DIRECTIVA 2003/4/CE DEL PARLAMENTO EUROPEO Y DEL CONSEJO de 28 de enero de 2003

The feedwater system encompasses that portion of the plant from the outlet of the deaerator to the

Figure 9AĆ4. The Design ET with Cavitrol trim is recommended for condensate pump recirculation applications. Its drilled hole trim gives protection from cavitation damage. Tight shutoff is also featured.

W3708Ć1

W2174

boiler inlet. It includes the feedwater pumps, the high pressure heaters, the economizer, and finally the boiler. In this section of the plant, the

feedĆwater pressure is raised to the full outlet pressure of the boiler feedpump, and it is also heated to prepare it for entry into the boiler. There are three important valves associated with this subsystemĆĆthe boiler feedpump recirculation valve, the feedwater regulator startĆup valve, and the feedwater regulator. The latter two are generally unique to drumĆstyle boilers and, depending on the boiler manufacturer, the A/E, or the utility, may or may not be control valves.

9A-5

CAUTION

Alloy 6 corrosion problems often occur in feedwater systems.

Typically, boiler feedwater is treated with ammonia, hydrazine or amine derivatives at the deaerator to eliminate excess oxygen. These chemicals attack the protective oxide films on Alloy 6 and allow initiation of an erosion/corrosion process that degrades Alloy 6 overlays. This phenomenon is responsible for many valve trim failures previously attributed to poor design or maintenance. Selection of solid 400 Series stainless steel trim or use of Colmonoyr overlays will eliminate the problem.

Feedwater recirculation valve. The boiler feedpump recirculation valve typically has the most severe service conditions of any control valve in a power plant. In order to protect the feedpump, there must be a recirculation system. The BFPR valve takes feedwater from the boiler feedpump discharge and recirculates it to the deaerator. Depending on the size and type of plant, the valve may be reducing an inlet pressure of as high as 5500 psig to outlet pressures of about 150 psig. These extremely high pressure drops cause very high energy cavitation that will destroy a standard trim control valve in a very short time. In some plants, the recirculation line runs to the condenser, which provides an even higher pressure drop for the valve to handle. There are three basic methods of providing feedpump recirculation. Each method employs a little different philosophy.

1. The modulating type of BFPR valve provides the most efficient method of recirculating. For each boiler feedpump, the manufacturer publishes a pump curve on which the required Net Positive Suction Head (NPSH) is given. For each power plant, there is a certain Net Positive Suction Head available as a function of the actual plant design and layout. To prevent cavitation from occurring in the pump, the NPSH available must be above the NPSH required. Therefore, the BFPR valve

Figure 9AĆ5. The Design EWNTĆ1 is typical of globe valves used for DALC service. Flow resistĆ ance is minimal at high loads and cavitation protection is provided for low load operation and startup.

W3310

modulates to provide the minimum amount of feedwater to the feedpump to insure that the NPSH available is above NPSH required. 2. An on/off method of recirculation provides a constant amount of feedwater flow to the feedpump, up to a specified plant load, at which time the BFPR valve will close. This on/off valve service is less efficient than the modulating recirculation, because some energy is wasted since feedwater is recirculated at higher plant loads.

3. Continuous recirculation might be found in an older power plant. This method recirculates a certain amount of feedwater regardless of plant load, feedwater pressure or feedwater

temperature. Since most feedpumps only require recirculation at low load conditions, this method is the most inefficient since excess recirculation is lost pumping energy. Also, since a certain amount

Figure 9AĆ6. The feedwater system includes three critical service valves: the feedwater recirĆ culation valve, the feedwater regulator, and the feedwater startup valve. While the valves handle the same fluid, the pressure drop and severity of each application vary greatly.

E0124 TO BOILERS FROM CONDENSATE PUMP DEAREATOR BOILER FEEDWATER STARTUP VALVE BOILER FEEDWATER REGULATOR VALVE BOILER FEEDWATER RECIRCULATION VALVE

BOILER FEEDWATER PUMP

of feedwater is always being recirculated, the feedpump will never obtain its maximum rated output capacity.

In older plants, before development of reliable cavitation control trims, the cavitation damage in the control valve was reduced by using a

backpressuring device downstream such as orifice plates or a system of capillary tubing. The valves in this type of system were exclusively on/off. This method cut damage by lessening the pressure drop, but did not solve the cavitation problem. The main problem with this type of recirculation valve combination occurs when the valve plug pulls off its seat. For a certain amount of time, the valve will not experience any backpressure and highĆenergy cavitation occurs. Also, since the backpressure plates are only designed for one set of service conditions, any other conditions will not provide the necessary backpressure to keep the valve from cavitating. If on/off transitions are quick and flow ranges are narrow, it works. Be cautious, however.

As power plant efficiency became a more

prevalent concern, need arose for a valve trim that would prevent cavitation altogether and be

capable of throttling. Cavitation abatement trim has developed fully and there is a Cavitrol trim to handle every cavitating situation.

Figure 9AĆ7. The CAV 4 is designed for boiler feedpump recirculation use with pressure drops to 6500 psi. It has fourĆstage, antiĆcavitation trim, which is designed to take the pressure drop away from seating surfaces.

W2700Ć1

9A-7 Typical service conditions are:

DrumĆStyle P1 = 2800Ć3200 psig ∆P = 2700Ć3100 psid T = 250Ć400°F OnceĆThrough P1 = 3700Ć4500 psig ∆P = 3800Ć4400 psid T = 350Ć450°F

Fisher's recommendation for the BFPR valve depends on the pressure drops involved. For drops less than 3000 psi, an appropriate size and pressure class HPT or EHT with twoĆ or

threeĆstage Cavitrol Ill trim may be used. Characterized trim should not be used since the pressure drop is relatively constant regardless of flow.

For pressure drops up to 4000 psi the Cavitrol III fourĆstage trim installed in an appropriate size HPT or EHT can be used.

For pressure drops greater than 4000 psi, a CAV 4 with Cavitrol IV trim is used. Typically, a 2Ćinch or a 4Ćinch valve is needed. The Design CAV 4 is the most proven valve on the market for these extreme conditions.

At these highĆpressure drops, the quality and care of seating surfaces is very important. Tight shutoff can be provided by use of metal seats, which are soft" and easily deformed and by the use of Tight Shutoff Trim (TSO). These soft seats avoid energy loss and provide longer trim life due to the elimination of wireĆdraw effects.

For dirty feedwater applications, the Dirty Service Trim (DST) has been used to allow the passage of particulate up to threeĆquarter inch in diameter and still provide cavitation protection up to pressure drops of 4000 psi.

In a drum style boiler, when the drum is being filled at startup, the feedwater flow control system must absorb the full pressure difference between

Figure 9AĆ8. The Design EHD is recommended for feedwater regulator service. It has high capacity and good rangeability with modified equal percentage characteristic.

W2992Ć1

the feedpump pressure and the unpressurized drum. The highĆpressure drops present in this situation offer a high potential for cavitation. Initial flow rates are usually very small. As the drum comes up to pressure, the pressure drop across the feedwater flow control system decreases and the flow increases. Once startup is complete the feedwater flow regulator experiences very small pressure drops. For this reason, feedwater flow control is generally handled with a parallel piped twoĆvalve system. A small cavitation protected valve handles startup and a high capacity feedwater regulator handles higher loads. Due to pressure conditions, both of these valves are highĆpressure globe style valves.

Feedwater startĆup valve. The startĆup valve will have much the same service conditions as the boiler feedpump recirculation valve. That is, high inlet pressure and full drop. This valve needs antiĆcavitation trim and tight shutoff. Unlike the boiler feedpump recirculation valve, the feedwater

Figure 9AĆ9. The main steam system features five critical service control valves.

E0125

startĆup valve generally does not need the same amount of cavitation protection at all valve travels since pressure drop changes with travel.

Therefore, a Cavitrol Ill characterized cage may be used.

Typical service conditions are: P1 = 2800Ć3200 psig

∆P=2700Ć3100 psid T = 300Ć400°F

The normal recommendation is a small HPT or EHT with Cavitrol III threeĆstage trim. Trim materials should be chosen to avoid use of Alloy 6.

Feedwater regulator valve. Depending on the A/E, utility and boiler manufacturer, the feedwater regulator valve will either be a large motor

operated valve or a large, high capacity control valve. In plants with constant speed, motor operated feedpumps, this application usually requires a control valve, and it controls feedwater flow. Those plants with steam driven, variable

speed pumps use the pump to control flow and use the valve to control drum pressure and maintain spraywater differential. The valve in this case is usually a motor operated valve, but it can be a control valve.

Typical service conditions are: P1 = 2800Ć3200 psig

∆P = 100Ć600 psid T = 300Ć400°F

Fisher designed the large, high capacity EH to handle this application (Figure 9AĆ8). The modified equal percentage characteristic gives the high capacity and large turndown ratio required. Depending on the specific utility, tight shutoff may or may not be required. Again, avoid use of Alloy 6 trim components.

In many smaller plants, A/E's and utilities are looking to combine the Feedwater Startup and Regulator application into one control valve. Depending on the actual service conditions, a Design HP or EH with characterized Cavitrol Ill trim can often provide the low travel cavitation

9A-9 protection along with the high capacity

requirement. Sizing is critical, however.