How to Judge this Power Station Valve Quality?
There are several criteria for judging the quality of valves in a power station: seal reliability, action responsiveness, strength, stiffness, and life expectancy. In addition, considering the power station valve as a basic unit in the whole thermal equipment system, there are also requirements for fluid-solid coupling vibration and vibration control.
The following are the four common problems in the application of valves in the power station.
1. The Vibration Problems of Power Station Valve：
Valve opening changes, the poor dynamic performance of the actuator, and the existence of Leakage in the valve body are the main reasons for vibration. Vibration is minimal damage to the valve itself, but the impact on the whole unit is excellent, as shown in the generation of low-frequency oscillations.
There are two types of low-frequency oscillations in the team:
One is oil film oscillation, which is generated by the oil film supporting the bearings during the ramp-up or no-load operation of the unit;
The other is steam oscillation, which is more complex than oil film oscillation and vibrates under the action of steam excitation force, often occurring when the unit is under load.
Valve opening changes and Leakage are important causes of steam oscillation. Some information shows that the United States and Germany have occurred in the steam oscillation of the destruction of the machine, China has also happened in the 50mw and 200mw turbine destruction of the device, due to the lack of real-time data records, so the cause of failure can not be determined, but suspected with two kinds of low-frequency oscillation related. This shows that it is essential to eliminate and reduce steam oscillations, which relies on a systematic study of valve opening variations and the excitation forces generated by leaks. The chance of steam oscillation can be reduced through the practical design of the valve opening and closing strokes.
2. The Control Issues of in Power Stations Valves
The main steam valve and reheat steam valve control system failure are five major turbine accidents. The valve opening degree does not match the design, including transmission mechanism failure, travel overrun, and lag, which affect the strength and vibration of the valve. The valve opening control directly affects the working condition of the steam engine, so it is highly valued and has become one of the core issues for research. In recent years, intelligent valves have been the main focus of research in valve reliability. Smart valves have the function of judging working conditions by themselves and are automated in real-time. The critical component in intelligent valves is the digital positioner, which uses a microprocessor to accurately position the valve’s actuator, monitor， and record the valve’s relevant data.
3. Strength of Power Station Valve
The frequent start-up of the unit has a particular impact on the strength of the valve and the valve’s service life, especially for turbines with regulating valves for speed regulation. In the past, research focused on the control of the valve, but now it seems that the strength problem should not be ignored. Although not all attention should be focused on control issues, attention should be paid to strengthening the strength of the valve, life, sealing research because they are the most basic conditions for the valve work.
(1). Frequent Start-up of Valve
Due to the frequent start-up of the unit, it is possible that the original main steam valve will not be able to meet the new operating requirements. Because the general main steam valve is designed according to the primary load, the design process is only according to its vital static pressure, temperature, and creep assessment. Therefore, there is no common circumference fatigue life problem. However, now the working conditions have changed, the original design will not necessarily meet the requirements. For this reason, it is necessary to consider the low circumference fatigue life design in the design process. Thus, the design working conditions and operating conditions are consistent with achieving the purpose of extending life.
(2). Low Accuracy of the Actuator Stroke Control
Due to the low accuracy of the actuator stroke control, the valve spool to the valve seat impacts load. As a result, some power plants have shattered valve seats and cracked blocks washed into the steam engine, causing a sharp drop in turbine output and severe rotor damage failure. In addition, for high-pressure valves, etc., there are cavitation phenomena, the original casting defects of the valve body, the life analysis and prediction of the valve body after the emergence of cracks, and other topics are worth further research.
4. Valve leakage problems in power stations：
(1). Leakage is not only the cause of vibration, and external Leakage will also cause pollution; internal Leakage will also cause energy loss. Therefore, solving the leakage problem can, to a certain extent, prevent the system from vibrating and also extend the life of the equipment and improve efficiency.
(1). The life of high-pressure valves in supercritical units is sometimes concise, and valves in power stations must be replaced with packing after a few starts. Therefore, research on new sealing filler or design new effective seal form to extend the life of such high-pressure valves and improve operational reliability is necessary.
Due to the unique nature of valves in power stations, we should fully address the above power station valve issues to safely use valves in the power station.