Leakages of High-temperature and High-pressure Valves

The valves of thermal power plants are used to close, open, regulate and protect the flow of media in high-temperature and high-pressure steam pipelines, drainage water and water supply pipelines. In the daily operation of valves, especially those in water drainage and sewage systems, defects such as difficulty in opening and closing, and internal leakage of the valve often occur due to frequent operations, while regulating valves are prone to steep rises and falls in the regulation curve or leakage flow exceeding the allowable rate. Even during operation, leakage may occur, causing major defects that endanger the safety of other equipment on site, operation inspections, and maintenance personnel, posing potential accident hazards to the surrounding environment, personnel, and equipment, and affecting the safety and economic operation of the unit. Therefore, understanding and mastering common valve failures and taking corresponding technical measures to deal with them is an important means to ensure key maintenance work and quality control during unit shutdown and major repairs. In the past, when valves or pipes leaked, online pressure plugging could be used to deal with it. However, this method has major hidden dangers for operators and equipment. Therefore, it has been banned in recent years, so we are not going to discuss this in this article.
 
A thermal power plant is a production conversion process that converts the chemical energy of fuel into thermal energy, thermal energy into mechanical energy, and mechanical energy into electrical energy. In the entire conversion process, the four pipes, namely main steam pipes, hot section reheat steam pipes, cold section reheat steam pipes and water supply pipes play a vital role in the process of converting fuel chemical energy into thermal energy and are responsible for the transportation and heat transfer of the medium within the heating surface and the transmission of the medium outside the heating surface. Therefore, installing safety valves, regulating valves and globe valves, on pipelines outside the heating surface are important accessories that control the flow of media in the pipeline by changing the internal interception area, and are important equipment in thermal power plants. Accidents caused by valve defects account for a large proportion of thermal power plants. In thermal power plants, common defects of high-temperature and high-pressure valves mainly include the following categories.
 
1. External leakages
1.1 Different leakages
There are three different leakages. The first is leakage from the packing. The seal between the moving part of the valve (valve stem) and the stationary part (valve bonnet) adopts a packing seal structure. When the valve is operated, the valve stem has two forms of movement: rotation around its axis and upward and downward movement in the direction of the axis. As the number of valve opening and closing times increases, the number of relative movements also increases, and the wear of the sealing packing increases. In addition, the packing is in a high-temperature environment, which makes it prone to aging and losing elasticity. The contact pressure between the packing and the valve stem gradually weakens and fails, resulting in leakages. If there is no opportunity for maintenance during this period, the leakage will further expand. To eliminate such defects, it is necessary to isolate the system; wait until the pressure in the system drops to zero; disassemble the packing gland, and replace the packing with new ones. If the system cannot be isolated, you can tighten the packing gland again to increase the contact pressure between the packing and the valve stem to see if the defect can be eliminated. This situation is suitable if the valve has been overhauled recently and the packing has been replaced during the overhaul. In addition, the valve can be fully closed to replace the packing when the medium is pressed against the lower part of the valve core and there is no pressure. In this case, it should be considered that the temperature and pressure are not high, and the sealing surface of the valve must be sealed well and leak-free. If the valve is installed so that it does not need to be closed during normal operation, the valve stem of the leaked valve can also be kept in a fully open state to form a seal between the undercut groove of the valve stem and the lower joint surface of the valve packing bin. Replace the packing after the device is shut down.
 
The second is leakage outside the valve bonnet or flange. The sealing of the valve bonnet or flange is achieved by tightening bolts and compressing gaskets. When the bolts are pre-tightened, the flange will undergo elastic or plastic deformation, and the gasket will fill the tiny unevenness on the flange surface to achieve sufficient sealing specific pressure and prevent interface leakage of the sealed fluid medium. The reasons for such leakage are that the bolts elongate after being heated, resulting in insufficient pre-tightening force; when tightening the bolts, the tightening force is uneven and the gaps between the joint surfaces are inconsistent, forming an opening and causing leakage; the gasket is improperly selected, and its hardness is higher than flange; in addition, aging failure of the gasket and mechanical vibration will cause leakage due to loose sealing of the joint surface with the flange; leakage will appear for radial grooves and cracks on the contact surface, and the gasket being corroded and penetrated by the medium. The size of the gasket is too small and the center is not properly located during installation, causing the gasket to be installed not in the right position and the partial tightening force to be excessive, exceeding the design limit of the gasket, resulting in insufficient partial sealing specific pressure and leakages. Such defects can only be eliminated by replacing new gaskets after the system is isolated and depressurized. It is worth noting that for valves sealed by flanges and valve bodies, when tightening the flange, the flange must be tightened diagonally; the tightening force cannot be too great at one time, and the tightening must be tightened step by step.
 
The third is leakage of valve bodies. Such defects are mainly caused by casting or forging defects in the manufacturing process of the valve body, such as blowholes, gas holes and cracks, as well as the scouring of the valve body by the medium. The preventive measures for such defects are that strengthen the acceptance after arrival; disassemble the valves, and check the quality of the internal turning process; randomly inspect the valves and conduct pressure tests to find whether there are leakages. However, it is difficult for general blowholes to cause leakages under cold hydraulic pressure testing. During operation, blowholes or gas holes may appear due to thermal stress caused by the influence of high-temperature and high-pressure media. Such defects can only be replaced after using an isolation system.