Selection of control valve in power station
Selection of control valve in power station
In the thermal power industry, the operating conditions of control valves are relatively special. From a simple point of view, the power process is only a water and steam cycle process, but the selection of control valves in this process is both simple and complex. The simple thing is that the process medium is simple, and there are only two kinds of water and steam. The complex thing is that the temperature and pressure of water and steam fluctuate widely, causing a series of problems, such as flash, cavitation, erosion, noise and corrosion. However, the safety and reliability of the control valve should be considered first.
1. Feed water valve (main and auxiliary boiler feed water valve, composite feed water regulating valve)
Control requirements: control the boiler water inflow, ensure the steam evaporation, and maintain the boiler water level (the main and auxiliary boiler water supply valve systems include two control valves, one for boiler startup and one for normal water supply.
Working condition: the differential pressure of the valve can reach 100-300 kg when the boiler is started, and the flow is small, while the differential pressure is small during normal operation, and the flow is large, and the adjustable ratio of flow is 75:1~100:1. Valve requirements: WCB/WC9. For the startup valve in the dual-valve system, cavitation resistance and Class V closing level are required. For the composite feedwater valve, the characteristic valve trim shall be used to meet the requirements of boiler startup and normal operation. Generally, the valve is selected as fault closed.
2. Small flow circulating valve of boiler feed pump
Control requirements: make the boiler feed pump start and stop operation safely, and eliminate the pump damage caused by too small flow, increased temperature rise and strong water cavitation. In old power stations, switch-type small flow control valves are used, and mechanical pump automatic circulation valves are also used.
Working condition: the pressure drop reaches 160-350 kg, which is the valve with large pressure difference in the power station system, and the flow is 30% of the normal flow.
Valve requirements: anti-cavitation, anti-blocking structure, Class V sealing grade, continuous operation of the valve when the pump is started and stopped, the valve is closed after the pump is started normally, and the valve is normally opened in fault.
3. High and low heater drain valves (feedwater heater drain, condensate heater drain)
Control requirements: control the position of condensate in the heater, timely discharge the condensate, discharge at high water level, and drain at normal water level.
Working condition: the differential pressure of control valve between saturated water and heater is generally 6-30kg, and the last stage drain valve is directly connected to the condenser or deaerator. Valve requirements: anti-flash scouring, Class V or VI sealing grade, the valve is usually open in fault.
4. Deaerator water level control valve
Control requirements: provide flow to maintain the normal water level of the deaerator, and increase the water temperature by adding steam to eliminate the dissolved gas in the water.
Working condition: the flow rate is small in the startup stage, the pressure difference is large and the temperature is high, and the flow rate is large in the normal condition, the pressure difference is small, and the working condition is similar to the water supply valve.
Valve requirements: effective cavitation resistance at low flow.
5. Desuperheating valve (desuperheater)
Control requirements: inject high-pressure water into the steam to make the steam temperature meet the control requirements.
Working conditions: water, large differential pressure valve requirements: various types: nozzle type, auxiliary atomization type, self contained type and wafer type.
6. Temperature and pressure reducing valve (high and low pressure bypass valve)
Control requirements: adjust the high temperature and high pressure steam to the steam with a certain temperature and pressure, provide a bypass channel for the steam turbine, and the boiler can work independently of the steam turbine. Working conditions: high-temperature and high-pressure steam, valve requirements: thermal shock resistance, noise reduction, rapid response.