Leakage of valve often occurs on packing, flange seal and valve body. Long time leakage of valve can cause erosion of valve stem and flange seal surface, and eventually make the valve scrapped, coupled with the loss of medium fluid, so that power plant consumption increases, cost increases, economic benefits decline. If the medium fluid is toxic, inflammable, explosive, corrosive and other external leakage, it is easy to occur poisoning, fire, explosion and other casualties and accelerate the corrosion rate of plant equipment, shorten its service life, seriously pollute the surrounding environment, damage power production, damage to people's health. The existence of leakage seriously threatens safety production and increases the number of unplanned downtime accidents. The following describes some valve leakage reasons and plugging methods as well as valve maintenance and maintenance methods for reference.
Two: the form and factors of valve leakage.
1: leakage and cause of valve packing
During the operation of the valve, there is a relative movement between the stem and the packing, which includes rotation and axial movement. With the increase of the number of switches, the number of relative movement also increases, as well as temperature, pressure and fluid medium characteristics, valve packing is the most vulnerable to leakage. It is due to the gradual weakening of the contact pressure of the filler, aging of the filler itself, loss of elasticity and other reasons. At this point, the pressure medium will leak out along the contact clearance between the packing and the valve stem, blowing away part of the packing and scouring the valve stem out of the groove for a long time, thereby expanding the leakage.
2: Flange Leakage
The flange seal of the valve mainly relies on the pre-tightening force of the connecting bolt to achieve enough sealing pressure through the gasket to prevent the leakage of the sealing pressure fluid medium. There are many reasons for its leakage, such as insufficient compressive force of gasket, roughness of joint surface does not meet the requirements, gasket deformation and mechanical vibration will cause leakage of gasket and flange joint surface. In addition, bolt deformation or elongation, gasket aging, resilience decline, cracking, etc. will also cause flange seal is not strict and leakage. Flange leakage can not be ignored by human factors, such as seal gasket offset, so that the partial seal pressure is not enough tight excessive, exceeding the design limit of the gasket, and flange tightening process uneven force or two flange center line offset, resulting in false tightening phenomenon are prone to leakage.
3: leakage and cause of valve body
The leakage of valve body is mainly caused by casting or forging defects in the process of valve production, such as sand holes, pores, cracks and so on. Fluid erosion and cavitation are also common factors causing valve body leakage.
Three: the principle and advantages of plugging with pressure.
1: the principle of plugging with pressure.
The principle of leak plugging under pressure is based on the sealing mechanism of solid sealing material in dynamic condition of liquid medium. The method is to install special equipment in the leaking part, use the cavity formed between the sealing part and the special equipment, use special high-pressure glue injection tool to inject the sealant into the cavity, and fill the whole cavity space, so that the pressure of the sealant and the pressure of the leaking medium are balanced, and establish a new sealing structure to plug the leak. Pore channels, blocking the leakage of media.
2: the advantage of plugging with pressure.
(1) there is no need to shut down or isolate the system.
(2) no pressure relief is required for the system.
(3) save a lot of energy and manpower.
(4) the power loss caused by equipment isolation or shutdown is greatly reduced.
(5) reduce social and economic losses.
Four: introduction of several common leak points with pressure plugging method
According to the specific conditions of the factory production site, can be used to replace the valve, replacement valve packing, replacement of flange gasket or welding holes to eliminate the general leakage. However, for the valves in production and operation, in the case of no isolation, it is necessary to take appropriate technical measures to eliminate leakage, in order to ensure the normal operation of safe production unit.
1: leakage treatment of valve packing chamber leakage in power plant
At present, it is a safe and reliable technology to use injection type pressure leak plugging technology. It uses special clamp and hydraulic injection tools to inject sealant into the sealing cavity formed by the external surface of the clamp and the leakage part, and quickly compensates for various complex leak defects. Under the condition that the injection pressure is greater than the leakage medium pressure, the leakage is forcibly stopped, and the injection itself transforms from plastic to elastic in a short time, forming an elastic sealing structure and maintaining a certain working sealing pressure to achieve the purpose of re-sealing. At present, sealants produced and used at home and abroad are generally divided into two categories: one is heat-cured sealants, which can only be changed from plastic to elastomer at a certain temperature, and the other is non-heat-cured sealants, which are suitable for dynamic sealing at normal temperature, low temperature and high temperature. Operating requirements, this kind of sealant is mostly made of stick-like solid or two-component greasy material, after it is installed in high-pressure injection gun, under a certain pressure with good injection process and filling, and does not lose the valve switch function. Two common methods are introduced below:
(1) When the wall thickness of the valve packing box is more than 8 mm or so, the valve packing box can be operated directly by opening the injection hole on the wall of the valve packing box when the defect is eliminated by injection pressure plugging under dynamic conditions. The sealing chamber is the packing box of the valve itself, and the effect of the sealing agent injected into the packing box of the valve and the packing action. The same effect. First of all, in the proper position of the outer wall of the valve packing box with a diameter of 10.5mm or 8.7mm drill hole can not be drilled through, about 1-3mm left, withdraw the drill bit, with M12 or M10 tap tapping, after the end of tapping, the injection of special plug valve screw on, and put it in the open position, using a diameter of 3mm long rod bit to the remaining valve In order to prevent high temperature, high pressure, poisonous or corrosive media from spouting and injuring people, a baffle plate can be used before drilling a small hole. First, a hole with a diameter of 5mm is drilled on the baffle plate with a drill bit so that it can be set over a long bit, and the baffle plate can be used to drill the rest of the hole. There is no danger of wall thickness. After drilling through the small hole, take out the drill bit, screw the special cock valve for injection to the closed position, cut off the medium to connect the high-pressure injection gun for injection sealing agent operation. If the medium pressure in the valve packing box is low, you can also use a long drill with a diameter of 3 mm to drill directly through the small hole, and then carry out the sealant operation.
In June 2003, the leak stoppage technology was used to solve the leak problem of the valve and avoid a non-stop operation.
(2) for the thinner valve with stuffing wall, auxiliary clamp can be used for dynamic seal operation. The auxiliary fixture is just to make up for the insufficiency of the wall thickness of the valve packing box, which is equivalent to a special joint fixed on the outer wall of the valve packing box to connect the high pressure injection gun. It is difficult to obtain an ideal local bonding surface by the machining method of the fixture. When the conditions permit, the outer wall of the packing box of the valve can be repaired appropriately to make it better fit with the auxiliary fixture. If the outer wall of the packing of the leakage valve is complex or the dressing condition is not allowed, an asbestos rubber plate or a rubber plate can be padded at the bottom of the auxiliary fixture, and the fixture bolt can be tightened so that the rubber plate padded below can plug the gap of the joint surface. The auxiliary fixture should have a thread patch matching the injection cock valve, and then the packing box with the wall thickness is operated in accordance with the whole sealing operation, do not immediately switch the valve after the completion of the sealing operation, and so on after the sealing agent solidified, can be put into normal use.
In the middle of November 2002, the balance valve flange of the high-pressure heater inlet valve of Pangang Power Plant leaked. Because the packing wall of the valve was thin, the auxiliary fixture was used to seal the valve dynamically, which solved the problem of valve leakage.
2: leakage treatment for leakage of valve flange in power plant
2.1 copper wire containment method
The utility model is suitable for sealing leakage under pressure with small clearance between two flanges, uniform clearance and low leakage medium pressure, and can directly place a bolt special injection joint on the dismantled bolt, generally not less than two. When installing the injection joint, one nut should be loosened, the injection joint should be installed, the nut should be re-tightened quickly, and then another injection joint should be installed. The required joint nut should not be loosened at the same time, so as not to cause a significant drop in the sealing pressure on the gasket, an increase in leakage, or even leakage media will blow the gasket away, resulting in Irreparable consequences. If the original leakage is large, the sealing pressure can be maintained by a G-shaped clamp. After the injection bolt is installed, the copper wire with a diameter or slightly smaller than the leakage flange gap is inserted into the flange gap with a tool, and the outer edge of the flange is punched out of the lip, so that the copper wire is fixed in the flange gap, thus forming a new sealing cavity. . Then the high-pressure injection gun can be connected for dynamic sealing operation. The direction of injection should be in turn from the opposite leakage point, and the end point should be near the leakage point.
In June 2003, the leakage of the vertical flange surface of the low-pressure connecting pipe of Unit 1 of Pangang Power Plant was solved by this leakage stopping technology, and a shutdown accident was avoided.
2.2 steel belt containment method
When the gap between the two flanges is slightly larger and not more than 8 mm, and the medium pressure is less than 2.5 MPa, the steel strip sealing method can be used for operation. It has higher requirements for the two flange's coaxiality, and the requirement for the uniformity of flange clearance is not high. The thickness of steel strip is 1.5-3.0 mm, and the width is 20-30 mm. Welding or riveting can be used when making the steel strip. Transition gaskets should be installed under the two joints, and the number of injection joints should be determined according to the flange size. When installing the steel strip, the steel strip should be located on the gap between the two flanges, the connecting bolts should be screwed a few buckles, and then the two transition gaskets should be added, all the flange gaps should be wrapped, and the bolts should continue to be tightened, eventually forming a complete sealing cavity, then dynamic sealing operation can be carried out.
Plug clamping method for 2.3 flange
When the leakage flange clearance is greater than 8 mm and the medium pressure is greater than 2.5 MPa, the flange clamp with accurate machining dimension, good overall sealing performance and high pressure resistance should be designed and manufactured in consideration of safety and reliability. Before operation, the plug valve should be installed on the clamp of the worker, and the plug valve should be opened. The operator should stand at the upper air outlet. If the leakage pressure or flow rate is very large, the leakage medium can be blown to one side by compressed air, or long rods are connected to the clamp, so that the operator can avoid and less contact with the leakage medium. When installing, the injection hole of the fixture should be in the middle of the two flange connecting bolts, and ensure that there is a injection hole near the leakage. Do not align the injection hole with the bolt, so as not to increase the resistance of the injection. After the clamp bolt is tightened, the maximum contact clearance between the clamp and the flange should not exceed 0.5mm. The injection should start from the farthest away from the leakage point and gradually approach the leakage until the leakage stops. This method can also be used for pipeline plugging with pressure.
This plugging method is widely used in daily maintenance of # 1, # 2, and # 3 units in Panzhihua Iron and Steel Power Plant. It is a kind of plugging method that must be mastered by pipe valve maintenance specialty. For example, in 2003, the feed pump of Unit # 1 and Unit # 2 was warming back water to drain leakage of flange of primary and secondary valves; in February 2003, leakage of flange from auxiliary steam of Unit # 2 to front flange of 70% electric isolation valve of deaerator drainage; in March 2003, leakage of flange of imported high pressure heater of Unit # 1, etc.
3: leakage treatment of leakage of valve body in power plant
The leakage treatment of the valve body can be applied to the leakage treatment of the pipeline. There are two ways.
3.1 bonding method
It is a method of plugging with pressure by using special properties of adhesives. For the pressure medium and the small leakage of the sand hole, the metal luster can be first polished around the leakage point, and then the taper pin is used to align the leakage point, the appropriate force into, so that the leakage is significantly reduced or temporary plugging. A new solid sealing structure was established by coating the pin around with adhesive in time to achieve the purpose of leak stopping. For the defects of high medium pressure and large leakage, sealing operation can be carried out by overhead pressure tools. When operating, the overhead pressure mechanism is fixed on the side of the valve. The high-speed overhead pressure screw makes the axial direction of the overhead pressure screw face the leakage point, rotates the overhead pressure screw, and uses the rivet at the end of the overhead pressure screw to press tightly on the leakage part to force the leakage. The leakage stops. If the top of the rivet is smaller than the area of the leak point, a piece of soft metal can be padded under the rivet. After the leak stops, the metal surface around the leak point can be cleaned up in time to remove rust and grease, and the prepared adhesive can be applied to the surrounding area. After the adhesive is fully solidified, the fixed screws of the overhead screw and rivet are removed, and the overhead mechanism is removed. In order to ensure the pressure resistance effect, glass cloth can be used to reinforce the leaking parts.
3.2 welding method
A) In the case of low leakage pressure and small leakage, a nut with an inner diameter more than twice the leakage point can be used to let the leakage medium flow out of the nut, weld the nut to the valve body, and match a bolt of the same specification as the nut, place a rubber pad or asbestos pad at the bottom of the nut, and wind the top of the bolt. The upper adhesive tape is screwed into the nut to prevent leakage. For valve bodies with high leakage medium and large leakage, drainage welding can be used. Firstly, an iron plate with a circular hole in the middle is used to weld an isolation valve with the same diameter as a circular hole on the circular hole of the iron plate. The isolation valve is opened, and the central hole of the iron plate is aligned with the leakage point to paste the valve body so that the leakage medium can flow out through the central hole of the iron plate and the isolation valve. If the bonding surface is not good, rubber or asbestos pads can be placed on the bonding surface, and then the iron plate around the body welded, and then close the isolation valve can achieve the purpose of re-sealing.
B) In the case of high temperature and high pressure of the leakage medium in the valve body, but the valve has a larger size and a small leakage, welding method can be used. First, the gap on the valve body related to the leakage point is welded to death (no leakage point), and then a section of pipe length in accordance with the working conditions (working temperature, pressure) to see the scene environment. And (usually take about 200 mm), its diameter should be greater than the leak point, a pipe to adapt to the valve welding in one end of the pipe, and all open the valve, and then take the other end of the pipe to the leak point welded, close the valve can stop the leakage.
4: Universal plugging method
No matter which part of the valve is leaking, if the above method is difficult to operate, you can also use the "wrapping method": use the working condition of the sheet (pipe) to make a valve body can wrap the whole valve or a leak point, weld it on the valve so that it can wrap the leak point, if it is difficult to weld the box body Method B: It can be easily sealed by adding exhaust valve. This welding method has been used many times in the drainage system of the main steam passage of Unit 1, the secondary valve of main steam passage of Unit 3, the secondary valve of the main steam passage, the high pressure heater of Unit 1, 2 and 3 in Pangang Power Plant. Satisfactory success has been achieved. This method is the most effective one in daily maintenance of # 1, # 2, # 3 units in Panzhihua Iron and Steel Power Plant, and it is also a kind of leakage stopping method that must be mastered by valve inspection and repair specialty.
There are other components in the production site with pressure plugging, if we can master some basic plugging knowledge, for the economic improvement of power plants is very helpful. Only one 100MW unit starts and stops, and the direct economic loss will reach more than 300 thousand yuan. Therefore, we can successfully plug the leakage site with pressure, reduce the number of unplanned downtime, the benefit is significant. In a word,
(1) plugging with pressure is a kind of emergency repair work. The leakage point of plugging with pressure is a temporary treatment measure, which has some limitations and timeliness. Under the conditions of the situation, we still need to thoroughly repair the leakage parts. The fundamental way to eliminate the phenomenon of "running, running, dripping and leaking" and improve the health level of equipment operation is to rely on the rationality of planned maintenance and improve the process of equipment maintenance.
(2) The working environment of leak plugging under pressure is harsh, the working time is long, the labor intensity is great, there are many uncertain factors in the operation, and the operation risk is great. Pre-work safety preparation is very important, pre-operation risk analysis must be adequate, the implementation of safety measures must be in place.
(3) Pressure leak plugging is a very professional technology, requiring the operator's on-site strain capacity, mastery of mechanical expertise and the use of special tools for pressure leak plugging have high requirements. It is difficult to popularize this technology in thermal power plants because of the high requirements for operators and special equipment. At present, the pressure plugging work is completed by some professional companies.
(4) Pressure leak plugging is a new technology, it also has a continuous improvement and improvement process, it has its own limitations and scope of application. Plugging with pressure can not solve all the leakage problems, and is still in the stage of exploration and perfection.