The advent of distributed control systems (DCS) has revolutionized power plant operation and control. Most fossil generating units today utilize dcss to one degree or another. These systems can monitor final steam temperature to plus or minus 0.5 F, show temperature trends with two-second updates and respond to a given variable change in less than one second. In most cases, however, these plants are still using 40-year old control valve technology. These designs operate in only 20 percent of the stroke, respond in three seconds to a 30 percent step change with a 5 percent error in positioning accuracy, and incur seat leakage that costs up to 10 F in final steam temperature.
“Smart” devices introduced in recent years are being touted as the next generation of control valve technology, enabling operators and engineers to remotely monitor and evaluate various valve characteristics. These smart devices, however, are using dated technology such as lever-arm mechanical position feedback and pilot or spool valves for air loading and venting. Modern technology is available for various power plant valve applications that offer significant cost savings related to leakage reduction, reduced erosion, extended life and improved control.
ATTEMPERATOR SPRAY CONTROL
Attemperator sprays are used to control steam temperature ahead of the turbine, thereby preventing damage that could result from exceeding material constraints and design clearances at higher temperatures. Attemperator spray control valves maintain steam temperatures between about 1,000 and 1,050 F, mitigating against fluctuations induced by changes in steam flow, surface area, burner tilts and heat release rates.
A T2 Aeroflow soothblower PRV valve in use at an Oklahoma power plant.
Supply water for attemperator spray valves usually comes from the boiler feed pump discharge. Superheat spray valves see a low differential pressure, 200 to 300 psi, making turndown and accuracy of positioning key concerns. Reheat spray valves encounter a higher differential pressure, making cavitation and seat leakage key concerns. The major attemperator valve problem relates to reheat sprays. High differential pressure causes excessive seat leakage that, along with wire draw and inaccurate control, results in a reduction in steam temperature and efficiency.
The typical spray valve control scheme uses block valves operated in series. This is a code requirement and represents the only way to stop leaks associated with conventional globe control valves. When the demand signal to the control valve opens the valve, the closed limit switch opens the block valve(s). This causes overspray and a temperature decrease from set point. When the demand signal closes the spray valve, the closed limit switch closes the block valve and again creates overspray.
A Florida utility has been using a control scheme based on Leslie Controls’ spray valve technology that increases the tightness of the control and doesn’t require the use of block valves in series. The solution depends on the use of high gain trim with fast response, optical positioning and zero leakage, with up to six stages of cavitation protection. Rangeabilities of up to 250 to 1 are possible by using a pilot for low flow conditions and the main plug for higher flows; this greatly tightens the normal band associated with this application.
Although this control valve scheme has only been in use for three years, and long-term data collection is necessary to confirm performance expectations, performance to date has been positive. The utility is collecting data from the new sprays to compare with operating data collected prior to the valve system replacement. The figure below presents preliminary data and shows that control has indeed been improved. Plant personnel contend that seat leakage of the old valves and response speed did not allow them to obtain optimum temperature. Average reheater outlet temperature was 950 F with the old system. The plant is now able to make 1,000 F outlet under most load conditions, resulting in the recovery of 45 to 55 F of lost energy.
Power plants that use steam for sootblowing often have difficulty with header pressure regulation. Each bank of sootblower lances has a header that supplies steam at 200 to 500 psi from the superheater outlet. The steam pressure reducing valves (PRVs) see high pressure drops during intermittent operation. With conventional globe valves, leakage can lead to problems with steam PRV lifting, precipitating trim erosion. Slow response rates to rapid load swings can result in poor control and extreme variations in header pressure. In some cases, leakage can necessitate a valve rebuild in as little as three or four months.
Providing pilot-balanced trim without seat leakage prevents excessive header pressure build-up when not blowing soot. The metal-to-metal seating technology used in Leslie’s Aeroflow PRVs, drawing from experience in more than 100 installations, has demonstrated header pressure control to ± 10 psi when reducing pressure from 2,400 psi to 300 psi. A Louisiana utility reports that their trim is starting to show signs of wear after four years of operation, but this compares to previous rebuild frequency rates of every six months.
The piloted trim provides 300 ms per inch travel using pneumatic actuation without boosters and exhausters. For a 300 psi header, an 8 to 10 psi pressure droop can typically be recovered in 15 to 20 seconds.
BOILER FEED PUMP RECURCULATION
The boiler feedwater recirculation valve maintains a minimum flow through the boiler feed pump to keep the constant-speed pumps from cavitating at low flow conditions. Because the highest pressure in a power plant is produced at the boiler feed pump discharge, this equipment has the highest damage potential. Discharge pressures are typically 10 to 20 percent above the boiler’s design pressure. Since most fossil units operate between 1,500 and 3,600 psi, feed pump discharge operates between about 1,750 and 4,400 psi at 350 to 400 F.
Aeroflow valve installed on a boiler feed water recirculation line at a Florida utility.
The boiler feed pump recirculation valve protects the pump during boiler start-up and shutdown. Recirculation flow either goes to the deaerator (fossil units) or the condenser (nuclear units). Due to valve design limitations, these valves are either open or closed. The system monitors flow and fully opens the valve when flow is below a minimum value, typically between 17 and 35 percent. As long as the pump is operating above the recommended minimum flow, the valve is closed against the highest pressure in the plant. The combination of head pressure and deaerator operation can produce a back pressure on the valve of 150 to 250 psi.
For many years, power plants utilizing constant-speed feedwater pumps have been plagued by flow valve problems. Valve leakage has precipitated excessive trim wear, inefficient pump operation and unit load limitations. Trim erosion in recirculation valves is accelerated when encountering a full differential pressure at initial lift, which releases the full energy of cavitation until the trim has enough flow to contain cavitation. Most utilities have opted to install a block valve to prevent leakage when recirculation is not required. This enables the unit to make load, but endangers the pump should the unit trip and the block valve not open. A new impeller for a multi-stage pump can cost $200,000 to install, and lost generation revenues can run even higher.
The typical valve specified for boiler feedwater recirculation is rated for Class V shut-off conditions. Using a valve design that relies on a Tri-Shear Plug, zero leakage can be achieved regardless of pressure drop and size. The Tri-Shear Plug prevents leakage and reduces trim erosion in three ways. First, the protected seat prevents high-velocity transients during valve opening or closing. Second, a five-stage pressure drop occurs near the seating position, promoting effective seating. Third, the plug/seat interface is withdrawn from the flow to prevent direct particle impingement or clamping. Power plant installations in Florida, Tennesse, Pennsylvania and California report more than four years of operation without any signs of leakage or lost pump protection.
Rick Spreter is the Power/Utility Sales & Marketing Manager for Leslie Controls Inc. He has 29 years’ experience in valve and instrumentation applications for the power industry, having worked for Bailey Controls, Masoneilan and Leslie Controls. Spreter holds a bachelor’s degree in mechanical engineering from Worcester Tech.