Electricity is one of the most important energy industries. Currently, there are steam, wind, hydro, geothermal, tidal power, and solar power. This article focuses on the selection of some typical control valves in fuel, kerosene, gas, and nuclear power plants. The common point of this type of power station is to vaporize water into steam, which uses steam to drive the gas turbine to drive the generator to generate electricity. The difference is that the energy for converting water into steam is different: natural gas, fuel oil, coal, and atomic reactor. The structure of various power stations is roughly the same. Let us briefly describe the selection of control valves by taking the combined cycle power plant as an example, and then briefly explain the different parts of each power station.
In simple terms, the power flow diagram is actually a circulation process of water and steam. The selection of the control valve in this process is simple and complicated. The simple process medium is simple, only water and steam. The complicated temperature and pressure fluctuation range of water and steam 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 flow to expand the control valve selection instructions.
1. Water supply valve main and auxiliary boiler water supply valve and compound water supply control valve Control requirements: control boiler water intake, maintain boiler liquid level and steam flow.
Working condition requirements: When the boiler starts, the valve pressure difference can reach 100-300 kg, the flow rate is small, the pressure difference is small during normal operation, the flow rate is large, and the flow rate adjustable ratio is 75:1-100:1.
Valve requirements: The valve body material is WCB/WC9. For the start valve of the double valve system, it is required to resist cavitation, and the V-class is closed. For the composite water supply valve, the characteristic valve trim should be used to meet the boiler start and normal operation. need. Generally, the valve is selected as the fault shutdown. (It should be noted that the general feed water will eliminate the dissolved oxygen and will be treated with hydrazine. It will adversely affect the SATELLITE alloy or other similar alloys, so it is best not to use 316+. STELLITE surfacing method for handling valve trim, can be directly selected from 416 stainless steel).
2. Minimum flow circulation valve of boiler feed water pump Control requirements: Make boiler feed water pump safely start and run, eliminate pump damage caused by excessive flow, increased temperature rise and strong cavitation of water. (Old-fashioned power plants have a switch-type minimum flow control valve, as well as a mechanical pump automatic circulation valve).
Working condition: The pressure drop reaches 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.
Valve requirements: anti-cavitation, anti-blocking structure, V-class sealing level, the valve works continuously when the pump starts and stops running, the valve closes after the pump starts normally, the valve is usually fault open.
3, high / low heater trap water supply heater hydrophobic, condensate heater hydrophobic control requirements: control the location of the condensate in the heater, timely discharge of condensed water, that is, high water level discharge, normal water level is hydrophobic.
Working condition: The medium is saturated water, the control valve pressure difference between the heaters is generally 6-30 kg, and the last stage steam trap is directly connected to the condenser or deaerator.
Valve requirements: anti-flash scouring, V or VI seal rating, the valve is usually fault open.
4. Deaerator water level control valve (DALC)
Control requirements: Provide flow to maintain the normal water level of the deaerator, increase the water temperature by adding steam to eliminate the gas dissolved in the water.
Working condition: The flow in the starting stage is small, the pressure difference is large, the normal flow is large, the pressure difference is small, and the working condition is similar to the water supply valve.
Valve requirements: effective against cavitation at lower flow rates,
5, desuperheater valve (desuperheater)
Control requirements: Inject high pressure water into the steam to bring the steam temperature to control.
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 opposite type.
6, desuperheating pressure 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 passage for the steam turbine, and the boiler can work independently.
Working conditions: high temperature and high pressure steam desuperheating and decompression.
Valve requirements: resistance to thermal shock, noise reduction, and rapid response.
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