DRAG Valves for Boiler Feedpump Recirculation

B DRAG® VALVES FOR
BOILER FEEDPUMP
RECIRCULATION

INTRODUCTION

In a power plant the boiler water is circulated in a closed loop by
a feedpump driven either by an electrical motor or a steam turbine.
The pump takes the water from the deaerator or high pressure
heaters and boosts the pressure as high as 5500 psi (387kg/cmz) in
supercritical, once-thru boilers. These pumps require a certain mini-
mum amount of flow to avoid overheating and cavitation problems.
In order to protect the pump when the boiler feed flow require-
ment is less than the minimum permissible flow through the feed-
pump, a recirculation system is used to return a portion of the high
pressure flow back to the condenser or deaerator and from there to
the pump. See Figure 1 for a system schematic.

Figure 1 - Schematic of typical once-thru, SUPERHEATER
universal pressure, steam-generating plant,
showing location of boiler feedpump recircu- ECONOMIZER

lation valve.

-P TURBINE

DEAERATOR L-P TURBINE

FiGGGil I I ( ) CONDENSER

HEATER PUMP 99

w;g;TER

DEMINERALIZER

Figure 2 - A CCI DRAG@ valve of globe Figure 3 - Actuator control schematic
configuration is shown located near pump shown is for 3-15 psi modulation signal, with
discharge with flow to deaerator. An angle increasing signal tending to close valve.
CCI DRAG valve is also available, should The actuator is fitted with a spring for fail
piping configuration dictate. Systems dis- open on loss of air signal. A snap-acting
charging to the condenser normally use relay is provided to ensure the valve is
angle valves. seated at maximum signal. The snap-
acting relay is set as shown so the valve
SIGNAL INLET modulates between 10% and 100% open.

SNAP ACTING RELAY HIGH SELECTOR RELAY

(10% OFF SEAT) POSITIONER

REGULATOR ’ ACT&OR V
SET Q 5 PSI OVER CLOSED SUPPLY INLET
ACTUATOR
POSITION SIGNAL 80 PSI MIN / 150 PSI MAX (FAIL OPEN)

r

B

TYP

AC7‘UATOR
A I R SUPPLY

-A--J GLOBE VALVE
ANGLE VALVE

TABLE 1

Face to Face/CL Actuator Dia.

Data listed is for comparison only. Actual numbers will vary based on flow conditions and other application requirements.
For P z 4100 PSI, a larger actuator may be required.

CC1 DRAG VALVE FOR BOILER FEEDPUMP
RECIRCULATION

Valve rating: ANSI Class 2500

Maximum allowable 6200 psig at -40 to +450F
working pressure:

Hydrostatic test: 9375 psig

Seat test: 6200 psig

MSS-SP 61 Class Leakage

cItem Description Material/Component Modulating On-Off
Std Opt Std Opt

1 Body

Seat Ring Gasket 400 Series SS XX
Seat Ring TFE & 300 Series SS XX
Disk Stack 400 Series SS/17-4 PH XX
Guide Bushing
Balance Seal
Plug Assembly
Bonnet/Pressure Bleed
Metal Seal
Bonnet Flange
Bonnet Stud
Bonnet Nut
Packing Spacer
Packing
Packing Follower
Actuator
Yoke
18 Positioner, Pneumatic
19 Limit Switches (not shown)
20 Packing Stud
21 Yoke Clamp
22 Packing Flange
23 Stem Connector
24 Stem Connector Bolting
25 Filter Regulator
26 Filter Regulator (not shown)
27 High Pressure Select Relay
28b I/P Transducer
29 Solenoid Valve
3oc Snap Acting Relay
31 Actuator Fittings/Tubing

Notes:
a Recommended spare parts.
b Required for applications where the modulation signal is 4-20 mA.
c The snap acting relay is used to prevent the valve from throttling less than 10% off the seat

The snap acting relay assures 100% actuator loading in the closed position.

OPERATION Determine:
Ideally the recirculation system would meter the flow in re- Cv = flow coefficient based on pressure drop at Qmax
Qmin = minimum controllable flow (GPM))
sponse to the pump’s requirements. This indicates the need for a
modulating control valve, which normally would be closed, but Procedure:
opens automatically on turbine trip. The valve would need to
handle water with thermodynamic conditions at the inlet of 2500- where: Po = FFP,, &ax = Flow (GPM)
5500 psi (175-387kg/cm2) pressure and 300°-500°F (141°-246°C) P, = Inlet Pressure
temperature and, at the outlet, saturation conditions in the range
of 1 IO°-240°F (41°- 109 °C) with corresponding pressures of - 12 psig The above equations require the flow rate be in gallons per
to 10 psig. Thus, during modulation of full flow conditions, the minute (GPM) and the nressure be in pounds per sauare inch (psi).
valve must handle high pressure drop and substantial flashing yet
manage to minimize erosion, cavitation, and noise problems. 1.0

When shut, the pressure could build as high as 5500 psig k
(387kg/cm2) and the valve be expected to hold “drop-tight” against .80 -
leakage. Should the valve leak, even the smallest amount of flashing
steam and water mixture would cut the seating surface like a rough 700
wire and in a short while the shutoff function of the valve would be 100 200 300 400 500
lost. Excessive leakage cuts into the efficiency of the plant both
through direct energy loss, and because the boiler cannot be fed the WATER TEMPERATURE - ° F
full rated flow. In the extreme case, a shut down of an entire power
plant may be necessitated because of a single leaking valve. Figure 5
These valves are designed for critical pressure steam plants, with
Clearly, there is a need for a Boiler Feedwater Pump Re-
circulation Valve that will: pump discharge pressures of 2500 to 5500 psi (175 to 387kg/cm2)
and fluid temperatures of 300” to 500°F (141° to 246°C). The
1. Break down water pressure from 5500 psi (387kg/cm2) to patented CCI D R A G disk stack in these valves has a special, com-
atmospheric or vacuum pressure and simultaneously neutralize plex pattern disk specifically designed for this service. The only
the effects of consideration required for sizing is to determine Cv required or flow
-trim erosion due to flashing; required in GPM. Select the valve from Table 1 which meets or
-cavitation; and, exceeds the required Cv or flow.
-mechanical vibration and noise.
For end connections, refer to Buttweld Ends, Figures 6 thru 9.
2. Remain leak tight for long periods of time.

3. Modulate automatically and open in case of failure of power to
the valve operator.

A NEW CCI DRAG@ VALVE HAS BEEN DESIGNED
TO MEET THESE STRINGENT REQUIREMENTS

Information required for sizing procedure:

l Maximum flow rate

l Maximum pressure drop

l Pressure drop at maximum flow rate

l Vapor pressure of fluid (Figure 4)
l Specific gravity at flowing temperature or water temperature

(Figure 5)

l Inlet pressure and outlet pressure

PRESSURIZED SEAT PLUG

The pressurized seat plug design is a special form of the balanced
plug that is balanced during modulation, but unbalanced when
shut off.

While the unbalanced and balanced plugs may be applied in
either over-the-plug (flow-to-close) or under-the-plug (flow-to-open)
configurations, the pressurized seat plug is applied only in a
flow-to-close configuration When the valve is completely shut off,
the balance hole is closed by the valve stem.

When the valve is shut, a tiny bleed path through the bonnet lets
pressure build up on top of the valve plug, creating very high seat
loading for superior shutoff with a metal-seated valve. This seat
loading is in addition to the actuator loading applied to the seat.

Once the stem is lifted to uncover the balancing hole, the main
valve plug can be opened as easily as any other balanced plug
configuration. This feature provides superior shutoff (MSS-SP-61).

Figure 4 TEMPERATURE (°F)

Buttweld Ends ANSI B 16.25

120

6" 160 8&9

xxs

100

120

8" 140 9

xxs

160

6"

31
R .25

f/-
.03

At DIIA FIGURE 8
-4

tlao _i $DlA*.O3 ’

GURE 6

i--_1.0+ f FIGURE 9
_ . _ _ . . . ?.

Note: Special weld ends are available. Include a sketch ot desired
weld end with the order.

FIGURE 7

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