Application Discussion
AD105
June 15, 2003

The boiler feedwater startup valve is used in many power plants to transition from feedpump recirculation to operation of the unit. Not only is the valve used to initially fill the steam drum, it is also used to protect the main feedwater regulator, if present, from cavitation damage. This valve must operate at high pressure drops initially and low flow rates.

Service conditions for the feedwater startup valve nearly match that of the feedpump recirculation valve with the exception of the flow rates. The startup valve will begin to transition the flow from the recirculation valve. During transition, the startup valve will open as the recirculation valve closes. The startup valve will continue to operate until adequate pressure is achieved in the steam drum to ensure cavitation does not occur in the feedwater regulator valve. Once optimum pressure and flow is achieved, the startup valve will begin to close and transition flow to either the feedwater regulator or the operation of the variable speed pump (See Figure 1). If flow is transitioned to the feedwater regulator, the regulator should be set to an initial position to limit the occurrence of low flow erosion of the valve trim components.

Figure 1: Typical Feedwater Startup System

The service conditions for the startup valve will vary depending on plant size and unit operation. Inlet pressures can reach as high as 5500 psig, but most plants see operation between 2500 psig and 3500 psig. Fisher’s recommendation for this service is a globe or angle valve utilizing Cavitrol protection. The three-stage design capable of withstanding 3000 psid is the most common offering. However, the four-stage design that can withstand pressure drops up to 4000 psid is also available.

In some instances, especially supercritical once-through units, Cavitrol IV trim or Dirty Service Trim (DST) is required. Cavitrol IV and DST can be used in globe or angle valves with DST providing cavitation protection up to 4000 psid and Cavitrol IV providing cavitation protection up to 6000 psid. These solutions also incorporate a protected seating surface that provides additional protection against clearance flow cavitation occurrence at the shutoff areas of the valve trim.

Tight shutoff is also recommended to ensure long lasting trim life. This is especially important when the boiler feedpump is operating in recirculation mode. High pressures will be seen at the valve inlet with minimal pressure on the backside. If any leakage does occur, the valve trim can be washed out in a short amount of time. This will cause a high level alarm in the steam drum that could lead to dumping treated feedwater to the wastewater system let alone affect the controllability of the valve. Because of this, all Cavitrol designs come standard with Class V shutoff protection.