figure 3-19A. Basic components of all mechanical seals. (By per-
I mission, Adams, W. H., Chemical Engineering Feb. 7, 1983, p. 48.)
· (3) (2)
't Shafi+-·--·--·-·
Internal Seal Exltrnol Seol
SPRING HOLDER
U:H HR M Figure 3-198. The three sealing points in mechanical seals. (By per-
SPRING-18·8 mission, T. J. Sniffen, Power and Fluids, Winter 1958, Worthington
WORK HAR ENED Corp.)
DRIVE COLI AR
L'.:.!.� CHROME.
P: Prtnurt of Liquid in Boa
p':Avtrage Pressure Across
Stol Facts
Closi�g Foret: P 1 1 Arta "A:: t, Spring Loading
Opening Force : P I Arto B
Figure 3-20. Area relationship for unbalanced seal construction. (By
Figure 3-18. Typical single mechanical seal inside pump stuffing permission, T. J. Sniffen, Power and Fluids, Winter 1958, Worthing-
box. (Courtesy Borg-Warner Co.) ton Corp.)
Pumping of Liquids 173
P = Pressure of Liquid in 801 all mechanical seals) and must not attack the material of
p': Averogt Prtnure Across
Stol Focn the 0-ring shaft packing. Many other designs are avail-
able, and the manufacturers should be consulted for
advice on specific sealing problems.
Centrifugal Pump Selection
The centrifugal pump is a versatile unit in the process
Closing Force = P � (Areo " � :- � rea"c") + Spring Loading
Oeening Force : P I Area B plant, since its ease of control, non-pulsing flow, pressure
'l\ ,"e" and "c" ore Vorloblt
limiting operation fits many small and large flow systems.
Figure 3-21. Area relationship for balanced seal construction. (By
permission, T. J. Sniffen, Power and Fluids, Winter 1958, Worthing-
ton Corp.)
pressures. Actually the maximum operating process pres-
sures are a function of the shaft speed and diameter for a
given seal design fluid and fluid temperature.
Figure 3-22 is an outside balanced seal designed for
vacuum to 150 psig and -40°F to +400°F. (See Table 3-4)
The process fluid must be free of solids (as for practically
Table 3-4
Requirements for Mechanical Seal Installations
Fig.
Feature Description Remarks No.
Cooling Water Jacketed Liquid must dead end 3-23 Figure 3-22. Outside balanced seal (single). (Courtesy Durametallic
Stuffing Box in stuffing box Corp.)
Cooling Gland Plate Efficient to cool 3-24
contact faces
Lubrication Dead-End Good under vacuum, 3-25 0
....
11>
mild abrasives metal- -
:::,
metal, dry seals >000
...
....
Lubrication Circulating Good cooling of 3-26 400 e
contact faces 3 GPM Cooling Wafer_ Cb
Flushing Inside Seal Good for volatile 3-27 3001-
liquids, sol'ns tending 5 GPM Cooling Wafer >C
0
to crystallize, steam 200CD
DI
c:
-
,
Flushing Outside Seal Heating to prevent 3-28 I II II , 100=
solidification �
Quenching Outside Seals For oxidizing and 3-29 0 100 • 2 300 400 soo 600 700 • oo en
(only) corrosive liquids, seal
liquid washes process
fluid, for high temp.
Vent and Inside Seal Safety feature, for 3-30
Drain venting to flare,
draining
Flushing Double Requires circulation 3-31A
system
Flushing Tandem Requires circulation 3-31B
system
Two Rotary Double For improved sealing 3-31C
Four Rotary Double For special sealing 3-310
Tandem problem Figure 3-23. Water jacketed stuffing box. (By permission, H. P. Hum-
mer and W. J. Ramsey, Bulletin SD, 752, O&T, Durametallic Corp.)
174 Applied Process Design for Chemical and Petrochemical Plants
2 N.P.T's for Circulating
Lubrication
- Figure 3-26. Circulating lubrication. (By permission, H. P. Hummer
.!!!
'S and W. J. Ramsey, Bulletin SD 752, O&T, Durametallic Corp.)
0
,::,
c: r---------7-
c:,
- ... Product Recirculating, Flushing, I External Fluid Flushing
or By-pass from Discharge
-=
,:r
c:
0
0
...
-
<...)
0
Figure 3-24. Gland plate cooling. (By permission, H. P. Hummer and
W. J. Ramsey, Bulletin SD 752, O&T, Durametallic Corp.) Figure 3-27. Flushing inside seal. (By permission, H.P. Hummer and
W. J. Ramsey, Bulletin SD 752, O&T, Durametallic Corp.)
I N.P.T. for Dead-End .-------7
Lubrication Product Recirculating, Flushing, I External Fluid Flushing
or By-pass from Discharge
r-----1.'_ I N.P.T. for Flushing
; Connection
,I
Figure 3-25. Dead-end lubrication. (By permission, H. P. Hummer Figure 3-28. Flushing outside seal. (By permission, H. P. Hummer
and W. J. Ramsey. Bulletin SD 752, O&T, Durametallic Corp.) and W. J. Ramsey, Bulletin SD 752, O&T, Durametallic Corp.)
Generally speaking the centrifugal pump has these char- 6. Develops turbulent conditions in fluids
acteristics: 7. Turbine type: (a) offers very high heads al low flows,
(b) self-priming, (c) limited Lo very clean, non-abra-
1. Wide capacity, pressure, and fluid characteristics sive fluids with limited physical properties, (d) clear-
range ances can be problem on assembly and maintenance.
2. Easily adapted to direct motor, V-belt or other drive
3. Relatively small ground area requirements Single-Stage (Single Impeller) Pumps
4. Relatively low cost
5. Difficult to obtain very low flows at moderate to high This type of pump (Figures 3-1, 3-2, 3-3) is the work-
pressures horse of the chemical and petrochemical industry. It also
Pumping of Liquids 175
2 N. P. T. 's for Quenching Inlet ond Outlet of a manufacturer's performance curve to fit the control
requirements of the system. If the curve is too steep, select
an impeller of necessary basic characteristics to move the
curve in the proper direction, providing the manufactur-
er has an impeller pattern to fit that pump casing, and
with the improved physical dimensions. This may require
changing the make of pump to obtain the necessary
range and characteristic.
For conditions of (1) high suction side (or inlet) fric-
tion loss, from suction piping calculations or (2) low avail-
Figure 3-29. Quenching outside seal. (By pennission, H. P. Hummer
and W. J. Ramsey, Bulletin SD 752, O&T Durametallic Corp.) able Net Positive Suction Head (l O feet or less), a large
open eye on the impeller inlet is necessary to keep the
inlet velocity low. NPSH is discussed in a later section. The
2 N.P.T. s for Vent and Drain manufacturer should be given the conditions in order to
1
Throttle properly appraise this situation.
-------- In most instances the manufacturer has a series of
Bushing
impellers to use in one standard casing size. The impeller
may be trimmed to proper diameter to meet head
requirements and yet stay within the power range of a
specified driver. It is not necessary to place a full size
impeller in a casing unless the system requires this per-
formance. It is good to know when larger impellers can be
placed in the casing, and. what their anticipated perfor-
mance might be in order to adequately plan for future
uses and changing loads on the pump.
Figure 3-30. Vent and Drain. (By permission, H. P. Hummer and W. J. Although the previous discussion has pertained to sin-
Ramsey, Bulletin SD 752, O& T, Durametallic Corp.)
gle impellers, the principles are the same for the multi-
stage units (impellers in series in the casing) and the cas-
serves important functions in petroleum refining and ing with double inlets. The latter pump is used for the
almost every industry handling fluids and slurries. higher flows, usually above 500 GPM, and this design
Although the performance characteristics may vary for serves to balance the inlet liquid load as it enters the
specific applications, the general fundamental features impeller, or first stage (if more than one) from two sides
are the same especially for manufacturers who standard- instead of one as in the single impeller. The double suc-
ize to some extent through the Hydraulic Institute [17] tion pump has the liquid passages as a part of the casing,
and American National Standards Institute. with still only one external suction piping connection.
Figure 3-32 indicates the relative relationship for three The axial and mixed flow impellers are used primarily
of the centrifugal type pumps, with curves labeled "cen- for very high capacities at relatively low heads as shown in
trifugal" referring to the usual process (open or enclosed Table 3-2. They are usually applied to services such as
impeller) type unit. A similar set of curves is shown in Fig- water distribution to a large system, waste water disposal,
ure 3-33 for the turbine unit. Note that the flat. head curve recirculating large process liquor flows, and the like.
of the centrifugal unit has advantages for many process Many applications can be handled either by a horizon-
systems, giving fairly constant head over a wide range of tal or a vertical pump. In the range usually associated with
flow. For some systems where changes in flow must be process plants and the associated services, Tables 3-5 and
reflected by pressure changes, the turbine characteristic is 3-6 are helpful guides in making the selection [I 2].
preferred. The centrifugal impeller provides an ever ris-
ing horsepower requirement with increasing flow, while Pumps In Series
the horsepower of the turbine pump falls off with increas-
ing flow (and decreasing head); hence it is "overloading" Sometimes it is advantageous or economical to use two
at low flows and must be operated with ample horsepow- or more pumps in series (one pump into and through the
er for these conditions. other) to reach the desired discharge pressure. In this sit-
The effects of impeller shape for the usual centrifugal uation the capacity is limited by the smaller capacity of any
process pump performance are given in Figure 3-34. The one of the pumps (if they are different) at its speed of
only part the process designer can play is in the selection operation. The total discharge pressure of the last pump is
176 Applied Process Design for Chemical and Petrochemical Plants
FLUSH
%"
PUMP END DRIVE
-----r---'ff'"""-�--..i,.,.....- END
VIKING GEAR SEAL OIL
PUMP TANK
(1.5-2 GPM/ 25 GALLON
PROCESS (LOW MINERAL
PUMP SEAL HOUSING PUMP) OR OTHER
ACCEPT ABLE LOW
VISCOSITY OIL)
Figure 3-31A. Typical seal flush arrangement for double mechanical seals.
PLANT VENT __ ........
_-(
HEADER SYSTEM
SIGHT GLASS SEAL OIL SUPPLY TANK
FLUSH 2-3 GAL. CAPACITY
%"
DISCHARGE
PIPE FROM
PUMP
DRIVE END
-- LOW PRESSURE SEAL (OUTSIDE)
PUMP SEAL HOUSING
Figure 3-31 B. Typical seal flush arrangement for tandem mechanical seals.
Pumping of Liquids 177
head is twice that of the rated pressure of one pump at the
designated flow rate (Figure 3-35). The pump casing of
Atmosphere each stage (particularly the last) must be of sufficient pres-
Side sure rating to withstand the developed pressure.
Pumps in Parallel
Figure 3-31C. Double mechanical seal, two rotary elements against
common stationary. (By permission, Fischer, E. E., Chem Process- Pumps are operated in parallel ta divide the load
ing, Oct. 1983 [24].) between two (or more) smaller pumps rather than a single
large one, or to provide additional capacity in a system on
short notice, or for many other related reasons. Figure 3-
, , (- 35 illustrates the operational curve of two identical pumps
Barrier Fluid In
in parallel, each pump handling one half the capacity at
Atmosphere the system head conditions. In the parallel arrangement of
Side two or more pumps of the same or different characteristic
curves, the capacities of each pump are added, at the head
of the system, to obtain the delivery flow of the pump sys-
tem. Each pump does not have to c?JTY the same flow; but
Figure 3-310. Tandem double seal. (By permission, Fischer, E. E., it will operate on its own characteristic curve, and must
Chem. Processing, Oct. 1983 [24].) deliver the required head. At a common tie point on the
discharge of all the pumps, the head will be the same for
the sum of the individual discharge pressures of the indi- each pump, regardless of its flow.
vidual pumps. For identical pumps, the capacity is that of The characteristic curves of each pump must be con-
one pump, and the discharge pressure of the last pump is tinuously rising (right to left) as shown for the single
the sum of the individual heads of each pump acting as a pump of Figure 3-35, otherwise with drooping or looped
single unit. Thus, for two identical pumps the discharge curves they may be two flow conditions for any one head,
l !
\
"{ .- __ -· - ·-..... ._tfficlency
_c
-
.....
Q) ,d I',._ ,Head, ft. a f Liquid ,,./ ... '!- -- -.-1 ...........
r-,
"C / :.,..-- -,
"'
:i: ......... I f'.....r--L -: / ... 7" A= Axial
>- M =Mixed
u ' ......... :----_ -; / '
c: l(._ ./ -.,L. C =Centrifugal
Q)
·.::; " i- M / -
.,--
iE -·- ·- ·- /;; -·- �
�
UJ �� ?2 -· - ·- :::,... _
c.: ·.-
I//
:i: ./ // Rating I ;:;:
co � Point � ·-. �
--- r-- � � / ira ke Horsepower t \ [\ I
,/
A
i-- u, :M:_
"7
/ � � _ ..... c -- - I
" /- -·-
0 -·--· r-.; I
� � r---....... I
'.// I I
Gallons per minute
Figure 3-32. Comparison of impeller types for centrifugal pump performance. (Adapted by permission from Pie-a-Pump, Allis-Chalmers Mfg.
Co.)
178 Applied Process Design for Chemical and Petrochemical Plants
' Table 3-5
50 1,000 -, Pump Selection Guide
� ..
--· ·-
...
or= - ·- =- .. �� � r-, � ... Feature Horizontal Vertical
;$'
Less floor area, more
....
..... v -, Capacity·heod • Space Requirements Less head room head room.
...
0
-
0 ' at 30ft. NPSH \ "' ... NPSH Requires more Requires less
\ ', � I I I 0 Priming Required* Usually not required
Capocity·head
-�·i-- --· � �:-' '-'at 9 ft. NPSH 1 :J: Flexibility (Relative to
....
a,
.... capacity-hecid, e future changes) Less More
1t 6 ft NP� H \ ' I��) \ 15.0ID Maintenance More accessible Major work project
5.0 --
!---.-...._ \ �o.d j Corrosion and Abrasion No great Can be considerable
.. �(Bffpl'K{) -
10 200 Ho '. s "�· � 10.0 problem problem
I Cost Less More (requires more
. I - � � alloy to handle
0 I corrosive Fluid)
15 20 25 30 35 40 45
Capacity - G. P. M. *For some conditions
--
Figure 3-33. Performance of turbine type centrifugal pump. (Cour-
300
tesy Roy E. Roth Co.) Note : Systems IHustrated
Assume Dupllcate
Pumps.
Enclosed Impeller Characteristics
240
..... , Wide Impeller
<,
'
..
" .. \ .....
-e ' \ ';; 180
,: \ Narrow Impeller
.s 150
...,
.,
� 120
Capacity , GPM --
Enclosed or Open Impeller Characteristics 60 H,
Vane;@ @ � ore o, 02
Less Wrap of Vanes
0 50 100 150 200
.. ·-·- .......... Capacity, Gpm.
"D <,
0
,: -, iore Pump in Series: Q = Constant
\ @ Less Vones {Same H (Total) = H 1 + H 2 + "'
Wrap of Vann
20 = S-R denotes Series-Rating
Point, Total
Wrap as B above}
Pumps in Parallel: H = Constant
Q (Total) = 0 1 + 02 + "' (at H for each
Single Pump Curve)
Capacily, GPM -
0 = P-R denotes Parallel-Rating
Figure 3-34. Impeller performance guide. Wrap refers to curvature of Point
vanes on impeller. (Adapted by permission, Pie-a-Pump, Allis- 1 0 = Single pump rating
Chalmers Mfg. Co.)
Figure 3-35. Operation curves of two duplicate centrifugal pumps in
series and parallel.
and the pumps would "hunt" back and forth with no
means to become stabilized. teristics of the pump's design, impeller entrance opening
and diameter, and the hydraulic operating efficiency of
Figures 3-36A, 3-368, and 3-36C represent typical and the pump at the fixed designated speed of the perfor-
actual performance curves showing discharge total head mance curves are shown on the chart. All of this perfor-
(head pressure at pump outlet connection for any fluid), mance is for one specific impeller diameter of the fixed
required minimum water horsepower (for pumping rotating speed (rpm), and the fixed impeller design pat-
water), and capacity or pumping volume of the pump (for tern proprietary to the manufacturer (number, shape and
any fluid) for several impeller diameters that would fit the spacing of vanes, and wrap or curvature of vanes).
same case (housing). In addition the important NPSHR Note that Figure 3-36B plots the NPSHR curve for this
(net positive suction head required by the pump) charac- "family" of impellers (different diameters, but exact same
Pumping of Liquids 179
Table 3-6
Type Selection Based on Liquid Handled
Liquid Basic Pump Type Type Impellers
Water and other clear non-corrosive Single or double suction Closed except for very small capacities
liquids at cold or moderate tempera-
tures.
Water above 250° F. Single or double suction. This is usually boiler Closed except for very small capacities
feed service at high pressures requiring multi-
stage pumps.
Hydrocarbons, hot Single suction, often of the special type called Closed with large inlets.
refinery pumps, designed particularly for high
temperature service.
Corrosives: Single or double suction
Mildly acid or alkaline
Strongly acid or alkaline Single or double suction with single suction Closed except for very small capacities or
probably less expensive if available for the where liquid tends to form scale on surfaces
rating. of moving parts.
Hot corrosives Single suction, with many refinery pump types
also used here because of high temperatures
and corresponding suction pressures.
Water with solids in suspension: Single suction with end clearance wearing fits. Open, which allows better application of the
Fine abrasives If all particles pass through Ya" mesh screen, rubber, except in larger sizes. Also made in
rubber lined pumps are available which will closed type.
give many times the life of metal pumps, pro-
viding no chemical action or excessive tem-
perature will deteriorate the rubber. Special
rubber compounds can be applied to improve
resistance to certain chemicals.
Coarse abrasives Single suction. Not available for full range of Closed.
ratings, that is, small capacities not too easily
obtained. Often have very large impellers
operated at slow speeds for use when solids
larger than l" diameter are the standard diet.
This would be of the type called dredge
pumps handling sizeable rocks.
Pulpy solids such as paper stock Single suction. Double suction only used on Closed. Open type used to be standard but
very slight solids concentrations and then with change to end clearance wearing fits made
special end clearance wearing fits. closed impellers better suited.
180 Applied Process Design for Chemical and Petrochemical Plants
design dimensions and features), while Figure 3-36A Figure 3-36A illustrates typical manufacturers' perfor-
shows the NPSHR numbers printed at selected points on mance curves for centrifugal pumps as a function of
the curve. capacity.
Figure 3-36C illustrates the change in performance for Pumps are normally selected to operate in the region
the exact same pump, same impellers, but for different of high efficiency, and particular attention should be
rotating speeds of 1750 and 3550 rpm. (Note that the given to avoiding the extreme right side of the character-
respective motor designated standard speeds are 1800 and istic curve where capacity and head may change abruptly.
3600 rpm, but the pump manufacturer cannot count on Total Head: the pressure available at the discharge of a
these speeds under load in order to provide performance pump as a result of the change of mechanical input ener-
information the customer needs for design of a system.) gy into kinetic and potential energy. This represents the
total energy given to the liquid by the pump. Head, pre-
Hydraulic Characteristics For Centrifugal Pumps viously known as total dynamic head, is expressed as feel
of fluid being pumped.
Capacity: the rate of liquid or slurry flow through a The total head read on the pump curve is the difference
pump. This is usually expressed as gallons per minute between the discharge head (the sum of the gauge reading
(GPM) by pump manufacturers and design engineers in on the discharge connection on the pump outlet, for
the chemical and petrochemical industries. A few conve- most pumps corrected to the pump centerline, plus the
nient conversions are: velocity head at the point where the gauge is attached)
and the suction head ( the sum of the suction gauge read-
ing corrected to the pump centerline and the velocity
1 imperial gal/min = 1.201 U.S. GPM
head at the point of attachment of the suction gauge)
1 barrel (42 gal)/day = 0.0292 U.S. GPM [25]. Note that the suction gauge reading may be positive
or negative, and if negative, the discharge head minus a
For proper selection and corresponding operation, a minus suction (termed lift) creates an additive condition.
pump capacity must be identified with the actual pumping (See laler discussion.)
Lemperature of the liquid in order to determine the prop- This is shown on the curves of Figure 3-36A. This head
er power requirements as well as the effects of viscosity. produced is independent of the fluid being pumped and is, there-
I I I I
Horsepower for
.,
0
Li quid of Sp. Gr. =1.0( ---- r--
:;::; 200 10 Hp . ....._
::, ._ I I l 7 112 H11._ --; Efficie!CY Values
.!?' 1.---� v
...I 6 112• lmoeller Diameters �r--..so- t,,... �
I
I
[160 s· Perfor monce Curve ..... .. v �r> f:::- - 1'
6�
5 Hpve'!
.....__ r-,
� ,=::::::
0
5 112• 3 Hp., /v r---.... ..... ..... � 70 �
.......
......___. .
:j 120 .. r:::, :""'Ill: ...___
r--,..__
- ,._
LL. - I I .1 ·� ,, K i.-- 1-1.t ........ L- " i
c � 718" 2 Hp. ,....,_ r,,c. r-.. v ..... v - '7'- e�
..
1: 80 4 112· ..... -� � A
:c N.P.s.,., ,_ - / � � t> 1 ........... I)<;. � --,
�
f.411y L.Required
1,.7
v/
"' Suctio11 "lU(d) t>i! rr-- L� v 60�
�
� 40 flttaximu L,,,., 'Feet 12 - �
�
Feet
I...£'
1" Wfte � a � ' � i·.ijsed 21 17 l'>c.. -
�
0 16 2i:
._ Pump Speed: 3,500 Rpm. 11
Pump Size: 2• X 2•
-
Maximum Impeller Diameter :s•
Minimum Impeller Diameter :4 112"
0 20 40 60 80 100 120 140 160 180 200 220
Capacity for any Liquid, Gpm.
Figure 3-36A. Typical centrifugal pump curves. (Adapted by pennission, Allis-Chalmers Mfg. Co.)
11, ....
l..�1�1'{/
-· J08 3500
EHGH:ER/
-- ··- - - ·- CONTRACTOR
� JT � BY/ IIIOOCL NO.
?ll.r .. _,. GP.W. 1£AO SP. GR. TEt.P. 41n-21
� ,� D• IIIAX. Sl'HCll[S
75 _ I ··�- 7/J2"
l lNIT E1! A I ...... J io• FLUD �.OIA. Sil[
..._
11/45
I "r-,... 17 ..._ ...... rw •A SUCTION CIA. 11/2" OISOi. CIA. 11/4' COCK 1''
.,.,, ' 11 2" >IA. I <, I J r-,;:i � ,- - I lill 11n
M_ LINT n ' I I <, �r,'. CODE
:-.....
�
119--975
J r � I I )"" I"',,,,, i,..,.___ -CLLU
... - I I """" � r- - I ... I'.. ......... � 9• 34233
l'l,
...
I
.. "- z ,..... ""- I "!'.... r,..... � <, I/ CAIIHG
4784
;;; 1A'1 . .. ' ... lnu.
•
........
-·
,.
...
" �
w
.,
� ... . -- - r-,;:� """- - I I I"' ..... ---, �� "tl
\
c
c
J.7" �
3
... r-, r- r- ........ <, , ...... � � 1C HF "C
0
3·
45. r-, (C
r-,....
, ... n "� \ ""'-- ...... r-, itJHr g,
r-
-
. ..,, ;-. ,..,., r-, ---.... r-, \ I\ I""--.. <, "- � v 1,1:> ii"
j
II,_
c
�
I>
- I\..
1/)
35 - IINT E1• r-, ' v a:
- 1--t--
<,
'r-- L--
,nr I""-, ,......... - --- r-, � 5 w v
25 _ r--,.. , .........
""'--
.........
r-,...._ 7.5
........
c,n _ .. 20 ::i
1S - �- -- ., �� ;
...
ii
w
I p, M l-.- : 2.5i.
. •
n I n
> 20 40 60 80 100 120 140 160 CIMVC NO.
CAJIACITY
U.S. l:Plil PCJ557
lll(Tt:11 � -,--- --··-·r I --·-r--- ' ' OAT[
CU. 4 8 12 1e 20 24 28 32 J6
9-1-88
"'" 1111.
.....
a,
.....
Figure 3-368. Typical periormance curve showing NPSH in convenient form. (By permission, Crane Co., Deming Pump Div.)
11, ....
l..�1�1'{/
-· J08 3500
EHGH:ER/
-- ··- - - ·- CONTRACTOR
� JT � BY/ IIIOOCL NO.
?ll.r .. _,. GP.W. 1£AO SP. GR. TEt.P. 41n-21
� ,� D• IIIAX. Sl'HCll[S
75 _ I ··�- 7/J2"
l lNIT E1! A I ...... J io• FLUD �.OIA. Sil[
..._
11/45
I "r-,... 17 ..._ ...... rw •A SUCTION CIA. 11/2" OISOi. CIA. 11/4' COCK 1''
.,.,, ' 11 2" >IA. I <, I J r-,;:i � ,- - I lill 11n
M_ LINT n ' I I <, �r,'. CODE
:-.....
�
119--975
J r � I I )"" I"',,,,, i,..,.___ -CLLU
... - I I """" � r- - I ... I'.. ......... � 9• 34233
l'l,
...
I
.. "- z ,..... ""- I "!'.... r,..... � <, I/ CAIIHG
4784
;;; 1A'1 . .. ' ... lnu.
•
........
-·
,.
...
" �
w
.,
� ... . -- - r-,;:� """- - I I I"' ..... ---, �� "tl
\
c
c
J.7" �
3
... r-, r- r- ........ <, , ...... � � 1C HF "C
0
3·
45. r-, (C
r-,....
, ... n "� \ ""'-- ...... r-, itJHr g,
r-
-
. ..,, ;-. ,..,., r-, ---.... r-, \ I\ I""--.. <, "- � v 1,1:> ii"
j
II,_
c
�
I>
- I\..
1/)
35 - IINT E1• r-, ' v a:
- 1--t--
<,
'r-- L--
,nr I""-, ,......... - --- r-, � 5 w v
25 _ r--,.. , .........
""'--
.........
r-,...._ 7.5
........
c,n _ .. 20 ::i
1S - �- -- ., �� ;
...
ii
w
I p, M l-.- : 2.5i.
. •
n I n
> 20 40 60 80 100 120 140 160 CIMVC NO.
CAJIACITY
U.S. l:Plil PCJ557
lll(Tt:11 � -,--- --··-·r I --·-r--- ' ' OAT[
CU. 4 8 12 1e 20 24 28 32 J6
9-1-88
"'" 1111.
.....
a,
.....
Figure 3-368. Typical periormance curve showing NPSH in convenient form. (By permission, Crane Co., Deming Pump Div.)
182 Applied Process Design for Chemical and Petrochemical Plants
OOULDe .. UM .. a, INC. CEHTRIFUGAL PUMP CHARACTERISTICS
@ e•NIICA PALL.a, N. Y. RPM 355 0 CDS 1733- 3
MODEL 3196
SIZE IY2X 3-13 STEEL
.. IMP.DWG. 100-538 100-53i
PATTERN 53971 53970
- EYE AREA 4.9 SO IN.
-
8 o, MAX. ALLOWABLE H.P. -120 -
7",11 ..
�·.
5.ru '.
..... . . 3550
��· - R.P.M.
aI_Y
32'
2.4.1
16L
8•
0 50 100 150 200 250 300 350 400 450
GALLONS PU MINUTI
(§ ::��:! ���· ��!: CENTRIFUGAL PUMP OIARACTERISTICS RPM 17 50 CDS 1680 NO. 2
Moon 3196
SIZE I� X 3-13 1 STEFi I
IMP.DWG. I00-5.�A 1100-5..�7
PATTERN 53971 153970
EYE AREA 4 9 �n IN
2··
180
160 1750
t::;
�140 R.P.M.
g
,_IOO
80
60
40
20
0 40 80 120 160 200 240 280
GALLONS 1111 MINUTI
Figure 3-36C. Illustrates exact same pump casing and impellers at two different shaft speeds. (By permission, Goulds Pumps, Inc.)
Pumping of Liquids 183
fore, the same Jar any fluid through the pump at a given speed of the suction and discharge sides of the pump. Refer to Fig-
rotation and capacity. ures 3-38 and 3-39.
Through conversion, head may be expressed in units
other than feet of fluid by taking the specific gravity of the (3-3)
fluid into account.
The sign of h, when a suction lift is concerned is nega-
(Head in feet), H = (psi) (2.31)/SpGr, for any fluid (3-1) tive, making H = hd - (-hs) = hd + h,
The three main components illustrated in the exam-
Note that psi (pounds per square inch) is pressure on ples are (adapted from [5]):
the system and is not expressed as absolute unless the sys-
tem is under absolute pressure. Feet are expressed as head, 1. Static head
not head absolute or gauge (see later example). Note the 2. Pressure head
conversion of psi pressure to feet of head pressure. 3. Friction in piping, entrance and exit head losses
A pump is acted on by the total forces, one on the sue-
or, (head in ft), H = (psi) ( 144/ p) (3-2)
Lion (inlet) side, the other on the discharge side. By sub-
tracting (algebraically) all the suction side forces from the
where p = fluid density, lb/ cu ft discharge side forces, the result is the net force that the
1 lb/sq in. = 2.31 ft of water al SpGr = 1.0
pump must work against. However, it is extremely impor-
1 lb/sq in. = 2.31 ft ofwater/SpGr of liquid= ft tant to recognize the algebraic sign of the suction side
liquid components, that is, if the level of liquid to be lifted into
1 in. mercury = 1.134 ft of water= 1.134/SpGr liq- the pump is below the pump centerline, its algebraic sign
uid, as ft liquid is negative ( - ) . Likewise, if there is a negative pressure or
vacuum on the liquid below the pump centerline, then
For waler, SpGr = 1.0 at 62°F, although for general use this works against the pump and it becomes a negative
it can be considered 1.0 over a much 'Wider range. For (-). (See discussion Lo follow.)
explanation of vacuum and atmospheric pressure, see
Chapter 2.
c)Butane
Example 3-1: Liquid Heads SpGr= 0.6
lr-
If a pump were required to deliver 50 psig Lo a system, b)Naphtha
for water, the feet of head on the pump curve must read, SpGr= 0.8
2.31 (50) = 115.5 ft
1
For a liquid of SpGr 1.3, the ft of head on the pump a)For water,
curve must read, 115.5/1.3 = 88.8 fl ofliquid. SpGr= 1.0 I d)Carbon
For liquid of SpGr 0.86, the ft of head on the pump 166.7' tetrachloride
curve must read, 115.5/0.86 = 134.2 ft ofliquid. I SpGr=1.50
If a pump were initially selected Lo handle a liquid 125'
where SpGr = 1.3 at 88.8 ft, a substitulion oflight hydro-
carbon where SpGr = 0.86 would mean that the head of
liquid developed by the pump would still be 88.8 feet, but 100' 66.6'
the pressure of this lighter liquid would only be
88.8/[(2.31)/(0.86)1 or 44.8 psi. Nole that for such a
change in service, the impeller seal rings, packing (or
mechanical seal) and pressure rating of casing must be
evaluated to ensure proper operation with a very volatile
fluid. For other examples, see Figure 3-37. a)= 43. .
/ 3 ps1g
The total head developed by a pump is composed of Pressure gauge attached at bottom
the difference between the static, pressure and velocity Figure 3-37. Comparison of columns of various liquids to register
heads plus the friction entrance and exit head losses for 43.3 psig on pressure gauge at bottom of column.
184 Applied Process Design for Chemical and Petrochemical Plants
-c:,
..
Atmospheric "" ...
Pressure ::c: Exit Loss
� I ;::
I
""
Suction Di.scharge iii D
..
I
Liquid Head Head ... Suction: h5 =S-hsL
�
hSL =Pipe, Fittings and
..
I s:; of her Friction Losses
Entrance I O_ i s_c_h_ a r..,_g_ e _..., u
Loss I Piping Cl Discharge: hd =D+hdL
I hdL =Pipe, Fittings and
Suction -·-·- other Friction Losses
Centerline
Piping Pump Note: Sw = Worst condition to
Pump
empty this tank, ft
Figure 3-38. Suction head system.
Exit Loss
0
1
"" ..
� .. �,M
-c:,
-::c:
o:;:
i,;;.� s s=u
a;;
�·.;:
U) *Suction: h. = -SL - hsL
hsL = Pipe, Fitting, Valves,
Exchanger and
other Friction
Losses
-hs = SL+ hsL
**Discharge: hd = D + hdL
hdL = Pipe, Fittings and
other Friction
Losses
*Suction: Worst Case = S' L (Substitute in
above)
**Discharge: (Worst Case) use (D + D1
Figure 3-39. Suction lift system
Static Head Pressure Head
For Figure 3-40C
This is the overall height to which the liquid must be
raised.
Discharge pressure head = 100 psig
Suction pressure head = 0 psig
For Figure 3-40A Total pressure head = 100 - ( +O) = 100 psig
= 100(2.31)* = 231 ft of
water
Discharge static head: H
Suction static head: L (actually - L) Note: The totals are differentials and neither gauge
nor absolute values.
Total system static head: H + L; *Applies to water only. For the other fluids use appro-
actually H -- (- L) (3-4) priate specific gravity conversion.
For Figure 3-40D
For Figure 3-40B
Discharge pressure head= 100 psig
Discharge static head: I-I (from centerline of pump) Suction pressure head = + 50 psig ( =64. 7 psia)
Total pressure head = 100 - ( +50) = 50 psi
Suction static head: S, (actually +S)
not gauge or absolute =
Total system static head: 1-1 - S; or H- ( +S) (3-5) 50 (2.31) = 115.Sftofwater
Pumping of Liquids 185
'
1
L
t Figure 3-400. Pressure head, positive suction. (Adapted by permis-
sion, Centrifugal Pumps Fundamentals, Ingersoll-Rand Co., Wash-
ington, N.J. 07882.)
Figure 3-40A. Static head, overall = H + L.. (Adapted by pennission,
Centrifugal Pumps Fundamentals, Ingersoll-Rand Co., Washington,
N.J. 07882.)
f
H.-
l f
Figure 3-408. Static head, overall = H - S. (Adapted by permission,
Centrifugal Pumps Fundamentals, Ingersoll-Rand Co., Washington,
N.J. 07882.)
Figure 3-40E. Pressure head with negative suction. (Adapted by per-
mission, Centrifugal Pumps Fundamentals, Ingersoll-Rand Co.,
ATMOSPH�AE. Washington, N.J. 07882.)
11
8 CHECK VALVE\
���� J ---,j()�--�
140' OF
11
The above examples purposely disregarded pressure head, 8 PIPE
friction, entrance, and exit head losses.
1500 GPM CAPACITY
Figure 3-40C. Pressure head. (Adapted by permission, Centrifugal
Pumps Fundamentals, Ingersoll-Rand Co., Washington, N.J. 07882.)
Note that both the discharge and suction pressw·es must
be on the same base/units. These illustrations are for stat-
ic head only, while overall the pump has to work against the Figure 3-40F. Pumping arrangement for Example 3-2. (Adapted by
static and the pressure heads. (To be discussed.) permission, Centrifugal Pumps Fundamentals, Ingersoll-Rand Co.,
Washington, N.J. 07882.)
For Figure 3-40E
Suction static head = -10 ft
Discharge pressure head = 100 psig = 231 ft water * Total suction head = +115 + (-10) = +105 ft
(system fluid) * Total head on pump = 281 - 105 = 176 ft fluid
Discharge static head = 50 ft
Total discharge head = 231 + 50 = 281 ft (*Note Friction Losses Due to Flow
that no flow friction losses
or entrance/exit losses are Friction, entrance and exit heads, valve losses
included in this example)
Suction pressure head +50 psig = + 115 ft water These losses and calculation methods were presented
(system fluid) in Chapter 2. Comments here will be limited. These loss-
186 Applied Process Design for Chemical and Petrochemical Plants
es are a function of the characteristics of the fluid flowing
in the piping systems and the velocities of flow. Entrance
and exit losses relate to the pipe and not the suction or
discharge connections at the pump. Usually they are very s
small, but cannot be ignored without checking. Velocity
heads at the pump connections are considered internal
losses. These are handled by the manufacturer's design of Pump
the pump and are not considered with the external losses
in establishing the pump heads. h5 = S- hSL + p
(al
h5 �-s-hSL+P
Example 3-2: Illustrating Static, Pressure, and Friction (bl
Effects Note: When P is expressed In absolute pressure units,
hs will be in absolute units. If P Is less than atmospheric
Refer to Figure 3-40F for basis of the example. pressure: Pis(-) If expressed as a gauge reading and
To aid in speed of computation, the friction figures will be a negative feet of liquid. P Is(+) If expressed In
absolute units. The friction loss hsL Includes any
are taken from the Cameron Hydraulic Tables in Chap- entrance or exit losses and other such fittings in the
ter 2 and use water, which is suited to these tables, as an system.
example fluid.
Discharge head = 60 ft Figure 3-41. Typical suction systems. (Adapted by permission,
Discharge pressure head = 26 psig Carter, A. and Karassik, "A.P.-477." Worthington Corp.)
(2.31 ft/psi) = 60 ft gauge
Discharge friction and exit head (at pipe/tank): pump centerline, and that it is decreased with an increase
140 ft of 8-in. pipe: 6.32 ft/100(140) = 8.8 ft in friction losses through the suction piping system. Thus,
3 8-in. goo ells: (6.32/100)(3)(20.2) 3.8
1 8-iu. gate valve 0.3 total suction head (TSH) = static head - hsL (3-6)
1 8-in. check valve 3.3
*Exit loss: Assume 8-in. pipe The total suction lift is defined as above except the
= vel hd 1.4 level of the liquid is below the centerline of the pump or
Subtotal, ft 17.6 the head is below atmospheric pressure. Its sign is nega-
Total discharge head 137.6 ft tive. Total Suction Lift (TSL) = static lift plus friction
Suction static head (lift) = -10.0 ft head losses.
Suction pressure head 0, psig (atmos) 0.0 In summary to clarify:
Suction friction and entrance head:
10 ft of 10-in. pipe, (2.1 ft/100) (10) 0.2 1. The pressure units (gauge or absolute) must be con-
1 l 0-in. suction go 0 ell; sistent for all components used in determining both
(2.l/100) (25.3) 0.5 suction side and discharge side conditions. Most
*En trance loss: l 0-in. pipe assume designers use gauge as a reference, but this is not
= vel head 0.6 necessary.
Subtotal -1.3 2. Static head is positive pressure of fluid on pump suc-
Total suction head = - 10 + (-1.3) = -11.3 ft tion above its centerline (S), ( +).
Total pump head r= 137.6 - (-11.3) = 148.9ft
3. Positive external pressure, P, on surface of fluid on
*These are not velocity heads at pump connections, pump suction is used as a positive integer, expressed
but are related to the piping connections. See earlier note as feet of fluid, ( +).
in this regard. 4. Partial vacuum, P, on the surface of liquid is a nega-
tive pressure. As a partial vacuum expressed as a
gauge reading as feet of liquid below atmospheric,
Suction Head or Suction Lift , h, the pressure is negative and would be designated by
a minus (-) sign. A partial vacuum, P, expressed as
The total suction head, Figure 3-41, is the difference in absolute vacuum or absolute pressure would be desig-
elevation between the liquid on the pump suction side nated by a positive ( +) sign. It is essential to be con-
and the centerline of the pump (plus the velocity head). sistent for all pressure units. If absolute units are
Note that the suction head is positive when above the used, the total suction head would be in absolute
Pumping of Liquids 187
units and the discharge head must be calculated in Discharge Head, hd
absolute units.
5. Suction lift is a negative suction head, S, used lo des- The discharge head of a pump is the head measured at
ignate a negative static condition on the suction of the discharge nozzle (gauge or absolute), and is com-
the pump (below atmospheric). The sign for suction posed of the same basic factors previously summarized; 1.
head is positive ( +), while its corresponding termi- static head 2. friction losses through pipe, fittings, con-
nology of suction lift is negative (-), since the term tractions, expansions, entrances and exits 3. terminal sys-
"lift" denotes a negative condition. Note that the only tem pressure.
difference in these terms is the difference in signs. Some typical discharge systems are given in Figure 3-
42. General practice is to express the terminal discharge
This applies because the total head for a pump is total pressure, P, at a vessel as in Figure 3-42 in terms of gauge
discharge head a(+), minus (-) the [suction head, pressure, and hence P = 0 for atmospheric discharge. If P
a(+)], or [suction lift, a(-)]. is less than atmospheric or otherwise expressed in
For general service the average centrifugal pump absolute units, then it must be added as equivalent feet of
should lift about 15 feet of water on its suction side. How- liquid to the value of hd ordinarily expressed as a gauge
ever, since each process situation is different, it is not suf- reading.
ficient to assume that a particular pump will perform the Figures 3-38 and 3-39 illustrate the use of siphon action
needed suction lift. Actually, certain styles or mode is of a in pump systems. Theoretically, the head in the siphon
manufacturer's pumps are often specially adapted to high should be recoverable, but actually it may not, at least not
lift conditions. On the other hand it is unnecessary to equivalent foot for foot. Usually not more than 20 feet of
select a high lift pump when pressure head or flooded siphon action can be included [ 4] even though 34 feet are
suction conditions prevail. Proper evaluation of suction theoretical at sea level. The siphon length is D' in the fig-
lift conditions cannot be over emphasized. ures [32]. For some systems the discharge head on the
The theoretical maximum suction lift at sea level for pump should be used as (D + D'), neglecting the siphon
water (14.7 psi) (2.31 ft/psi) = 34 ft. However, due to flow action. In any case, if air can be trapped in the loop, (and
resistance, this value is never attainable. For safety, 15 feet it usually can) it must be vented during start-up, otherwise
is considered the practical limit, although some pumps the pump will be pumping against the head established
will lift somewhat higher columns of water. When sealing using (D + D'). On start-up the flow can be gradually
a vacuum condition above a pump, or the pump pumps increased, making more head available from the pump to
from a vessel, a seal allowance to atmosphere is almost overcome the higher starting head of the system. This
always taken as 34 feet of water. High suction lift causes a should not be overlooked nor underestimated in deter-
reduction in pump capacity, noisy operation due to mining the specifications for the pump.
release of air and vapor bubbles, vibration and erosion, or
pitting (cavitation) of the impeller and some parts of the Velocity Head
casing. (The extent of the damage depends on the mate-
rials of construction.) Velocity head is the kinetic energy of a liquid as a result
of its motion at some velocity, v. It is the equivalent head
(EL)
Atmospheric
Pressure
(: __
Entran:: } -·-t-·-·t --t- t::l1
Pump Pump Pump
hd : D + hdL + p hd = D + hdL hd : D + hdL
(al (bl (cl
Note: '.or a system evaluation, includino suction and discharge, the units of P must be the same ,
either gage or absolute ,expressed as feet of fluid.
The friction losses from the pump to the vessel include ony entrance or exit losses. Unless
velocities ore high, these losses are usually negligible.
Figure 3-42. Typical discharge systems.
188 Applied Process Design for Chemical and Petrochemical Plants
in feet through which water would have to fall to acquire then the true total head = ( 45.5 + 4.5) - (8.6 + 1.4) =
the same velocity, expressed as foot-pounds per pound of 40 ft, and the difference in gauge readings would be 45.5
liquid. - 8.6 = 36.9 ft, or an error of 7.8%.
Most designers ignore the effects of velocity head, but
2
h, = v /2 g. feet of fluid 3-7 the above brief examples emphasize that the effect varies
depending on the situation and the degree of accuracy
where h, = velocity head, ft desired for the head determinations.
v = liquid velocity, ft/ sec
g = acceleration of gravity, fl/ sec-sec Friction
As a component of both suction and discharge heads, The friction losses for fluid flow in pipe valves and fit-
velocity head is determined at the pump suction or dis- tings are determined as presented in Chapter 2. Entrance
charge flanges respectively, and added to the gauge read- and exit losses must be considered in these determina-
ing. The actual pressure head at any point is the sum of tions, but are not to be determined for the pump
the gauge reading plus the velocity head, the latter not entrance or discharge connections into the casing.
being read on the gauge since it is a kinetic energy func-
tion as contrasted to the measured potential energy. The NPSH and Pump Suction
values are usually (but not always) negligible. Present
practice is for these velocity head effects at the pump suc- Net positive suction head (in feet of liquid absolute)
tion and discharge connections to be included in the above the vapor pressure of the liquid at the pumping tem-
pump performance curve and pump design, and need perature is the absolute pressure available at the pump
not be actually added to the heads calculated external to suction flange, and is a very important consideration in
the pump itself [5]. selecting a pump which might handle liquids at or near
It is important to verify the effects of velocity head on their boiling points, or liquids of high vapor pressures.
the suction and discharge calculations for pump selec- Do not confuse NPSH with suction head, as suction
tion. In general, velocity head (kinetic energy) is smaller head refers to pressure above atmospheric [17]. If this
for high head pumps than for low head units. Sometimes consideration of NPSH is ignored the pump may well be
the accuracy of all the other system calculations does not inoperative in the system, or it may be on the border-line
warrant concern, but for detailed or close calculations and become troublesome or cavitating. The significance
velocity head should be recognized. The actual suction or of NPSH is to ensure sufficient head of liquid at the
discharge head of a pump is the sum of the gauge reading entrance of the pump impeller to overcome the internal
from a pressure gauge at the suction or discharge and the flow losses of the pump. This allows the pump impeller to
velocity heads calculated al the respective points of gauge operate with a full "bite" of liquid essentially free of flash-
measurement. ing bubbles of vapor due to boiling action of the fluid.
Regardless of their density, all liquid particles moving The pressure at any point in the suction line must
at the same velocity in a pipe have the same velocity head never be reduced to the vapor pressure of the liquid (see
[ 11]. The velocity head may vary across a medium to large Equation 3-6). Both the suction head and the vapor pres-
diameter pipe. However, the average velocity of flow, that sure must be expressed in feet of the liquid, and must both
is, dividing the total flow as cu ft/sec by the cross-section- be expressed as gauge pressure or absolute pressure. Cen-
al area of the pipe is usually accurate enough for most trifugal pumps cannot pump any quantity of vapor, except
design purposes. possibly some vapor entrained or absorbed in the liquid,
Using the example of Reference [25], for a pump han- but do not count on it. The liquid or its gases must not
dling 1500 GPM, having a 6--inch discharge connection vaporize in the eye/ en trance of the impeller. (This is the
and 8-inch suction connection, the discharge velocity lowest pressure location in the impeller.)
head is 4.5 ft and the suction is 1.4 ft, calculated as shown For low available NPSH (less than 10 feet) the pump
above. If the suction gauge showed 8.6 ft, the true head suction connection and impeller eye may be considerably
would be 8.6 + 1.4 = 10.0. If the discharge head showed oversized when compared to a pump not required to han-
105.5 ft head, the true total head would be 105.5 + 4.5 = dle fluid under these conditions. Poor suction condition
110.0 ft, less (8.6 + 1.4) or 100 ft. The net true total head due lo inadequate available NPSH is one major contribu-
would be 110 ft - 10 ft = 100.0 ft. Looking only at the tion to cavitation in pump impellers, and this is a condi-
gauge readings, the difference would be 105.5 - 8.6 = tion at which the pump cannot operate for very long with-
96.9 ft, giving an error of 3.1 % of the Lota! head. As an out physical erosion damage to the impeller. See
alternate example, if the discharge head were 45.5 ft, References [11] and [26].
Pumping of Liquids 189
Cavitation of a centrifugal pump, or any pump, devel- worst possible operating conditions (see pump
ops when there is insufficient NPSH for the liquid lo flow curves Figures 3-36A, B, C) with pump curve values
into the inlet of the pump, allowing flashing or bubble for NPSH expressed as feet of liquid handled. These are
formation in the suction system and entrance to the the pump's required minimum NPSHR. The pump's
pump. Each pump design or "family" of dimensional fea- piping and physical external system provides the
tures related to the inlet and impeller eye area and available NPSHA.
entrance pattern requires a specific minimum value of
NPSH to operate satisfactorily without flashing, cavitating, NPSHA must be > NPSHR (3-8)
and loss of suction flow.
Under cavitating conditions a pump will perform 2. Internal clearance wear inside pump.
below its head-performance curve at any particular flow 3. Plugs in suction piping sysLem (screens, nozzles,
rate. Although the pump may operate under cavitation etc.).
conditions, it will often be noisy because of collapsing 4. Entrained gas (non-condensable).
vapor bubbles and severe pitting, and erosion of the 5. Deviations or fluctuations in suction side pressures,
impeller often results. This damage can become so severe temperatures (increases), low liquid level.
as to completely destroy the impeller and create excessive 6. Piping layout on suction, particularly tee-intersec-
clearances in the casing. To avoid these problems, the fol- tions, globe valves, baffles, long lines with numerous
lowing are a few situations to watch: elbows.
7. Liquid vortexing in suction vessel, thus creating gas
1. Have NPSHA available at least 2 feet of liquid greater entrainment into suction piping. Figure 3-43 sug-
than the pump manufacturer requires under the gests a common method to eliminate suction var-
Air/Vapor (non-condensed)
entrained
Desired Liquid Level
Liquid Level
Control
Actual Liquid
Flow Pattern �-�•'t-__.���,--J
---------- To Pump Suction
Note: (a) Dimension, "h", min.
of 5" to "h" = 1.25 x Nozzle Dia.
(b) Dimension "L" approx.
3.5 to 5 times Nozzle Dia.
Clearance, 2" min. to
usual 4-6", except
large nozzies require
more clearance.
(c) Bottom of vortex breaker may be attached to bottom or raised up 2" to 4". Vortex "cross" must be sturdy, welded of
heavy plate (not light sheet metal). Vortex breaker must not restrict liquid flow into nozzle opening, but prevent swirling
of liquid.
Figure 3-43. Liquid vortex in vessel and suggested design of vortex breaker.
190 Applied Process Design for Chemical and Petrochemical Plants
texing. Since the forces involved are severe in vor- Total Suction Lift (as water at 70°F) = NPSHA (calcu-
texing the vortex breaker must be of sturdy con- lated for fluid system) - 33 feet. The vapor pressure of
struc�i�n, firmly anchored to the vessel. water at 70°F is 0.36 psia,
8. Nozzle size on liquid containing vessel may create
severe problems if inadequate. Liquid suction veloc- Example 3-3: Suction Lift
ities in general are held to 3-6.5 feet per second.
Noz;le l�sses ar� important to recognize by identify- What is the Suction Lift value to be used with the pump
ing the exit design style (see Chapter 2). Usually, as curves of Figure 3-36A, if a gasoline system calculates an
a guide, the suction line is at least one pipe size larg- NPSH of 15 feet available:
er than the pump suction nozzle.
Total Suction Lift (as water) = 15 - 33 = -18 feet.
Therefore, a pump must be selected which has a lift of at
The NPSHA available from or in the liquid system on least 18 feet. The pump of Figure 3-36A is satisfactory
the suction side of a pump is expressed (corrected to using an interpolated Suction Lift line between the dotted
pump centerline) as: curves for 16 feet and 21 feet of water. The performance
of the pump will be satisfactory in the region to the left of
NPSHA = S + (p', - p'vp) - hsL (3-9) the new interpolated 18-foot line. Proper performance
should not be expected near the line.
NPSHA = S + (Pa - P,p) (2.31/SpGr) - hsL (3-10)
If the previous system were at sea level, consider the
same pump with the same system at an altitude of 6000
Where p'a or Pa represent the absolute pressure in the ves- feet. Here the barometric pressure is 27.4 feet of water.
sel (or atmospheric) on the liquid surface on the suction This is 34 - 27.4 = 6.6 feet less than the sea level instal-
side of the pump. lation. The new NPSHA will be 15 ft - 6.6 ft = 8.4 feet
P' or P represent the absolute vapor pressure of the available. Referring to the pump curve of Figure 3-36A it
,·p
vp
liquid at the pumping temperature. is apparent that this pump cannot do greater than 21 feet
hsL is the suction line, valve, fitting and other friction suction lift as water or 12 feet NPSHR of liquid (fluid).
losses from the suction vessel to the pump suction flange. Total Suction Lift as water = 8.4 - 33 = -24.6 feet.
Smay be ( +) or (-) depending on whether static head The pump curves show that 21 feet suction lift of water is
or static lift is involved in the system. all the pump can do, hence the 24.6 feet is too great. A
This available value of NPSHA (of the system) must different pump must be used which can handle this high
always be greater by a minimum of two feet and preferably a suction lift. Such a pump may become expensive, and it
three or more feet than the required NPSH stated by the may be preferable to use a positive displacement pump
pump manufacturer or shown on the pump curves in for this high lift. Normally lifts are not considered rea-
order to overcome the pump's internal hydraulic loss and sonable if over 20 feet.
the point of lowest pressure in the eye of the impeller.
The NPSH required by the pump is a function of the phys- Example 3-4: NPSH Available in Open Vessel System at
ical dimensions of casing, speed, specific speed, and type of Sea level, Use Figure 3-38
impeller, and must be satisfied for proper pump perfor-
mance. The pump manufacturer must always be given com- Conditions: at sea level, atmospheric pressure, Pa =
plete suction conditions if he is to be expected to recom- 14.7 psia.
mend a pump to give long and trouble-free service.
Assume liquid is water at 85°F, vapor pressure == P,'P =
As the altitude of an installation increases above sea
level, the barometric pressure, and hence p' a or Pa o.s psia.
decreases for any open vessel condition. This decreases Assume tank liquid level is 10 feet above center line of
the available NPSH. pump, then S = + 10 feet.
Figure 3-36A represents a typical manufacturer's per- Assume that friction losses have been calculated to be
formance curve. The values of NPSHR given are the min- 1.5 feet, hsL = l .5
imum values required at the pump suction. As men-
tioned, good practice requires that the NPSHA available Then: NPSHA available = S + (P. - P,,,) (2.31/SpGr) - hsL
be at least two feet of liquid above these values. It is impor- = + 10 + (14.7 - 0.6) (2.31/0.997) - 1.5
tant to recognize that the NPSHR and Suction Lift Values = 41.2 ft (good) (3-10)
are for handling water at about 70°F. To use with other liq-
uids it is necessary to convert to the equivalent water suc- Note: For worst case, which is an empt)' tank, "S"
tion lift at 70°F and sea level. becomes S.v on the diagram.
Pumping of Liquids 191
Example 3-5: NPSH Available in Open Vessel Not at Sea Then: NPSH available= S + (P 2 - P,'P)(2.31/SpGr) - hsL
Level, Use Figure 3-39 = -8 + (60 - 44) (2.31/0.58) - 12 = +43.8 feet (3-10)
Conditions: vessel is at altitude 1500 ft, where atmos- This presents no pumping problem.
pheric pressure is 13.92 psia = Pa,
Example 3-8: Closed System Steam Surface Condenser
Liquid: water at 150°F, vapor pressure P"P = 3.718 psia
SpGr = 0.982 NPSH Requirements, Use Figure 3-44
Assume vessel liquid level is 12 ft below centerline of This is a closed steam surface condenser system with
pump, SL= -12. condensate being pumped out to retreatment facilities.
Friction losses: assume calculated to be 1.1 ft of liquid. From the conditions noted on the diagram,
Then: NPSHA available = S + (Pa - Pvp) (2.31/SpGr) - hsL Friction loss in suction line side = 2.92 ft
= -12 + (13.92 - 3.718) (2.31/0.982) - 1.1 Absolute pressure in condenser= p' = 1.5 in. Hg Abs
= + 10.88 ft (3-10) = 1.5(1.13 ft/in. Hg)
= 1.71 ft water
The worst condition case should be calculated using Water from steam tables at saturation = 1.5 in. Hg Abs
S' L, since this represents the maximum lift. @ 91.72°F
Vapor pressure, p'vp, at 1.5 in. Hg Abs= l.5(1.13)
= 1. 71 ft water
Example 3-6: NPSH Available in Vacuum System, Use NPSH..,. available= + 10 + (1.71 - 1.71) - 2.92
Figure 3-41A
= +7.08 ft
Conditions: vessel is liquid collector at 28 in. Hg Vacu- The suction head or lift for the pump (separate calcu-
um (referred to a 30 in. barometer). This is 30 - 28 = 2 lation from NPSHA) is:
in. Hg abs, or Pa = [ ( 14. 7 /30)] (2) = 0.98 psia.
The 28.42 in. vacuum Hg (gauge) is equivalent to 1.5
Liquid: water at 101.2°F, vapor pressure = 0.98 psia. in. Hg Abs
Assume vessel liquid level is 5 feet above centerline of
pump, S = + 5', worst case, S,,. = 2' 28.42 in. vacuum (1.137) = 32.31 ft water
Static submergence = JO.O (see figure)
Friction losses: assume to be 0.3 foot of liquid
Friction zentrance losses = 2.92 ft
Net static submergence = 7.08 7.08 ft
Then: NPSHA available = S + (Pa - P,p) (2.31/SpGr) - hsL Equivalent suction lift = 25.23 ft [Note: 32.31
= + 5 + (0.98 - 0.98) (2.31/0.994) - 0.3
= + 4.7 ft (3-10) - 7.08]
( = vacuum effect less net submergence)
Worst case = l. 7 (not practical design)
The pump selected for this application (water boiling CONDENSER
at 0.98 psia) must have a required NPSH less than 4. 7 ft,
preferably about 3 tc 3.5 ft. This is a difficult condition. If Abs E 1.50" Hg
possible the vessel should be elevated to make more head Vacuum = 28.42" Hg
(S) available, which will raise the available NPSH.
Condensate
Example 3-7: NPSHA Available in Pressure System, Use 91.72° F
Figure 3-41 (b)
Conditions: vessel contains butane at 90°F and 60 psia
system pressure. Pa= 60 -
Butane vapor pressure, P,p at 90°F = L!4 psia, SpGr
= 0.58.
Assume liquid level is 8 feet below pump centerline, Figure 3-44. Surface condenser condensate removal. Closed sys-
S = -8. tem steam surface condenser NPSH requirements. (By permission,
Cameron Hydraulic Data, 16th ed. Ingersoll-Rand Co., 1979, p.
Friction losses: assume to be 12 ft of liquid. 1-12.)
192 Applied Process Design for Chemical and Petrochemical Plants
Note that the equivalent suction lift must be added High altitude venting
to the total discharge head for the pump system to
obtain the total system head. Keep in mind that the
work the pump must accomplish is overcoming the suc- P = 13.2 PSIA
tion losses ( + or - ) plus the discharge losses, that is, + l Low pressure water
discharge loss (all) - ( + if head, or - if lift on suction
losses, all). Thus, the suction lift becomes a (-) (-) or r
a ( +) to obtain the total system head. Keep in mind
that a vacuum condition on the suction of a pump P = 11.5 PSIA
200°FWater
never helps the pump, but in effect is a condition that
..
the pump must work to overcome. 0
II
(/)
Example 3-9: Process Vacuwn System, Use Figure 3-45
-
Friction/entrance type losses= 1', hsL
For this process example, again using waler for conve- .
nience, a low pressure, low temperature water is emptied
into a vented vessel, and then pumped lo the process at a
location at about 3000 feet altitude (see Appendix A-6)
where atmospheric pressure is approximately 13.2 psia.
Water SpGr is at 200°F = 0.963. Figure 3-45. High altitude process vacuum system, NPSH require-
ments.
Determine the NPSHA for pump:
Reductions in NPSHR
Limitations for use of the Hydraulic Institute NPSH
NPSHA = +S + (p. -- P"P)(2.31/SpGr) - h,1 reduction chart (Figure 3-46) are [17]:
=+IO+ (13.2 - 11.5)(2.3)/.963 - 1.0
NPSHA = + 13.07 ft available
1. NPSH reductions should be limited to 50% of the
NPSHR required by the pump for cold water, which
For hydrocarbons and water significantly above room is the fluid basis of the manufacturer's NPSHR
temperatures, the Hydraulic Institute [17] recommends curves.
the use of a correction deduction as given in Figure 3-'16. 2. Based on handling pure liquids, without entrained
This indicates that the required NPSH as given on the air or other non-condensable gases, which adversely
pump curves can be reduced for conditions within the affect the pump performance.
range of the curve based on test data. 3. Absolute pressure at the pump inlet must not be low
enough to release non-condensables of (2). If such
release can occur, then the NPSHR would need to be
If the pump given in the curve of Figure 3-36A were increased above that of the cold water requirements
being used to pump butane at 90°F and 0.58 gravity, to avoid cavitation and poor pump performance.
the correction multiplier from the NPSH curve is 4. For fluids, the worst actual pumping temperature
about 0.99 by interpolation. This means that the values should be used.
of Figure 3-36A should be multiplied by 0.99 to obtain 5. A factor of safety should be applied to ensure that
the actual NPSH the pump would require when han- NPSH does not become a problem.
dling a hydrocarbon of these conditions. The correction 6. Do not extrapolate the chart beyond NPSH reduc-
does not apply to other fluids. tions of 10 feet.
If the system pressure were 46 psia, then NPSHA Example 3-10: Corrections to NPSHR for Hot Liquid
available = -8 + (46 - 44) (2.31/0.58) - 12 = -12 Hydrocarbons and Water
feet, and this is an impossible and unacceptable condi-
tion. This means liquid will flash in the line and in the In Figure 3-46, use the dashed example lines at a tem-
impeller, and cannot be pumped. NPSH must always be perature of 55°F for propane [17), and follow the vertical
positive in sign. line to the propane vapor pressure dashed line, which
Pumping of Liquids 193
1000 .. � , ,
... ., ,
/ , � I
500 )' / � ... , ,J , '
II
400 , , i, , I/
,· ,J , i
,
300 r , �' �
,
f
., '/ / .JI i J ........ 10
...... � �, . I ... ... 9.5
A
200 r'
7 � � - ..... ..... -.� - - --- _ ... 8
/
-
__
150 • i - - ..... .... l..i'- ... Ir �� 6
z
"lt,. -
i., .....- � i- i.- v I'-- i.- ..... ...... I,.- � ..... i.,,i- """"' �loo - .. � 5 ....
....
-
- -�-
LL.I
......
- -�-
�
- --
100 ·- � � � - / ........ .... - loo.- -� '7' ,_i.,, _L.., """" 1, ,. ... .. i- � 4 LL.I
--
- - �
.......-
""' -
IJ...
,
""""
,
-
,__
L...-
-
z
L.--"' - -7 1.- � ":; , r i...,, - I.- .... -- -- -· ch
-
......--
......
�
� -
;:::
< ......-- __... - i- � � ..... - - v loo.- - - J _ ... j j """" ..... 3 0
--
u5 -- ""'� --- -: - -i,, ........ o
i,....--
__....
0. - � - - - , i-- - - - -'- ' ...... 2 =>
,J
,JO
c
,.
LL.I 50 _ .... � - ..,t ...... ...... ' i LL.I
___.J
I
0:: � - -- , I, i - J. --- '--"" ... 0::
-
=> 40 :..-- - .... 1.5
-
U) . ,_ �"' - , __ :I:
en - L. - � -?4 - - _., ' U)
LL.I 30 -- , ,/ - - - r - - -- 0.
-
,
0:: t.- - I- - - ...... ........ -� 1.0 z
« /
0. i--- �· -- � .....- -- - I _ ... .... - ..... .....
-
J. - -
....
/
0:: 20 ;}>� - ' ,� - i.- - ..... ... -
� -
� -� / ..... - I - - ,
-� ,/
< ,Y"'."A - ').: -
> 15 '/ -- -- r -- - - ' ........ ..
r -- -- .f� I ., I -- ....... ...... -- 0.5
��
�
�A
/
/
10 -- ,.. -A ,. ' - , --
-�-.,
-
,JO ,, . - - v
� --
1,11"' , / - - � ,, '
I.,..-
-
�
5 --- , ,I -- �v t '
v
'
�-
J
4 �· , i ,,
3 �, I
v /
I � •
2 - ... ,
1.5 , I , '
�' J I
1.0 I
0 50 100 150 200 250 300 400
TEMPERATURE F
Figure 3-46. NPSH reductions for pumps handling hydrocarbon liquids and high temperature water. (Note: do not use for other fluids.) (By
permission, Hydraulic Institute Standards for Centrifugal, Rotary, and Reciprocating Pumps, Hydraulic Institute, 13th ed., 1975.)
194 Applied Process Design for Chemical and Petrochemical Plants
reads 100 psia vapor pressure. Then follow the slant lines have similar (not necessarily identical) performance char-
(parallel) to read the scale for NPSH reductions, that is, acteristics. The three main characteristics of capacity,
feet at 9.5 ft. head, and rotative speed are related into a single term
Now the pump selected reads NPSHR on its pump per- designated "specific speed" [25]. The expression for spe-
formance curve of 12 feet for cold water service. cific speed is the same whether the pump has a single or
double suction impeller.
Now, ;1 of 12 ft = 6 ft The principle significance of specific speed for the
Figure 3-46 reads = 9.5 ft reduction process engineer is to evaluate the expected performance
Corrected value of NPSHR to use = 6 ft, since 9.5 ft is of a second pump in a particular manufacturer's series
> � the cold water value while basing it on the known performance (or curve) at
the point of optimum efficiency of a first and different
Example 3-11: Alternate to Example 3-10 size pump. In effect the performance of any impeller of a
manufacturer's homologous series can be estimated from
Assume that a boiler feed water is being pumped at 180 the known performance of any other impeller in the
F. Read the chart in Figure 3-46 and the water vapor pres- series, at the point of optimum efficiency. Figures 3-48
0
sure curve, and follow over to read NPSH reduction = and 3-49 represent the standardized conditions of essen-
0.45 feet. A pump selected for the service requires 6 feel tially all pump manufacturers.
cold water service NPSHR:
A typical "operating specific speed" curve is shown
� of 6 = 3 ft in Figure 3-50 and represents a technique for plotting
Value from chart for 180°F = 0.45 ft reduction the specific speed on the operating performance
Then correct NPSHR to use = 6 ft - 0.45 ft = 5.55 ft curve. Figure 3-50 represents a 6-inch pump operating
required by the pump at 1760 rpm, with maximum efficiency at 1480 GPM
for this service and 132 feet head [25]. The operating specific speed is
zero at no flow and increases to infinity at the maxi-
mum flow of 2270 gpm and zero head. Stable opera-
Specific Speed
tions beyond about 1600-1700 gpm cannot be planned
The specific speed of a centrifugal pump correlates the from such a curve with a sharp cutoff drop for head
basic impeller types as shown in Figure 3-47. capacity.
The formula for specific speed index number is: "Type specific speed" is defined as that operating spe-
cific speed that gives the maximum efficiency for a specif-
ic pump and is the number that identifies the pump type
N s = n{Q_/H 3/4 (3-11)
[25]. This index number is independent of the rotative
speed at which the pump is operating, because any change
where: Q is the GPM capacity at speed n in rpm and head H. in speed creates a change in capacity in direct proportion
H is the total head per stage, in feet. and a change in head that varies as the square of the speed
The principle of dynamical similarity expresses the fact [25]. Practice is to "true type" the specific speed of the
that two pumps geometrically similar to each other will pump reasonably close to the conditions of maximum effi-
0 0 0 0 00 0 0 8 0 0 0 0 0 00 0 0
0 0 0 0 00 0 0 0 0 0 0 0 00 0 0
an <D I'- a, a, q, 0 0 q 0 0 00
� <'i ,,; � 0 s ,..:- aS O'i cS' 0 0
iri'
�
0
Values of Specific Speed N
Impeller Shrouds
Radial -Vane Field Mixed- Flow Field Axial-Flow Field
Figure 3-47. Impeller designs and corresponding specific speed range. (By permission, Standards of the Hydraulic Institute, 10th ed.) Also
see [17], Hydraulic Institute, 13th ed., 1975.
Pumping of Liquids 195
600 400 300 200 150 100 80 60 50 40 30 20
, I J I I I
/ / I I ' I(
4000 / / I I /
Cl) I / / j I
Q. I I I I
:E I / i I I I
I
I I
I
:, / I I I I I I I I
Q. 3500 I I ' I I I I I i/
a: �, I I I )
.�, I
w VS/ I I I I I I
...I I(�,
..I _<:l I I �7 I I I I
w 3000 ., ,, ... I I I I
<,J I r:t
Q.
s --- s ) r;;J I rr I/ :::;1 I/ I
-
I �I
4. I
I
,._j �,
e ,_"/ lh 7 /� �v ) I
z 1.,."/ ) I ' (JJ I ./
::> v
:c 2500 [Y /. �I I �/ I .::,, '/ I
-
a: ' I' If ... , I >) I/ .t5j
w
> I I I �1 I ) � j ,i '/
0 h/ /"" I/ I 9 I/ '1
z v I ) / I ,._
J�l
j
0 /l I I I i:-I v I i{;v
-:,;,
� 2000 JW v I v I l
:, 1,/ J ii':'/ I
'"'
en I{� I I I w I !/
w /
I
...I 14 v v v, ...., J fl v
e �"f:J I/ ..
'
z :1 [/ I/ I I g I/
cii , .;:'J
a: v l v ,.t-'1 I/
v
0 1500 I/ II / ) I
� II / ,{)
}
J/ i) , II I J v I/
v
' 1111 II I I v ' I I I -
I/
J
v v
'#
:c
I/
j J
:E
Q.
'I
a:
II J I J v J I I I l I I
•
z v I I I I
Q 1000 / / J I I
w
w , I , I
Q. I I I I I
(I)
I I I
(.) 900 I I I I I ;
i:i:
u / I I ;
U,I 800 / I / I
Q. I/ i
(I) / J i
/ / i
/ t
700 I I
400 300 200 150 100 80 60 50 40 30 20
H = TOTAL HEAD IN FEET (FIRST STAGE)
Figure 3-48. Upper limits of specific speeds for single suction overhung impeller pumps handling clear water at 85°F at sea level. (By per-
mission, Hydraulic Institute Standards for Centrifugal, Rotary, and Reciprocating Pumps, Hydraulic Institute, 13th ed., 1975.)
196 Applied Process Design for Chemical and Petrochemical Plants
20000 lOO 50 40 30 20 15 10 8 7 6 5 4
I v I I I I v I
I I I I / I J
I v/ I I I
I I
I I/ V I I I I I
-
I I I I I I I
15000 I j j
I I I I I
' I I I I I
en ·; ;, I I I I I I/
a. �
:E � v � I I/ I /
::) �/ /� !:-- I
a. ' i.r: I �/ I �/ j I
3 ! 1:-i
0 I SI � '� v I � IJ
.... 1; I I ,:i; i-fli) I I f/ j
.J
.J ,:;, / i4'J
-e "#. I 'i; ,; ) � '/ j
)( "> :c � I
-e :E 10000 l?l-« I ii;
a.
I
Ci a: I I ,, I r, I
z II. ,� �
""
-e II "' J -,, , .. I I ·- I
I
1
•
3 z 9000 I I I I I I I .... i I I
0 I I , I I
Q
.... .... J J I I j .... I
I
.J
t, I
....
.... a. 8000 I I I I I j I j I I
Ci
I- I
)( en . ,.; I I I I
- u II ,I /11.,,i I
'
:E
z i. '4fl T7 'Vi I I I I
I
0 � 7000 I I ;. :, I I j
I/
ti a. � i/ I I I
en
::) �) I ) " _, I I
en ,� I I 1
"
w 'r..i'l I c �) j .. ,
�/
.J 6000 �,,
(!J I J. ) I I J.::.'/ I
z ) v I
en � J .. , I I I 4 I 0
:,
r�
�, / I I -ii: I :::5/
II. I b1 I � J
5000 /
I I I I - I I � I
ii I I I ) ,. '-1 j tj
I ...;, I
I I �/
� I
I I j � if
I I I I
4000 / !/ J
,___ ---
------
--- - I- - -1/.-- ------ - --- -
I I __ ,_ -
t--1/ ---- - - i,.--- ·-
. -J f--- --- ---
3500 I/ I -·- ...._ - -- -- ------ �-- ... -
100 50 40 30 20 15 10 8 7 6 5 4
H = TOTAL HEAD IN FEET (FIRST STAGE)
Figure 3-49. Upper limits of specific speeds for single suction, mixed and axial flow pumps handling clear water at 85°F at sea level. (By per-
mission, Hydraulic Institute Standards for Centrifugal, Rotary, and Reciprocating Pumps, Hydraulic Institute, 13th ed., 1975 [17).)
Pumping of Liquids 197
fixed condition of suction lift, and relates speed, head
' I ii and capacity. This index is a valuable guide in establishing
"'1" I the maximum suction lifts and minimum suction heads to
1e t-r avoid cavitation of the impeller with resultant unstable
I
hydraulic performance and physical damage. For a given
--;;�icSl'f.f.0
� - - set of conditions on the suction and discharge of a pump,
01,-- I a slow rotative speed will operate safer at a higher suction
lift than a pump of higher rotative speed.
IIO - Rotative Speed
HEAD-CAPACITY ,,. ,........,
llO ' \\""
The rotative speed of a pump is dependent upon the
--p< r--,.....
.., l40 .>./ \ impeller characteristics, type fluid, NPSH available and
�
...
...
l'OIIIT �
i,.,,-- '�
:! L "' ,.� #-- IIAill- � � I \ other factors for its final determination. The most direct
["1CI[..,
,,.,.....
;,::." ·-- 4J, ' \ method is by reference to manufacturer's performance
d
.., J ,,,,, ..., K"' "'" '\ curves. When a seemingly reasonable selection has been
I
c
I
..,
:r:. IO L.- I I " \ 40 made, the effect of this selected speed on the factors such
e eo I I I L as NPSH required, suction head or lift, fluid erosion and
c
40 J 11 I ZO.:- corrosion, etc., must be evaluated. For many systems these
z
to I I I 10• factors are of no concern or conseqt,ence.
I I Normal electric motor speeds run from the standard
0 I 0 induction speeds for direct connection of 3600, 1800 and
0 ! 4 I (! 10 12 14 II 111 ZO 22
1200 rpm to the lower speed standards of the synchro-
CAPACITY, !N 100 CiPM
nous motors, and then lo the somewhat arbitrary speeds
Figure 3-50. Typical centrifugal pump characteristic curve with aux- established by V-belt or gear drives. For some cases, the
iliary specific speed curve. Double-suction, single-stage, 6-in. pump speed is set by the type of drivers available, such as
pump, operating at 1760 rpm constant speed. (By permission, a gasoline engine.
Karassik, I. and Carter, R., Centrifugal Pumps, McGraw-Hill Book Electric motors in pump application never run at the
Co., inc., 1960, p. 197.)
"standard" rotative design speeds noted above, but rotate
at about (with some deviation) 3450, 1750, and 1150 rpm,
ciency. Figure 3-4 7 illustrates the range of typical specific which are the speeds that most pump manufacturers use
speed index numbers for particular types of impellers. for their performance curves. If the higher numbers were
used (motor designated or name plate) for pump perfor-
Example 3-12: "Type Specific Speed" mance rating, the pumps would not meet the expected
performance, because the motors would not be actually
In Figure 3-50, where the pump operates at 1760 rpm rotating fast enough to provide the characteristic perfor-
(a standard motor speed under load) and has maximum mance curves for the specific size of impeller.
efficiency at 1480 GPM and 132 feet head, the "type" spe-
cific speed is Pumping Systems and Performance
It is important to recognize that a centrifugal pump will
(3- 11) operate only along its performance curve [10, 11). External
conditions will adjust themselves, or must be adjusted in
order to obtain stable operation. Each pump operates within
[1760'\ � ] a system, and the conditions can be anticipated if each
N 1740
s
(132)0,o component part is properly examined. The system con-
sists of the friction losses of the suction and the discharge
piping plus the total static head from suction to final dis-
Figure 3-47 indicates the general type of impeller charge point. Figure 3--51 represents a typical system head
installed.
curve superimposed on the characteristic curve for a 10
The specific speed of a given type pump must not by 8-inch pump with a 12-inch diameter impeller.
exceed the specific speed values presented by the Depending upon the corrosive or scaling nature of the
Hydraulic Institute [17]. This is based on a known or liquid in the pipe, it may be necessary to take this condi-
198 Applied Process Design for Chemical and Petrochemical Plants
45
I I I I
10 Hp.'\ 15 Hp.
I "l
40 '\ I Size Pump'. IO"X 8" -
Speed: 860 Rpm.
'\I'\. Efficiency "-."'
'\ 10 � - 7_o --....._ -,
35 I'\._ I i-.00,
/
12" Dia. Impeller, New Condition � /,' 1 ,operating Points with
·r
Possible Opttalion ott-l--!-S:::::� �-}- � ew 1lmp � lle � H 1 1
Worn Impeller I �� ::::-hJ. I
""'1,..,......_
e q'
I
.!: I.��� • ;:�, � -Operating Points,with Worn Impeller
..,
: 25 ""'' •\\� '.#' I ...... --
� �
::c % \<>�. N <, <,
< r-,
c, <y\l\ 11t"r I I ._ ... ,.
\e\l'l � e\l'I �,. Suction plus Discharge Friction ·, .........
� 20
.---""-1�Jr I I
'""::::;; � System Static Head
15
10
5 o 200 400 600 800 1,000 1,200 1,400 1,600 1,800 2,000 2,200
Capacity, Gpm.
Figure 3-51. System head curves for single pump installation.
tion into account as indicated. Likewise, some pump ing valves, adding control valves, or decreasing resis-
impellers become worn with age due to the erosive action tance by opening valves or making pipe larger, etc.
of the seemingly clean fluid and perform as though the
impeller were slightly smaller in diameter. In erosive and For the system of Figure 3-39, the total pumping head
other critical services this should be considered al the requirement is
time of pump selection.
Considering Figure 3-39 as one situation which might H = (D + hod - [-SL+ (-hsLJJ = (D + hod
apply to the system curve of Figure 3-51 the total head of + (SL+ hsd (3-13)
this system is:
The total static head of the system is [D - (-S)] or (D +
H = D + hoL - (-SL - hsd (3-12) S) and the friction loss is still hoL + hsL, which includes
the heat exchanger in the system.
The values of friction loss (including entrance, exit loss- For a system made up of the suction side as shown in
es, pressure drop through heat exchangers, control valves Figure 3-41 (a) and the discharge as shown in Figure 3-
and the like) are hsL and hoL· The total static head is D - 42(a), the total head is
SL, or [ (D + D') - (-SL)] if siphon action is ignored, and
[ (D + D') - (S' L)] for worst case, good design practice. (3-14)
Procedure:
where P2 is used to designate a pressure different than P1.
1. Calculate the friction losses hsL and hoL for three or The static head is [ (D + P1) - (S + P2)], and the friction
more arbitrarily chosen flow rates, but rates which head is hoL + hsL·
span the area of interest of the system. Figure 3-52 illustrates the importance of examining the
2. Add [hsL + hoL + (D = S)] for each value of flow system as it is intended to operate, noting that there is a
calculated. These are the points for the system head wide variation in static head, and therefore there must be
curve. a variation in the friction of the system as the GPM deliv-
3. Plot the GPM values versus the points of step 2, above. ered lo the tank changes. It is poor and perhaps erro-
4. The intersection of the system curve with the pump neous design to select a pump which will handle only the
impeller characteristic curve is the operating point average conditions, e.g., about 32 feet total head. Many
corresponding to the total head, H. This point will pumps might operate at a higher 70-foot head when
change only if the external system changes. This may selected for a lower GPM value; however, the flow rate
be accomplished by adding resistance by partially dos- might be unacceptable to the process.
Pumping of Liquids 199
Example 3-13: System Head Using Two Different Pipe
Sizes in Same Line 50 I I I
45 Condition
The system of Figure 3-53 consists of the pump taking 40 , Impeller Operating Head Curve ,,,, .. Friction + 22' Static Head _
suction from an atmospheric tank and 15 feet of 6-inch 35 lL .._ - / t .r 1__, __ 1 .1_
Friction + 15' Static Head
pipe plus valves and fittings; on the discharge there is 20 J 30 - � ,::;.- _..-.::::: Operating Points for System
20 -
feel of 4-inch pipe in series with 75 feet of 3-inch pipe plus .5 �..,..,... t- ... t <, . I
a control valve, block valves, fittings, etc. The pressure of i 25 :...- -- .,....,.. ...
:r
the discharge vessel (bubble cap distillation tower) is 15 15 - � vi"
i.--
psig. Using water as the liquid at 40°F
Suction: 6-inch pipe (using Cameron Tables-Table 2- 110 ......- t> I'" System Friction (See Figure 3-51)
22). To simplify calculations for greater accuracy, use 5 � .,....,.. I I I I I I I i
�
detailed procedure of Chapter 2. 0 - !
0 10 20 30 40 50 60 70 80 90 100 110 120 130 uo 150 160
System Flow, Gpm.
Pipe or Figure 3-52. System head curves for variable static head.
Fitting Loss 200 GPM 300 GPM
Loss, ft/100 ft 0.584 1.24 15 psi;
For 15 ft 15.0 fl
Two, 900 ell, eq. 22.8 A tmos!)heric Bubble Cop
Gate valve, open __1,L Vent Distillation
Total 41.0 ft 0.24 ft. 0.51 ft Tower
The total suction head = h, = +7 - 0.24 = +6.76 at
200 gpm
h, = +7 ·- 0.51 = +6.49 at 300 GPM 150 Discrepancy due lo ....,
Variation in Friction, .,.,-'
Discharge: Lou Doto.,�
/
.... " ,it'
1:,
�<t°',"' ,_,...,\
�.... 'ti:,
100 x C:,'\ � ':-_ .,._\ � °''t\CIC)j
4 inch pipe 3 inch pipe :: -<,o.'°'
200 GPM 300 GPM 200 GPM 300 GPM ....
c:
..
Loss, ft/ l OC ft 4.29 9.09 16.1 34.1 "O 3"
0
For 20 ft 20.0 20.0 :I: 50
For 75 fl 75.0 75.0
Two, 3", 90° ells, eq. 8 8
One, 4", 90° ell, eq. 4.6 4.6
\Oischorgel .;"
friclion
One Gate Valve open L. 7 fl 1.7 ft o���==::::::::::::===�
Total, equivalent ft 24.6 24.6 84.7 84.7
Friction loss, ft fluid 1.06 2.23 i3.6 28.8 0 100 200 300
Control valve al 60% Gallons per Minute
of total, ft fl 1.59 3.34 20.4 43.2 Figure 3-53. System head using two different pipe sizes in same line.
Total discharge friction
loss, ft 2.65 5.57 34.0 72.0
Total head on pump at 300 GPM =
Total static head= 45 - 7 + 15(2.31) = 72.6 ft, SpGr = 1.0 H = 45 + 15(2.31) + 5.57 + 72.0 - 7 + 0.51 = 150.68 ft
Composite head curve
at 200 GPM: head = 72.6 + 0.24 + 2.65 + 34.0 = 109.49 ft The head at 200 GPM (or any other) is developed in the
at 300 GPM: head = 72.6 + 0.51 + 5.57 + 72.0 = 150.68 ft same manner.
200 Applied Process Design for Chemical and Petrochemical Plants
Example 3-14: System Head for Branch Piping with curves. The final Total System curve is the friction of (B-
Different Static Lifts P-C) + (C-E) + (C-D) plus the head, a. Note that liquid
will rise in pipe (C-D) only to the reference base point
The system of Figure 3-54 has branch piping discharg- unless the available head is greater than that required to
ing into tanks at different levels [13]. Following the dia- flow through (C-E), as shown by following curve (B-P-C)
gram, the friction in the piping from point B to point C is + (C-E) + a. At point Y, flow starts in both pipes, at a rate
represented by the line B-P-C. At point C the flow will all corresponding to the Yvalue in GPM. The amounts flow-
go to tank E unless the friction in line C-E exceeds the sta- ing in each pipe under any head conditions can be read
tic lift, b, required to send the first liquid into D. The fric- from the individual System Curves.
tion for the flow in line C-E is shown on the friction curve, The principles involved here are typical and may be
as is the corresponding friction for flow through C'.,-D. applied to many other system types.
When liquid flows through both C-E and C-D, the com-
bined capacity is the sum of the values of the individual
curves read at constant head values, and given on curve
(C-E) + (C-D). Note that for correctness the extra static
head, b, required to reach tank D is shown with the fric- Relations Between Head, Horsepower, Capacity, Speed
tion head curves to give the total head above the "refer- Brake Horsepower Input at Pump
ence base." This base is an arbitrarily but conveniently
selected point.
BHP= QH(SpGr)/(3960e) (3-15)
The system curves are the summation of the appropri-
ate friction curves plus the static head, a, required to where e is the pump efficiency, fraction.
reach the base point. Note that the suction side friction is
represented as a part of B-P-C in this example. It could be \i\Tater or liquid horsepower [25)
handled separately, but must be added in for any total
whp = QH(SpGr)/3960 (3-16)
The difference between the brake horsepower and the
water or liquid horsepower is the pump efficiency. The
requirement in either case is the horsepower input to the
shaft of the pump. For that reason, the brake horsepower
represents the power required by the pump, which must
g·feet of l" m·feet of n"pipe be transmitted from the driver through the drive shaft
c through any coupling, gear-box, and/or belt drive mech-
anism to ultimately reach the driven shaft of the pump.
Therefore, the losses in transmission from the driver to
the pump itself must be added to the input requirement
of the driven pump and are not included in the pump's
System Curves
brake horsepower requirement.
Pump efficiency [ 17) =
liquid HP (energy delivered by pump to fluid)
Curves Developed ot Points (3-17)
of Equol head when brake HP (energy to pump shaft)
Combining Individual Ports
of System
Overall efficiency [ l 7] =
\VI-IP ( energy delivered by pump to fluid)
(3-18)
el-IP (energy supplied to input side of pump's driver)
a
where eHP = electrical horsepower
WHP = liquid horsepower
Gallons per Minute
For the rising L)'Pe characteristic curve, the maximum
Figure 3-54. System head for branch piping with different static brake horsepower required to drive the pump over the
lifts. entire pumping range is expressed as a function of the
Pumping of Liquids 201
brake horsepower at the point of maximum efficiency for This condition would require a brake horsepower from
any particular impeller diameter [7]. the pump curve between 7.5 and 10, that is, about 9.25
BHP for the pump's input shaft (for water calculates at
BHP (max.) = 1.18 (BHP at max. efficiency point) (3-19) 9.03 BHP), estimating the spread between 7.5 and l 0.
Thus a 10 hp (next standard size motor) would be
Unless specifically identified otherwise, the BHP values required, and this would satisfy the original condition and
read from a manufacturers performance curve represent the the second condition for water. It would still be satisfacto-
power only for handling a fluid of viscosity about the same ry for any fluid with a specific gravity < I.O, but if pump-
as water and a specific gravity the same as water, i.e., SpGr = ing a liquid of 1.28 SpGr ( ethyl chloride, for example),
1.0. To obtain actual horsepower for liquids of specific then (1), the original BHP would need to be 1.28(5.75) =
gravity other than 1.0, the curve values must be multiplied 7.36 BHP, and (2), the second condition would require
by the gravity referenced to water. Viscosity corrections 1.28(9.25) = 11.84 BHP (calculates 11.56). Whereas, a 10-
are discussed in another section. Good design must allow hp motor would be non-overloading for the water pump-
for variations in these physical properties. ing case, it would require a 15-hp (next standard above a
10 hp) motor direct drive to satisfy the ethyl chloride case
Driver Horsepoioer under the 160 GPM condition.
If you do not select a non-overloading motor, and vari-
The driver horsepower must be greater than the calcu- ations in head and/ or flow occur, the motor could over-
lated ( or value read from curves) input BHP to the shaft heat and stop operating. Study the pump-capacity curve
of the pump. The mechanical losses in the coupling, V- shape to recognize the possible variations.
belt, gear-box, or other drive plus the losses in the driver
must be accounted for in order that the driver rated Important note: Any specific pump impeller operating in
power output will be sufficient Lo handle the pump. a physical (mechanical) system will only perform along its
Best practice suggests the application of a non-over- operating characteristic curve. If there is a change in the
loading driver to the pump. Thus a motor rated equal to system flow characteristics (rate or friction resistance or
or greater than the maximum required BHP of the pump, pressure head), the performance will be defined by the
assuming no other power losses, would be non-overload- new conditions and the pump performance will "slide"
ing over the entire pumping range of the impeller. It is along its fixed curve. Thus, the designer cannot arbitrari-
important to examine the pump characteristic curve and ly pick a point and expect the pump to 'Jump" to that
follow the changes in power requirements before select- point. Refer to Figure 3-36A. Using a 6-inch impeller
ing a driver. curve, for example, the designer cannot make this pump
For example, referring to Figure 3-36A, if your pump operate at a point of 100 GPM and 150 feet head. This
were selected with a 6-inch diameter impeller for a rated would require about a 6X-inch diameter impeller. The 6-
normal pumping of 100 GPivl, the pump would put out inch curve will only put out 138 feet (approx.) at the
about 138 feet of head of any fluid ( neglecting viscosity intersection of 100 GPM and the 6-inch curve.
effects for the moment). The intersection of the 100-GPM A driver selected to just handle the power require-
vertical line with the 6-inch performance curve would ments of the design point ( other than maximum) is usu-
indicate that 5.75 brake horsepower (hp) would be ally a poor approach to economy. Of course, there are
required/or water (between 5 hp and 7.5 hp). Therefore, applications where the control system takes care of the
to be non-overloading (that is, the motor driver will not possibilities of power overload.
overheat or lose power) at this condition would require a
7.5 horsepower motor (if no other losses occur between
driver and pump), because there is no standard motor for Affinity Laws
direct connected service between the standard 5 and 7.5 The affinity laws relate the performance of a known
hp. Now, if you know or project that you may need at pump along its characteristic curve to a new performance
some time to pump 160 GPM of any fluid with this pump curve when the speed is changed. This would represent
at 160 feet head, then (l) this pump could not be used the same "family" of pump curves. As an example, see Fig-
because it will not physically take an impeller larger than ures 3-36A, B, and C.
6.5-inch-diameter. However, recognizing this, (2) if you
change the external physical piping, valves, etc., and l. For change in speed with a geometrically similar
reduce the head Lo fit the 6.5-inch impeller curve, at 160 family of fixed impeller design, diameter and efficiency,
GP�·I, you could handle 152 feet head ( estimated from the following conditions and characteristics change simul-
the curve for a 6.5-inch impeller). taneously [25]:
202 Applied Process Design for Chemical and Petrochemical Plants
�-tr�.,, 'J <,
(3-20) 60
�·
I
I\
>,
vs.
(3-21) �� �<:/. \2\ Efficiency 50 �
u
>---- ��� Cl ' ' Capacity >----- 40 �
/, ,�
:�
I '
(3-22) � I ' \ 30 �
20
Ill
For a fixed speed [25]:
100
�o
(3-23) 90 "'9.o.,,
80
(3-24) "'" \
70 � 11.5:
�·
� ,i> ' I Head
vs .
°:: 60 .o.,, ,-
(3-25) II. Capacity
c; K I \
; 50 '�2 \
"' ...
,__11.,-0
For geometrically similar impellers operating at the :i:: 40 -
same specific speed, the affinity laws are [25,11]: � \ \
30 I 7.5 j:
I I_\- f--1 ..
0
Q.
20 - 1'50 ?.\l"'" Broke - - �
5
0
(3- 26) � � :I:
450�
\,
10 ---- ""W'")-- 2.5 !:
-
Coprity
0 o - � O ID
120 160 200 240 280
40 80
(3- 27) Capacity, Gpm.
Figure 3-55. Relation of speed change to pump characteristics.
BHP 2
(3- 28)
BHP 1
2. For changes (cut-down) in impeller diameter (not
design) at fixed efficiency: [ 11]
where: condition of subscript (2) represents the new non-
cavitating or desired condition, and condition of sub-
script (I) represents the condition for which a set of con- (3- 29)
ditions are known.
These relations do not hold exactly if the ratio of speed
change is greater than 1.5 to 2.0, nor do they hold if suc-
tion conditions become limiting, such as NPSH. (3- 30)
Figure 3-55 illustrates the application of these perfor-
mance laws to the 1750 rpm curves (capacity, brake horse-
power, and efficiency) of a particular pump to arrive at (3- 31)
the 1450 rpm and 1150 rpm curves. Note that the key
value is the constant efficiency of points (1) and (2).
When the speed drops to 1450 rpm, capacity drops: where d, is the original impeller diameter in inches, and
subscript (2) designates the new or desired conditions cor-
Q 2 = 204(1450/1750) = 169 GPM responding to the new impeller diameter, d 2. All perfor-
mance changes occur simultaneously when converting from
condition (1) to condition (2), no single condition can be
The head also drops:
true unless related to its corresponding other conditions.
An impeller can be cut from one size down to another
H2 = 64(1450/1750) 2 = 44 ft
on a lathe, and provided the change in diameter is not
greater than 20 percent, the conditions of new operation
and: can be described by the type of calculations above. A cut to
reach 75-80 percent of the original diameter may adverse-
(BHP) 2 = 6.75(1450/1750) = 3.84 BJ-JP ly affect performance by greatly lowering the efficiency [ 4].
3
Pumping of Liquids 203
Most standard pump curves illustrate the effect of When the performance of a pump handling water is
changing impeller diameters on characteristic perfor- known, the following relations are used to determine the
mance (Figure 3-36A). Note change as reflected in the performance with viscous liquids [17]:
different impeller diameters. However, the slight change
in efficiency is not recorded over the allowable range of (3-32)
impeller change.
Recognizing the flexibility of the affinity laws, it is bet- (3-33)
ter to select an original pump impeller diameter that is
somewhat larger than required for the range of anticipat- (3-34)
ed performance, and then cut this diameter down after
in-service tests to a slightly smaller diameter. This new per- BHP,i, = (Qvi,) (l-Ivis) (SpGr)/3960(£,i,) (3-35)
formance can be predicted in advance. Once the impeller
diameter is too small, it cannot be enlarged. The only Determine the correction factors from Figure 3-56 and
solution is to order the required large impeller from the Figure 3-57, which are based on water performance
manufacturer. because this is the basis of most manufacturer's perfor-
mance curves (except, note that the "standard" manufac-
Example 3-15: Reducing Impeller Diameter at Fixed turer's performance curves of head vs GPM reflect the
RPM head of any fluid, water, or other non-viscous). Do not
extrapolate these curves!
If you have a non-cavitating (sufficient NPSH) operat- Referring to Figure 3-56 [ 17]:
ing 9-inch impeller producing 125 GPM at 85 feet total
head pumping kerosene of SpGr = 0.8 at 1750 rpm using 1. The values are averaged from tests of conventional
6.2 BHP (not motor nameplate), what diameter impeller single-stage pumps, 2-inch to 8-inch, with capacity at
should be used to make a permanent change to 85 GPM best efficiency point of less than 100 GPM on water
at 60 feet head, at the same speed? performance.
2. Tests use petroleum oils.
Q2 = Q1 (cl2/d1) (3-23) 3. The values are not exact for any specific pump.
85 = 125(cl2/9)
d2 = 6.1 in. diameter (new) Referring to Figure 3-5 7 [ 17]:
The expected head would be 1. Tests were on smaller pumps, l-inch and below.
2. The values are not exact for any specific pump.
1)
H2 = Ht(d2/d 2 (3-24)
H2 = 85(6.1/9)2 The charts are to be used on Newtonian liquids, but
= 39.0 ft (must check system new total head Lo deter- not for gels, slurries, paperstock, or any other non-uni-
mine if it will satisfy this condition.) form liquids [17].
Figure 3-56 and 3-57 are used to correct the perfor-
The expected brake horsepower would be mance to a basis consistent with the conditions of the
usual pump curves. In order to use the curves, the fol-
2
1
BHP2 = BHP (d /d 1)3 (3-25) lowing conversions are handy:
BHP2 = 6.2(6.1/9) 3
= 1.93 BHP (use a 2- or 3-hp motor) Centistokes = centipoise/SpGr
t]fects of Viscosity SSU = Saybolt Seconds Universal
= (Centistokes) ( 4.620) at 100°F
When viscous liquids are handled in centrifugal pumps, = (Centistokes) ( 4.629) at 130°F
the brake horsepower is increased, the head is reduced, = (Cernistokes) (4.652) at 210°F
and the capacity is reduced as compared to the perfor-
mance with water. The corrections may be negligible for Example 3-16: Pump Performance Correction For
viscosities in the same order of magnitude as water, but Viscous Liquid
become significant above 10 centistokes ( l O centipoise for
SpGr = 1.0) for heavy materials. While the calculation When the required capacity and head are specified for
methods are acceptably good, for exact performance a viscous liquid, the equivalent capacity when pumping
charts test must be run using the pump in the service. (text continued on page 206)
204 Applied Process Design for Chemical and Petrochemical Plants
�I 80
u,
a: 60
0 >
.... (J
(J z 100
� LI.I
u. u
z i:i:
0 u.
j::: w 80
(J Q
a: z
LI.I
a: �
0
(J � 60
u
�
A.
�
(J 40
-
LI.I
0
�
....
u,
....
u,
a:
i:i:
tu 600
400
LI.I 300
u. 2og
- 100
15
z
80
Q 60
� 40
LI.I
::c lg
1 2 4 6 8 10 15 20 40 60 80 100
CAPACITY IN 100 GPM
Figure 3-56. Viscosity performance correction chart for centrifugal pumps. Note: do not extrapolate. For centrifugal pumps only, not for axial
or mixed flow. NPSH must be adequate. For Newtonian fluids only. For multistage pumps, use head per stage. (By permission, Hydraulic Insti-
tute Standards for Centrifugal, Rotary. and Reciprocating Pumps, 13th ed., Hydraulic Institute, 1975.)
Pumping of Liquids 205
l.00
.90
.80 §
en .70.
Q: �-
.60
�
z .50
0
j::
(.) .40
l&I
Q: Reproduced with permission of
Q:
8 .30 INGERSOLL-RAND COMPANY .
. 20
.10
b
.,(fl
?,(fl
,tfl
.... .,o.
'\.(fl
t:i ¥P
... ffJ
l&I
� t,O
Q ?,0
<
l&I
::c ,f)
'-"
'\.o
ii
b
I
10 15 20 25 30 50 60 70 80 90 100
CAPACITY-GALLONS PER MINUTE (at bep)
Figure 3-57. Viscosity performance correction chart for small centrifugal pumps with capacity at best efficiency point of less than 100 GPM
(water performance). Note: Do not extrapolate. For small centrifugal pumps only, not for axial or mixed flow. NPSH must be adequate. For
Newtonian fluids only. For multistage pumps, use head per stage. (By permission, Hydraulic Institute Standards for Centrifugal, Rotary, and
Reciprocating Pumps, 13th ed., Hydraulic Institute, 1975.)
206 Applied Process Design for Chemical and Petrochemical Plants
(text contin ued from page 203) 7. Calculate Brake Horsepower for viscous liquid
water needs to be determined using Figure 3-56 or 3-57 in
order to rate pump selection from manufacturer's curves.
Qvi, H ,;, (SpGr)
Determine proper pump selection and specifications (BHP),.i, 3960(e vis ) (3- 35)
when pumping oil with SpGr of0.9 and viscosity of25 cen-
tipoise at the pumping temperature, if the pump must (125) (86) (0. 9)
deliver 125 GPM at 86 feet total head (calculated using ------ = 4. 3 B. horsepower
(3960) (0.568)
the viscous liquid).
Viscosity conversion: Example 3-17: Corrected Performance Curves for
Viscosity Effect
Centistokes = 25/0.9 = 27.8
When a pump performance is defined for water, the
corrected performance for a viscous fluid can be devel-
Referring to Figure 3-56: oped using Figure 3-56 or 3-57. In order to develop the
curves for viscosity conditions of 100 SSU or 1,000 SSU as
shown in Figure 3-58, the following general procedure is
1. Enter capacity at 125 GPM, follow vertically to 86 used [17].
feet of head, then to right to viscosity of 27.8 centi-
stokes, and up to correction factors:
1. Starting with performance curve based on pumping
water:
Efficiency, CE = 0.80 a. Read the water capacity and head at peak efficien-
Capacity, CQ = 0.99 cy. This capacity is the value of (1.0 Q.,w).
Head, CH= 0.96 (for 1.0 QN),
QN = head at best efficiency point b. Using this value of GPM, calculate 0.6, 0.8 and 1.2
times this value, giving 0.6 Qnw, 0.8 Qnw and 1.2
Qnw respectively, and read the corresponding
Note this represents a flow rate using water under maxi- heads and water efficiencies.
mum efficiency conditions [ 1 7].
2. Using Figure 3-56 or 3-57 enter GPM at value corre-
sponding to peak efficiency, 1.0 Qnw, and follow up
2. Calculate approximate water capacity: to the corresponding head value, Hw, then move to
the viscosity value of the liquid, and up to the cor-
Qw = Q.i,/CQ (3-32) rection factors CE, CQ, CH.
Qw = 125/0.99 = 126.3 3. Repeat step 2 using GPM and head values of step
(lb).
3. Calculate approximate water head: 4. Correct head values:
(3-33)
I-Iw = H,i,/CH = 86/0.96 = 89.6 ft (3-33)
5. Correct efficiency values:
4. A pump may now be selected using water as the
equivalent fluid with capacity of 126.3 GPM and (3-34)
head of 89.6 feet. The selection should be made at
or very near to the point ( or region) of peak perfor- 6. Correct capacity values:
mance as shown on the manufacturer's curves.
5. The pump described by the curves of Figure 3-36 fits (3-32)
these requirements. The peak efficiency is 71 per-
cent using water. 7. Calculate the viscous BHP as indicated in the previ-
6. Calculate the viscous fluid pumping efficiency: ous example.
8. Plot values as generally indicated on Figure 3-58 and
evi, = Cc(e,J (3-34) obtain the performance curves corresponding to
= (0.80) (71) = 56.8% the viscous liquid conditions.
Pumping of Liquids 207
! 00 2 0 0 ...-...--..--.....---.---.---.----.---.----,.---,--,--,.--...--....-......-"T"-1
90 180 � ..... -==-+--+--+--+--+--+--+--+--+--+---+---+-+--+---l
80
...
Q.I
3
a 20
Q.I
s 15
:x: IO
Q.I
....:
� 5
CD
0
100 200 300 400 500
Capacity, Gpm.
Figure 3-58. Typical curves showing the effect on a pump designed for water when pumping viscous fluids. (By permission, Pie-a-Pump,
1959, Allis-Chalmers Mfg. Co.)
Temperature Rise and Minimum Flow or, alternate procedure [33,6]. For low capacity:
When a pump operates near shut-off (low flow) capac- 1-1(1 - e)
ity and head, or is handling a hot material at suction, it 778(c )(e) (3-37)
P
may become overheated and create serious suction as well
as mechanical problems. To avoid overheating due to low
flow, a minimum rate (GPM) should be recognized as where H, 0 = total head of pump at no flow or shutoff or at any
necessary for proper heat dissipation. However, it is not flow rate with corresponding efficiency from
necessarily impossible to operate at near shutoff condi- pump curve, ft
tions, provided (l) it does not operate long under these
conditions, as temperature rises per minute vary from less e = pump efficiency at the flow capacity involved (low
than I °F to 30-40°1·� or (2) a by-pass is routed or recycled flow), decimal
from the discharge through a cooling arrangement and
back to suction to artificially keep a minimum safe Dow Another alternate procedure [10]
through the pump while actually withdrawing a quantity
below the minimum, yet keeping the flowing temperaLure �Tr= (GPM) (H, 0) (SpGr)/3960 (3-38)
down [31].
See Figure 3-59 and Figure 3-60 for a graphical solution
to the equation above for temperature rise. Figure 3-59
1. Temperature rise in average pump during operation illustrates the characteristics of a boiler feed water pump
[6].
set to handle 500 GPM water at 220°F for a total of 2600
feet head. The temperature rise curve has been superim-
posed on the performance chart for the pump, and values
42.4 P.
�T --- ' 0 -,°F/min [25] (3- 36) of �Tr are calculated for each flow-head relationship. Note
r
W 1 cP how rapidly the temperature rises at the lower flows. This
heating of the fluid at low flow or no flow ( discharge valve
shut, no liquid flowing through the pump) can be quite
where [25] rapid and can cause major mechanical problems in the
�Tr= temperature rise, °F/min pump's mechanical components. The maximum temper-
ature rise recommended for any fluid is 15°F (can be a bit
P, = brake horsepower at shutoff or no flow higher at times for the average process condition) except
0
when handling cold fluids or using a special pump
W 1 = weight of liquid in pump, lbs
designed to handle hot fluid, such as a boiler feed water
cp = specific heat of liquid in pump pump of several manufacturers.
208 Applied Process Design for Chemical and Petrochemical Plants
50 36 1 00
45 34 I 90
/8
40 32 HEAD CAPACIT)' 80 800
.... ..... r---o
EFFICIENCY
0 .... i-t"-- """-... - - 70 1- 700
...i 35 030 J I I z
v, 0 / <; ....
,... -TEMPERATURE RISE ,J ....
� 30 ci28 v ... ........ 60 � 600 a:
....
� � r-, 50: 500 i
� 25 � 26 �v -
a: - ' � ....
� o9!> � � �
.... :I! \ pO� �--
� 20 �24 v 4oG 400 �
p-,
..... � A' �� 11-it'I\ �� -- Li:: ....
ffi 15 � 22 ....-.....- 30 !!; 300 �
� 20 x. .--� I �� �.�· ...,.,.,,. Iii
j 10 20 200
1\/ c � ............ �
5 18 »: -...... - 10 100
L,
0 16 0 0
0 100 200 300 400 500 600
CAPACITY, GPM
Figure 3-59. Typical temperature rise for boiler feed water pump. (By permission, Transamerica De/aval Engineering Handbook, 4th ed., H. J.
Welch, ed., 1983, Transamerica Delaval, Inc., IMO Industries, Inc., Div.)
50
Temp. rise, °F/min = 40
(BHP at shutoff) ( 42.4) (3- 36)
(weight of liquid in pump) (c P ) 30
.... 20
or w
J:
z 15
w
a:
J:
cf 10
(BHP - WHP) (2545) ""'
c.?
Temp. rise ° F /min (3-39) w 8
( pump capacity ) 0
� 6
w
a:
::,
2. Minimum Flow (Estimate) [6] .... 4
cf
a:
w 3
CL
2
w
....
The validity of the method has not been completely 2
established, although it has been used rather widely in set-
ting approximate values for proper operation [10]. For
multistage pumps use only the head per stage in temper-
ature limit by this method. 300 500 1,000 2,000 3.000 5,000
TOTAL HEAD, IN FEET
a. Determine NPSHA available at pump suction Figure 3-60. Temperature rise in centrifugal pumps in terms of total
head and pump efficiency. (By permission, Karassik, I. and Carter,
b. Add the NPSH value lo the vapor pressure of the R., Centrifugal Pumps, McGraw-Hill Book Co. Inc., 1960, p. 438.)
liquid at suction conditions. This represents the
vapor pressure corresponding to the temperature of d. Approximate minimum safe continuous flow effi-
the liquid at the flash point. Read temperature, t2, ciency:
value from vapor pressure chart ofliquid.
c. Allowable temperature rise = t 2 - (actual pumping H, 0, at shutofffrom curve
temperature). Boiler feed water practice uses 15°F 778(6.Tr )cp + Hso, at shutoff (3 - 4-0)
rise for average conditions [10].
Pumping of Liquids 209
where eM = minimum safe flowing efficiency, overall fluid handled and the calculated NPSHA condition. For
pump, fraction NPSHR refer to corrections discussed earlier.
H, 0 = head at no flow or shutoff, ft
cp = specific heat of liquid, BTU /lb/°F Centrifugal Pump Specifications
Kf R = temperature rise in liquid, °F
Figure 3-61 presents specifications for a centrifugal
e. Read minimum safe now in GPM from pump per- pump. Although the process engineer cannot or should
formance curve at value of minimum efficiency cal- not specify each item indicated, he must give the perti-
culated in (d). nent data to allow the pump manufacturer to select a
pump and then identify its features. Pumps are selected
Example 3-18: Maximum Temperature Rise Using Boiler for performance from the specific characteristic curves
Feed Water covering the casing size and impeller style and diameter.
Often the process fluids are not well known to the pump
Using the example of Reference [6], assume a pump manufacturer, therefore the materials of construction, or
with characteristic curve and added temperature rise data at least any limitations as to composition, must be speci-
as shown on Figure 3-59 is to handle boiler feed water at fied by the engineer.
220°F, with a system available NPSHA = 18.8 feet. The
vapor pressure of water at 220°F is 17.19 psia from steam Example 3-19: Pump Specifications, Figure 3-61
tables and the SpGr = 0.957. Correcting the 18.8 feet
NPSHA: psia = 18.8 (1/ [2.31/0.957)] = 7.79 psia at The pump specified identifies the design data, key por-
220°F. tions of the construction materials and driver data as
The vapor pressure to which the water may rise before required information for the pump manufacturer. If the
it flashes is 17.19 psia + 7.79 psia = 24.98 psia. pump is to be inquiried to several manufacturers this is all
From steam tables (or fluid vapor pressure tables), that is necessary. The individual manufacturers will iden-
read at 24.98 psia (for waler of this example), tempera- tify their particular pump selection and details of con-
ture = 240°F. struction materials and driver data. From this information
Therefore, allowable temperature rise of the water a pump can be selected with performance, materials of
(this example) = 240° - 220°F = 20°F. construction, and driver requirements specified.
A plotted curve as shown on Figure 3-59 [33] shows In the example the manufacturer has been specified
that at point A a rise of 20°F on the temperature rise from available performance curves, and the details of
curve corresponds Lo a flow of 47 GPM minimum safe for construction must be obtained. The pump is selected to
the pump handling 220°F, with NPSHA of 18.8 feet. operate at 22 GPM and 196 to 200 feet head of fluid, and
An alternate estimate for minimum flow [ 11]: must also perform at good efficiency at 18 GPM and a
Minimum flow (for water) through pump, head which has not been calculated, but which will be
close to 196 to 200 feet, say about 185 feet. Ordinarily, the
QM = 0.3 ?, 0, GPM (3-41) pump is rated as shown on the specification sheet. This
insures adequate capacity and head at conditions some-
where P, = shutoff horsepower what in excess of normal. In this case the design GPM was
0
determined by adding 10 percent to the capacity and
For cold liquids, general service can often handle ti Tr of allowing for operation at 90 percent of the rated efficien-
up to 100°F, a rule with approximately 20% factor of safety: cy. Often this latter condition is not considered, although
factors of safety of 20 percent are not unusual. However,
Q, 1 = 6 P, 0 /,1.T" GPM (3-42) the efficiency must be noted and the increase in horse-
power recognized as factors which are mounted onto nor-
Li.Tr= permissible temperature rise, °F. mal operating conditions.
Sometimes the speed of the pump is specified by the
The NPSHR required at the higher temperature may purchaser. However, this should not be done unless there
become the controlling factor if cavitation is not Lo occur. is experience to indicate the value of this, such as packing
The minimum flow simply means that this flow must cir- life, corrosion/erosion at high speeds, and suspended
culate through the pump casing (not recirculate with no particles; as the limitation on speed may prevent the man-
cooling) back to at least the initial temperature of the feed, ufacturer from selecting a smaller pu!.Tip. In some cases it
if excessive temperatures are not to develop. The best prac- must be recognized that high heads cannot be reached at
tice is to request the manufacturer to state this value for the low speeds in single stage pumps. Table 3-7 presents sug-
210 Applied Process Design for Chemical and Petrochemical Plants
.. ate often should be placed in service on a regular sched-
Pa- 1 af 1 P1111u ule just to be certain they are in working order.
Unit P,ic• 2 (one a.a SP!l e}
IIMNo.------ If solids are carried in the fluid, this can present a diffi-
CENTRIFUGAL PUMP SP!CIFICATIOHS No. Unit, P-(P A & B
cult problem if they are not properly flushed from the
DESIGN OAT.A pump on shutdown. Some spare or second pumps are
Senl,;e Forwarding to T-98 Llq .. ld �7-'%� · � C= au=s � t= 1c � ------r
"Solid, Nppe Colc-.GPM_� 1= 8---- Dn.GPM_.- 22�-----< selected for 100 percent spare; others are selected so that
Ve,.rPr.H. PSIA T•mp, � °F Vi,. � Cp Sp.Gr. 1.0f each of two pumps operate in parallel on 50 percent of the
HUD SUCTION DISCHARGE DIFFERENTIAL flow, with each being capable of handling 67 to 75 percent
+ 4.1 •.. + 12.1 F,.
Stalic Ft. Ab .. -
==:=2�g:'.:; : g;::=== F;;, Ab,. + 199.R •.. 11'.R . .. of total load if one pump should fall off the line. This then
3,4
+
--�o� .1�-••· + 199.9 . .. only reduces production by about 25 percent for a short
•..
Tatal �+--'lc,,2� .1�-F,. Ab,. + 207.3 Ft. Abs. + 195.2 . ..
NPSH Availab/111 9.5 Ra1in9H•ad 196 to 200 F•, period, and is acceptable in many situations. These pumps
PUMP SELECTION are usually somewhat smaller than the full size spares.
Mfgt. "A" Siu & r,,..Centr1fupal, 2 x 'Jilodel AAA Ca •• Ty,.-V � rt ·-�
St19H One Imp. TypaEnOl0§9d Da1l11n l111p, Dia. 1� 1/f" Maic.Jmp. Oia. _ __o1�4�-----! When it is necessary to plan several pumps in parallel,
0,1... Motor RPM 1750 Dul;n En. 13% Ratal/vn• Counter Clockwise the pump manufacturer must be advised, and care must
Ra1lftf BNP�Nvn•OHrlOGd BHP-1.5..____ Shul•aff P,.,,, 14} ps1p; NPSH R•quirad,_-,�,--=-ft�·----!
S1,1e. b�otlon End Disch. Locotion VertlC{ll Top eu,.,. No. C - 12� be taken in arranging suction piping for the pumps, oth-
CONURUC'TK>N,.MA'JERIAiU
erwise each may not carry its share of the flow.
There are many flow conditions, and pumps should be
Packing ihdt S.al: lnt•rnml C Eat11rn•I O Ph.,111•d LJ
Slufflng Bo:ir s-1 Cag• Bu•hln111 Glcrnd Bolt. ----I selected to operate as efficiently as possible over the
MechanlcalS.al1 Mak• � Type � ; Coolant�;l1111deC Our•ld•l:!'J Sin;l•CI DoubleC
V
Bcr•911lete Cast Iron Cawp!in11:Mf11. '1 A" Typo � _ e __ GuQ1d_--"""e•.._ __ widest range of capacity.
Bearln1•1 Type Bell, double MQ\ie CoclQnt --�-----<
l111P•ll•rNu1 N!Ckel Tl,ru•t B•crring1: T1p• � br!Ca.ted M11ke_•_. •z � •-----l If the flow is expected to vary during the system opera-
eo.. Srud, Nl eke) ea •• End CQWlllr'l _�N= ! c=k=e� l _ tion, the high and low GPM (and corresponding heads)
DHFu •• ,a Nickel Oiopl,rcrgrn ----------
Plpl"11 _llike_,,_ l should be given to allow proper evaluation.
0���------,,�.�-
� ·=·• � tl• � n �� n:!,;;; � '="====== � ·= ·" � ''..:;1S=Q � #:;;::;5t � 4 � ,;;;;; � Dl � •<h � · � ·· � S1 � u:::=::::;;;;;:;;:;:..:•: •• � • �� = l' �� n..O � • � • � :;;;;; �
DlllYER
r,,.. TEFC Mfgr, __ --"B----c-c�---- Hp - � 1 � � Fluid Conditions
-c-c-----1
'
•
RPM 1750 Fra- Yolt,�4�4� 0 __ PhcrH __ � · __ Cyde GO
Elec. Clau_. CoN1eclion: Dit•d�.;-- Belh c;.., ��
l•l•tS,._ PSIG� __ .,. Ellh11U1tStca,,, PSIG�-----...c"F. The manufacturer must be told the conditions of the
St•-Rohl
RE.WAR KS liquid, percent suspended solids, physical properties, cor-
• facing P•,np rrll!l'I Drivar En.,.
rosive nature and maximum and minimum temperature
.. , •... , •... ranges. For extremely hot liquids, special hot pumps must
be used, and temperature effects taken into account.
o ... I la.,•, . I I Rev, I !
P.O. Ta:---------------------------'
System Conditions
Figure 3-61. Centrifugal pump specifications.
The manufacturer must know if the suction side of the
pump is associated with vacuum equipment, or is to lift
gestions for materials of construction for pump parts in the liquid. This can make a difference as to the type of
the services indicated. The effect of impurities, tempera- impeller suction opening he provides. If the system oper-
ture, analysis variations and many other properties make ates intermittently it should be noted. A piping diagram is
it important to obtain specific corrosion service data in often helpful in obtaining full benefit of the manufactur-
the specific fluid being pumped. Sometimes this is not er's special knowledge.
possible, and generalized corrosion tables and experience
of other users must be relied on as the best information Type of Pump
for the materials selection.
If there is a preference as to horizontal or vertical split
casing, it should be stated. Also the suction and discharge
Number of Pumping Units connections should be stated as to top or end, or special,
together with the preference as to flanged (rating) or
A single pump is the cheapest first-cost installation. screwed. Small pumps are commonly furnished with
However, if downtime has any value such as in lost pro- screwed connections unless otherwise specified.
duction, in hazards created in rest of process, etc., then a
stand-by duplicate unit should be considered. A spare or Type of Driver
stand-by can be installed adjacent lo the operating unit,
and switched into service on very short notice, provided it Pumps are usually driven by electric motors, steam or
is properly maintained. Spare pumps which do not oper- gas turbine or gas (or gasoline) engines either direct or
Pumping of Liquids 211
Table 3-7
Pump Materials of Construction
Table materials are for general use, specific service experience
is preferred when available
I
Casing & Impeller & Shaft Type of
Liquid Wear Rings Wear Rings Shaft Sleeves Seal Seal Cage Gland Remarks
Ammonia, Cast Iron Cast Iron Carbon Steel Carbon Steel Mechanical ........... ?IIall. Iron NOTE:
Anhydrous & Materials of
Aqua Construction
shown will be
revised for
some jobs.
Benzene Cast Iron Cast Iron Carbon Steel Nickel Moly. Ring Cast Iron Mall. Iron
Steel Packing
Brine (Sodium Ni-Res!st* Ni-Resist* K Mone! K l\fonel Ring Ni-Resist** Ni-Resist** "Cast Iron
Chloride) Packing acceptable.
**Malleable
Iron
acceptable.
Butadiene Casing: C. Impeller: Carbon Steel 13% Mechanical . . . . . . . . ... Carbon Steel
Steel-Rings: C.1.-Rings: Chrome
C.J. C. Steel Steel
Carbon Cast Iron Cast Iron Carbon Steel Carbon Steel Mechanical ........ . . . Mall. Iron
Tetrachloride
Caustic, 5 0% Mi�co C Misco C 18-8 Misco C Ring Misco C Carbon Steel Misco C
(Max. Temp. Stainless Packing manufactured
200° F.) Steel by Michigan
Steel Cast-
ing Com � any.
29 Cr-9 i
Stainless
Steel, or
equal.
Caustic, 50% Nickel Nickel Nickel or Nickel Ring Nickel Nickel
(Over 200° F) 18-8 Stain- Packing
& 73% less Steel
Ca us tic, 10% Cast Iron 23% Cr. 23% Cr. 23% Cr. Ring Cast Iron ........... Specifica-
(with some 52% Ni 52% Ni 52% Ni Packing tions for 50%
sodium Stainless Stainless Stainless Caustic
chloride) Steel Steel Steel (Maximum
Temperature
200° FJ also
used.
Ethylene Cast Steel Carbon Steel Carbon Steel Carbon Steel Mechanical Cast Iron Mall. Iron
Ethylene Cast Iron Cast Tron Steel K Mone! Mechanical . . . . . . . . . . . K Mone!
Dichloride
Ethylene Glycol Bronze Bronze 18-8 18-8 Ring . . . ' . . . . . . Bronze
Stainless Stainless Packing
Steel Steel
Hydrochloric Impregnated Impregnated 18-8 Impregnated Mechanical . . . . . ...... Impregnated
Acid, 32% Carbon Carbon Stainless Carbon Carbon
Steel
Hydrochloric Rubber Hard Carbon Steel Rubber or Ring Rubber Rubber
Acid, 32% Lined Rubber Plastic Packing
(Alternate) C. Iron
Methyl Chloride Cast Iron Cast Iron 18-8 18-8 Mechanical . . . . . . . . . . . Mall. Iron
Stainless Stainless
Steel Steel
Propylene Casing: Imp.: Carbon Steel Carbon Steel Mechanical Cast Iron Mall. Iron
C. Steel- C.I .-Rings:
Rings: C.l. C. St!
Sulfuric Acid, Hard Special Carbon Steel 1-lastelloy C Ring Special Special
Below ,55% Rubber Rubber Packing Rubber Rubber
Lined C.I.
Sulfuric Acid, Cast Si-Iron Si-Iron Type 316 Si-Iron Ring Teflon Si-Iron
55 to 95% Stn , Stl. Packing
Sulfuric Acid, Cast Iron Cast Iron Carbon Steel 13% Cr Mechanical . . . . . . . .... Mall. Iron
Above 95%
Styrene Cast Iron Cast Iron Carbon Steel 13% Ring Cast Iron Mall. Iron
Chrome Packing
Steel
Water, River Cast Iron Bronze 18-8 Bronze Mechanical Cast Iron Mall. Iron
Stainless
Steel
Water, Sea I Casing, Impeller: K Mone! K Mone! or Ring Mone! or Monel or
Alloy 20 SS
1-2% Ni,
Rings:
Cr 3-0.5% Mon el (Aged) Alloy 20 SS Packing Alloy 20 SS
Cast Iron S-Monel
Rings: Ni-
Resist, 28
� .
212 Applied Process Design for Chemical and Petrochemical Plants
through V-belts or gears. The pump manufacturer should
know the preferred type of drive. If the manufacturer is to Water Vertical Pumps
furnish the driver, the data on the specification sheet Flow
under Driver should be completed as far as applicable. If
a gas or gasoline engine is to be used, the type of fuel and
its condition must be stated. Engine cooling water (if air d=Bd/3
not used) must be specified.
Sump Design for Vertical Lift
Partition
The proper design of sumps for the use of vertical lift (see Ref. [17])
pumps or horizontal pumps taking suction from a sump
is important to good suction conditions at the pump [2,
3, 14].
The arrangement and dimensions indicated in Figure
3-62 or Figure 3-63 are satisfactory for single or multiple 0
pump installations. (For more details, refer to Reference
[17]). A few key points in sump-pump relationships for
good non-vortexing operation are:
Figure 3-63. Acceptable sump arrangement for multiple pumps.
1. Avoid sudden changes in direction or elevation of
flow closer than five bell diameters to pump.
2. Avoid sump openings or projections in water path 3. Have water flowing parallel to sump walls as it enters
close to pump. pump. Water should enter pump suction with as low
a turbulence as possible.
4. Water velocity in sump must be low, 1J4 feel/second
is good practice.
- 5. Inlet channel width to each pump is considered opti-
mum at 2 Bct to prevent secondary turbulence
effects. [3]
6. Avoid placing several pumps in one open channel
--- removing water in series fashion. If this must be
done, velocity at each pump must be kept at same
value as for single pump. The channel width at each
pump would be taken from Reference [17].
A suction bell on the inlet of a vertical pump ( or the
inlet pipe of the suction side of a horizontal pump) is not
necessary as far as pump or sump operation is concerned.
If a bell is omitted, the entrance losses due to flow will be
Pump higher with only a straight pipe, and this must be consid-
ered in pump operation. An economic comparison will
help decide the value of the bell. Strainers should not be
placed on suction bells unless this is the only arrange-
Water Level ment. Inlet water should be screened with trash racks,
-- -- bars and screens to keep the sump free of debris.
--- ---
Submergence,Sb Water
.. Depth Submergence of the inlet pipe column or bell inlet
B • :· in Pump,! below the water level is necessary for good operation and
Diadmet8rl .":.·:
• I if ell �,, to prevent vortexes and entrained air. The minimum sub-
.. : ::. :::··;.: \'-, °.: :°::: :; : ·: :·: :':":: ":: :-. : __ : .·.:/ � -Bd/3 lo Bd/2 mergence as recommended by the manufacturer must be
maintained at all times. Generally, for 70°F water, each
Figure 3-62. Sump design. Note: S = (1 '.4 to 2) a, I 000 feet of elevation above sea level adds 14 inches to the
Pumping of Liquids 213
required submergence. If the water is al l00°F at sea level, used for metering. For specific performance characteris-
approximateiy 17 inches must be added to the 70°F sub- tics of any type consult the appropriate manufacturer.
mergence value [14].
These pumps are low in cost, require small space, and
are self priming.
Rotary Pumps
Some can be rotated in either direction, have close
clearances, require over-pressure relief protection on dis-
There are many different types of positive displace- charge due to positive displacement action, and have low
ment rotary pumps [29] as illustrated in Figure 3-64 and volumetric efficiency [8].
Figures 3-65A, B, C.
Performance Characteristics of Rotary Pumps:
The majority of this type are capable of handling only
a clean solution essentially free of solids. The designs
using rubber or plastic parts for the pressure device can 1. Flow proportional to speed and almost independent
handle some suspended particles. In general, these of pressure differential.
pumps handle materials of a wide range of viscosity (up to
500,000 SSU), and can develop quite high pressures ( over (a) Internal slip reduces efficiency, and increases
1000 psi). In addition, the units can handle some vapor or with pressure and decreasing viscosity.
dissolved gases mixed with the liquid being pumped. The (b) Entrained gases reduce liquid capacity and cause
capacity is generally low per unit, and at times, they are pulsations.
Suction
Four-Lobe Pump Sliding Vane Pump
External Gear Pump Internal Gear Pump Three -Lobe Pump Inlet Discharge
i t Discharge
Drivi;g Gear fil__�,0/-� I I
��
Single Screw Pu mp Swinging Vane Pump
Com or Roller Pump Cam-and-Piston Pump
Su�tion Flexible
Rubber Tu�e
·, t
Discharge
Three-Screw Pump Shuttle Block Pump Squeegee Pump Flexible Vane
Figure 3-64. Rotary pumps. (By permission, Dolman, R. E., Chemical Engineering, Mar. 1952, p. 159.)
214 Applied Process Design for Chemical and Petrochemical Plants
DIM Diaphragm FLOWS TO 1480 GPH, PRESSURES TO 5000 PSI
SUCTION DISCHARGE
Figure 3-65A. Diaphragm metering pump, "Pulsa" series. One of several styles/types. (By permission, Pulsafeeder, Inc.)
(c) Liquid displacement [6]: 3. Pump power output (whp) [ 17]
d "(1 - En) whp, = (Q'P,ct)/1714 (3-45)
d'= ----------, cu ft/min (3-43;
(1 - E 11) + E,, (P/P 1)
where P,d = differential pressure between absolute pres-
where P is theatmospheric pressure, and P 1 is the sures at the outlet and inlet to pump, psi
inlet absolute pressure to the pump. whp, = power imparted by the pump to the fluid
discharged (also liquid HP)
d"= theoretical displacement, cu ft/min Ev = volumetric efficiency, ratio of actual pump
d' = liquid displacement, cu ft/min capacity to the volume displaced/unit time
En= percent entrained gas by volume at atmospheric
pressure E,. = 231 Q'(lOO)/(D"n) (3-46)
2. Volume displaced [ 17]
4. BHP varies directly with pressure and speed.
5. For speed and pressure constant, BHP varies direct-
Q' = D "n - S" GPM ly with viscosity.
231 '
(for no vapor or gas present ) (3- 44)
where Q' = capacity of rotary pump, fluid plus dissolved gases/ Selection
entrained gases, at operating conditions, GPM
D" = displacement (theoretical) volume displaced per
revolution (s) of driving rotor, cu in./revolution Suction and discharge heads are determined the same
n = speed, revolutions per minute of rotor(s), rpm as for centrifugal pumps. Total head and capacity are used
S" = slip, quantity of fluid that leaks through internal in selecting the proper rotary pump from a manufactur-
clearances of pump per unit time, GPM er's data or curves. Since viscosity is quite important in the
Pumping of Liquids 215
Choice of leak-resistant packing or
standard mechanical seal (illustrated).
Special mechanical seals available to
Positive-lock thrust control for preci- fit equipment.
sion rotor and shaft positioning to Integral safety relief valve works to
protect system against excessive
maintain original perfonnance and pressure.
minimize wear for extended pump life.
Cushioned, positive-flow action of Specially designed and machined
rotor and idler combination provides ....____ revolvable casing provides eight
non-pulsating, low-shear transmis- port positions to suit application.
sion of liquid.
Figure 3-658. Typical rotary gear pump. (By permission, Viking Pump, Inc., Unit of ldex Corporation.)
selection of these pumps, it is sometimes better to select a Significant Features in Reciprocating Pump
larger pump running at low speeds than a smaller pump Arrangements
at high speeds when dealing with viscous materials.
I. Liquid Pump End
As a general guide, speed is reduced 25-35 percent A. Pump Pressure Component
below rating for each tenfold increase in viscosity above
1000 SSU. Also, generally, the mechanical efficiency of I. Piston
the pump is decreased 10 percent for each ten fold 2. Plunger
increase in viscosity above 1000 SSU, and referenced to B. Types
a maximum efficiency of 55 percent at this point. [1] 1. Simplex, one piston
2. Duplex, two piston (Figure 3-66)
3. Triplex, three piston (not used as steam driven)
Reciprocating Pumps
C. Piston or Plunger Action
1. Single acting, one stroke per rpm
Reciprocating pumps are positive displacement pis-
ton units driven by a direct connected steam cylinder 2. Double acting, two strokes per rpm, cylinder
or by an external power source connected to the crank- fills and discharges each stroke (Figure 3-67)
shaft of the pump piston. Being positive displacement, D. Packing for Piston or Plunger
these pumps can develop very high pressures ( 10,000 1. Piston packed: packing mounted on piston and
psi and higher) for very low or high capacities (up to moves with piston; applied to comparatively low
1000 GPM). pressures
216 Applied Process Design for Chemical and Petrochemical Plants
FULL FLOW PARTIAL FLOW NEUTRAL OR NO FLOW REVERSE FLOW
The hner (grey areal rs in full-capacity Now the liner has been rotated counter- Rolahng the liner snn further. a pomt rs When the hner rs rotated past the "no flow"
position with the seal po,nt at the ton and clockwise. which opens the seal pomt al reached where displacement volumes point. the volume above lhe rotor exceeds
!he pumping chamber at the bottom AU lhe lop This allows par1 of lhe liquid lo be above and below the rotor are equahzed lhat below. and net flow reverses d1rect1on
hqurd coming into the pump at the left rs recirculated, reducing the net flow by a This causes as much l!Qu1d to be returned even though pump speed and relation have
moving out of 11 at lhe nght. proportionate amount over the lop as rs brought forward across nol changed 1L1m1led lo approx 30'I, of
the bottom resull+ng m zero flow full forward ttow on all models I
Figure 3-65C. Sliding vane rotary pump. (By permission, Blackmer Pump, Dover Resources Co.)
Pumping of Liquids 217
R, .. o •• ble ttun cl.nt
cou,, P4naftfln! quld1
•«•• to ll"m .... R" ..
Rr,1d nd Iron cradl ..
of Hrnlcircul., Hction.
Hturln 11 li;:::::h .nd
Twin llquld cyllncl1tt m•·
Figure 3-66. General service duplex steam driven cMned IA dwpfu botint •ill
au•ri111 cotnd ccn19f'I
piston pump. (Courtesy Worthington Corp.)
2. Cylinder packed: packing stationary; plunger Application
moves; applied to high pressures; more expen-
sive than piston packed. Piston Type: used for low pressure light duty or inter-
II. Drive End: Steam mittent service. Less expensive than the plunger design,
A. Steam Cylinders but cannot handle gritty liquids.
1. Simple: single cylinder per cylinder of liquid Plunger Type: used for high pressure heavy duty or con-
pump; uses more steam than compound. tinuous service. Suitable for gritty and foreign material
service, and more expensive than the piston design.
2. Tandem Compound; high and low pressure
cylinder on same centerline; usually requires 80
psi or greater steam to be economical. Performance
3. Cross Compound: high and low pressure cylin-
der arranged side-by-side with cranks 90° apart. The performance of reciprocating pumps provides for
Need for crank and flywheel arrangement only; ease of operation and control. Depending upon the type
usually requires 80 psi or greater steam to be of piston action, the fluid may be subject to pulsations
economical. Percentage gain in compounding unless accumulator or surge drums are provided.
steam cylinders varies from 25-35 percent for The slip of a pump is fraction or percent loss of capac-
non-condensing, and 25-40 percent for con- ity relative to theoretical. Slip is (1 - ev 01), where e, 01 is the
densing [] 8]. volumetric efficiency. Volumetric efficiency is the actual
B. Cylinder Action liquid pumped (usually considered water) relative to that
l. Direct: steam piston direct connected to liquid which should theoretically be pumped based on piston
piston or plunger through piston rod. displacement.
2. Crank and Flvwheel: flvwheel mounted on The NPSH required is approximately 3-5 psi of liquid
/ /
crank shaft driven by steam cylinder. above the vapor pressure of the liquid.
III. Drive End: Power The capacity of a pump is given in manufacturers
General features same as steam, except drive always tables as actual, after deducting for volume occupied by
through crankshaft; speed gear increasers or reduc- piston rod and slippage. Slip varies from 2-10 percent of
ers; Vbelts, or direct coupling connection to drive displacement, with 3 percent being a fair average.
shaft. Capacity: actual, for single acting pumps, single cylinder
IV. Designation
Units are identified as: (steam cylinder diameter,
(l2a t) (e"
01)
inches) (liquid cylinder diameter, inches) (length Q= ------'-'- (3- 47)
of stroke, inches). (231) (2) 0.0204 ct/ tevoi' GPM
218 Applied Process Design for Chemical and Petrochemical Plants
Tota I ly - Enclosed
Dust-Proof Oil-TiQhl
Power End
Solid Forged Steel
Cylinder-no Gaskets
Under Discharge Pressure
Positive Flood Lubrication
Provided by Oil DistribufinQ Wing Guided Flange and Screw
('"' Take-up on Pocking
Pump Type Gland- Even
Figure 3-67. Duplex double-acting plunger pump, power driven. (Courtesy Worthington Corp.)
For double acting pumps, single cylinder Horsepower
Q = (two times value for single acting) - 0.0204 d\ t, Hydraulic
GPM (3-48)
For multiple cylinders, multiply the capacities just HHP = Q(actual)H/3960 (3-49)
obtained by the number of cylinders. If the piston rod
does not replace pumping volume as in some arrange- Brake
ments, the last term of the double acting capacity equa-
tion is omitted.
BHP= HHP/e (3-50)
Discharge Flow Patterns
where e represents the total overall efficiency, and is
Figure 3-68 shows the discharge flow patterns for sev- e = em ( e,. 0 1), and em is the mechanical efficiency.
eral reciprocating power pump actions which are essen-
tially the same for steam pumps. The variations above and
below theoretical mean discharge indicate the magnitude Mechanical efficiencies of steam pumps vary with the
of the pulsations to be expected. Although not shown, the types of pump, stroke and the pressure differential. Some
simplex double acting discharge would follow the action representative values are 55 to 80 percent for piston
of one piston on the duplex double acting curve from O pumps with strokes of 3 inches and 24 inches respectively,
to 360°. Its variation or pulsing is obvious by inspection, and pressure differential up to 300 psi. For the same
and accumulator bottles would be required to smooth the strokes a plunger design varies from 50 to 78 percent, and
flow. The simplex single acting discharge would be one at over 300 psi differential the efficiencies are 41 to 67
pumping stroke from Oto 180°, then no pumping from percent [9). Steam required is approximately 120
180° to 360°; and here again the pulse action is obvious. lbs/hour per BHP.
Pumping of Liquids 219
QUINTUPLEX SINGLE-ACTING PUMP
Variation Above Mean, 1.8%
Variation Below Mean, 5.2%
Total Variation, 7. %
Variation Above Mean, 4.82%
Variation Below 1\-foan, 9.22%
Total Variation, 14.04%
oo 60° 120° 180° 240° l00° 3600
TRIPLEX SINGLE-ACTING PUMP
Vari a lion Above Mean, 6.1%
Variation Below Mean, 16.9%
Total Variation, 23. %
oo 120° 240° 360°
QUADRUPLEX SINGLE-ACTING PUMP
Variation Ab1>Ye Mean, I 1. %
Variation Bdow i\J,.an, 21.5�{ 1
Total Variation, 32.5%
DUPLEX DOUBLE-ACTING PUMP
Variation Above Mean, 24.1%
Variation Below Mean, 21.5%
Total Variation, 45.6%
Figure 3-68. Reciprocating pump discharge flow patterns. (Courtesy the Aldrich Pump Co.)
220 Applied Process Design for Chemical and Petrochemical Plants
HORIZONTAL DIRECT ACTING STEAM PUMP
ITEM-----·
OR. POWE:R PUMP
APPARATUS- ..
...
_
E--- ----BY--------
OPERATE AT----# GA; MAX WP----# GA le MAX T---"P'--------- DAT ...
BASED ON STEAM A-:t----LBS,/SQ, IN. GA AND---- -------DATKO
ITEM NO,---·
SIZE AND TYPE---------
A. CONDITIONS OF SERVICE DATA
OPIRATION
PUMPED MATERIAL--------------
0API AT 60/60° F-------------------------------------------------
SPECIFIC GRAVITY AT 60° F----------------------------------
SPECIFIC GRAVITY AT P. T,---------------------------------
VISCOSITY AT P, T.-
PUMPING TEMPERATURE-°F--------------------------------
U. S. G. P. M. AT 60° F--------------------------------
U. S, G. P, M, AT P. T.
DISCHARGE PRESSURE-LBS/SQ. IN. GA--
SUCTION PRESSURE-LBS/SQ. IN. GA----------------
APPROX. VAPOR PRESSURE OF LIQUID-------------------------------
EXHAUST STEAM
B. PUMP SPECIFICATIONS
SUCTIONVALVES-NUMBER------------------------------
SUCTION VALVES-SIZE; INCHES-----
---------
SUCTION VALVES-AREA EACH, SQ, IN,---------------------
DISCHARGE VALVES-NUMBER-------------------
DISCHARGE VALVES-SIZE, INCHES-------------------- -------
DISCHARGE VALVES-AREA EACH, SQ, IN----------------------------
SUCTION CONNECTION---------SIZE:----------- --SERIES; _
DISCHARGE CONNECTION--------SIZE:----------- SERIES:-----·------------
TYPE OF RINGS-STEAM END:------------------PUMP END:----
C. MATERIALS: STEAM END LIQUID END
CYLINDERS---------·--------------
LINERS -------------·---------·------·
PISTONS
PISTON RODS
VALVES--
VALVE SEATS
VALVE SPRINGS
PACKING-- ------------------------
0. PERFORMANCE
PISTON SPEEO-FT/MIN,-- ----�R���G��-P-�H=·�P�----�����--�X�N�Si�A�L.f�D��H�-��- · ��-
............
R.P.M. . .... Q<..CB-I ...... Yc...o.f..,,R-�R.c...-&P_M . ------------·
STALLING PRESSURE-LBS/SQ. IN. GA--------------------------------------
STEAM CONSUMPTION,--------------------------------------
COPYTO--------------.DATo:.------CHECKEw.-------DAT�E-----APPROVED -----DAT!i'.-----
REMARKS:
Figure 3-69. Horizontal direct-acting steam pump or power pump.
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APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition
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