High-temperature and high-pressure valves are important accessories in the thermal system of modern thermal power plants. They play very important functions such as cutting off, regulating, and protecting. They are mainly used to control the operation of various media. If they fail, leakages may happen, resulting in a waste of working fluid, reducing the reliability and economy of unit operation. In severe cases, non-stop emergency repairs or even explosions of pressure vessels or pipelines may happen, causing injuries to equipment and personnel. Therefore, it is very important to understand the defects that are prone to occur in the operation of high-temperature and high-pressure valves and how to deal with common failures of pipeline valves. The causes of failures and common treatments are introduced in this article.
 
1.2 Early detection or prevention of leakages
The leakage of valves is nothing more than the ones mentioned above: leakages caused by failures of packing, defects in valve bonnets and flanges, and leakage occurring in the manufacturing process of the valve. In fact, these types of defects can be discovered or prevented early, so that timely measures can be taken, and corresponding elimination methods can be formulated according to the type of defects.

(1) Develop a complete equipment patrol inspection system and enhance daily inspections. During the operation of the equipment, maintenance personnel and operating personnel must strengthen inspections of on-site equipment in accordance with relevant inspection systems. During the inspection of valve equipment, observe whether there is water vapor in the valve packing chamber, and judge whether the valve packing leaks based on this. There will be a squealing sound coming from the leakage of high-temperature and high-pressure media. In this case, the inspection personnel need to be careful not to get too close to the equipment where the sound is coming from. After reporting to the person in charge in the relevant department, gradually approach the equipment making the squealing sound by reducing the force to determine the location of the leak. 

(2) Develop a maintenance plan for equipment. According to the plan, when the equipment is out of service, comprehensive disassembly inspection and maintenance of key equipment will be carried out. During this process, the packing, grinding valve stem, and packing chamber cavity are replaced to the quality standards required by the maintenance regulations; the valve body is ground, and the metal supervision is contacted to conduct metal supervision on the valve body to detect the safety and reliability of the valve body. Check the straightness and hardness of the valve stem. For unqualified ones, if they can meet the standards through maintenance and reprocessing, they can continue to be used, otherwise, they should be replaced.
 
(3) In the maintenance process, focus on checking the corrosion of the sealing surface for the joint surfaces of the valve, valve bonnet and valve flange, and whether the material, quality and size of the sealing gasket meet the requirements of the operating medium. During normal maintenance work, all disassembled sealing materials should be replaced as a whole and must not be used again.
 
2. Internal leakages of valves
2.1 Causes of internal leakages of valves
(1) Original defects: During manufacturing, the valve sealing surface was not heat-treated during surfacing or the surfacing layer was defective, resulting in cracks, falling, and getting stuck inside the pipeline during operation. This will cause insufficient medium flow or medium flow rate to change, causing internal leakages and greater scouring of the sealing surfaces of pipes and valves.

(2) Frequent opening and closing of valves: cause grooves on the sealing surface due to scouring of media, which causes the sealing of the valve seat and valve core to fail and internal leakages. Such defects are more common in regulating valves and blowdown valves.

(3) Foreign matter between the valve seat and the valve core: causes the valve’s sealing surface to be damaged or not closed well in the closing process. This situation often occurs after the four pipes are replaced; the debris inside the pipes has not been flushed, and the sewage hasn’t been discharged for a long time.

(4) The valve not being closed well: When the manual valve is closed, due to the great torque, it is judged to be closed well when it is not closed well and internal leakage occurs; the stroke adjustment of the electric valve including the regulating valve is incorrect or the motor torque is too small; the valve cannot be closed well. These two situations not only cause leakages of the valve but also cause the sealing surface to be thinned or to form grooves under high-speed scouring of the medium with a small flow rate for a long time, reducing the maintainability of the equipment and affecting the service life.
 
2.2 Treatment and prevention of internal leakages of valves
(1) Different types of internal leakages can be eliminated in different ways. For example, internal leakage of an electric globe valve is mainly caused by the valve not being closed well due to changes in the electronic zero operating point of the electric door travel switch. The elimination method is the valve the maintenance personnel cooperated with the thermal maintenance personnel to re-adjust and position the valve switch stroke so that the valve can be opened or closed flexibly. The valve is not closed well due to the small torque of the electric actuator or pneumatic actuator of thermal equipment, which can be eliminated by manually continuing to close the valve at the production site to ensure that the valve is well closed and there is no leakage. Reposition the off-stroke and eliminate the problem. After the output of the electric or pneumatic actuator reaches the maximum and cannot be adjusted, consider replacing the electric or pneumatic actuator with a higher power to solve the defect. After replacing the new electric or pneumatic actuator, the switch position of the valve should be repositioned.
 
(2) The internal leakage or curve of the regulating valve is not ideal. For such defects, the valve should be completely disassembled and the sealing surface should be ground. Focus on checking whether the scouring of the valve cage and regulating valve stem seriously exceeds the standard. If they don’t meet the requirements, they should be replaced. For imported control valves, relevant after-sales service personnel should be contacted in advance before equipment maintenance, and they will conduct inspection and processing on site.
 
(3) When sewage valves discharge sewage, they must be fully opened according to the requirements of the sewage discharge in the operating procedures, and the sewage discharge time must reach the specified time. Insufficient opening of the valve such as slightly open or half-opening and insufficient discharge time are prohibited, which will cause dirt to accumulate on the sealing surface, causing the sealing surface to be squeezed or the valve core and valve seat to be stuck in the closing process of the valve. If the four pipes are replaced in a large area, in addition to flushing the pipes, it is also necessary to increase the number of sewage discharges and the sewage discharge time after the unit is started to ensure that the dirt can be fully discharged and make sure the safety of equipment operation.