Abstract: Electricity is an important energy guarantee for the sustained growth of the national economy. It is critical for all sectors of society to continuously improve the demand for electricity and maintain the safety of thermal power plants. As the main device in thermal power plants, high-pressure valves in thermal power plants may have high-pressure Valve leakage problems due to long-term wear, aging, or human factors, which affects the safety of thermal power plants. Based on this, we should comprehensively analyze and consider the causes of high-pressure valve leakage accidents in thermal power plants, and seek reasonable measures to deal with them in order to improve the quality and safety of power supply.
Key words: thermal power plant; high pressure valve; treatment strategy; leakage CLC number: TM621 Document code: A Article ID: 1671-2064(2018)20-0162-02
During the operation of thermal power plants, there are many high-pressure valves applied. It is a single device that regulates the management medium and plays an important role in flow regulation and blocking. In practical applications, the valve body, packing or flange seal is easily affected by objective factors, causing valve leakage accidents, laying a safety hazard for the production and operation of thermal power plants, threatening the safety of personnel life and property. At the same time, if the long-term valve leakage problem is not solved in time, it may cause damage to the single equipment and affect the stability of the process system. If the medium is added, it will bring different levels of energy consumption and increase the production of thermal power plants. The cost has a far-reaching impact. Strengthening research on leakage of high-pressure valves in thermal power plants will help promote innovation and improvement of treatment methods and provide reference for subsequent production.
1 Causes of leakage of high-pressure valves in thermal power plants 1.1 Leakage of high-pressure valves at filling and gland In the long-term use, due to frequent start-stop movement, the stem and packing are highly frictional. In particular, the continuous increase of working time, the contact pressure of the valve stem and the packing is reduced, the gap is increased, and the high temperature medium leaks along the gap between the packing and the valve stem. In addition, valve stem surface roughness, self-quality and component aging problems may lead to leakage accidents at the packing and gland, threatening production safety and personnel safety [1].
1.2 The bonnet and flange joint surface leakage valve bonnet and flange joint leakage accidents are more common, the reasons for such problems are more diverse, mainly in the following points:
(1) The sealing surface is asymmetrical, the bolts are not tight; (2) The surface roughness of the sealing surface is large, and the sealing is poor; (3) The material quality is low, which does not meet the requirements of high-pressure valves. Aging, the flange joint surface and the gasket are poorly sealed; (4) The valve body and the bonnet joint surface are affected by long-term scouring, and the groove may increase the medium leakage and cause serious safety accidents. At the same time, the installation and maintenance of the maintenance personnel is not proper, or the level of process mastery is insufficient. It is easy to cause media leakage accidents due to human factors such as bolts not tightening and bolt asymmetry [2].
1.3 Valve body leakage High pressure valve In the long-term operation, the fluid medium will wash the inside of the valve, and it will be affected by external impact or corrosion. Cracks and blisters may occur. If it is not treated in time, serious leakage will occur. accident. During the forging process, valve manufacturers may form cracks and stomatal hazards due to improper production processes, which may also aggravate high-pressure valve leakage accidents.
2 Treatment methods for leakage of high-pressure valves in thermal power plants The handling of leakage problems of high-pressure valves in thermal power plants should be combined with the leakage of pipelines and equipment for the current treatment, the method of stopping some pipelines or replacing parts, and timely replacement of gaskets and fillers. Effective welding treatment for leak problems.
2.1 Leakage treatment methods at filling and gland In the long-term operation of thermal power generator sets, valve packing leakage accidents are more common, which will cause serious casualties and property losses. High-pressure and high-temperature equipment have corresponding desuperheating water to understand the valve, and the boiler regularly drains the electric main gate stem packing medium leakage problem. For such problems, the leakage accident can be solved by tightening the packing gland bolts. After the high-pressure valve inspection activity is completed, a gap is left between the packing chamber and the gland, and the heat is tight during operation to ensure that the gland is pressed against the packing [3]. In the long-term operation of some high-pressure valves of thermal power plants, due to the lack of disassembly and repair, the gluing bolts may be rusted and a series of safety hazards may be buried. The specific treatment method can close the valve through the welding connection, replace the rusted gland bolt with the isolation system; the flange connection can replace the isolation system in time. If there is no corresponding type of valve, the original valve can be removed and the new valve replaced. In order to eliminate leakage accidents [4].
2.2 bonnet and flange leakage treatment methods The bonnet and flange leakage problems are handled in a variety of ways. It is necessary to select the corresponding method according to the actual situation, in order to improve the solution, mainly including the following:
(1) High pressure valve with pressure trapping method. The practical application of the method of pressure trapping is outstanding, and it can be realized without system isolation. Although it has a wide range of use and high cost, it is more reasonable compared with the start-stop cost of the unit [5]. This method has high technical requirements and requires the operator to have professional capabilities and a stronger on-site resilience. The knowledge can be flexibly applied to practice. In the operation of high-pressure valves in thermal power plants, the number of times of pressure-filling leakage is small each year, and it is difficult to promote and apply them in a wide range. It should be noted that in order to improve the effectiveness of pressure plugging, it is necessary to hire a professional team to deal with the problem of valve leakage, avoid frequent frequent start and stop problems, and minimize power plant losses. (2) Fastening bolt method. This method is suitable for leaks that have just appeared, and the amount of leakage is small. It is possible to organize experienced personnel to perform maintenance work and deal with them according to technical requirements to avoid unnecessary losses. For example, the valve of the power plant found a slight steam problem, which can be tightened by means of a torque wrench to avoid leakage accidents as much as possible [6]. (3) The bonnet and flange joint surface are flushed and surfacing. In practical application of this method, the leakage of the flange joint surface and the bonnet is mainly caused by long-term flushing. The surfacing treatment in the scouring area can be selected, and the convex portion can be treated by the angle grinder to ensure that the smoothness of the joint surface meets the actual requirements. After checking the isolation system, after the flange and bonnet joint surface is washed for a long time, a groove may be formed, and the metal winding pad is replaced to deal with the leakage problem; the metal winding pad is replaced, the sealing glue is applied on the bonding surface, and then the operation is performed for a while. External leakage; the groove is simply polished by means of argon arc welding, and the new metal winding pad can be replaced after normal operation.
2.3 Valve Body Leakage Treatment Method The valve body leakage problem should be handled in the best way according to the actual situation, so as to control the hidden danger within a reasonable range. Specifically in the following categories:
(1) Making a mold. In the long-term use of high-pressure valves, due to the flushing action of the flow medium of some equipments or the defects of the valve body itself, the leakage amount is large, which affects the smooth progress of production activities. For such problems, it can be combined with the actual situation of the job site to process the mold to reduce the amount of media leakage and control it within a reasonable range. The top pressing mold is made, and the pressing mechanism is firmly fixed on the valve side to ensure that the pressing mold corresponds to the leakage point in the forward direction, and the sealing material is used for the treatment at the contact point to avoid leakage stop. Another method can be used to make a sealed box, which combines the specific mounting position and valve size, makes an outer sealed box and a throttled diverter valve, reduces the medium pressure, and closes the valve after tightening. This method can reasonably control the direction of media leakage, minimize the leakage, ensure the orderly development of production activities, and avoid the adverse effects on the safety of people's lives and property as much as possible [7]. For example, the thermal power plant found that there was a void in the body of the sewage door, resulting in a large amount of slurry being directly discharged to the ground, covering a wide area, seriously polluting the surrounding ecological environment, threatening the safety of people's lives and property. For such problems, a 400mm × 300mm × 300mm sealed box can be made, and a media diverting valve can be installed to control the leakage direction while effectively reducing environmental pollution and damage, and meeting the development needs of thermal power plants. (2) Welding method. The welding method is suitable for the case where the amount of leakage of the medium is small, and the threaded short tube whose inner diameter is twice as large as the leakage point can be selected to cause the medium to flow out of the short tube. The short tube of the valve body is welded with the plug corresponding to the short pipe thread, which can effectively improve the prevention and control of the leakage problem after welding. Short tubes reduce the amount of leakage, control the direction of leakage, and select materials in combination with specific needs. It should be noted that in the application of this method, the current is reasonably controlled to avoid the solder joints and the leak points being too close, so as to avoid large leakage problems. For example, a 3mm diameter trachoma in the valve body of a thermal power plant, combined with the specific production requirements of the site, selects a 6mm diameter nut to deal with defects, which helps to avoid the leakage of high pressure valves. During the welding, comprehensive consideration of the adverse effects caused by external currents is an effective method for handling leakage accidents. (3) Riveting method. There is a leakage accident at the position of the valve body. The leakage area should be polished to ensure smooth surface, reduce surface roughness, and seek a reasonable process to deal with the leakage point, which will help improve the short-term sealing results.
3 Conclusion In summary, in the long-term operation of high-pressure valves of thermal power plants, the wear and aging may aggravate the problem of failure. If the valve leakage problem is not solved in time, it will not only aggravate the damage of the single equipment, but also affect the damage. The production activities are carried out in a stable manner, which requires seeking reasonable methods to deal with them in a timely manner, reducing the production costs of thermal power plants and bringing greater economic benefits.
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[3]Zhong Aimin. Analyzing the Technical Points of Outer Valve Leakage and Pressure Sealing Method in Power Plants[J].Science and Technology Vision,2016,(19):252.
[4]MA Junheng, XUE Minghua, DAI Kunpeng. On-line monitoring of internal leakage of steam traps in power plants based on wireless ZigBee technology[J]. Energy Research and Information, 2016, (2): 113-117.
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