205 Drag Valves for Boiler Feedpump Recirculation

7/28/2019 205 Drag Valves for Boiler Feedpump Recirculation

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BDRAG VALVES FOR

BOILER FEEDPUMP

RECIRCULATION


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INTRODUCTIONIn 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 pressureheaters and boosts the pressure as high as 5500 psi (387kg/cmz) insupercritical, 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 tothe pump. See Figure 1 for a system schematic.

Figure 1- Schematic of typical once-thru,universal pressure, steam-generating plant,showing location of boiler feedpump recircu-lation valve.

DEAERATOR

SUPERHEATER

ECONOMIZER

-P TURBINE

L-P TURBINE

FiGGGil I I ( ) CONDENSER

HEATER PUMP 9 9w;g;TERDEMINERALIZER

Figure 2 - A CC I DRAG@ valve of globeconfiguration is shown located near pumpdischarge with flow to deaerator. An angleCC I DRAG valve is also available, should

piping configuration dictate. Systems dis-charging to the condenser normally useangle valves.

Figure 3 - Actuator control schematicshown is for 3-15 psi modulation signal, withincreasing signal tending to close valve.The actuator is fitted with a spring for failopen on loss of air signal. A snap-actingrelay is provided to ensure the valve isseated at maximum signal. The snap-acting relay is set as shown so the valvemodulates between 10% and 100% open.

SIGNAL INLET HIGH SELECTOR RELAY

POSITIONER

SNAP ACTING RELAY

(10% OFF SEAT)

REGULATOR

SET Q 5 PSI OVER CLOSEDPOSITION SIGNAL

ACT&ORSUPPLY INLET

80 PSI MIN / 150 PSI MAX

V

ACTUATOR

(FAIL OPEN)


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-A--JANGLE VALVE

AC7UATOR

A I R SUPPLY

T

GLOBE 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.


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CC1 DRAG VALVE FOR BOILER FEEDPUMPRECIRCULATION

Valve rating: ANSI Class 2500

Maximum allowable

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

Hydrostatic test: 9375 psig

Seat test: 6200 psig

MSS-SP 61 Class Leakage

cItem1

18

19

20

21

22

23

24

25

26

27

28b

29

3oc

31

Description Material/Component

Body

Seat Ring Gasket

Seat Ring

Disk Stack

Guide Bushing

Balance Seal

Plug Assembly

Bonnet/Pressure Bleed

Metal Seal

Bonnet Flange

Bonnet Stud

Bonnet Nut

Packing Spacer

Packing

Packing Follower

Actuator

Yoke

Positioner, Pneumatic

Limit Switches (not shown)

Packing Stud

Yoke Clamp

Packing Flange

Stem Connector

Stem Connector Bolting

Filter Regulator

Filter Regulator (not shown)

High Pressure Select Relay

I/P Transducer

Solenoid Valve

Snap Acting Relay

Actuator Fittings/Tubing

Modulating

Std Opt

On-Off

Std Opt

400 Series SS

TFE & 300 Series SS

400 Series SS/17-4 PH

X X

X X

X X

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


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O P E R A T I O N

Ideally the recirculation system would meter the flow in re-

sponse to the pumps requirements. This indicates the need for a

modulating control valve, which normally would be closed, but

opens automatically on turbine trip. The valve would need to

handle water with thermodynamic conditions at the inlet of 2500-

5500 psi (175-387kg/cm2) pressure and 300-500F (141-246C)

temperature and, at the outlet, saturation conditions in the range

of 1 IO-240F (41- 109 C) with corresponding pressures of- 12 psigto 10 psig. Thus, during modulation of full flow conditions, the

valve must handle high pressure drop and substantial flashing yet

manage to minimize erosion, cavitation, and noise problems.

When shut, the pressure could build as high as 5500 psig

(387kg/cm2) and the valve be expected to hold drop-tight against

leakage. Should the valve leak, even the smallest amount of flashing

steam and water mixture would cut the seating surface like a rough

wire and in a short while the shutoff function of the valve would be

lost. Excessive leakage cuts into the efficiency of the plant both

through direct energy loss, and because the boiler cannot be fed the

full rated flow. In the extreme case, a shut down of an entire power

plant may be necessitated because of a single leaking valve.

Clearly, there is a need for a Boiler Feedwater Pump Re-

circulation Valve that will:

1. Break down water pressure from 5500 psi (387kg/cm2) to

atmospheric or vacuum pressure and simultaneously neutralize

the effects of-trim erosion due to flashing;

-cavitation; and,

-mechanical vibration and noise.

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

T O M E E T T H E S E S T R I N G E N T R E Q U I R E M E N T S

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

Figure 4 TEMPERATURE (F)

Determine:

Cv = flow coefficient based on pressure drop at Qmax

Qmin = minimum controllable flow (GPM))

Procedure:

where: Po= FFP,, &ax = Flow (GPM)P, = Inlet Pressure

The above equations require the flow rate be in gallons per

minute (GPM) and the nressure be in pounds per sauare inch (psi).

1.0

k.80-70000 200 300 400 500

WATER TEMPERATURE - FFigure 5

These valves are designed for critical pressure steam plants, with

pump discharge pressures of 2500 to 5500 psi (175 to 387kg/cm2)

and fluid temperatures of 300 to 500F (141 to 246C). Thepatented CC I D R A G disk stack in these valves has a special, com-

plex pattern disk specifically designed for this service. The only

consideration required for sizing is to determine Cv required or flow

required in GPM. Select the valve from Table 1 which meets or

exceeds the required Cv or flow.

For end connections, refer to Buttweld Ends, Figures 6 thru 9.

P R E S S U R I Z E D S E A T P L U G

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 aflow-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 let

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).


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Buttweld Ends ANSI B 16.25

6"

31

f/-R .25

.03

tAD-4

GURE 6

tlao _i $DlA*.O3

IIA

F I G URE 7i--_1.0+ f

120

6" 160 8&9

xxs

100

120

8" 140 9

xxs

160

F I G URE 8

F I G URE 9

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

Control Components Inc.We Solve Control Valve Problems.

22591 Avenida Empresa l Rancho Santa Margarita l California 92688 USA

Tel.: (714) 858,1877 Fax: (714) 858-1878

Sales & Service Offices Throughout The World.MC-BFR-3195

205 Drag Valves for Boiler Feedpump Recirculation

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