Electricity is one of the most important energy industries. At present, there are steam, wind, water, geothermal, tidal power, and solar power. This article focuses on the selection of some typical control valves in oil, kerosene, gas and nuclear power plants. The common point of this type of power station is to vaporize water into steam, and use steam to drive a gas turbine to drive a generator to generate electricity. The difference is that the energy sources for converting water into steam are different: natural gas, fuel oil, coal, and atomic stacks. The structure of various power stations is roughly the same. Let us first take a combined cycle power station as an example to briefly describe the selection of control valves, and then briefly explain the different parts of each power station.
In simple terms, the electricity flow chart is actually a cycle of water and steam. It is simple and complicated to select a control valve in this process. The simple is that the process medium is simple, with only water and steam. The complicated is that the temperature and pressure of water and steam fluctuate widely, which brings a series of problems such as flashing, cavitation, erosion, noise, and corrosion. But the first thing to consider is the safety and reliability of the control valve.
We follow the system process to expand the control valve selection instructions.
1. Feed water valve main and auxiliary boiler feed water valve, compound feed water regulating valve
Control requirements: control the boiler water inlet to maintain the boiler liquid level and steam flow.
Working condition requirements: When the boiler starts, the valve pressure difference can reach 100-300 kg, the flow is small, the pressure difference is small during normal operation, the flow is large, and the flow rate is 75:1-100:1.
Valve requirements: The valve body is made of WCB/WC9. The start valve in the double valve system requires anti-cavitation and a V-level closing level. For the composite feedwater valve, it is necessary to use characteristic valve trims to meet the requirements of the boiler during startup and normal operation. need. Generally, the valve is selected as fault closed. (It should be noted that in order to eliminate the dissolved oxygen in the water, hydrazine will be added for treatment. It will have adverse reactions to SATELLITE No. 6 alloy or other similar alloys, so it is best not to use 316 +STELLITE surfacing method to process valve trims, 416 stainless steel materials can be directly used). 2. The minimum flow circulation valve of the boiler feed water pump.
Control requirements: make the boiler feed water pump start and run safely, and eliminate the damage to the pump due to low flow, increased temperature rise, and strong water cavitation. (The old power station has a minimum flow control valve that uses an on-off type, as well as an automatic circulation valve that uses a mechanical pump).
Working conditions: The pressure drop is 160-350 kg, which is the valve with the largest pressure difference in the power station system. The flow rate is generally 30% of the normal flow rate.
Valve requirements: anti-cavitation, anti-blocking structure, V-class sealing level, the valve works continuously when the pump starts and stops, the valve is closed after the pump is started normally, and the valve is usually open in failure. Imported pump
3, high/low heater trap, feed water heater drain, condensate heater drain
Control requirements: control the position of the condensate in the heater, and discharge the condensate in time, that is, discharge at high water level, and drain at normal water level.
Working conditions: The medium is saturated water, the pressure difference between the control valves between the heaters is generally 6-30 kg, and the last-stage trap is directly connected to the condenser or deaerator.
Valve requirements: anti-flashing erosion, V or VI sealing level, the valve is usually open for failure.
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 the gas dissolved in the water.
Working condition: The flow is small at the start-up stage, and the pressure difference is large. Under normal conditions, the flow is large and the pressure difference is small. The working condition is similar to a water valve.
Valve requirements: effective anti-cavitation at lower flow rates,
5, desuperheating valve (desuperheater)
Control requirements: Inject high-pressure water into the steam to make the steam temperature meet the control requirements.
Working conditions: the medium is water, the pressure difference is large, the valve requirements: various types: nozzle type, auxiliary atomization type, self-contained type and wafer type
6, temperature reducing valve (high and low pressure bypass valve)
Control requirements: adjust the high temperature and high pressure steam to a certain temperature and pressure steam; provide a bypass channel for the steam turbine, and the boiler can work independently of the steam turbine.
Working condition: high temperature and high pressure steam reduces temperature and pressure.
Valve requirements: thermal shock resistance, noise reduction, and rapid response.
