Control valves reduce boiler feedwater pump problems
Southwestern Public Service Co.`s (SPS) Harrington Station in Amarillo, Texas, switched to high gain control valves applied in series for an upgraded material valve seat and a piloted valve operation. This installation is expected to lessen valve seat erosion and reduce throttling when the valves are opened. “In power plant operation, the propensity for seat leakage on high-pressure control valves, used on the deaerator steam supply lines, is always a genuine maintenance concern. If the normally closed valves fail, the worst-case scenario is loss of boiler feedwater pump suction pressure, which can literally tear up the pump. That`s why we switched to new high gain control valves,” said James Lower, Harrington Station controls superintendent. “The Aeroflow control valves, used on the steam supply from heat exchangers to the deaerators, have been in service for more than one year. We anticipate that the valves will more than double the service life of previous valves. During a recent outage, inspection of the valve seats revealed no seat erosion.” The valves were supplied by Leslie Controls Inc., Tampa, Fla.
SPS is a regional electric utility serving a population of about 1 million people in a 52,000 square-mile area including parts of New Mexico, Texas, Oklahoma and Kansas. Harrington Station is the company`s first modern, coal-fired power plant. The station has three steam turbines with a total electrical generating capability of 1,100 MW. An average of seven, 110-car trains a week deliver a yearly total of approximately 4 million tons of low-sulfur, sub-bituminous coal from Wyoming. For cooling purposes, the Harrington facility uses about 10 million gallons of recycled sewage effluents a day from the city of Amarillo. “The control valves are in an analog system for our deaerators,” Lower said. “Deaerators for the heat exchangers serve three primary functions. First, they de-gas feedwater to help reduce equipment corrosion. Second, they heat feedwater regeneratively and improve thermodynamic efficiency. Last, they provide storage, positive submergence and surge protection on the boiler feed pump.”
Two high gain control valves are installed in series to help avoid throttling action when the valves are called into action. SPS personnel refer to the two-valve setup as series, split-range control. The first valve opens at a 3 psi control signal, the second at a 6 psi control signal. Deaerator steam pressure, maintained at 300 psi, is controlled by regulating the admission of high-pressure steam–2,700 psi–to the deaerator. A fast reset, voltage-adjusting type controller is applied as the pulse-to-pressure converter drive unit. The deaerator outlet pressure measurement is compared to the deaerator steam pressure setpoint, which is a function of the deaerator outlet temperature, usually 700 F. Any deviation is processed through the controller`s proportional integration actions to position the steam supply flow control valves pulse to pneumatic signal converter. Deaerator steam pressure setpoint is developed by a function generator. When deaerator steam pressure is high, the control valve is closed and the control signal is overridden.
Control valves, applied in series, are operated by E-P positioners, providing accurate, repeatable valve stem position. Valves are in analog system for deaerators that de-gas feedwater, heat feedwater regeneratively and improve thermodynamic efficiency.