In most power generation processes, the system operates through a cycle of water and steam. While the medium itself is simple—just water and steam—the selection of control valves in this process can be both straightforward and highly complex. The simplicity lies in the fact that the fluid involved is only water and steam, but the challenges come from the wide fluctuations in temperature and pressure, which can lead to issues such as flashing, cavitation, erosion, noise, and corrosion. Therefore, the primary concern when selecting control valves is ensuring their safety and reliability.
Water supply valves, including main boiler feed valves and composite feedwater regulating valves, play a crucial role in maintaining proper boiler operation. Their main function is to regulate the amount of water entering the boiler, ensuring stable steam generation and maintaining the correct water level. In many systems, the feedwater control consists of two valves: one for boiler startup and another for normal operation. This dual-valve setup allows for better control during different operational phases.
Under varying conditions, these valves face significant challenges. During boiler startup, the pressure difference across the valve can reach 100–300 kg, with relatively low flow rates. However, under normal operation, the pressure difference decreases, and the flow rate increases dramatically, requiring a large adjustable flow ratio of up to 75:1 or even 100:1. For these applications, materials like WCB or WC9 are commonly used. The starter valve in a dual system must have anti-cavitation features and a V-level shutoff rating, while the composite feedwater valve needs specialized internal components to handle both start-up and steady-state conditions. Typically, these valves are designed to fail in the closed position for safety.
Another critical component is the boiler feed pump minimum flow circulation valve. Its purpose is to protect the pump during startup and shutdown by preventing excessive flow, which could cause overheating, cavitation, and mechanical damage. Older power plants often used switch-type or mechanically controlled valves for this purpose. The working conditions for this valve are extreme, with pressure drops reaching 160–350 kg—making it the valve with the highest pressure differential in the entire power system. It typically handles about 30% of the normal flow rate. To cope with these conditions, the valve must be designed with anti-cavitation and anti-blockage features, along with a V-class sealing rating. It should operate continuously during pump startup and shutdown, then close once the pump is running normally. These valves are usually set to fail in the open position to ensure continued flow and prevent damage.
Lastly, high and low heater traps, such as those used in feedwater heaters and condensate heaters, are essential for removing condensate and maintaining efficient heat transfer. These valves require precise control to avoid inefficiencies and potential damage to the system. For more information on Valve Products, please visit: http://www.chinawj.com.cn
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