Analysis of Types and Selection of Control Valves in the Power Industry
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2017-03-07
Simply put, the power flow diagram is actually a cycle process of water and steam, and selecting control valves in this process is both simple and complex. The simple thing is that the process medium is simple, with only two types: water and steam. The complex thing is that the temperature and pressure fluctuations of water and steam are large, causing a series of problems such as flash evaporation, cavitation, erosion, noise, and corrosion. But the first thing to consider is the safety and reliability of the control valve.
We will follow the system process to provide instructions for selecting control valves.
1. Main and auxiliary boiler feed water valves and composite feed water regulating valves:
Control requirements: Control the boiler inlet water, maintain the boiler liquid level and steam flow.
Working condition requirements: When the boiler is started, the valve pressure difference can reach 100-300 kilograms, and the flow rate is small. During normal operation, the pressure difference is small, and the flow rate is large. The adjustable ratio of flow rate is 75:1 to 100:1.
Valve requirements: The valve body material is WCB/WC9. For start-up valves in dual valve systems, resistance to cavitation is required, with a V-class closure rating. For composite feed water valves, characteristic valve trims are required to meet the needs of boiler start-up and normal operation. Generally, the valve is selected as fault closed. (It should be noted that in order to eliminate dissolved oxygen in the water supply, hydrazine is added for treatment, which can cause adverse reactions to SATELLITE 6 alloy or other similar alloys. Therefore, 316STELLITE surfacing welding method is not used to treat the valve internals, and 416 stainless steel material can be directly used.).
Control requirements: Ensure the safe start and operation of the boiler feedwater pump, and eliminate pump damage caused by low flow rate, increased temperature rise, and strong water cavitation. (Old power plants used switch type minimum flow control valves, as well as mechanical pump automatic circulation valves.).
Working condition: The pressure drop reaches 160-350 kilograms, and it is the valve that bears the largest pressure difference in the power plant system. The flow rate is generally 30% of the normal flow rate.
Valve requirements: anti cavitation, anti clogging structure, V-class sealing grade, continuous operation of the valve during pump start and stop operation, and closing of the valve after normal pump start. The valve is usually open due to malfunction.
3. High/low heater feed water heater drainage, condensate heater drainage:
Control requirements: Control the position of condensate water in the heater, and timely discharge the condensate water, i.e. discharge at high water level and drain at normal water level.
Working condition: the medium is saturated water, the pressure difference of control valves between heaters is generally 6-30kg, and the last stage drain valve is directly connected to the condenser or deaerator.
Valve requirements: Anti flash erosion, Class V or VI sealing rating, and the valve is usually open due to malfunction.
4. Deaerator water level control valve (DALC):
Control requirements: Provide flow to maintain the normal water level of the deaerator, and increase the water temperature by adding steam to eliminate dissolved gases in the water.
Working condition: During the startup phase, the flow rate is small and the pressure difference is large. Under normal circumstances, the flow rate is large and the pressure difference is small, and the working condition is similar to that of a water supply valve.
Valve requirements: Effective resistance to cavitation at low flow rates.
5. Desuperheating valve (desuperheater):
Control requirements: Inject high-pressure water into the steam to ensure that the steam temperature meets the control requirements.
Working condition: The medium is water, with a large pressure difference. Valve requirements: diverse types: nozzle type, auxiliary atomization type, self contained type, and wafer type.
6. Temperature and pressure reducing valves (high and low pressure bypass valves):
Control requirements: Adjust the high-temperature and high-pressure steam to a certain temperature and pressure of steam; Provide a bypass channel for the steam turbine, and the boiler can operate independently of the steam turbine. Working condition: High temperature and high pressure steam temperature and pressure reduction.
