AEP saving $100,000annually with custom valves
American Electric Power`s (AEP) Big Sandy Unit 2, an 800 MW once-through supercritical unit, incorporated a vintage superheater bypass system which created operational problems and was very expensive to maintain. The original system consisted of six valves, two for initial cold water circulation which incorporated capillary tubing and four for the later stages of startup to handle the steam. The angle-style valves were noisy, leaked and their erratic pressure control caused subcritical and low flow boiler trips. These valves required some maintenance during every shutdown of the unit and delayed startup on numerous occasions.
When AEP replaced the valves with two modern valves which were custom designed with cavitation control trim, the problems disappeared. The valves perform smoothly throughout the entire operation from initial cold circulation, 2,000 psi at 150 F, to transfer to the once-through system, 3,500 psi at 940 F. The system is quiet, startups are quicker and the potential for trips caused by these valves has been greatly reduced. The valves have gone through five startups to date.
Compared to the other option considered–a four-valve system incorporated in other 800 MW units–the two-valve system reduced installation costs by $200,000 which is more than the expense for the two valves used. The valves are expected to require a minimum of maintenance which will add to the overall cost savings for the new system design. The superheater bypass system was designed to handle 1.3 million pounds per hour of fluid ranging from cold water to superheated steam. In the early stages of startup, two valves at the beginning of the run of capillary tubing were opened to handle the cold water flow.
The design was intended to be such that the pressure drop from the steam generator to the flash tank takes place along the entire length of the tubing resulting in quiet, vibration-free pressure reduction with no cavitation in the valve. It never happened. This was a crude means of handling cavitation and noise and did not prevent damage to the valves. As startup progressed and fluid temperature increased, the two water valves were closed and the four steam valves opened. As the steam was heated to higher temperatures and its specific volume increased, the four valves were unable to handle the full flow. The two capillary water valves were then reopened to provide additional capacity.
When the proper conditions are reached, flow transfer to once-through operation begins. At this state, the superheater bypass valves had to begin closing to maintain steam generator pressure. Pressure control was erratic due to margin actuator performance related to the elimination of lower friction asbestos packing, later replaced with higher friction graphite packing. If the valves did not close fast enough in response to the pressure controller, the unit was tripped on a subcritical pressure trip. At times, the valves had to be manually jacked to get them to start closing. If the unit tripped at this point, it required several hours and thousands of gallons of fuel oil to get back to this point in the cycle.
In addition to operational problems, the valves were expensive to maintain. The valves had a 2,500 psi differential across them when the unit was on line, causing wire drawing of the seats and plugs and erosion of the valve bodies. Seat leakage, operation vibration and erratic valve response was the norm.
When AEP decided to redesign the superheater bypass system and replace the antiquated valves, engineers from A.gif. Ehrke Co. in Columbus, Ohio, were called in. Working together, AEP employees, a Fisher Controls representative and others incorporated two 6-inch globe-style valves designed to operate in tandem. The Fisher Special Products Group custom designed the two control valves with anti-cavitation trim packages that met the bypass system requirements. The valves are 6-inch class 2500 EH design upgraded to handle the pressure/temperature requirements of the system. Below the seat ring, in the valve outlet, there is an overlay to prevent flashing damage to the valve. The valve design uses a bonnet spacer to accommodate 4-inch travel.
Port size is 4 3/8 inches. Class III shut off is obtained through use of a size 8, type 470 piston actuator which provides more than enough power for smooth valve performance. Control is accomplished using a type 3570 positioner. A trip system with volume tank is used to provide fail safe operation. The new valves` trim design is required to handle cavitation during the initial stages of startup, then flashing as boiler temperature rises and ultimately noise issues created by high-pressure, saturated steam flow. When the two bypass valves initially open to throttle high-pressure boiler feedwater, flow occurs through drilled orifices located in the bottom section of the trim cage. These orifices effect a pressure drop in three stages, which avoids creating cavitation. As boiler firing continues, the valves are called on to meet increased bypass capacity needs. It`s also at this point that flashing typically occurs, adding to the control difficulty imposed by