Ejectors and Mechanical Vacuum Systems 369
1 s P < 10 Lorr; wa =, 1tD8P0.3 4 (6-12) flow transition occurs from critical (sonic) to subsonic flow
through any particular leak. These factors can be calculat-
2
10 � P < 100 torr; w, = 1.2 1tD8Po 6 (6-13) ed by conventional fluid flow methods if the size of the
leak is estimated, which then is a real problem, or by using
100 � P < 760 torr; w, = 3.98 1tD8 (6-14) the data of Table 6-8 and applying fluid flow head losses.
The hydraulic effects for the submerged portions of
where D = sealed diameter, in. (estimates of nominal the vessel or system can be ignored when the total static
diameter, acceptable) head on the submerged portion is not greater than 0.53
w, = acceptable air-leakage rate assigned to a times atmospheric pressure plus the hydraulic head [22],
system component, lb/hr i.e., r,
8 = specific air leakage rate, lb/hr/in.
P = system operating pressl!re, torr
W10 = Wr = total calculated air inleakage, lb/hr P, = (p/760) + (hLPd34) (6-16)
3. Calculate the total acceptable air leakage rate, WT, where P, = static pressure, atm
lb/hr by adding D,V' a to the sum of the leak rates P = atmospheric pressure, mm Hg
assigned to the individual system components, w3 hL = liquid height, fl, below liquid surface
PL = specific gravity of liquid, relative to water = 1.0
'Wr = l:W' a + LW lb/hr (6-15)
0,
Acceptable Air /nleakage Rates {24}
To determine the capacity of the vacuum pump, the val-
ues of "\,VT above should not be used. It is necessary to In order to estimate an acceptable air inleakage rate
apply over-design or safety factors to ensure reliability [26] for sizing a vacuum pump for use in the medium to high
because pump capacity decreases with time and wear and vacuum system, consider:
air inleakage surges can occur due to a wide variety of leak
developing situations that result in more air or surges of
air inleakage. The over-design factor should be applied 100 Microns (0.10 Torr) to 1.0 Torr Range
only to the pump inlet throughput specification. The rec-
ommended [22] over-design factor should be 1.5 to 2.0 Estimate W' a:
times the air inleakage rate [22], and should also be
applied to saturated vapors entering the vacuum equip- 0.1 � P < 1 torr; w', = 0.026P0.6 "0).60 (6-17)
4
ment at the suction conditions required. Do not put a safe-
ty factor on tile suction condition of temperature or pres-
sure, provided the worst expected conditions of operation Estimate air inleakage for individual system specific
are specified. This requires a close examination of the leak rates, 8, from Table 6-8 [22] and from (w � 5 lb/hr)
process flow sheet range of operation. A safety factor of 2.0
is recommended for multistage steam jets with compres- 0.1 � p < 1 torr: Wa = 7t D8P0.6 4 (6-18)
sion ratio above 6:1; while a 1.5 factor is adequate for most
mechanical pumps and single-stage jets with a compres- Note that estimating maximum acceptable differs from
sion ratio of under 4: 1 [22]. The above procedure can be the design equations for Wa and W'a·
simplified for preliminary inleakage calculations [22]: Calculate total acceptable air inleakage rate, WT.
Multiply W'a X 2
(6-19)
For equipment with rotary seals allow additional 5
lb/hr for each conventional seal (packing type) and 2 A simplified alternate to the previously cited proce-
lb/hr for each mechanical seal and 0-ring. dures is suggested by Gomez [29] for calculating air
To account for air inleak a for vessels containing a liquid inleakage, but it is not presented in detail here.
level (portion of vessel submerged), the following applies.
If there is a large pinhole leak a few inches below the
liquid surface, it will behave like a leak above the liquid. Total Capacity at Ejector Suction
However, a small pinhole leak in the same location may
have zero inleakage due to capillary effects. The problem The Lota) capacity is the sum of all the expected con-
becomes complicated as the depth becomes large, and densables and non-co nde nsable flow quantities (in
370 Applied Process Design for Chemical and Petrochemical Plants
pounds per hour) which will enter the suction inlet of the steam jet refrigeration systems, degassing of liquids, high
ejector. It consists of the following vacuum distillation, evaporation, vacuum cooling and vac-
uum drying, or other systems where large volumes of con-
1. Air leakage from surrounding atmosphere. densable materials are to be removed at high vacuum. Fig-
2. Non-condensable gases released from gases original- ure 6-23 illustrates one application.
ly injected into the process for purge, products of
reaction, etc. Evacuation Ejector
3. Non-condensable gases, usually air, released from
direct contact water injection. An evacuation booster or "hogging" ejector is some-
4. Condensable vapors saturating the non-condensables. times used to remove air from a system on start-ups. Its
capacity is set to bring the system pressure down to near
Reasonable factors of safety should be applied to the operating conditions before the continuous operating
various loads in order to insure adequate capacity. Excess ejector system takes over. Figure 6-23 illustrates the instal-
ejector capacity can be handled by pressure control and lation of such a unit.
some adjustment in steam flow and pressure, but insuffi-
cient capacity may require ejector replacement. Factors of When an extra jet for this purpose is not desirable, the
2.0 to 3.0 are not uncommon, depending upon the par- secondary jet of a multiple system is often sized to have
ticular type of system and knowledge of similar system sufficient air removal capacity to pump down the system
operations. in a reasonable time.
Capacities of Ejector in Multistage System Load Variation
When the ejector system consists of one or more ejec- Figure 6-24 illustrates three different multistage ejector
tors and intercondensers in series, the volume as pounds designs, A, B, and C, which indicate that design A is quite
per hour of mixture to each succeeding stage must be eval- sensitive to changes in load above the design point.
uated at conditions existing at its suction. Thus, the sec- Designs B or C are less sensitive. The curve extended
ond stage unit after a first stage barometric intercon- toward point D indicates the capacity of the primary or
denser, handles all of the non-condensables of the system first stage when all the vapor is condensed in the inter-
plus the released air from the water injected into the inter- condenser; or if handling air or an air-vapor mixture, the
condenser, plus any condensable vapors not condensed in performance when the secondary jets have sufficient
the condenser at its temperature and pressure. Normally capacity to take all the non-condensables.
the condensable material will be removed at this point. If
the intercondenser is a surface unit, there will not be any The curve labeled A indicates performance at overload
air released to the system from the cooling water. when the air-handling capacity of the secondary stage is
limited. This condition arises as a result of design for
steam economy. If the capacity of the secondary jets is
Booster Ejector
larger, the performance along curve B or C can be expect-
ed. When the secondary jet capacity is limited as curves A,
Booster ejectors are designed to handle large volumes
of condensable vapors at vacuums higher than that B, or C indicate, a capacity increase brings a rise in suc-
obtainable with standard condensers using cooling water tion pressure when the load increase is mainly air or non-
at the maximum available temperature. They are usually condensables. The increase in pressure is less when the
used with a barometric (or surface) condenser and stan- load increase is due to condensables. This emphasizes the
dard two-stage ejectors. The booster picks up vapors from importance in sizing the secondary jets for ample non-
the process system at high vacuum (low absolute pressure, condensable capacity, and the importance of specifying
around 0.5 in. Hg abs) and discharges them together with the range and variety of expected conditions which may
its own motivating steam to a lower vacuum condition confront the system.
(compresses the mixture) where the condensable vapors Once a system has been evacuated to normal operating
can be removed at the temperature of the condenser conditions, it is possible for capacity to fall to almost zero
water. The non-condensable vapors leave the condenser, when the only requirement is air inleakage or small quan-
passing to the two-stage ejector system. This overall system tities of dissolved gases. Under these conditions, it is
allows a constant vacuum to be maintained in the process, important to specify an ejector system capable of stable
unaffected by the temperature of the cooling water. operation down to zero load or "shut-off' capacity. The
Booster ejectors are used with barometric and surface curve of Figure 6-24 represents such a system.
Ejectors and Mechanical Vacuum Systems 371
�-00------ INLET
MOTIVE STCAM
IOOSTU UECTOlt
...
•
:!!•
.. o
... 1 -- INTElt • COND •
::, <>
cw
z ..
...
DIIAIN LOOP
IIHAUIT
Figure 6-23. Drying-special three-
stage assembly serving two high vac-
uum dryers. E,y permission, C. H. vacuu• SHELF DltYEltl ltEMOYAL
Wheeler Mfg. Co. PUMP
Steam and Water Requirements 260
I
240
Figure 6-25 presents estimated steam requirements for o/
several ejector systems. Exact requirements can be 220 /
obtained only from the manufacturers, and these will be l v
C..;
based on a specific performance. 200 I v
Figures 6-26A and B give typical good estimating selec- 180 I/
tion curves for single-stage ejectors. Table 6-9 gives evacu- r _.,,.-
-
ation factors. � 160 y
......--
>,
Size selection: locate size at intersection of ejector suc- 'g 140 I ./
tion pressure and capacity on Figure 6-26A. Q. r -
0
.J-
Steam consumption: read values on curves or interpo- � 120 I,- --
""
late. ·;;;
� 100
J Design Po int
(6-20) 80 I
60
Evacuation: I
40
W'm=EV/t (6-21) J
20
I
0 I 2 3 4
Example 6-10: Size Selection. Utilities and Evacuation Absolute Pressure, In. Mercury
Time for Single Stage Ejector
Figure 6-24. Effects of overloads on ejector operation. By permis-
Total mixture to be handled = 60 lbs/hr sion, C. H. Wheeler Mfg. Co.
Suction pressure: 4 in. Hg abs
Steam pressure: 125 psig Evacuation: system volume = 300 cu ft
Size selection: Figure 6-26A: 2 inch L E = 1.3 (Table 6-9)
V = 300
Steam consumption: 440 lbs/hr at 90 psig
\V'm = 60
60 = 1.3 (300)/t
at 125 psig, F = 0.88 (Figure 6-26B) t = 6.5 minutes lo evacuate the volume with the 2-inch L
W, = 440 (0.88) = 397 lbs/hr ejector
372 Applied Process Design for Chemical and Petrochemical Plants
Ejector Suction Pressure, mm. Hg. Abs.
100 --===:::::J==::r:::r::::::i::r:1::::r:i::::i:::====:i::::::J::::::J � ;::J::r:r: � oto1====0jo=2=0jot3:::to=ot5rojoj1joj1=====0E2:::::oi3:::::J=oi5:rot1rii 1 0
1- �+-f-Plj---+--+-+-+-+-Hl+l---+--�-+-+-l--�-+-l l,000
I
70
1--,, __ ....1....._�--4---4---l--+-Jl..-l--l----4--.j.._.--i---l--l..-l--'�' " ........i-1-� ' ---'=--- 1 -.l.._1,-+-+-+4-+-l---+--�-+---l--+--l--l--l--l700
' ,
Four Stage ,cl6)
, Six Stage c (6)
50 ,_ /100%Air lwo Stage,nc(3)(5) r--, " 500
j \ Three Stage, Borometri· � <J..\ c � r-,
30 'lntercondensers(5) .... .,..,"- �"- r,.:- 300 ...
1--t...... ) I I I II II '% � , 0- 1 -....... ...
r-,
Single Stog_e, "' .
'"
20 I ---......... V 1\.,,100% Air in Mixture f\. � " I A' nc (1)(2 )15) +..: �� ...- .' ,1--+-J�++++---+---1-l---4---+-+-l-+-I 200-:
q-? ��
\ \ ·
�
...
e I'\ � � I'._ I � I\ Five Stage, c (6) ::::E
..
�
= \ " r'\r,.. r-, r,.., �'$ � " <
� 10 " '- ,---l----1f--l--4- � +-1- � ---1-----1"1-1,Four Stage ,c (6) 100 ;e
...
'
"
E
\
�
< 7 l------1---+-���'>i-1-+-P,,..o;:--'�..,....-+-l-+-1--P..i-l����--�-+--l---+4-�' .�+-' ... ......._:-----+�---+-lr,+-+-+-+-+-1-l 70 ..
...
"""'
........_
---.
,s I'\ "
..
' 5 ' I'll'- i"",.NL.-IQ9%�ir'\_ \_ f '-� '!'\. 50 iii
e
.,;
...
vi � V � tur· � � . � � 2\.---+--+-+-+--+-+4j,1n-1--......,_, � ' All Curv:S �-., .&:,
e
.,; 3 "r0-. 30 ,::,
.&:, "�10%A�........._ [f. Two Stage, ,, 11 far 100% Air
I ........._ in Mixtur)'""" ,
Barometric
..., M 2 1----4--+--l--l--+-H-+-l-----".;.:-.........:..:.:;:=.=+ � 1-l'-..."""4 � tercondenser (4)(5) I'....,"\ ,"\ � 20 .,,.
'"
...
...
a,
"�
.,,.
·5 � ..... t-...t-...t,.,.- 10%Air in I�" � �' a:
e
...
c:,
a: 1 'r-......._ "' Mixture I ,, �
e 1.0 10 en
...
c:, t----+--+--+-+-+-t-i-+-t-----+--+--+-+-+-+-t-+t---+- , -+--+--+-+-t-+++---1-----11--+-+-..P.�-
vi 0.7 t-----+--+---+--+1 Steam Pressure = JOO psig.
t-----+--+---+--+1 Condensing Curves based on 85° F Water, Correction is < 10% for 70 • F.
0.5 1-----+--+---+--+1 For Air Quantities between 10% and 100%, Requirements Located by Proportion. 1-1---4---1---1---+.4-1--+-<
1----+---+--l--H Mixtures Assumed to be Air and Water Vapor, if other Vapors or Gases Convert
to Air Equivalent.
0.3 1-----4----1---+.---+-1 c =Condensing, with Barometric lntercondenser.
nc = Non-Condensing.
0.2
2 3 5 7 10 20 30 50 70 100 200 300 500 700 1,000
Ejector Suction Pressure, mm. Hg. Abs.
Figure 6-25. Estimating steam requirements for ejectors.
Rating of the two stage non-condensing ejectors is han- Example 6-11: Size Selection and Utilities For Two-Stage
dled in the same manner as for a single stage ejector, Ejector With Barometric Intercondenser
using Figures 6-27 A and B, and Table 6-10.
Figures 6-28 A, B, C, D and E give representative esti- Total mixture to be handled = 40 lbs/hr
mating data for two stage ejectors with barometric inter- Pounds of air in mixture = 14 lbs/hr
condenser. Suction pressure = 1.5 in. Hg abs
Steam pressure at ejector nozzle = 150 psig
Size selection: locate size at intersection of ejector suc-
tion pressure and capacity on Figure 6-28A.
Size selection, using Figure 6-28A: 2 inch (inlet and
Steam consumption: Use Figure 6-28B; for Kuse Fig- outlet connections)
ure 6-28C
Steam consumption:
(6-22)
W', = 5.55 (Figure 6-288)
'\<\\n = 40
Water consumption: K == 0.67, (Figure 6-28C) at W /vVm = 15/40 = 0.375
3
F == 0.88 (Figure 6-28D)
GPM (approximate) = 0.06 W, w, == 5.55 ( 40) (0.67) (0.88)
Minimum GPM = 10 == 131 lbs steam/hr
Ejectors and Mechanical Vacuum Systems 373
- -
1,300 I I I '-- - '--
I
1,000 I I 'uf _ �s90_ -
-
( \-\0 - •
800 ,eo 1t\ 9e�� �- - --
- --
••
4
600 \ S -- _...- - f-'f"" - 1- - -
500 \'f:,t::.· o_....... .,. - ..... ...... -
,_ �(.,)� ./'" ........... � � f-1-"" - - -
:::, 400 -
0 .:'i.?(.,)(.,)b .. ./I,, ............... - .......... � - - - -
...
::c 300 -
- - -
-
,,
'o\J
....
-
.... .... -i-
.....
- -
I,,.........
Cl) l/ v .. ,, '1,<::F:; ..... - - - - - - - - -
-
c. 200 / I/ ,, \, 00 __...... __ i- --· - -
...
1-
,.....-i--,.... - _,..
� ��
Cl)
-
- v / "' � ..,.... , oO"" ,....i., ,.....1-""· '---- - - - - - - - -
l/
-
_,....
:::,
V/ ./'
l/ /
- 100 v � v �/ ./,..... f:JOO u-r - ..... - - - -
�· V ..,....
><
'ooO ........ _,.... .... "i1'L
:E
--
,., ,
�
o.OO
0
en
�
�
-
.Cl 80 ,, / v ., .,,, I/ �oo - -- - -- - -
-
f:JO..,
c 60 / v " ,.... v I,," .,i .... - 2 M - - .. -
v I/ �
-
- 50 v / , ..... 'l.o? - ,.... ..... ....... - - .. ......
v I/ �"
�
.
-
>,. 40 , v v , ,.. ...... ... .. ... - - --
:'!:: \�oo --
-
<..) / / �' :/ ./ - - i
c 30 L,,' .. �· ,..\'?> I - _ .......
i-
......
..... .....
- -
c. l/ / I" ..... - - - "l
-
/
c
- -
,...
u / / / i"' ,ao z's �
-
l/ 1./
20 v / / ....... ,.. L,,' ...... � - - -
-
I v v v / ..... .'i, a-- ' - -
v
..... ......
I l/ ,I " / L,,' ...... L,,' le,'b - I l
v
10 , I/ /
3 4 5 6 7 8 9 10
Ejector Suction Pressure, Inches Hg. Abs.
Figure 6-26A. Size ejector for 20 psig steam consumption, single stages, 3"-1 O" Hg abs. By permission, Worthington Corp.
Water consumption:
1
�200 GPM = 0.06 (131) == 7.85
� 175 l
Ql) \ Use 10 GPM minimum
�
::::ll 150
VJ \ Ejector System Specifications
en
(l.'I 125 Figure 6-29 is helpful in summarizing specifications to
�
a.. \ conclensers to serve a specific process application. Most
the ejector manufacturer for rating of ejectors and inter-
- 100 � conditions require a detailed evaluation with the manu-
E
0
90
G.:l
facturer's test curves, as very few complicated systems
en 80 \ requiring high vacuum can be picked from stock items.
�
- 70 \ The stock items often fit single-stage vacuum require-
Q.;,
ments for process and such standardized situations as
N
N ' � pump priming.
0
z 60 I'\ Figure 6-29 is also adaptable to air and water ejector
. 8 1.0 1.2 applications .
In all cases it is important to describe the system, its
F=Steam Pressure Factor requirements, control and method of operation in the
specifications. The manufacturer needs complete data
Figure 6-268. Steam pressure factor for Figure 6-26A. By permis- concerning the motive steam (air or water) and the con-
sion, Worthington Corp. densable and non-condensable vapors.
374 Applied Process Design for Chemical and Petrochemical Plants
Table 6-9 Table 6-10
Evacuation Factors for Single Stage Jet Evacuation Factors For Two Stage Non-Condensing
Ejectors
Final Suction Pressure Evacuation Factor,
Final Suction Pressure Evacuation In. Hg. Abs. E
In.Hg.Abs. Factor, E
0.5 0.48
10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.9 1.0 0.67
8 1.0 1.5 .•••.•..•.••...••••.......•••....• 0.81
2.0 0.92
6 1.5
2.5 .....•......................•..... 1.00
5 1.0 3.0 ..•.•.•••..••••........•.........• I.IO
4 1.3 *By permission, Worthington Corp. Bui. W-205-SlOA.
*By permission, Worthington Corp. Bulletin 'W-205-SlA.
Ejector Selection Procedure
Utility unit costs as well as any preference for maxi- As a guide, the following is a suggested procedure for rat-
mum opera.ting economy or minimum first cost should be ing and selecting an ejector system for vacuum operation.
stated if the manufacturer is to make a selection to best fit
the plant system and economics. 1. Determine vacuum required at the critical process
point in system.
2. Calculate pressure drop from this point to the
Tables 6-11, 6-12, 6-13, and 6-14 provide useful refer- process location of the suction flange of the first
ence data. stage ejector.
200 I I I -
150 I I t-::. is90_ - -
.__
et \-\OUn;..-
s,eo�.2--- - "'"-" - - -
-
,��- -
... 100 \ � (j::.,..,-r l _.__ ,_ - -
0
·- -
:::, 80 ...... - --3" -
0 1....,, 1--- - -
i- ....... -
::i:: . I,"' I r::l:J'"' 1.,.- ,_ ·- ..... -
... 60 . _ ....
cu i> I., ,� o·i.- i- ·-
i.-
0. 50 1./v i..,.,,�o l,...,, i.- i.- i.- ....... -
...
t-r v
- 40 l/ i- ...... 1....... ._�oo L.,.,-,_ ,_ i.- i.- - .__ i..- ....... i- i- i-
cu
I
_.a.-
O
,,
,
i., GO
:::,
i.-
-�
1.,.-
- v ,, v ., � � v 0tOO _ i.- i.- i.- i.- i- i..- ,_
,_
i.-
I/
:E 30
v
I
211
0
I/ v
en I/ I/ v I,; vi.,.,,i...- .so 0"' i.- i.- -
.
.Q 20 v .... l,...,, .... i.- i.-
I/ 1./ i> .., L,..,- I i.- i.- _.._ i..- ....- i- i-
IC 15 v / 7..00- i.- ....--
;§ i> I' L.,.,- i....,,,- I i.- i.- i.- .._ .._
l/ i> � (.,..,,,' v .... ,1oi.- ....- i.- i.- i..i- i.- ·- ....- i- i.- i-
i.-
>,
- ... 10 ,; I/ � - i- ,-� 40-
IC 8 ,, . ..... , _ ... _,,o '--"
c.. v L. .....
IC
u ,/ .., ..... , i.,., _L.--
6 v i.,.,
5 l/ ... I' L..,.,
.6 .8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2
Ejector Suction Pressure, Inches Hg. Abs.
Figure 6-27A. Size ejector for 90 psig steam consumption, two-stage, non-condensing 0.6"-3.0" Hg abs. By permission, Worthington Corp.
Ejectors and Mechanical Vacuum Systems 375
c200 I \ 3. At ejector suction conditions, determine:
a. Pounds/hour of condensable vapor
� 175 b. Pounds/hour of non-condensable gases
.... 150 ( 1) dissolved
Q) \
::,
(/)
(/) (2) injected or carried in process
.... 125
Q)
a. \ (3) formed by reaction
( 4) air inleakage
E 100
- 90 4. Prepare specification sheet, Figure 6-29 and forward
c
Q)
U) 80 \ to manufacturers for recommendation.
4) 70 ' 5. For checking and estimating purposes, follow the
N \
N guides as to the number of stages, utility require-
0
:z 60 \ ments, etc. presented herein.
.8 1.0 1.2
F=Steam Pressure Factor Barometric Condensers
Figure 6-278. Steam pressure factor for Figure 6-27A. By permis- Barometric condensers are direct contact coolers and
sion, Worthington Corp. condensers. They may be counter flow or parallel flow.
Good contact direct cooling is an efficient inexpensive
5 ., design, being considerably cheaper and more efficient
/ / / I/ than indirect surface or tubular coolers.
� 4 ,, I I
<[ / 1v;, 1,/ 6" II
�4� -
o 3 2 - -7
::c / v II y
/
i 2 v / Temperature Approach
<>
-= / �v v /
.. / / 1111 / / When serving vacuum equipment, the temperatures
/
le
::,
s 1.0 ,,
.§ .8 ., ,, ., ,, / II" I / I ., / I are usually set as follows when the non-condensables do
c,
not exceed one percent of the total water vapor being
�
j .6 / � / v / condensed. See Figures 6-20 A, B, C, and D.
2 .5 ., / / ., /
.... .,
<> 4 /
� Terminal Difference, steam temperature corresponding to
20 30 40 60 BO 100 200 300400 600 8001,000 vacuum less outlet water temperature = 5°F.
Capacity , Total I bs. of Mixture per Hour
Figure 6-28A. Ejector size, single stage-typical. By permission, Exit Air or Non-Condensables, temperature to be 5°F
Worthington Corp. higher than inlet water temperature to barometric.
.,; I
..0 \
ct 4
c:i, \ I
::c
..,
"'
.J::. \
(.)
=_3
�
� \
:::, I
"' .., '\,, I
"'
a: 2 !'.... ....... I I
- ...........
c:
.s <,
(.)
:::,
en � -- --- .__ - I
-
0
Figure 6-288. Steam consumption fac- 3 4 5 6 7 8 9 10 II
tor. By permission, Worthington Corp. W� = Pounds of Motive Steam per lb. of Mixture Handled
376 Applied Process Design for Chemical and Petrochemical Plants
.e
g 1.0 � ,.....
LC - i- �
g .9 I i--
..J .8 i-
Cl> _i..-- t..,.,-'
:i5 7
i• ,....,..... i.,.,,,,-
� .6 ......
"8l /
8 .5 �
g � ./
z /
II 3
�·o .3 .4 .5 .6 .7 .8 .9 1.0
Pounds per Hour of Non - Condensable Gases
Figure 6-28C. Non-condensable load factor.
By permission, Worthington Corp. Pounds per Hour of Total Mixture Handled
,
1.2
- \
....
0
u
if I.I \
.... i'\.
Cl>
.,, <,
.,,
::,
f 1.0
0. r---.. r-.....
e
0 r-, r--
Cl>
t; .9 -..... ..__ -
II --
LI..
60 80 100 120 140 160 180 200 220
Figure 6-280. Steam pressure factor. By per-
Nozzle Steam Pressure -:ti: Go. mission, Worthington Corp.
5
1.2 -------------- GPM cooling water required = W L/ (L'l t.., 500) (6-23)
where W, is the pounds of steam to condense and L is the
.... latent heat of vaporization, usually taken as 1000 BTU/lb
.......
Q) I.I t-----1----+--- �---1 for process applications and 950 BTU/lb for turbine
::, - exhaust steam [6, 12].
�
Suction Pressure
� 4.0 Hg. Abs �--,c:;;;..+---� Example 6-12: Temperatures at Barometric Condenser
11
c: 1.0
... on E:jector System
0
(.) A barometric condenser is to condense 8,500 pounds
.... per hour of steam at 3.5 in. Hg abs using 87°F water. The
Q)
� 0. 9 ..,_ __ ,,t1:;..__;�...L-----f----1 non-condensables are 43 pounds/hr. Note that the non-
(.)
condensables are less than one percent of the steam.
Steam temperature (steam tables)
0·8 so 70 80 90 100 at 3.5 in. Hg abs 120.6°F
Cooling Water Temperature,°F Terminal difference 5
Outlet water temperature
Figure 6-28E. Steam requirement correction for two-stage unit with from barometric l 15.6°F
barometric intercooling. By permission, Fondrk, V. V., Petroleum Inlet water temperature 85
Refiner, V. 37, No. 12, 1958 [3]. 'Nater temperature rise 30.6°F
Ejectors and Mechanical Vacuum Systems 377
SPE;.C, D\"i'G, NO.
A.
Job No, --------------
Page of Pag<-s
B/M No. Unit Price
STEAM EJECTOR SPECIFICATIONS No. Uni ts I
Item No.
PERFORMANCE
-
Make -- Type
Servi ce Condenser: D Borometri c OSurface
No. of Stages No, of Ejectors per Stage --
Suet. Pr e s s MM HgAbs. Suet. Temp OF Max. Di sch. Press MM HgAbs.
Ste cm: Min. Pre!.s PSIA. Temp. __ oF Ouality %
Water! Sou re" Mox. Press _____ PSIA. Mox. Temp. OF
Vol. of Evccuated System Cu. Ft.
Expected Air Leakage L bs/H,.
Max, Evacuating Time Min.
Ejector Load -- Lbs/Hr. -- -- Mol. Wt. - _Cp, BTU/Lb-CF - __ Latent Ht., BTU/Lb. -
Condensobles
Nc n- Condensobles
---
DESIGN I
-·
1 st. Stage 2nd. Stage 3rd. Stage 4th. Stage 5th Stage
P rope I ling Steam, L.bs/H,. - I
Sfecm: !nlet Size --
Press. Class & Facing �----
Water, GPM
- -·· -- i--------------. -- �-- --
Water .'\ T, ° F -
Water: Inlet Size
Press. Class & Facing
Water: Exit Size
� Pre.ss. Class & Facing
� hamber Pr e s s, MM HgABS. .
Suet. Chamber Temp.° F
Condensers: Pre .. Inter .. After I
Barometric: No.Contact Stages I
Sur lc ce: Outside Tube Area Sq. F,. I
MATERIALS OF CONSTRUCTION
Ejector: Steam Chest Steam Nozzles
Di Huser: Inlet Dischorge Su ct. Chamber
1st. Stage Suet. Chamber Inlet (Siz:e x Pr. Cl. x Facing} x x
Barometric Condens.er: Shell Baflleo • Nozzles
I Surface Condenser: Shell x x Heed Material
Tubes (0.0. x !lWG x L)
Tube Sheet Baffles
Steam Strainer Shut Off Valves
�
c
n
3 " �
REMARKS .
�
n
Toil Pipes Furnished by
Interconnecting Piping by .
0
�
0
�
r
�
� I Chk'd. I App. IRev. _ ]S __ t:__ z c
Date I "' "
Figure 6-29. Steam ejector specifications.
378 Applied Process Design for Chemical and Petrochemical Plants
Table 6-11 Table 6-13
Air Density Table Temperature-Pressure-Volume of Saturated Water
Vapor Over Ice
.. -
Density. Lbs./Hr./CFM at
Temp. °F. Lbs./cu. ft. 30" Hg. Abs. Specific
ABSOLUTE PRESSURES Volume
30 . 0.08105 4.86 Temp. ------ Cu. Ft.
40 . 0.07943 4.76 •F. Inches Hg M.M.Hg Microns Per Lb.
50 . 0.07785 4.66
60 . 0.07635 4.58 32 0.1803 4.580 4580 3,306
70 . 0.07493 4.50 30 0.1645 4.178 4178 3,609
80 . 0.07355 4.42 25 0.1303 3.310 3310 4,508
90 . 0.07225 4.34 20 0.1028 2.611 2611 5,658
100 . 0.07095 4.25
110 · 0.06966 4.18 15 0.0806 2.047 2047 7,140
120 . 0.06845 4.10 10 0.0629 1.598 1598 9,050
130 . 0.06730 4.04 5 0.0489 1.242 1242 11,530
140 . 0.06617 3.97 0 0.0377 0.958 958 14,770
150 . 0.06510 3.91
-5 0.0289 0.734 734 19,040
-10 0.0220 0.559 559 24,670
-15 0.0167 0.424 424 32,100
-20 0.0126 0.320 320 42,200
Table 6-12
Gas Constants -25 0.0094 0.239 239 55,800
-30
74,100
0.180
0.0071
180
-35 0.0051 0.130 130 99,300
PV = WRT R = 1544/Molecular Weight -40 0.0038 0.097 97 133,900
I
For Use With Units Of:
Cubic Feet, Lbs./Sq. Ft. Abs., R., Pounds Values obtained from Keenan & Keyes-"Thermodynamic
0
Properties of Steam". John Wiley & Sons, 1936, by permission.
Mol. Gas
Gas or Vapor Formula Weight Constant, R
Hydrogen ............... I-h 2 772 suspended solids and slurries. Sizes range from l'2 inch to
Carbon Monoxide ........ co 28 55.l 24 inches. The ejectors are usually used in pumping air or
Oxygen ................. 02 32 48.3 gases while the eductors are used in pumping liquids.
Methane ................ CH. 16 96.5
Ethylene ................ C2H• 28 55.1
Nitrogen ................ N2 28 55.1
Ammonia ............... NH a 17 90.8 Steam Jet Thermocompressors
Carbon Dioxide ...... ' ... C02 44 35.l
Steam (Water Vapor) ..... H20 18 85.8
Sulfur Dioxide ........... S02 64 24.1 Steam jet thermocompressors or steam boosters are
Air ..................... ... ' 29 53.3 used to boost or raise the pressure of low pressure steam
to a pressure intermediate between this and the pressure
of the motive high pressure steam. These are useful and
economical when the steam balance allows the use of the
Air temperature leaving barometric = 85 + 5 = 90°F
necessary pressure levels. The reuse of exhaust steam
from turbines is frequently encountered. The principle of
8500 (1000)
en.,[ cooling water required = = 556 operation is the same as for other ejectors. The position
(30.6) (500)
of the nozzle with respect to the diffuser is critical, and
Water Jet Ejectors care must be used to properly position all gaskets, etc.
The thermal efficiency is high as the only heat loss is due
Ejectors using water as the motive fluid are designed to radiation [5].
for reasonable non-condensable loads together with large
condensable flows. Water pressures as low as 10-20 psig Ejector Control
are usable, while pressures of 40 psig and higher will
maintain a vacuum of 1-4 inches of Hg absolute in a sin- Ejectors do not respond to wide fluctuations in operat-
gle stage unit [1]. Combinations of water and steam ejec- ing variables. Therefore, control of these systems must
tors are used to efficiently handle a wide variety of vacu- necessarily be through narrow ranges as contrasted to the
um situations. The water ejector serves to condense the usual control of most other equipment.
steam from the steam ejector. For the single stage ejector, the motive steam flow can-
\,\Tater ejectors and water jet eductors are also used for not be decreased below critical flow in the diffuser [2],
mixing liquids, lifting liquids, and pumping and mixing (Figure 6-30). Units are usually designed for stable opera-
Ejectors and Mechanical Vacuum Systems 379
Table 6-14
Pressure-Temperature-Volume of Saturated Steam
Absolute Absolute Absolute
Pressure Temperature Volume Pressure Temperature Volume Pressure Temperature Volume
Ins. Hg oF. cu. ft./lb. Ins. Hg OF. cu. ft./lb. Ins. Hg OF. cu. ft./lb.
------
0.1803 32 3306 3.0 115.06 231.6 20.0 192.37 39.07
3.1 116.22 224.5 21.0 i94.68 37.32
3.2 117.35 217.9 22.0 196.90 35.73
3.3 118.44 211.8 23.0 199.03 34.28
0.20 34.57 2996.0 3.4 119.51 205.9 24.0 201.09 32.94
0.25 40.23 2423.7 3.5 120.56 200.3 25.0 203.08 31.70
0.30 44.96 2039.4 3.6 121.57 195.1 26.0 205.00 30.56
0.35 49.f)6 1761.0 3.7 122.57 190.1 27.0 i 206.87 29.50
0.40 52.tH 1552.8 3.8 123.53 185.5 28.0 208.67 28.52
0.45 55.89 1387.7 3.9 124.49 181.0 29.0 210.43 27.60
0.50 58.80 1256.4 29.922 212 26.80
0.60 63.96 1057. l 30 212.13 26.74
0.70 68A.1 913.8
0.80 72.32 805.7
0.90 75.84 720.8 Lb. Per Sq.
4.0 125.43 176.7 In. Abs.
4.5 129.78 158.2
5.0 133.76 143.25 14.696 212 26.80
5.5 137.41 131.00 15 213.0:3 26.29
1.00 79.03 652.3 6.0 140.78 120.72 20 227.96 20.089
1.10 81.96 596.0 6.5 143.92 112.00 30 250.33 13.746
1.20 84.64 549.5 7.0 146.86 104.46 40 267.25 10.498
1.30 I 87.17 509.1 7.5 149.63 97.92 50 281.01 I 8.515
1.40 89.51 474.9 8.0 152.24 I 92.16 60 292.71 7.175
1.50 91.72 444.9 8.5 154.72 87.08 70 302.92 6.206
1.60 93.81 418.5 9.0 157.09 82.52 i 80 312.03 5.472
1.70 95.78 395.3 9.5 159.48 78.48 90 320.27 4.896
1.80 97.65 374.7 100 327.81 4.432
1.90 99.43 356.2
125 344.33 3.587
150 358.42 I 3.015
2.00 101.14 339.2 10.0 161.49 74.76 175 370.75 ! 2.602
2.10 102.77 324.0 11.0 165.54 68.38 200 381.79 2.288
2.20 I 104.33 310.3 12.0 169.28 63.03 225 391.79 2.0422
2.30 105.85 297.4 13.0 172.78 58.47 250 400.95 1.8438
2.40 ! 107.30 285.8 14.0 176.05 54.55 275 409.43 1.6804
2.50 108.71 274.9 15.0 179.14 51.14
2.60 ll0.06 265.0 16.0 182.05 48.14
2.70 111.37 255.7 17.0 184.82 45.48 300 417.33 1.5433
2.80 112.63 247.2 18.0 187.45 43.11 350 431.72 1.3260
2.90 113.86 239.1 19.0 189.96 40.99 400 444.59 1.1613
I I
Values obtained directly or by interpolation from Keenan & Keyes-"Thermodynamic Properties of Steam," John Wiley & Sons, 1936
by permission and Courtesy C. H. Wheeler Co., Philadelphia, Pa.
tion at zero suction flow with the motive fluid maintaining tion of air ( 1) overloads the aftercondenser, the discharge
the required volume and energy to produce the necessary mixture can be recycled to control the pressure [3].
diffuser velocity. This is "shut-off" operation. A decrease in Figure 6-32 illustrates ejector systems with large con-
motive pressure below the stability point will cause a dis- densable loads which can be at least partially handled in
continuity in operation and an increase in suction pres- the precondenser, Controls are used to maintain constant
sure. If the motive fluid rate increases, the suction pressure suction pressure at varying loads (air bleed), or to reduce
will increase or capacity will decrease at a given pressure. the required cooling water at low process loads or low water
Figure 6-31 illustrates control schemes for the single temperatures [2]. The cooler water must not be throttled
stage unit wl ich allow greater stability in performance. As below the minimum (usually 30%-50% of maximum) for
the load changes for a fixed suction pressure, the process proper contact in the condenser. It may be controlled by
fluid is replaced by an artificial load (usually air; Figure 6- tailwater temperature, or by the absolute pressure.
31, item 1) to maintain constant ejector operation. An The controls for larger systems involve about the
artificial pressure drop can be imposed by valve (2), same principles unless special performance is under
although this is not a preferred scheme. Wben the addi- consideration.
380 Applied Process Design for Chemical and Petrochemical Plants
Absolute Pressure where Pct = piston displacement cu fl/min
Controller
0 V = system volume, cu ft
t = evacuation time, min
Steam P 2 = absolute discharge pressure of pump, psia
Pc = absolute intake pressure of pump with closed intake
P 1 = absolute intake pressure of pump
Process
§�
The relation above is theoretical, and does not take
Discharge into account any inleakage while pumping. It is recom-
mended that a liberal multiplier of perhaps 2 or 3 be used
Figure 6-30. Single-stage ejector control. to estimate closer to actual time requirements.
An alternate relation for calculating evacuation time is
from the Heat Exchange Institute [11]:
(6- 25)
Steam
Alternate Pumpdown to a Vacuum Using a Mechanical
Process Pwnp
§_ystem
For large process systems of vessels, piping, and other
Discharge equipment, the downtime required to evacuate the system
before it is at the pressure (vacuum) level and then to
Figure 6-31. Single-stage ejector control with varying load.
maintain its desired vacuum condition, can become an
important consideration during start-up, repair, and
Absolute Pressure 'Atmospheric Air Bleed restart operations.
or Tem�eroture Waler I
Controller I Reference [26] suggests an improved calculation:
Absolute O I
Pressure -- - --'----Steam
Controller �----Pressure-- Low Vacuum
i -
; si� (6- 26)
-,-- Discharge
cJndenser
i
where t, = pumpdown time, sec
0
Process p" = final pressure in vessel or system, torr
§.rstem Toil pipe
P" 0 = starting pressure in vessel or system, torr
Rps= pump speed, rotations (or strokes)/sec
V' = volume of vessel or system, liters
Figure 6-32. System handling large quantities of condensable V' 0 = volume of pump chamber, liters
vapors. S = pump speed, liters/sec
S 0 = pump speed, at P" liters/sec
0,
Time Required For System Evacuation Sn= pump speed at P" liters/sec
0,
It is difficult to determine the time required to evacu- Example 6-13: Determine Pump Downtime for a System
ate any particular vessel or process system including pip-
ing down to a particular pressure level below atmospher- Calculate the pump downtime for a system of vessels
ic. When using a constant displacement vacuum pump and piping with a volume of 500 liters. The final pressure
this is estimated by O'Neil [31]: is to be 0.01 torr, starting at atmospheric. From the speed-
pressure curve of a manufacturer's pump at 0.01 torr,
0
v p - p c speed is 2.0 liters/sec. At atmospheric pressure, S = 2.75
2
Pd= -log (6-24) liters/sec with P" 0 = 760 torr. From the manufacturer's
t c p - p
I c data, Rps = 15 and V' 0 = 0.5 liters.
Ejectors and Mechanical Vacuum Systems 381
( • Determine operating steam pressure, psig.
u"
In�- J • Determine/ establish required final suction pressure
p"
0 in vessel/system, inches Hg abs.
Solving: t = (6- 26)
In[ ,v J • Establish discharge pressure required (usually to
Rp, v' � � atmosphere), psig.
• Note that the performance is specific Lo the ejector
ln(Q.01) used.
760
(15 ) ln ( 500 ) Example 6-14: Evacuation of Vessel Using Steam Jet for
500 + 0.5 Pumping Gases
The performance and procedure use the data of Pen-
ln (0.0000131579) -11. 23841862
t= berthy for this illustration (by permission):
(15) In (0. 999000999) 15 (-0.0009995003)
t = 749.6 sec= 12.49 min Evacuating-Selection Procedure
Evacuation With Steam Jets Refer to U Evacuation Time chart.
Rough Estimate of System Pumpdown Using Steam jets [24 j Step 1. Determine evacuation time in minutes per
hundred cubic feel.
The remarks presented earlier regarding the use of
steam jets for pumping down a system apply. The method Step 2. Go to the left-hand column in table, final Suction
of power [35] presented by Reference [24] is: Pressure (hs). Read across to find evacuation time equal
to or less than that determined in Step 1. Read to the top
t = [2.3 - 0.003 (I'',)] V /wj (6-27) of table and note unit number. See Table 6-15.
where t = lime required to evacuate a system from atmos- Step 3. Read Steam Consumption of unit selected off
pheric pressure Lo the steady state operating pres- Capacity Factor Chart. See Table 6-16.
sure, min
P', = design suction pressure of ejector, torr Evacuating-�LE:
V = free volume of the process system, cu ft
"'.i = ejector capacity, 70°F dry air basis, lb/hr To evacuate 3000 cubic foot vessel full of air at atmos-
pheric pressure:
This assumes dry air with no condensables and negligi- Operating Steam Pressure, PSJG (hm) 100
ble pressure drop through the system to the ejector. Also, Final Suction Pressure, inches Hg abs (hs) 5
the jet air handling capacity is assumed approximately Time to evacuate, hrs 2.5
twice the design capacity, and air inleakage during evacu- Discharge Pressure (hd) atmosphere
ation is negligible.
When considering time to evacuate a system using a Step 1. Determine evacuation time in minutes per
steam jet, first recognize that securing reasonable accura- hundred cubic feet.
cy is even more difficult than for a positive displacement
pump. The efficiency of the ejector varies over its operat-
2.5 hr X 60
ing range; therefore as the differential pressure across the ------- = 5 min I 100 cu ft
unit varies, so will the volume handled. Consequently, 30 (hundred) cu fl
evacuation time is difficult to establish except in broad Step 2. Go to the final pressure on left of Evacuation Time
ranges. The above relations can be adopted to establish chart (5 in. Hg hs). Read across and find evacuation time
the order of magnitude only. equal lo or less than 5 minutes. See Table 6-15.
A recommended evacuation calculation is given in Ref:
erence [ 19]. This is specific to Penberthy equipment, but is The U-2 will evacuate the tank in 5.33 minutes per hun-
considered somewhat typical of other manufacturers. dred cubic feet and the U-3 will complete the evacuation
in 3.42 minutes per hundred cubic feet.
• Establish suction load of air to be evacuated in cubic
feet volume of vessel/ system. Step 3. Read steam consumption of selected unit off
• Establish the required time to evacuate, in minutes. Capacity Factor Chart. See Table 6-16. The unit to select
382 Applied Process Design for Chemical and Petrochemical Plants
Table 6-15
Example Using Penberthy Model U Ejector for Evacuation Time
U MODEL EVACUATION TIME (in minutes per 100 cu ft at 100 PSIG Operating Steam Pressure)
Model Number
Suction Press
In. Hg abs. (hs) U-lH U-2H U-3H U-4H U-5H U-6H U-7H U-8H U-9H U-lOH U-llH U-12H U-13H U-14H U-15H U-16H
12" 4.68 3.08 1.98 1.37 1.01 .769 .610 .494 .409 .343 .293 .253 .206 .171 .145 .123
11" 5.06 3.32 2.14 1.48 1.09 .830 .657 .532 .441 .370 .316 .273 .222 .185 .156 .133
10" 5.44 3.57 2.30 1.59 1.17 .894 .707 .572 .474 .398 .340 .293 .239 .198 .168 .143
9" 5.85 3.84 2.46 1.71 1.26 .960 .760 .615 .510 .427 .365 .315 .257 .213 .180 .151
8" 6.29 4.14 2.66 1.84 1.35 1.04 .818 .662 .549 .460 .393 .339 .276 .230 .194 .165
7" 6.76 4.45 2.86 1.98 1.46 1.12 .880 .771 .590 .495 .423 .365 .297 .247 .209 .178
6" 7.35 4.84 3.10 2.15 1.58 1.21 .955 .774 .640 .537 .460 .396 .323 .268 .227 .193
5" 8.10 5.33 3.42 2.37 l.74 1.33 1.06 .853 .706 .592 .507 .437 .356 .295 .250 .213
4" 9.32 6.13 3.94 2.73 2.01 1.54 1.22 .981 .813 .683 .584 .504 .410 .340 .288 .245
3" 11.6 7.60 4.87 3.38 2.48 1.90 1.50 1.22 1.01 .845 .721 .623 .507 .422 .356 .304
By permission, Penberthy Inc.
Table 6-16
Example Ejector Capacity Factor and Steam
U AND L CAPACITY FACTOR AND STEAM CONSUMPTION
MODEL L-11-1 L-2H L-3H L-4H Ir51-I L-61-1 L-71-1 L-81-1 L-9I-I L-lOH L-llH L-12H L-131-1 Irl4H L-151-1 L-161-1
NUMBER U-1!-1 U-21-1 U-3I-l U-4I-I U-5H U-6H U-7H U-8H U-9H U-lOH U-1 lI-I U-121-1 U-13H U-14H U-15H U-16I-I
CAPACITY
FACTOR OPER- .293 .445 .694 1.00 1.36 1.78 2.25 2.78 3.36 4.0 4.69 5.4.3 6.66 8.03 9.49 11.12
ATING STEAM
CONSUMPTION 85 125 195 270 370 ·180 610 755 910 1090 1280 1480 1820 2190 2580 3030
LB. PER HOUR
(Qm) (Valid at standard nozzle pressure of 80, 100, 120, 140, 160, 180 or 200 PSlG.)
By permission, Penberthy Inc.
would be the U-3 in this case and its steam consumption These units are mechanical compressors but are
is 195 pounds per hour. designed to operate at low suction pressures absolute.
They require special seals to prevent inleakage of air or
There are often useful operations performed by jet
equipment, such as pumping air or gases, exhausting sys- other vapors that could create suction performance prob-
tems, heating liquids, mixing of liquids, priming (removal lems. They also require special clearances between the
of air) for centrifugal pumps, and many others (See Fig- housing and the pressure producing elemenr(s). Figures
ures 6-9B and 6-10). 6-9A and 6-10 present representative diagrams of operat-
ing ranges of vacuum pumps and ejectors.
Mechanical Vacuum Pumps The chapter on Compression in Volume 3 of this series
presents details of several mechanical vacuum units, and
The process designer or mechanical engineer in a this information will not be repeated here. However, more
process plant is not expected to, nor should he, actually specific vacuum units and system related data is given.
design a mechanical vacuum pump or steam jet, but
rather he should be knowledgeable enough to establish Figure 6-33 diagrams vacuum system arrangements for
the process requirements for capacity, pressure drops, process systems. It is important to examine the plant eco-
etc., and understand the operation and details of equip- nomics for each system plus the performance reliability
ment available. for maintaining the desired vacuum for process control.
Mechanical vacuum pumps are eight to ten times more The most used mechanical vacuum pumps or com-
efficient users of energy than steam jets; although, steam pressors are reciprocating, liquid-ring, rotary-vane, rotary
jets are reliable and cost less [23]. See Table 6-17. blower, rotary piston, and diaphragm.
Ejectors and Mechanical Vacuum Systems 383
STEAM STEAM STEAM STEAM
NASH
SEPARATOR
RECYCLE
TOWER
TOW�R
Three-stage jet ejector. Ejector with vacuum pump. Pump vacuum system.
Figuro 6-33. Typical vacuum systems holding vacuum on a vessel. By permission, Nash Engineering Co.
Table 6-17 Although the thermal efficiencies of various mechanical
Typical Operating Range of Vacuum Generating vacuum pumps and even steam jet ejectors vary with each
Equipment manufacturer's design and even size, the curves of Figure
(For Ejectors, See Figure 6-9A and Figure 6-25) 6-34 present a reasonable relative relationship between the
types of equipment. Steam jets shown are used for surface
Absolute Pressure Range intercondensers with 70°F cooling water. For non-condens-
(Discharge, then Lower or Suction) ing ejectors, the efficiency would be lower.
Type Equipment mm Hg Abs mm Hg Abs
--------·---·
Rotary piston (Liquid) Combinations of steam jet ejectors operating in con-
Single-stage 760 250 junction with mechanical pumps can significantly improve
Two-stage 250 150 the overall system efficiency, especially in the lower suction
Rotary Lobe, two impellers pressure torr range of l torr to l.00 torr. They can exist
Single-stage 760 250 beyond the range cited, but tend to fall off above 200 torr.
Two-stage 250 75 Each system should be examined individually to deter-
Helical rotary lobe mine the net result, because the specific manufacturer and
Single-stage 760 200 the equipment size enter into the overall assessment.
Two-st.age 200 75 Some effective combinations are:
Centrifugal 760 130
Rotary sliding vane
Single-stage 760 75 • Steam jet-liquid ring vacuum pump
Two-stage 75 10
Reciprocating • Rotary blower-liquid ring vacuum pump
Single-stage 760 30
Two-st.age 30 10 • Rotary blower-rotary vane compressor
Rotary piston, oil sealed Microns, Abs
Single-stage 760 60 • Rotary blower-rotary piston pump
Two-stage 60
Rotary vane, oil sealed
Single-stage 760 7 Liquid Ring Vacuum Pwnps/Compressor
Two-stage 7 0.08
"Varies with manufacturer's equipment. Figure 6-35 provides a pictorial cross section of this
Compiled from various published references and manufacturer's lit-
erature. type unit along with an actual photograph of disassem-
bled major components.
384 Applied Process Design for Chemical and Petrochemical Plants
How it Works: Typical of This Class of Pump (By
permission of [27])
0.60
'The Nash vacuum pump or compressor has only one
> 0.50 moving part-a balanced rotor that runs without any
o internal lubrication. Such simplicity is possible because all
c
"'
� 0.40 functions of mechanical pistons or vanes are performed
by a rotating band of liquid compressant.
Q)
§ 0.30 ' I While power to keep it rotating is transmitted by the
Q)
s: ' I rotor, this ring of liquid tends to center itself in the cylin-
f- 0.20 For one-stage drical body. Rotor axis is offset from body axis. As the
liquid-ring
schematic diagram in Figure 6-35 shows, liquid cornpres-
0.10 sant almost fills, then partly empties each rotor chamber
Multistage steam jet
during a single revolution. That sets up the piston action.
Stationary cones inside the rotor have closed sections
Suction pressure, torr between ported openings that separate gas inlet and dis-
charge flows.
Figure 6-34. Rough estimates of thermal efficiency of various vacu-
um producing systems. By permission, Ryans, J. L. and Croll, S., A portion of the liquid compressant passes out with dis-
Chem. Eng., V. 88, No. 25, 1981, p. 72 [22]. charged air or gas. It is usually taken out of the stream by
O IN THIS secroa LIQUID MOVES
OUTWARD - DRAWS GAS FROM
INl.El PORlS INTO ROTOR
CHAM8EAS
LIOUID
Nash vacuum pump schematic.
Disassembled view shows
appearance of rotor, body
and ported cones.
Figure 6-35. Diagram of liquid ring vacuum
pump features. By permission, Nash Engineer-
ing Co.
Ejectors and Mechanical Vacuum Systems 385
8,000
:E I
:::, CONDENSATION BONUS
:::, -
� 904-R2 at 240 rpm 70of SEAL
;::i
� 7,000 I OF sA.i. ""poR
<( 20
1 VAPOR 70°F SEAL
� 1100F SAT.
LL. 60oFSEA
0 aooF SAT. VAPOR �
> .. ---
1- 6,000 -........_
� - 60°F DRY AIR 60oF SEAL
0:
<)
I I I I I I I I I I I
15 20 24
VACUUM, INCHES OF MERCURY
Figure 6-36. Typical capacity increases gained when a vacuum pump sealed with relatively cool water handles air saturated with water vapor.
By permission, Nash Engineering Co.
a discharge separator furnished by the manufacturer. inlet and outlet ports or openings are ported so the rotary
Makeup is regulated by orifices and manual valve adjust- rotor with its stationary cones are closed between the port-
ments. There is an optimum flow rate, but performance is ed openings that separate the gas inlet and discharge gas-
not seriously affected by variations. liquid mixture to a mechanical separator.
Moisture or even slugs of liquid entering the inlet of a
liquid-ring vacuum pump will not harm it. Such liquid The vapor or gas becomes separated and flows out
becomes an addition to the liquid cornpressanl. Vapor is either to the atmosphere (if air or environmentally accept-
often condensed in a vacuum pump. The condensate is able) or a condensable vapor can be condensed inside the
also added to the liquid cornpressant. pump by using recirculated chilled coolant directly as the
circulating liquid. The excess liquid from the condensa-
In a typical closed-loop system, liquid from the separa- tion can be drawn off the separator.
tor is cooled in a heat exchanger then recirculated. Any
excess liquid. added by mist and vapor flows out through These types of units can accept wet vapors or gases com-
a level-control valve."
ing into the suction, as well as corrosive vapors when the
The liquid ring pump/compressor is available from sev- proper circulating liquid compressan t is selected. Some
eral manufacturers, with about the same operating princi- types of entrained solids can be pumped through the unit,
ple, but differing in mechanical assembly and sealing while abrasive solids will naturally do some erosive dam-
details as well as ranges of operation. This type unit has age. An important significance of this type unit is that the
only one moving part, i.e., a balanced rotor (See Figure 6- liquid used to compress the incoming vapors can be select-
35) that does not require internal lubrication, because the ed to be compatible with the vapors, and does not have to
circulating sealing liquid inside the unit provides cooling be water. For dry air applications, these units normally
and lubrication (even when the liquid is water). There is a operate with 60°F seal compressing water. This keeps the
ring of rotating liquid in the case that is circulated by the waler loss low. For saturated air or other vapors, the chilled
rotor. To accomplish the pumping action, the rotor is off recirculating liquid reduces the volume that the pump
set from the axis of the case or body. The rotating liquid must handle, and thereby, increases pump capacity. Figure
fills and empties the chamber/case of the rotor during 6-36 illustrates for a particular pump the relative increase
each revolution. There are no inlet or outlet valves. The (text continued Dtl page 393)
Liquid Ring Vacuum Pump
NOD[L•s21osc1••••• EN 1311-l•S
STANDARD PERFORMANCE OF SC6 VACUUM PUMP
�
30• HG IAAONET[A. 10 DEG r SEAL WATEA 'O
12..
ro·
PEAFOAMANCE TO HEI STANDARD A£V A ltll c.
'ti
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(1)
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VI
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0 5 10 15 20 25 30
Vacuum-Inches of Mercury Gauge
Liquid Ring Vacuum Pump
NOD[L•s21osc1••••• EN 1311-l•S
STANDARD PERFORMANCE OF SC6 VACUUM PUMP
�
30• HG IAAONET[A. 10 DEG r SEAL WATEA 'O
12..
ro·
PEAFOAMANCE TO HEI STANDARD A£V A ltll c.
'ti
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Vacuum-Inches of Mercury Gauge
Ejectors and Mechanical Vacuum Systems 387
5
I/
..... �
, / 10
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/ 20
v
v 30
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v 40 .,
s
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f'
/
l/ 400
/ 500
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0 2 3 4 5
Factor - f
Total volume to be extracted Volume of vessel x f
(f at the working vacuum from the above chart)
Evacuating time = Total volume to be extracted
Capacity of the vacuum pump at the working vacuum
Exampie: Vessel size 200 ft3
Working vacuum = 60mm Hg. abs.
Evacuating time = 3 minutes
Total volume to be extracted = 200 x f = 200 x 2.5 = 500 ft3
Capacity to be extracted = 500 = 167 CFM at 60mm Hg. abs;
3
Pump selection = V5212 (from chart p.6) at 1750 rpm.
Figure 6-38. Chart for liquid ring vacuum pump to estimate the total volume to be displaced to evacuate a closed vessel to a predetermined
vacuum. By permission, Graham Manufacturing Co., Inc.
388 Applied Process Design for Chemical and Petrochemical Plants
1. Oil Mist Eliminator
2. Exhaust
3. Gas Ballast Valve (RA)
4. Inlet
5. Inlet Screen
® 6. Anti-Suckback Valve
7. Vane
8. Rotor
9. Oil Return Line
10. Spin-on Oil Filter
11. Oil Sight Glass
12. Exhaust Valve (RA)
R5 uperatmg Prmc1p1e
Rotation of the pump rotor, which is mounted eccentrically in the pump cylinder, traps entering vapor between rotor vane segments. As rota-
tion continues, vapor is compressed then discharged into the exhaust box. Vapors then pass through four stages of internal oil and smoke
eliminators to remove 99.9% of lubricating oil from the exhaust. Oil is then returned to the recirculating oil system.
The four stage exhaust box includes the oil box separator, the demister pad, the oil mist eliminator, and the synthetic oil baffle. Additional fea-
tures include an automotive type spin-on oil filter, a built-in inlet anti-suckback valve that prevents oil from being drawn into the system when
the pump is stopped, and a built-in gas ballast, available on the RA version, which permits pumping with high water vapor loads.
Figure 6-39. Rotary vane-type vacuum pump without external cooling jacket. By permission, Busch, Inc.
INLET PRESSURE - INCHES HG
.,, 29.9 29.5 26.0 0
cE' 2000 I I I .--r,· I I I
c: I
;a I
1600
0) -- I
� 1000 - �-· - � I
-
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p �I i
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0.1 1 10 100 760
INLET PRESSURE - TORR
INLET PRESSURE - INCHES HG
.,, 29.9 29.5 26.0 0
cE' 2000 I I I .--r,· I I I
c: I
;a I
1600
0) -- I
� 1000 - �-· - � I
-
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"C ,
c;· -· --- -- ,........ � ....
----
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c: . IJ /' I / . .-� ... -- 0
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INLET PRESSURE - TORR
390 Applied Process Design for Chemical and Petrochemical Plants
POSITION 1 POSITION 2 POSITION 3 POSITION 4
Figure 6-41. Rotating lobe vacuum blowers showing lobe rotation as gas moves through the unit. By permission, Roots Division, Dresser
Industries, Inc.
Figure 6-42. Helical four fluted gate blower rotors/two lobes main rotor intermeshing. See Figure 6-44. By permission, Gardner-Denver Indus-
trial Machinery-Cooper Industries.
Figure 6-43. Screw-type rotors for rotary lobe blower. By permission, Roots Division, Dresser Industries, Inc.
Ejectors and Mechanical Vacuum Systems 391
Agure 6-44. Rotary blower with screw-type rotors. By permission, Gardner-Denver Industrial Machinery-Cooper Industries.
Figure 6-45. Cut-away view of internal assembly of rotary lobe vacuum pump. By permission, Tuthill Corp., M.D. Pneumatics Division.
392 Applied Process Design for Chemical and Petrochemical Plants
1 . Cylinder and headplates
2. Impellers
3. Shafts
4. Bearings
5. Timing gears
6. Lubrication
Figure 6-46A. Exploded view of rotary lobe gas pump with mechanical seals and pressure lubrication for bearings. By permission, Roots Divi-
sion; Dresser Industries, Inc.
Detail - Mechanical seal
PRESSURE IN THIS GAS AREA
NOT TO EXCEED OIL PRESSURE
AT SEAL FACE
SEALING
FACE
" OIL THROntlNG
PASSAGE AND
BEARING FEED
Figure 6-468. Mechanical seal to be used with Figure 6-46A. By permission, Roots Division, Dresser Industries, Inc.
Ejectors and Mechanical Vacuum Systems 393
SEAUl!Cl
FLUID OUT
SU.UNG
FLUIDlH
PROCESS INDUSTRY REQUIREMENT DOUBLE MECHANICAL SEAL
Figure 6-46C. Double mechanical seal used for special gas sealing requirements in Figure 6-46A, and substitutes for the single seal of Fig-
ure 6-468. By permission, Roots Division, Dresser Industries, Inc.
(uxt conli11ued from page 385) Performance capacity curves are based on standard
in expected capacity as recirculated liquid (seal) inside the dry air with 60°F water as the liquid compressant or seal
pump casing is varied in temperatures. liquid. The pumps operate on a displacement or volu-
metric basis; therefore, the CFM capacities are about the
Materials of construction for this type of unit are usu- same for any particular pump for any dry gas mixture. To
ally modular cast iron rotors on steel shafts and cast iron calculate pounds/hours of air or gas mixture, the appro-
casings or bodies. For special requirements in corrosive priate calculation must be made.
situations that cannot be remedied by changing the seal
liquid, pumps can be furnished in Type 316 stainless steel Figure 6-37 presents a typical performance curve for
or other alloys (expensive). this type of vacuum pump. Note that it is specific to 60°F
394 Applied Process Design for Chemical and Petrochemical Plants
Figure 6-47. Cut-away view of internal assembly of rotary lobe vacuum pump. By permission, Tuthill Corp., M.D. Pneumatics Div., Bull. A-
5/888.
seal or compressant water, and that the capability to hold in sealing the moving vane against the casing. The rotary
a vacuum with higher seal water temperature is reduced. shaft is off-center in the casing to provide a continuously
Typically, for a 95°F seal water, the vacuum may only be 26 decreasing volume from suction to discharge of the
to 27 inches Hg compared to the 28-inch Hg vacuum machine. Other styles of rotary vane units do not have
gauge shown. Also, care must be used to recognize that suction or discharge valves. Note that the volume pumped
the curves represent inches of mercury, vacuum gauge, is expressed as free air, which is measured at 60°F and 14.7
not absolute. To convert, use the 30-inch Hg barometer psia. Figure 6-40 illustrates a typical performance curve.
less the vacuum reading to attain absolute vacuum, inch-
es Hg abs. The estimated pump-down capacity perfor- These pumps are relatively simple mechanical units
mance for a typical liquid-ring vacuum pump is given in compared to rotary-piston pumps. The "pumping com-
Figure 6-38. partment vanes" are spring loaded to hold them against
the off-centered position in the casing/housing. Some
designs do not use springs on the vanes, but rely on cen-
Rotary Vane Vacuum Pumps trifugal force to position the sliding vanes sealing against
the casing wall. These pumps are used for medium vacu-
um ofless than I torr [20]. Also see Parkinson [34].
Figure 6-39 illustrates a single-stage rotary vane vacuum
pump, without external cooling jacket. The sliding vanes Units without an oil pump rely on once-through oiling
(No. 7 in illustration) are oiled by a closed system to aid for vacuum sealing. The oil usage is low for most units.
Ejectors and Mechanical Vacuum Systems 395
20 30
' i I ............
c, 20
18 iii I r,,,.,.,,.
l c..
I Pressure capability l
E 10
16 ::, for
! Ill
Ill
14 5- 5 rotary lobe blowers
.5! Cl)
� e>
al
c: 12 .c:
e
·;;; I "
Ill
Ill c
� 10 I
a. I 1 .
E 50 100 500 1,000 5,000 10,000 50,000
0
o 8 Inlet volume - CFM
high vacuum
6 boosters 22 I
I Ii I I I E 20
::,
4 i:l 18
i � 16
i \
2 b, 14 Vacuum capability
I
50 100 500 1,000 5,000 10,000 30,000 ! 12 I I I I II f l I I II '
11 or
Gross displacement - CFM 1! 10 rotary lobe blowers ,
"
-= 8
Figure 6-48A. Typical performance of high vacuum booster lobe- 6 11 I I . I
I
I
type high volume draw-down for evacuating vacuum systems before 50 100 500 1,000 5,000 10,000 50,000
use of higher vacuum (lower absolute pressure) pump. By pennis- Inlet volume - CFM
sion, Roots Division, Dresser Industries, Inc.
Figure 6-48C. Typical application rotary lobe vacuum blower perfor-
mance. By permission, Roots Division, Dresser Industries, Inc.
28
26
E 24 Rotary Blowers or Rotary Lobe-Type Blowers
::,
::, 22
"
"'
:,. ' These units are useful in both vacuum and pressure
"'
:c 20 water-sealed ranges and can usually develop compression ratios of 3:1
Ill 18 to 10:1, depending on the inlet absolute pressure. The
"
.c: vacuum pumps
" 16
-= units are positive displacement in performance.
14 The units use meshing balanced lobes (Figures 6-41, 6-
I 42, 6-43, 6-44, 6-45, 6-46 A, B, C and 6-47) or screw-type
12
100 500 1,000 5,000 10,000 30,000 rotors that are synchronized by timing gears and do not
lnlet volume - CFM touch each other or the housing with very close toler-
ances. The shafts of the driving shaft and driven shaft are
Figure 6-486. Perfcnnance of lobe-type vacuum pump using water
spray internally to reduce slip of gases from discharge to suction. By sealed with labyrinth type seals for a minimum leakage.
permission, Roots Division, Dresser Industries, Inc. Purged labyrinth or mechanical type seals are available.
The rotors do not require lubrication per se because
they do not touch; however, some designs add a small
These units cannot handle iiquid slugs or dirt particles; amount of sealing liquid such as water or other compati-
hence, they require gas/vapor cleaning before entering ble fluid to reduce the "slip" or backflow as the lobes
the unit. For units with cooiing jackets and automatic rotate. Depending on the type of design, i.e., lobe or
temperature regulation of the process gas, the tempera- screw-type cycloidal or helical rotors, the discharge pres-
ture can range as high as 302°F [20].
sure may have some pulsation, such as -60% to 140+%
of gas discharge pressure, or it may be essentially
"smooth," almost pulsation free. The shafts of the rotors
rotate .in opposite directions by means of the drive gears.
By adjusting the jacket temperatures, condensation or The air or process gas/vapor is drawn .into the suction or
polymerization can be avoided inside the pump. The vol- inlet cavity of the intersecting and rotating "lobes" by the
ume handled Figure 6-40 is often expressed as "free air, rotor mesh. As the rotors continue to rotate, the cavity of
CFM." This is simply the mechanical displacement of the suction gas is sealed from the inlet by the moving and/ or
pumping volume of the unit for the particular driven advancing "lobes" as they pass the fixed boundary of the
speed of the unit. inlet opening of the casing. AI; the rotors rotate, the gas is
396 Applied Process Design for Chemical and Petrochemical Plants
(.045 CFR DISPL.)
250
200
6T'
12 150
10 100
B
50
5
2 0
AIRFLOW BASED UPON "Hg VAC
�,so INLET: 70" F 2
::. t+H,..........., DISCHARGE: 29.92" Hg Abs. 5
Li.
S12s B
1- 10
w 12
...J 100
�CFM 15
�75
�
g 50 8
Li. "H VAC.
0:: 15
< 25 7
12
0 6
10
5
B
4
5 BHP
3
2
0::
� 2
0
0..
w
(/)
0::
0
:r: 0
1000 1500 2000 2500 3000 3500 4000
RPM
Figure 6-49. Typical performance curves for rotary lobe vacuum pump. By permission, Tuthill Corp., M.D. Pneumatics, Division.
compressed against the discharge headplate, similar in are referenced to the indicated vacuum, not absolute pres-
concept to a piston compressor against a piston compres- sure. (See the beginning of this chapter for clarification).
sor cylinder head [21].
Booster vacuum pumps are used to shorten the pump- Rotary Piston Pumps
down on evacuation (time) of a vacuum system before
switching to the smaller vacuum pump to maintain the These units are also positive displacement oil-sealed
system opening vacuum and to handle the air inleakage pumps. The pumping action is shown in Figure 6--50. The
to the system. action of the rotating piston assembly draws in a set vol-
A typical performance range of capacities of rotary lobe ume (depending on the pump's size/capacity) of gas,
vacuum pumps is shown in Figures 6-48A, 6-48B and 6- compresses it in the eccentric rotating cylinder against
48C. Another set of curves for rotary lobe pumps, shown the inside wall of the casing, and exhausts it to atmos-
in Figure 6-49, provides the brake horsepower, airflow at phere, as do most other vacuum pumping devices. AE. the
inlet (CFM) (referred to as their standard volume at 70°F oil-sealed piston revolves, it opens the inlet port, draws in
and 29.92 inch Hg abs discharge pressure-essentially gas, and traps it in the fixed suction space. As the piston
atmosphere), and the temperature rise through a non- rotates, the gas is compressed, and the discharge valve
cooled (no internal or external cooling) vacuum. All data opens to discharge the gas. A single-stage pump can have
Ejectors and Mechanical Vacuum Systems 397
hL = Liquid height, ft
K = Non-condensable load factor
L = Latent heat of vaporization of steam, BTU/lb
M = Average mol weight of system vapors
M, = Molecular weight of non-condensable gas
M, = Molecular weight of conclensable vapor
P = Total absolute pressure, lbs/sq in. absolute (or
other consistent units), or system operating pres-
sure, torr
Pa= Partial pressure of air in mixture, lbs/sq in. abs
Pc = Absolute intake pressure of pump
Pct = Piston displacement, cu ft/min
P n = Partial pressure of non-conclensable gas; pounds
per square inch absolute (or other absolute units)
P, = Static pressure, atm
Figure 6-50. Typical rotary displacement vacuum pump, oil sealed, P,, = Vapor pressure of condensable vapor, pounds per
single-stage. By permission, Kinney Vacuum Co.
square inch absolute (or other absolute units)
P' = Partial pressure of a vapor in a mixture, psia
compression. ratios up to 100,000:1 when discharging to P,' = Design suction pressure of ejector, torr
atmosphere [22]. P n" = Final pressure in vessel or system, torr
Mechanically, the pump operates in an oil bath, with P 0" = Starting pressure in vessel or system, torr
the seaiing oil lubricating the pump and seals against P = Atmospheric pressure, mm Hg
back flow from the exhaust to the intake/suction. P 1 = Intake pressure of pump, psia or, initial pressure in
sys., in Hg abs. (Eq. 6-25)
These pumps cannot effectively handle condensation P 2 = Discharge pressure of pump, psia
of vapors inside the unit, because the capacity is reduced Pc= Intake pressure of pump with closed intake, psia
and the condensate creates lubrication problems, which P 2 = Final pressure in system, in. Hg abs
in turn leads to mechanical breakdown. R = Gas constant, = 1544/mol weight
To prevent/reduce the undesirable condensation in Rp, = Pump speed, revolutions ( or strokes) per second
S = Pump speed, liters/sec
the pump, a small hole is drilled in the pump head to S 11 = Pump speed at P n", liters/sec
admit air or other process non-condensable gas (gas bal- S 0 = Pump speed at Po", liters/sec
last) into the latter portion of the compression stroke. This T = Temperature, R = 460 + °F
0
occurs while the vapor being com pressed is sealed off from t = Evacuation pump downtime, min
the intake port by the piston. By reducing the partial pres- t, = Evacuation pump downtime, sec
sure of the vapor's condensables, the condensation is t,. = Ambient air temperature, °F
avoided. Obviously, this can reduce the capacity of the tm = Temperature of mixture at ejector suction, °F
t, = Temperature of steam on downstream side of noz-
pump, as the leakage past the seals allows the gas ballast to zle, °F
dilute the intake volume of base suction gas. For most Lltw = Temperature rise of water, °F
process applications, the effect of this leakage is negligible, V = Volume of tank or system, cu ft
unless the vacuum system suction is below 1 torr [22]. V' = Volume of vessel or system, liters
Huff [23] found that reciprocating and rotary piston V 0' = Volume of pumping chambers, liters
pumps were the most economical mechanical systems for W or Wa = Flow rate, lbs/hr
Wn = Weight of non-condensable gas, lbs/hr
their range of application. Obviously, the economic dis- Wm = Total pounds of mixture handled per hour
cussions are dependent on the vacuum expected and the vV, = Total steam consumption, lbs/hr
local utility costs, plus the cost of maintenance. '"-'v = Weight of condensable vapor, lbs/hr
W, = Total weight of gas, lbs
WTa = Total calculated air inleakage, lbs/hr
N omenclature or, WT = Total calculated air inleakage, lbs/hr
w; = Air inleakage resulting from metal porosities and
Cpa = Specific heat of air at constant pressure (0.24 cracks along weld lines, lbs/hr
approx.) Wm' = Ejector capacity at final evacuation suction pres-
cp, = Specific heat of steam at constant pressure corre- sure, lbs/hr
sponding to downstream absolute pressure (0.45 w; = Pounds of motive steam per pound of mixture han-
approx.) dled
D = Sealed diameter, inches (estimates of nominal Y.,T,,' = Pounds water vapor per pound air
diameter acceptable) w = Constant flow rate, lbs/min
E = Evacuation factor, at final evacuation suction pres- w. = Acceptable air leakage rate assigned to a system
sure, Tables 6-9 and 6-10 component, lbs/hr
F = Steam pressure factor wj = Ejector capacity, 70°F dry air basis, lbs/hr
398 Applied Process Design for Chemical and Petrochemical Plants
Greek Symbols 23. Huff, G. A., Jr., "Selecting a Vacuum Producer," Chem. Eng.,
V. 83, No. 6, 1976, p. 83.
p = Density, lbs/ cu fl 24. Ryans,]. L. and D. L. Roper, Process Vacuum System Design and
PL = Specific gravity of liquid, relative Lo water = 1.0 operation, 1986, McGraw-Hill Book Co.
8 = Specific air leakage rate, lb/hr/in.
re= Pi= 3.1418 25. Gibbs, C. W., editor, Compressed Air and Gas Data, 1969, Inger-
soll-Rand Co.
Subscripts 26. Consiantinescu, S., "Alternative to Gaede's Formula for Vac-
uum Pumpdown Time," Chem. Eng., V. 94, No. 7, 1987.
a= air 27. Nash Engineering Co., Technical Catalogs.
1, 2, etc. = Refers to components in system, or initial and final 28. Meyerson, E. B., "Calculate Saturated-Gas Loads for Vacuum
system pressures, psia Systems," Chem. Eng. Prog., V. 87, No. 3, 1991.
90 = Steam pressure, psia
v = Condensable vapor 29. Gomez, J. V., "Calculate Air Leakage Values for Vacuum Sys-
n = Non-condensable gas/vapor tems," Chem. Eng .. V. 98, No. 6, 1991.
30. Ketema, Schutte and Koening Div., Bul.J2-15Ml79B, 1979.
References 31. O'Neil, F. W., Ed., Compressed Air Data, 5th Ed., Compressed Air
Magazine, 1939, p. 73.
I. Berkeley, F. D., "Ejectors Have a Wide Range of Uses," Pet.
Ref. 37, No. 12, 1958, p. 95. 32. Wintner, B., "Check the Vacuum Ratings of Your Tanks,"
2. Blatchley, C. G., Controlling Ejector Performance, 1956, Schutte Chem. Eng., V. 98, No. 2, 1991, p. 157.
and Koerting Co. 33. Birgenheicr, D. B., Butzbach, T. L., Bolt, D. E., Bhatnagar, R.
3. Fondrk, V. V., "Figure What an Ejector Will Cost," Pet. Ref. K., and Aglitz, J. "Designing Steam Jet Vacuum Systems,"
37, 1958, p.lOL. Chem. Eng., V. 100, No. 7, 1993, p. 116.
1. Freneau, Philip, "Steam-]et. Air Ejectors," Power; May, June, 34. Parkinson, G. and Ondrey, G., ''Vacuum Pumps Are Running
July, 1945, Worthington Corp., RP-284. Dry," Chem. Eng., V. 99, No. 6, 1992, p. 121.
5. Freneau, Philip, "Steam-Jet Thermocompressors Supply
Intermediate-Pressure Processes," Power;Jan. 1945, Feb. 1945. 35. Power, R. B., Hydrocarbon Proc. v, 43, No. 3, (1964) pg. 138.
6. Graham Manufacturing Co., Bulletin, 1955.
7. Hammond, N. B., 'The Jet Compressor as Applied to the Gas
Industry," presented before the New England Gas Association, Bibliography
Operating Division Meeting, Providence, R. !.,June 1958.
8. Ketterer, S. G. and C. G. Blatchley, Steam jet Vawum Pumps, Matthews, J., "Low-Pressure Steam EjecLors," Chem. Eng., V. 94,
SchutLe and Koerting Co. No. 9, 1987, p. 155.
9. Linck, C. G., "Selecting Ejectors for High Vacuum," Chem.
Eng., Jan. 13, 1958, p. 145. Croll-Reynolds Co., Vamum Systems, Bulletin E68A, Croll-
10. Standards for Direct Contact Barometric and Low Level Condensers, Reynolds Co., Westfield, NJ.
Llth Ed., 1957, Heal Exchange Institute, Cleveland, Ohio. Croll-Reynolds Co., Design and Application of Steam Jet Vacuum
11. Standards for Steam Jet Ejectors, 3rd. Ed., 1956 and Standards Ejectors, Croll-Reynolds Co., Westfield, NJ.
for Steam ]et Vacuum Systems, 4Lh Ed., 1988, Heat Exchange
Institute. Steuber, A., "Using Steam Jet Ejectors in Vacuum Systems," Plant
12. Standards for Steam Surface Condensers, 4th Ed., 1955, Heal Engineering,June 26, 1975.
Exchange Institute, Cleveland, Ohio. Jackson, D. H., ·when to Use Steam jet Ejectors, Bulletin CR-722,
13. Steam jet Ejectors, Form 9046, Ingersoll Rand Co. Croll-Reynolds Co.
14. Steam-jet Ejector Application Handbook, Bulletin W-205-E21,
1955, Worthington Corp. Roper, D. L., and Ryans,]. L., "Select the Right Vacuum Gauge,"
15. Thejet-Vac Corp., Bulletin, 400 Border St., East Boston, Mass. Chem. Eng., V. 96, No. 3, 1989.
16. Steam Jet Ejector Manual, W-205, May 1947, WorthingLOn Steuber, A., "Consider These Factors for Optimum Vacuum Sys-
Corp. tem Selection," Hydro Proc., V. 61, No. 9, 1982, p. 267.
17. Water Jet Exhausters and Compressors, Bui. 4P, 1957, Schulle
and Koerting Co. Pu111:ps, Vacuum Pumps and Compressors '87, series of technical arti-
18. Graham Manufacturing Co., Ejector Operating Maintenance cles, published by Dr. Harnisch Verlagsgesellschaft mbH,
and Installation Manual, Bui OMI:JEl-184 (no date). UBERSEE-POST ORGAl"\/ISATION, Postf., D-8500 Nurnberg
19. Pumping Gases, Bul 1300, Seel. 1000, July 1987, Penberthy 1/FRG.
Inc. Myerson, E. B., "Calculate Saturated-Gas Loads for Vacuum Sys-
20. Patton, P. ,,v., ''Vacuum Systems in the CPI," Chem. Eng: Prag., terns," Chem. Eng. Prag .. \T. 87, No. 3, 1991, p. 92.
December 1983, p. 56.
21. Bulletin S-5627, LAL Spiraxial Compressors, Roots-Dresser LaPelle, R. R., Practical Vacuum Systems, 1972, McGraw-Hill Book
Industries, Roots Blower Operation, Connerville, Ind. Co., 1st Ed.
22. Ryans.]. L. and Croll, S., "Selecting Vacuum Systems," Chem. Sanders, Roy, "Don't Become Another Victim of Vacuum,"
Eng., V. 88, No. 25, 1981, p. 72. Chem. Eng. Prog., V. 89, No. 9, 1993, p. 54.
Chapter
7
Process Safety and
Pressure-Relieving Devices
The subject of process safety is so broad in scope that possible injury to personnel, the loss or equipment can be
this chapter must be limited to the application design, rat- serious and an economic setback.
ing, and specifications for process over-pressure relieving Most states have laws specifying the requirements
devices for flammable vapors and dusts; process explo- regarding application of pressure-relieving devices in
sions and external fires on equipment; and the venting or
flaring of emergency or excess discharge of gases to a vent process and steam power plants. In essentially every
instance at least part or the reference includes the
flare stack. The subject of fire protection per se cannot be A.S.1\;1.E. Boiler and Pressure Vessel Code, Section VIII, Divi-
adequately covered because partial treatment would be sion 1 (Pressure Vessels) and/or Division 2 [1]; and Sec-
"worse" than no treatment; therefore, the engineer is tion VII, Recommended Rules for Care of Power Boilers [2]. In
referred to texts dealing with the subject in a thorough addition the publications of the American Petroleum
manner [l, 30, 31, 32, 33, 34]. The important topic or Institute are helpful in evaluation and design. These are
steam boiler safety protection is not treated here for the
sarne reason. API-RP-520 [ 10 J, Design and Installation of Pressure-Relieving
Systems in Refineries; Part 1-Desig;a; Part !I-Installation;
and API-RP-521 [13], Guide for Pressure Relief and Depres-
The possibilities for development of excess pressure surizing Systems, Ai"JSI/ASME B31.l Power Piping; Bl6.34;
exists in nearly every process plant. Due to the rapidly and NFPA [27], Sections 30, 68, and 69.
changing and improved data, codes, regulations, recom-
mendations, and design methods, it is recommended that The ASME Code requires that all pressure vessels be
reference be made to the latest editions of the literature protected by a pressure-relieving device that shall prevent
listed in this chapter. While attempting to be reliable in the internal pressure from increasing more than 10%
the information presented, this author cannot be respon- above the maximum allowable working pressures of the
sible or liable for interpretation or the handling of the vessel (MA',,VP) lo be discussed later. Except where multi-
information by experienced or inexperienced engineers. ple relieving devices are used, the pressure shall not
This chapter's subject matter is vital to the safety of plants' increase more than 16% above the Mi-\WP or, where addi-
personnel and facilities. tional pressure hazard is created by the vessel being
exposed to external heat (not process related) or fire,
It is important to understand how the over-pressure supplemented pressure relieving devices must be installed
can develop (source) and what might be the eventual to prevent the internal pressure from rising more than
results. The mere solving of a formula to obtain an orifice 21 % above the MAWP. See Ref. [l] sections U-125 and
area is secondary to an analysis and understanding of the UG-126. The best practice in industrial design recom-
pressure system. Excess pressure can develop from explo- mends that (a) all pressure vessels of any pressure be
sion, chemical reaction, reciprocating pumps or com- designed, fabricated, tested and code stamped per the
pressors, external fire around equipment, and an endless applicable ASME code [ 1] or American Petroleum Insti-
list of related and unrelated situations. In addition to the tute (API) Codes and Standards, Ref. [33] and (b) that
399
400 Applied Process Design for Chemical and Petrochemical Plants
pressure relieving devices be installed for pressure relief Types of Valves/Relief Devices
and venting per codes [1, 10, 13] [33].
Although not specifically recognized in the titles of the There are many design features and styles of safety
codes, the rupture disk as a relieving device, is, neverthe- relief valves, such as flanged ends, screwed ends, valves fit-
less, included in the requirements as an acceptable ted internally for corrosive service, high temperature ser-
device. vice, cryogenic service/low temperatures, with bonnet or
Usual practice is to use the terms safety valve or relief without, nozzle entrance or orifice entrance, and resis-
valve to indicate a relieving valve for system overpressure, tance to discharge piping strains on body. Yet most of
and this will be generally followed here. When specific these variations have little, if anything to do with the actu-
types of valves are significant, they will be emphasized. al performance to relieve overpressure in a system/vessel.
A few designs are important to the system arrangement
and relief performance:
Types of Positive Pressure Relieving Devices
(see manufacturers' catalogs for design details)
Conventional Safety Relief Valve
Relief Valve: a relief valve is an automatic spring loaded This valve design has the spring housing vented to the
pressure-relieving device actuated by the static pressure discharge side of the valve. The performance of the valve
upstream of the valve, and which opens further with upon relieving overpressures is directly affected by any
increase in pressure over the opening pressure. It is used changes in the back pressure on the valve ( opening pres-
primarily for liquid service [1,10] (Figure 7-lA and 7- sure, closing pressure, relieving capacity referenced to
lB). Rated capacity is usually attained at 25 percent over- opening pressure) [35]. See Figures 7-3, 7-6, and 7-6A.
pressure. When connected to a multiple relief valve manifold, the
Safety Valve: this is an automatic pressure-relieving performance of the valve can be somewhat unpredictable
device actuated by the static pressure upstream of the from a relieving capacity standpoint due to the varying
valve and characterized by rapid full opening or "pop" backpressure in the system.
action upon opening [l,10], but does not reseat. It is used
for steam or air service (Figure 7-2). Rated capacity is Balanced Safety Relief Valve
reached at 3%, 10% or 20% overpressure, depending
upon applicable code. This valve provides an internal design (usually bellows)
Safety-Relief Valve: this is an automatic pressure-relieving above/on the seating disk in the huddling chamber that
device actuated by the static pressure upstream of the minimizes the effect of backpressure on the performance
valve and characterized by an adjustment to allow reclo- of the valve (opening pressure, closing pressure and
sure, either a "pop" or a "non-pop" action, and a nozzle relieving capacity) [35]. See Figures 7-4, 7-6, and 7-6A.
type entrance; and it reseats as pressure drops. It is used
on steam, gas, vapor and liquid (with adjustments), and is
probably the most general type of valve in petrochemical
and chemical plants (Figures 7-3, 7-3A, and 7-4). Rated
capacity is reached at 3% or 10% overpressure, depend-
ing upon code and/or process conditions. It is suitable
for use either as a safety or a relief valve [1,10]. It opens
in proportion to increase in internal pressure.
Pressure Relief Valve
The term Pressure-relief valve applies to relief valves,
safety valves or safety-relief valves [10].
Piwt Operated Safety Valves
When properly designed, this type of valve arrange-
ment conforms to the ASME code. It is a pilot operated
pressure relief valve in which the major relieving device is
combined with and is controlled by a self-activating auxil- FLOW
iary pressure relief valve. See Figures 7-5A and B. Figure 7-1A. Relief valve. Courtesy of Crosby-Ashton Valve Co.
Process Safety and Pressure-Relieving Devices 401
Pr988ure-Rellevlng Devices
Spindle
Test Gag Spindle Lift Nu!
Cap Forked Lever Pin
.....,._,,,,,, Spindle ���
Adjusting Bolt Lever Adjusting Boll
Adj. Bolt Nut Adj. Ball Lock Nut
Gasket Cap Set Screw
Bor,net
REGULAR llftlMG
SCREWED CAP GEAR
Type A (Std.) type C
SCREWED CAP AND
TEST ROD
Type B
PACKED LIFTING GEAR
PACKED LIFTING ANDTESTROO
GEAR (Voi...e not ohowo uo;;..t)
Type D Typ• E
Figure 7-18. Accessories for all types of safety relieving valves. Courtesy of Crosby-Ashton Valve Co.
Special Valves-
a. Internal spring safety relief valve
CAP --------�..;. b. Power actuated pressure relief valve
SPINDLE NUT
ADJ BOLT BEARING c. Temperature actuated pressure relief valve
These last three are special valves from the viewpoint of
chemical and petrochemical plant applications, but they
can be designed by the major manufacturers and instru-
mentation manufacturers as these are associated with
instrumentation controls. Care must be taken in the sys-
tem design to make certain it meets all ASME code
requirements.
Rupture Disk
DISC NUT----..._loi-k-·-""
DISC HOLDER
DISC INSERT A rupture disk is a non-reclosing thin diaphragm
GUIDE (ADJ) RING (metal, plastic, carbon/graphite (non-metallic)) held
between flanges and designed to burst at a predeter-
mined internal pressure. Each bursting requires the
NOZZLE RING ----- installation of a new disk. It is used in corrosive service,
....
NOZZLE------- toxic or "leak-proof' applications, and for required burst-
BODY-------.
ing pressures not easily accommodated by the conven-
Figure 7-2. Safety valve. Courtesy of Crosby-Ashton Valve Co. tional valve such as explosions. It is applicable to steam,
402 Applied Process Design for Chemical and Petrochemical Plants
gas, vapor, and liquid systems. See Figures 7-7B, 7-8A An explosion rupture disc is a special disc (or disk
through R, and 7-9A through F. There are at least four designed to rupture at high rates of pressure rise, such ,
basic types of styles of disks, and each requires specific run-away reactions. It requires special attention from th
design selection attention. manufacturer [35].
Other rupture devices suitable for certain applicatior
are [35]:
a. breaking pin device
b. shear pin device
c. fusible plug device
Pressure-Vacuum Valves: See page 458
Bill of Materials-Conventional
ITEM PART NAME MATERIAL
28( )A10 SA-218 GR. Ytt8.
thru 26( )A26 C1r110n Steel
1 Body
26( )A32 SA-217 GR. WC6, Allor SI.
thru 26( )A36 (1'4 CR-Y, Moly)
26( )A10 SA-216 GR. WCB,
thru 26( )A26 Carbon Steel
2 Bonnel
26( )A32 SA-217 GR. WC6, Ala/ St
thru 26( )A36 (1'4 CR-Y, Mott)
3 Cap. Plain Screwed Cal1>on Steel
4 Disc Stainless Sl!el
5 Nozzle 316 St. St.
6 Disc Holder 300 Series St. St.
7 Blow Down Ring 300 Series St. SI.
8 Sleew Guide 300 Serias St. SI.
9 Stem Stainless Steel
10 Spring AdjusUng Screw Stainless Steel
11 Jam Nut (Spr. Adj. Ser.) Stainless Steel
12 Lock Screw (B.D.R) Stainl11$1 Steel
13 Lock Screw Stud Statnleu Steel
14 Stem Retainer Stainless St?et
Clrl>On Sllel
15 Spring Button Rust Proofed
ASTMA193
16 Body Stud
Gt 87. Alloy St.
ASTMA194
17 Hex Nut (Body)
Gt 2H, Alloy St.
26( )A10 Calbon Steel
thru 26( )A16 Rust Proofed
18 Spring
26( )A20 High 11"". Al/af
thru 28( )A36 Rust Proofed
19 Cap Gasket Son lrvn or Steel
20 Body Gasket Soft Iron or Sllel
21 BometGasklll Soft Iron or 51111
22 Lock Screw Gasllat Soft Iron or SINI
23 Hex Nut (B.D.R.L.S.) S1llnllll 511111
24 Lock ScllJW (D.H.) Stalnless SINI
25 Pipe Plug (Bonnel) SINI
26 Pipe Plug (Body) SINI
Also suitable for liquid service where ASME Code
certification is not required.
Figure 7-3. Conventional or unbalanced nozzle safety relief valve. By permission, Teledyne Farris Engineering Co.
Process Safety and Pressure-Relieving Devices 403
SPINDLE
SPINDLE ------
SEAT RETAINER
...._ __ NOZZLE
Figure 7-3A. Safety relief valve with rubber or plastic seats. By permission, Anderson, Greenwood and Co.
Definition of Pressure-Relief Terms (See Figures (7-3, Overpressure: pressure increase over the set pressure of
7-3A, 7-4, and Ref. [35J) the primary relieving device is overpressure. It is the same
as accumulation when the relieving device is set at the
maximum allowable working pressure of the vessel [ 10].
Set Pressure: the set pressure, in pounds per square inch
gauge, is the inlet pressure at which the safety or relief valve
is adjusted to open [10,13]. This pressure is set regardless Accumulation: pressure increase over the maximum
of any back ?ressure on the discharge of the valve, and is allowable working pressure of the vessel during discharge
not to be confused with a manufacturer's spring setting. through the safety or relief valve, expressed as a percent
404 Applied Process Design for Chemical and Petrochemical Plants
Figure 7-3a cont.
MATERIAL
ITEM PART NAME
81C,83C 81S,83S
1 BODY CARBON STEEL 316 S.S.
2 DRIVE PIN 316 S.S. 316 S.S.
3 SPRING WASHER CARBON STEEL 316 S.S.
4 SPRING CARBON STEEL 316 S.S.
5 GUIDE 303 S.S. 3165.S.
81C-17-4 S.S. 815-17-4 S.S.
6 SPINDLE
83C-303 S.S. 835-316 S.S.
81C-TEFLON, KEL-F, 81S-TEFLON,KEL-F,
7 SEAT ORVESPEL ORVESPEL
83C-BUNA-N 835-BUNA-N
81 C-17-4 S.S. 815-17-4 S.S.
8 NOZZLE
83C-303 S.S. 835-316 S.S.
9 LOCKNUT CARBON STEEL 3165.S.
10 PRESS.ADJ. CARBON STEEL 3165.S.
11 CAP CARBON STEEL 316 S.S.
12 SEAL, SLOWDOWN TEFLON TEFLON
13 SLOWDOWN ADJ. CARBON STEEL 3165.S.
14 JAM NUT CARBON STEEL 316 S.S.
81C-17-4 S.S. 815-17-4 S.S.
15 RETAINER SCREW
83C-CARBON STEEL 835-316 S.S.
16 RETAINER 303 S.S. 3165.S.
81C-BUNA-N" 81S-TEFLON""
17 SEAL, NOZZLE
83C-BUNA·N" 835-BUNA-N"
18 SPLIT RING CARBON STEEL 316 S.S.
19 BONNET FLANGE CARBON STEEL 316 S.S.
20 FLANGE BOLTS CARBON STEEL 3165.S.
81C-BUNA-N• 81S-TEFLON ••
21 SEAL, BONNET
83C-BUNA-N" 835-BUNA-N"
22 BONNET CARBON STEEL 3165.S.
23 PRESSURE SEAL TEFLON TEFLON
'Teflon or Viton also available.
"Buna-N or Viton also available.
of that pressure, pounds per square inch, is called accu- ing from the pressure in the discharge system of the
mulation [10]. installed device [35]. This pressure may be only atmos-
pheric if discharge is directly to atmosphere, or it may be
Blowdown: blowdown is the difference between the set some positive pressure due to pressure drops of flow of
pressure and the reseating pressure of a safety or relief discharging vapors/gases (or liquids where applicable) in
valve, expressed as a percent of the set pressure, or the pipe collection system, which in turn may be con-
pounds per square inch [ 10]. nected to a blowdown or flare system with definite back-
pressure conditions during flow, psig (gauge). The pres-
Back Pressure: pressure existing at the outlet or dis- sure drop during flow discharge from the safety relief
charge connection of the pressure-relieving device, result- valve is termed "built-up Back pressure."
Process Safety and Pressure-Relieving Devices 405
Burst Pressure: the inlet static pressure at which a rup- plate) (see Par. UG-A22) but exclusive of any corrosion
ture disk pressure-relieving device functions or opens to allowance or other thickness allowances for loadings (see
release internal pressure. ASME Par.-UG-22) on vessels other than pressure (for
example, extreme wind loadings for tall vessels). The
Design Pressure: the pressure used in the vessel design to
establish the minimum code permissible thickness for
containing the pressure.
Maximum Allotuable Working Pressure (MAWP): the maxi-
mum pressure pounds per square inch gauge permissible
at the top of a completed vessel in its operating position
for a specific designated temperature corresponding to
the MAWP pressure. This pressure is calculated in accor-
dance with the ASME code (Par. UG-98) [I] for all parts
or elements of the vessel using closest next larger to cal-
culated value nominal thickness ( closest standard for steel
em of Materials-BalanSeal
rrBI MllT MAME lllmlllAL
26( )910 s.\-218 GR. �6.
!IIIIJ 26( )826 Cart>on S!ffl
1 Bod';
26( )832 sP..217 GR. WC6, Alloy SL
lllru 26( )836 (1V, ca-w Moly)
26( )810 SA-21& GR. wca.
mru 26( )B26 Carlloo Steel
2 Bonnet
26( )832 SA-2T7 GR. WC6. Alloy St.
Hlrll 26( )836 (1\11 CR-YI Mel\/)
3 Clp.P\aJn� Carbon S!HI
• [Jisc S!ilnlns Sb!ef
5 Nonie 316 St. SL
6 [),s,: Holdv 300 Senes SI. St.
7 Blow Down Flin� 300 Senn St. St.
! SI-Guide 300 Sertllll St. St.
9 Slllm StllnllH SIMI
10 SPllng hl11Jstino Screw Stalnless Sl!!JI
11 Jim Nu! (Spr. Adj, Ser) Stainless Stoel
12 IJJck ScrM (S.U R) Slllinlas SINI
,� LDl:kScrNSDJd Stal,i1%s S!Mt
13
Stirn Retainer
Stainless Steel
15 Bellows 316l St. SL
16 S.llows Gasqt Fll!x, ble Graplllte
Camon Steil
17 St>nnq Bultlln Aust Prooted
ASTM Al9J --
18 Sod)' Stud Gr. 87, Moy St
ASTM A194
19 Ho Nut (Bo�} Gr 2H, Alloy St.
26( )810 Carbon Steel
!liru 26( 1816 Rusi Prool!d
20 Spnnq
26( )820 Hfqn Temp. Alloy
!liru 26( )636 Au!I Proofed
21 C:i.p Gasket Soft lrcn or St•el
22 lkxfy G..sllrt SllN Iron or Steel
23 Bonnel G ul<et Solt Iron or Steel
24 Loek ScrN Gasket Soft Iron or Steel
2!i Hex Nul {B.O.RLS.) Stainlm S1eel
26 Lock Screw (0 H.) Suinleu St&el
'1T Pip, Plug U!ody) Ste&I ·-
Also suitable for liquid service where ASME Code
certification is not required.
Figure 7-4. Balanced nozzle safety relief valve, Balanseal®. By pennission, Teledyne Farris Engineering Co.
406 Applied Process Design for Chemical and Petrochemical Plants
With no system pressure, the pilot inlet seat is open and As inlet pressure rises above set pressure, dome pressure
outlet seat is closed. As pressure is admitted to the main reduction will be such as to provide modulating action of
valve inlet, it enters the pilot through a filter screen and is the main valve piston proportional to the process upset.
transmitted through passages in the feedback piston, past The spool/feedback piston combination will move, re-
the inlet seat, into the main valve dome to close the main sponding to system pressure, to alternately allow pressure
valve piston. in the main valve dome to increase or decrease, thus mov-
ing the main valve piston to the exact lift that will keep sys-
As system pressure increases and approaches valve set tem pressure constant at the required flow. Full main valve
pressure, it acts upward on the sense diaphragm, with the lift, and therefore full capacity, is achieved with 5% over-
feedback piston moving upward to close the inlet ssat, thus pressure. As system pressure decreases below set pres-
sealing in the main valve dome pressure, as the outlet seat sure, the feedback piston moves downward and opens the
is also closed. A small, further increase in system pressure inlet seat to admit system pressure to the dome, closing the
opens the outlet seat, venting the main valve dome pres- main valve.
sure. This reduced dome pressure acts on the unbalanced
feedback piston to reduce feedback piston lift, tending to Due to the extremely small pilot flow, the pilot on gas/vapor
"lock in" the dome pressure. Thus, at any stable inlet pres- valves normally discharge to atmosphere through a
sure there will be no pilot flow (i.e. zero leakage). weather and bug-proof fitting. Pilots for liquid service
valves have their discharge piped to the main valve outlet.
Figure 7-5A. Pilot operated safety relief valve. By permission, Anderson, Greenwood and Co.
MAWP is calculated using nominal standard steel plates Assume calculated thickness per ASME code Par. UG-
(but could be other metal-use code stresses) thickness, 27: 0.43 in.
using maximum vessel operating temperature for metal Closest standard plate thickness to fabricate vessel is
stress determinations. See Ref [l] Par. UG-98. 0.50 in. with - 0.01 in. and + 0.02 in. tolerances at mill.
Example 7-l: Hypothetical vessel design, carbon steel
grade A-285, Gr C Then
Normal operating: 45 psig at 600°F 1. Using 0.50 in. - 0.01 in. (tolerance) = 0.49 in. min.
Design pressure: 65 psig at 700°F corres. to the 65 psig. thickness.
Process Safety and Pressure-Relieving Devices 407
Seat
Pressure Pickup
Figure 7-58. Safety relief valve mechanism as connected to a non-flow (zero flow) pilot safety relief valve. By permission, Anderson Green-
wood and Co.
A-CONVENTIONAL SAFETY VALVES B- BALANCED SAFETY VALVES
Bonnet Vented to Atmo1ph1r1 Balanced Disk and Vented Piston Type Bellows Type
.....-vented
Bonnet
/Vented ,vented
Bonnet Bonnet
Set .,..,.._ I\ , !!_ , Spring ,ore,
P1 A• • '• • Pa CAo • .Ail P1 AN• ,, + Pi AN AN No11l1 Seat Areo
Bock Pressure. Dea-eases Bock Pressure Increases
Set Pressure Set Pressure Bock Pressure Has Very Little Effect on Set Pressure
Note: P1 = Valve Set-pressure
P2 = Vent Line Bock-pressure
Fs = Force of Valve Spring
AN= Nozzle Seat Area
Ap= Piston Face Area
Ae= Bellows Area
Ao= Valve Oise Area
Figure 7-6. Effect of backpressure on set pressure of safety or safety relief valves. By permission, Recommended Practice for Design and
Construction of Pressure-Relieving Systems in Refineries, API RP-520, 5th Ed. American Petroleum Institute (1990) (also see Ref. [33al).
408 Applied Process Design for Chemical and Petrochemical Plants
Opening Presssure
.... 100 100 100
Q)
::,
(J) 80 80 80 Closing Pressure
(J)
....
Q)
o,
Q)
Cl)
......
0
'#
Conventional Valve Conventional Valve Balanced Valve
(a) Backpressure Decreases Set Pressure (b) Backpressure Increases Set Pressure (c) Backpressure Has Little
Effect on Set Pressure
% of Backpressure
Figure 7-6A. Diagram of approximate effects of backpressure on safety relief valve operation. Adapted by pennission, Teledyne Farris Engi-
neering Co.
2. Using 0.50 in. + 0.02 in. (tolerance) = 0.52 in. max. P d = 2SEt/ (R - 0.4t) (7-3)
thickness.
Generally, for design purposes, with this type of toler- t = PR/[2SE + 0.4P] (7-4)
ance, nominal thickness = 0.50 in. can be used for calcu-
lations. The vessel shell wall thickness shall be the greater of
Now, using Par. UG-27, 0.50 in. thickness and ASME Equations 7-2 or 7-4, or the pressure shall be the lower of
code stress at 750°F (estimated or extrapolated) per Par. Equation 7-1 or 7-3 [l].
UCS-23 at 750°F, the maximum allowable stress in tension For above example assume calculated MAWP (above) =
is 12,100 psi. 80 psig. This is the maximum pressure that any safety relief
Recalculate pressure (MAivP) using Par. UG-27 [l] valve can be set to open.
For cylindrical shells under internal pressure: For pressure levels for pressure relief valves referenced
( 1) Circumferential stress (longitudinal joint) to this MAWP, see Figures 7-7A and B.
Operating Pressure: the pressure, psig, to which the ves-
sel is expected to be subjected during normal or, the max-
Pc1 = SEt/(R;+ 0.6t), psi= psig (7-1)
imum probable pressure during upset operations. There is
a difference between a pressure generated internally due
t = PR/ [SE - 0.6P] (7-2)
to controlled rising vapor pressure (and corresponding
temperature) and that generated due to an unexpected
where t = minimum actual plate thickness of shell, no runaway reaction, where reliance must depend on the
corrosion, = 0.50" sudden release of pressure at a code conformance pres-
P d = design pressure, for this example equals the sure/ temperature. In this latter case, careful examination
!vIAWP, psi of the possible conditions for a runaway reaction should
R, = inside radius of vessel, no corrosion be made. This examination is usually without backup data
allowance added, in. or a firm basis for calculating possible maximum internal
S = maximum allowable stress, psi, from Table vessel pressure to establish a maximum operating pres-
UCS-23 sure and from this, a design pressure.
E = joint efficiency for welded vessel joint, plate Design Pressure of a Vessel: the pressure established as a
to plate to heads. See ASME Par. UW-12, nom- nominal maximum above the expected process maximum
inal = 85% = 0.85 operating pressure. This design pressure can be estab-
t = required thickness of shell, exclusive of cor- lished by reference to the chart in Chapter 1, which is
rosion allowance, inches based on experience/practice and suggests a percentage
(2) Longitudinal stress (circumferentialjoints). increase of the vessel design pressure above the expected
Process Safety and Pressure-Relieving Devices 409
Pressure Vassel Requirements Vessel Typical Characteristics of
Pressure Pressure Rellel Valves
Maximum allowable Maximum relieving
accumulated pressure 121 pressure for
(fire exposure only) - 120- fire sizing
- -
- -
Maximum allowable - - Multiple valves
accumulated pressure 116 Maximum relieving
for multiple-valve installation pressure for
(other than lire exposure) - 115- process sizing
,- - - Single valve
...... - Maximum relieving
,- - pressure for process sizing
Maximum allowable -� - Maximum allowable set pressure
::,
accumulated pressure ccs ' for supplemental valves
for single-valve installation � 110 (fire exposure)
(other than fire exposure) ,- Q) - '
:i
,- "' -
"' �
c. - t � Overpressure (maximum)
c:
- c:
� :i: -
5
...... 3: 105 Maximum allowable set pressure
Q) for additional valves (process)
�� -
3:
- .Q -
<ii
...... E -
::,
E
Maximum allowable - ·;. -
working pressure ccs 100 t Maximum allowable set pressure
E
or design pressure - 0 - Simmer for single valve
(ty
E - pical)
Start to open
...... Q) t
-� - Slowdown (typical)
�
- - (see Note 6)
- 95-
- -
- - Closing pressure
- - for a slngle valve
- -
Maximum expected
operating pressure 90 Leak test pressure (typical)
(see Notes 5 and 6) - -
- -
-
...... -
-
- 85-
Notes:
I. This figure conforms with the requirements of Section VIII of the 4. The maximum allowable working pressure is equal to or greater
ASME Boiler and Pressure Vessel Code. than the design pressure for a coincident design temperature.
2. The pressure conditions shown are for pressure relief valves in- 5. The operating pressure may be higher or lower than 90.
stalled on a pressure vessel. 6. Section VIII, Division 1, Appendix M, of the ASME Code should
3. Allowable set-pressure tolerances will be in accordance with the be referred to for guidance on blowdown and pressure differentials.
applicable cedes.
Figure 7-7A. Pressure level relationship conditions for pressure relief valve installed on a pressure vessel (vapor phase). Single valves (or more)
used for process or supplemental valves for external fire (see labeling on chart). Reprinted by permission, Sizing, Selection and Installation of
Pressure Relieving Devices in Refineries, Part 1 "Sizing and Selection," API RP-520, 5th Ed., July 1990, American Petroleum Institute.
410 Applied Process Design for Chemical and Petrochemical Plants
Vessel Typical Characteristics of a
Pressure Veuel Requirements Pressure Rupture Disk Device
Maximum allowable Maximum relieving
accumulated pressure 121 pressure for
(fire exposure only) - 120 - fire sizing
- -
- -
Maximum allowable Maximum relieving
accumulated pressure - - pressure for
for installation of a 116 process sizing
multiple rupture disk
device (other than - 115- t Multiple rupture disk device
fire exposure) - - - Single rupture disk device
- -
- -
Maximum allowable - ' ,
accumulated pressure - al' Maximum allowable burst pressure
0
for installation of a :, 110 for supplemental (fire exposure)
ca
single rupture disk .9! - j I rupture disk device
device (other than - � (see Note 6)
:,
fire exposure) -111 -
Cl) , ....,...__ Overpressure (maximum)
41
-a -
0
-c - Maximum allowable burst pressure
�
-� - (see Note 6)
-o 105
for additional rupture disk device
J
ii
-J -
0
� ii -
Maximum allowable -§ - I , Maximum allowable burst pressure
working pressure E
.i 100
or design pressure _e for single rupture disk device
-- -
(see Note 3) - (see Note 6)
0
c
- fl -
- 95-
-£ -
-
-
-
-
-
-
Maximum expected - -
operating pressure 90-
(see Notes 5 and 6) - -
- -
- -
- -
- 85-
Notes:
I. This figure conforms with the requirements of Section VIII of the 4. The allowable burst-pressure tolerance will be in accordance with
ASME Boiler and Pressure Vessel Code. the applicable code.
2. The pressure conditions shown are for rupture disk devices in- 5. The operating pressure may be higher or lower than 90 depending
stalled on a pressure vessel. on the rupture disk design.
3. The margin between the maximum allowable working pressure 6. The stamped burst pressure of the rupture disk may be any pres-
and the operating pressure must be considered in the selection of a sure at or below the maximum allowable burst pressure.
rupture disk.
Figure 7-78. Pressure level relationships for rupture disk devices. Reprinted by permission, Sizing, Selection and Installation of Pressure
Relieving Devices in Refineries, Part 1 "Sizing and Selection," API RP-520, 5th Ed., July 1990, American Petroleum Institute.
Process Safety and Pressure-Relieving Devices 411
Figure 7-BA. Metal type frangible disk (above) with cross-section
(below) Courtesy of Black, Sivalls and Bryson Safety Systems, Inc.
maximum process operating pressure level. There is no
code requirement for establishing the design pressure.
(See chart in Chapter 1.) Good judgment is important in
selecting each of these pressures. See operating pressure
description in above paragraph. Depending on the actual
operating pressure level, the increase usually varies from
a minimum of 10% higher or 25 psi, whichever is greater,
to much higher increases. For instance, if the maximum Figure 7-88. Standard rupture disk. A prebulged rupture disk avail-
expected operating pressure in a vessel is 150 psig, then able in a broad range of sizes, pressures, and metals. Courtesy of
experience might suggest that the design pressure be set B.S. & B. Safety Systems.
for 187 to 200 psig. Other factors known regarding the
possibility of a run-away reaction might suggest setting it Figure 7-BC. Disk of Figure 7-88 after rupture. Note 30° angular
at 275 psig. A good deal of thought needs to enter into seating in holder is standard for prebulged solid metal disk. By per-
this pressure level selection. (Also see section on explo- mission, B.S.&B. Safety Systems, Inc.
sions and DIERS technology this chapter [55] [67] .)
Figure 7-BD. Disk of Figure 7-88 with an attached (underside) vacu-
Relieving Pressure: this is the pressure-relief device's set um support to prevent premature rupture in service with possible
pressure plus accumulation or overpressure. See Figures 7-7A less than atmospheric pressure on underside and/or pulsation ser-
and 7-7B. For example, at a set pressure equal to the max- vice. By permission, B.S.&B. Safety Systems, Inc.
imum allowable at the MAWP of the vessel of 100 psig,
and for process internal vessel pressure, the pressure this chapter and Figures 7-7B, 7-31A, B for these allowable
relief device would begin relieving at nominal l 00 psig pressure levels) and in no case do the figures apply to a
(actually begin to open at 98 psig, see figures above) and sudden explosion internally.
the device (valve) would be relieving at its maximum con-
ditions at 110 psig (the 10 psig is termed accumulation
pressure) for a single valve installation, or 116 psig, for a Resealing Pressure: the pressure after valve opening
multiple valve installation on the same vessel. These are under pressure that the internal static pressure falls to
all process situations, which do not have an external fire when there is no further leakage through the pressure
around the vessel (See External Fire discussion later in relief valve. See Figure 7-7A.
412 Applied Process Design for Chemical and Petrochemical Plants
Popping Pressure: the pressure at which the internal
pressure in a vessel rises to a value that causes the inlet
valve seat to begin to open and to continue in the open-
ing direction to begin to relieve the internal overpressure
greater than the set pressure of the device. For compress-
ible fluid service.
Materials of Construction
Safety and Relief Valves; Pressure-Vacuum Relief Valves
For most process applications, the materials of con-
struction can be accommodated to fit both the corrosive-
Figure 7-SE. Rupture disk (top) with Teflon® or other corrosion-resis- erosive and mechanical strength requirements. Manufac-
tant film/sheet seal, using an open retaining ring. For positive pres-
sure only. By permission, Fike Metal Products Div., Fike Corporation, turers have established standard materials which will fit a
Blue Springs, Mo. large percentage of the applications, and often only a few
parts need to be changed to adapt the valve to a corrosive
service. Typical standard parts are: (See Figures 7-3, 7-3A,
and 7-4)
Option 1 (typical only)
Body carbon steel, SA 216, gr. WCB
Nozzle 316 stainless steel
Disc/Seat stainless steel
Blow Down Ring 300 Ser. stainless steel
Stem or Spindle stainless steel
Spring C.S. rust proof or high temp.
Figure 7-SF. Rupture disk (top), similar to Figure 7-SE, except a alloy, rust proof
metal vacuum support is added (see Figure 7-SF(A)). By permission, Bonnet SA-216, Gr. WCB carbon steel
Fike Metal Products Div., Fike Corporation, Blue Springs, Mo. Bellows 316L stainless steel
Option 2 (typical only)
Body 316 stainless steel
Nozzle 17-4 stainless steel or 316 stain
less steel
Disc/Seat Teflon, Kel-F, Vespel or Buna-N
Blow Down Ring 316 stainless steel
Stem or Spindle 17-4 stainless stee I or 316 SS
Spring 316 stainless steel
Figure 7-SF(A). Cross section of disk assembly for Figure 7-SF. By Bonnet 316 stainless steel
permission, Fike Metal Products Div., Fike Corporation, Blue
Springs, Mo. Bellows
For pressure and temperature ratings, the manufactur-
ers' catalogs must be consulted. In high pressure and/or
temperature the materials are adjusted to the service.
Closing Pressure: the pressure established as decreasing For chemical service the necessary parts are available
inlet pressures when the disk of the valve seats and there in 3.5 percent nickel steel; monel; Hastelloy C; Stainless
is no further tendency to open or close. Type 316, 304, etc.: plastic coated bellows; nickel; silver;
nickel plated springs and other workable materials.
Simmer: the audible or visible escape of fluid between The designer must examine the specific valve selected
the seat/ disk of a pressure-relieving valve at an inlet static for a service and evaluate the materials of construction in
pressure below the popping pressure, but at no measur- contact with the process as well as in contact or exposed
able capacity of flow. For compressible fluid service. to the vent or discharge system. Sometimes the corrosive
Process Safety and Pressure-Relieving Devices 413
FEATURES:
• Isolates Safety Relief Valves
• No Fragmentation
• Operates up to 90% Rated
Pressure
• Can Withstand Full Vacuum
without Supports
• Available in Sizes 1" thru 36"
• Wide Material Availability
• U.S. Patent Number
3,294,277
Unburst Burst
Figure 7-SG. Reverse buckling® disk, showing top holder with knife blades (underside) that cut the disk at time of rupture. By permission,
B.S.&B. Safety Systems, Inc.
RB-90 reverse buckling disk. Pressure on CON-
VEX side of disk and patented seating design
puts compression loading on disk metal.
Figure 7-8G(A). Reverse buckling® disk. Pressure on convex side of Figure 7-81. Flat disk used for low pressure and for isolation of cor-
disk and patented seating design puts compression loading on disk rosive environments. Usual pressure range is 2 to 15 psig with ± 1 psi
metal. By permission, B.S.&B. Safety Systems, Inc. tolerance. Stainless steel disk with Teflon® seal is usually standard.
By permission, Fike Metal Products Div., Fike Corporation. Catalog
73877-1, p. 35.
nature of the materials in the vent system present a seri-
ous corrosion and fouling problem on the back or dis-
charge side of the valve while it is closed.
For these special situations properly designed rupture
disks using corrosion-resistant materials can be installed
Figure 7-SH. Special metal disk holder for polymer systems using a both before the valve inlet as well as on the valve dis-
smooth disk surface to reduce polymer adherence, and a smooth charge. For these cases, refer to both the valve manufac-
annular sealing area. Usually thick to avoid need for vacuum support
and to allow for corrosion attack. By permission, Fike Metal Prod- turer and the rupture disk manufacturers. See later dis-
ucts Co. Div., Fike Corporation, Inc. cussion of code requirements for this condition.
414 Applied Process Design for Chemical and Petrochemical Plants
pre-assembly
screw pressure
gauge
holddown __
rupture disc -
base :"
Figure 7-SJ. Exploded view of double disk assembly. Usually burst
pressure is same for each disk. Used for corrosive/toxic conditions
to avoid premature loss of process and at remote locations. Note
the use of tell-tale between disks. By permission, Fike Metal Prod-
ucts Div., Fike Corporation, Inc.
r- W' NPT CONDUIT CONNECTION Figure 7-SL. Rupture disk indicator alarm strip breaks when rupture
disk breaks and alarms to monitor. FM system approved. By per-
�:ff � ---- - 1 �
/ mission, Continental Disc Corporation.
� iURSTINDICATORUNIT
...............
_._._..__P_
!
".:: ELECTRICAL
CONNECTIONS
Figure 7-SK. Rupture disk with burst indicator. Several other tech-
niques available. By permission, Fike Metal Products Div., Fike Cor-
poration, Inc.
Rupture Disks
Rupture disks are available in:
1. Practically all metals that can be worked into thin
sheets, including lead, Mone), nickel, aluminum, sil-
ver, Inconel, 18--8 stainless steel, platinum, copper,
Hastelloy and others.
2. Plastic coated metals, lead lined aluminum, lead
lined copper. Figure 7-SM. Ultrex® Reverse Acting Rupture Disc providing instan-
3. Plastic seals of polyethylene, Kel-F®, and Teflon® taneous full opening, non-reclosing. By permission, Continental
4. Graphite, impregnated graphite or carbon. Disc Corporation.
Process Safety and Pressure-Relieving Devices 415
Figure 7-SN. Quick Change™ for quick rupture disk changeout. By permission, Continental Disc Corporation.
OUTLET NUT The selection of the material suitable for the service
(MUFFLED)''--- ��� -:::,,,,-
.... depends upon the corrosive nature of the fluid and its
bursting characteristics in the pressure range under con-
sideration. For low pressure, a single standard disk of
some materials would be too thin to handle and maintain
its shape, as well as give a reasonable service life from the
corrosion and fatigue standpoints. See section on Selec-
tion and Application.
General Code Requirements [I]
HOLD DOWN
RING-+-,11�
It is essential that the ASME code requirements be
understood by the designer and individual rating and
specifying the installation details of the safety device. It is
not sufficient to merely establish an orifice diameter,
INLET since process considerations which might cause overpres-
(BASE)
sure must be thoroughly explored in order to establish
the maximum relieving conditions.
An abbreviated listing of the key rating provisions is
given in paragraphs UG-125 through 135 of the ASME
code, Section 8, Div. 1, for unfired pressure vessels [ 1].
Figure 7-80. Reusable screw type holder (30° seat) for smaller disks. 1. All pressure vessels covered by Division 1 or 2 of Sec-
By permission, Fike Metal Products Div., Fike Corporation, Inc. tion VIII are to be provided with protective over-
416 Applied Process Design for Chemical and Petrochemical Plants
3. Pressure relief must be adequate to prevent internal
pressures from rising over 10% above the maximum
allowable working pressure, except when the excess
pressure is developed by external fire or other
unforseen heat source. See design details in later
paragraph. Papa [85] proposes an improved tech-
nique for relief sizing. Also see Figures 7-7 A and 7-7B.
4. When a pressure vessel is exposed to external heat
or fire, supplemental pressure relieving devices are
required for this excessive pressure. These devices
must have capacity to limit the overpressure to not
more than 21 % above the maximum allowable work-
Assembly UA-2M ing pressure of the vessel. (See Figures 7-7 A and 7-
7B.) A single relieving device may be used to handle
the capacities of paragraph UG-125 of the code, pro-
Figure 7-SP. Typical union type disk holders. They are not all avail- vided it meets the requirements of both conditions
able. By permission, B.S.&B. Safety Systems, Inc. described.
pressure devices. There are exceptions covered by
paragraph U-1 of the code, and in order to omit a
protective device this paragraph should be exam- 5. Rupture disks may be used to satisfy the require-
ined carefully. For example, vessels designed for ments of the code for conditions such as corrosion
above 3000 psi are not covered; also vessels with <120 and polymer formations, which might make the safe-
gallons of water, vessels with inside diameter not over ty/relief valve inoperative, or where small leakage by
6 inches (at any pressure), vessels having internal or a safety valve cannot be tolerated. They are particu-
external operating pressures not over 15 psig larly helpful for internal explosion pressure release.
(regardless of size), and a few other conditions may
not be subject to this code.
6. Liquid relief valves should be used for vessels that
2. Unfired steam boilers must be protected. operate full of liquid.
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APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition
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