1 Preface
Valves in power plants are mainly used to control the operation of fluid media on various equipment and pipelines. Valve leakage often occurs on packing, flange seals and valve bodies. Long-term valve leakage can cause impact on the valve stem and flange sealing surface. Corrosion will eventually cause the valve to be scrapped, and the loss of the medium fluid will increase the consumption of the power plant, increase the cost and decrease the economic benefits.If the medium fluid is toxic, flammable, explosive, corrosive, etc., it is prone to accidents such as poisoning, fire, explosion and other casualties, and accelerates the corrosion rate of plant equipment, shortens its service life, pollutes the surrounding environment, and destroys electricity. Production harms people’s health.The existence of leakage seriously threatens production safety and increases unplanned shutdown accidents of power plants.The following introduces some of the reasons for valve leakage, plugging methods, and valve repair and maintenance methods for reference.
2 Forms and factors of valve leakage
2.1 Leakage and causes of valve packing
During the operation and use of the valve, there is relative movement between the valve stem and the packing, which includes rotation and axial movement.With the increase of the number of switches, the number of relative movements also increases, as well as the influence of temperature, pressure and the characteristics of the fluid medium. The valve packing is the most prone to leakage.It is caused by the gradual weakening of the contact pressure of the filler, the aging of the filler itself, and the loss of elasticity.At this time, the pressure medium will leak outward along the contact gap between the packing and the valve stem, which will blow away part of the packing and flush the valve stem out of the groove for a long time, thereby expanding the leakage.
2.2 Leakage of flange
The flange sealing of the valve mainly relies on the pre-tightening force of the connecting bolts, and the gasket achieves sufficient sealing specific pressure to prevent the leakage of the sealed pressure fluid medium.There are many reasons for its leakage. The pressure of the gasket is insufficient, the roughness of the joint surface does not meet the requirements, the deformation of the gasket and the mechanical vibration, etc. will cause the sealing gasket and the flange joint surface to leak tightly. .In addition, deformation or elongation of bolts, aging of gaskets, decreased resilience, cracks, etc. will also cause leakage of flange surface due to inadequate sealing.There are also human factors that cannot be ignored in the flange leakage, such as the offset of the sealing gasket, the partial sealing pressure is insufficient, the tightening force is excessive, the design limit of the sealing gasket is exceeded, and the uneven force or two methods in the flange tightening process Leakage is prone to occur due to the deviation of the center line of the orchid, causing false tightening.
2.3 Leakage of the valve body and its cause
The leakage of the valve body is mainly caused by casting or forging defects in the production process of the valve, such as sand holes, pores, cracks, etc., and the erosion and cavitation of the fluid medium are also common factors that cause the valve body to leak.
3 Principles and advantages of leak plugging under pressure
3.1 Principle of leak plugging under pressure
The principle of leak plugging under pressure is based on the sealing mechanism of the solid sealing material under the dynamic conditions of the liquid medium.The method is to install special equipment at the leaking part, use the cavity formed between the sealing part and the special equipment, and use a special high-pressure glue injection tool to inject the sealant into the chamber, and fill the entire chamber space to make the sealant squeeze The force is balanced with the pressure of the leaking medium, and a new sealing structure is established to block the channels of the leaking pores and block the leakage of the medium.
3.2 Advantages of leak plugging under pressure
(1) There is no need to shut down or isolate the system.
(2) There is no need to depressurize the system.
(3) Save a lot of energy and manpower.
(4) Greatly reduce the power loss caused by equipment isolation or shutdown.
(5) Reduced social and economic losses.
4 Introduction of several common leak plugging methods under pressure
According to the specific conditions of the power plant's production site, the method of replacing the valve, replacing the valve packing, replacing the flange gasket or welding holes can be used to eliminate general leakage.
However, for valves in production and operation, if they cannot be isolated, corresponding technical measures must be taken to eliminate leakage to ensure the normal operation of the safe production of the unit.
4.1 Treatment of under-pressure plugging for leaks in power plant valve packing chambers
The injection-type leak-stopping technology under pressure is currently a relatively safe and reliable technical means. It uses special fixtures and hydraulic injection tools to inject the sealant into the sealed cavity formed by the fixture and the outer surface of the leaking part, quickly To make up for various complex leakage defects.Under the condition that the pressure of the injection is greater than the pressure of the leaking medium, the leakage is forcibly stopped, and the injection itself is transformed from plastic to elastomer in a short time, forming an elastic sealing structure and maintaining a certain working seal specific pressure , To achieve the purpose of resealing.At present, the sealing injections produced and used at home and abroad are roughly divided into two categories: one is heat-curing sealing injections, which can transform from plastics to elastomers only when they reach a certain temperature, and are solid at room temperature; One type is non-heat-curing sealant, which is suitable for dynamic sealing operation requirements at room temperature, low temperature and high temperature. This kind of sealant is mostly made of rod-like solid or two-component greasy material, which is installed in a high-pressure injection gun After that, it has good injection processability and filling performance under a certain pressure, and does not lose the valve switching function.Two common methods are described below:
(1) When the wall thickness of the valve stuffing box is greater than about 8mm, when the injection type is used for leak plugging under dynamic conditions to eliminate defects, the injection hole can be directly opened on the wall of the valve stuffing box, and the sealed cavity is The valve stuffing box itself, the role of the sealing injection agent injected into the valve stuffing box is the same as that of the packing.First, use a drill with a diameter of 10.5mm or 8.7mm to make a hole at the appropriate position on the outer wall of the valve stuffing box. The hole cannot be drilled. Leave about 1-3mm. Withdraw the drill and tap with an M12 or M10 tap. The tapping is finished. Then, screw on the special plug valve for injection and put it in the open position. Use a 3mm diameter long-stem drill to drill through the remaining valve stuffing box wall. At this time, the leakage medium will be ejected along the direction of the drill bit row. , In order to prevent the high temperature, high pressure, toxic or corrosive medium from spraying and hurting people during drilling, a baffle can be used before drilling small holes. First, drill a 5mm diameter hole on the baffle with a drill bit to make it fit. If the drill bit is too long, the remaining wall thickness of the baffle drill will not be dangerous.After drilling through the small hole, take out the drill bit, turn the special plug valve for injection to the off position, cut off the medium and connect the high-pressure injection gun to inject the sealing agent.If the medium pressure in the valve stuffing box is low, a long drill with a diameter of 3mm can also be used to directly drill through the small holes, and then perform the seal injection operation.
In June 2003, the self-sealing gland leakage of the electric main steam valve of Pangang Power Plant 6# unit used this kind of plugging technology to solve the leakage problem of the valve, avoiding a problem that cannot be solved by stopping the machine.
(2) For valves with thin stuffing box walls, auxiliary clamps can be used for dynamic sealing operations.The auxiliary clamp is only to make up for the insufficient wall thickness of the valve stuffing box, and is equivalent to a special joint fixed on the outer wall of the valve stuffing box to connect the high-pressure injection gun.It is difficult to obtain an ideal partial bonding surface by the mechanical processing method of the clamp. If conditions permit, the outer wall of the valve stuffing box can be properly repaired to make it fit better with the auxiliary clamp.If the shape of the packing outer wall of the leakage valve is complicated or the trimming conditions are not allowed, an asbestos rubber plate or rubber plate can be placed at the bottom of the auxiliary fixture, and the fixture bolts can be tightened so that the rubber plate underneath can well block the gap of the fitting surface.There should be a threaded block on the auxiliary fixture that matches the injection plug valve, and then the operation is the same as the stuffing box with the same wall thickness. After the entire sealing operation is completed, do not open and close the valve immediately, and wait for the sealing injection to solidify before it can be put into normal operation. use.
In mid-November 2002, the flange of the balance valve of the inlet gate of the high pressure heater of Panzhihua Iron and Steel Power Plant leaked. Because of the thinner wall of the stuffing box of the valve, auxiliary fixtures were used for dynamic sealing operation, which solved the problem of valve leakage.
4.2 Treatment of under-pressure plugging of valve flange leakage in power plants
4.2.1 Copper wire containment method
It is suitable for pressure plugging with small gap between two flanges, uniform gap, and low leakage medium pressure. Special injection joints for bolts can be directly placed on the removed bolts, generally no less than two.
When installing the injection joint, loosen a nut, install the injection joint, quickly re-tighten the nut, and then install another injection joint. The required joint nuts cannot be loosened at the same time, so as to avoid the sealing pressure on the gasket. Significant decrease, increase in leakage, and even leakage of medium will blow away the gasket, resulting in irreparable consequences.If the original leakage is large, you can use a G-clamp to maintain the balance of the seal specific pressure. After installing the injection bolt, use a tool to insert a copper wire with a diameter or slightly smaller than the leakage flange gap into the flange gap. The outer edge of the flange punches out of the lip, so that the copper wire is fixed in the flange gap, thus forming a new sealed cavity.Then the high-pressure injection gun can be connected for dynamic sealing operation. The injection direction should be from the opposite of the leak point, and the end point should be near the leak point.
In June 2003, Panzhihua Iron and Steel Power Plant #6 unit low pressure interconnection pipe vertical flange surface leakage is the use of this plugging technology to solve the leakage problem of the interconnection pipe flange surface and avoid a shutdown accident.
4.2.2 Steel belt containment method
When the gap between the two flanges is slightly larger and does not exceed 8 mm, and the medium pressure is less than 2.5 MPa, the steel belt containment method can be used for sealing operations.It has high requirements for the coaxiality of the two flanges, but not high requirements for the uniformity of the flange gap.The thickness of the steel belt is generally 1.5-3.0mm, and the width is 20-30mm. Welding or riveting can be used during production. A transition gasket should be added under the two joints. The number of injection joints is determined according to the flange size.When installing the steel belt, the steel belt should be placed on the gap between the two flanges, the connecting bolts should be tightened a few times, and two transition gaskets should be added to cover the flange gap. Continue to tighten the bolts to form a complete seal. The cavity can be dynamically sealed at this time.
4.2.3 The convex fixture plugging method of flange
When the leakage flange gap is greater than 8mm and the medium pressure is greater than 2.5MPa, considering safety and reliability, it should be designed and manufactured into a flange fixture with accurate processing dimensions, good overall sealing performance, and high pressure resistance. Its dynamic sealing operation has a higher success rate. High, is a widely used sealing technology.Before the operation, install the plug valve on the jig and keep the plug valve in the open state. The operator should stand on the upper air vent. If the leakage pressure or flow is large, compressed air can be used to blow the leakage medium to one side, or A long rod is connected to the fixture to prevent the operator from contacting the leaked 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 an injection hole near the leak. Do not align the injection hole with the bolt, so as not to increase the resistance of the injection.After the clamp bolts are tightened, the maximum contact gap between the clamp and the flange should not exceed 5mm. The injection should start from the farthest point from the leakage point, and gradually approach the leakage until the leakage stops.This method can also be used to plug the pipeline under pressure.
这种堵漏方法在攀钢发电厂＃1、＃2、#3机组日常维护中普遍应用，是管阀检修专业必须掌握的一种堵漏方法。例如，2003年＃1、＃2机组给泵正暖回水疏水一、二次阀法兰的泄漏；2003年2月份，＃2机辅汽到除氧器疏水70%电动隔离阀前法兰的泄漏；2003年3月份＃1机高压加热器进口法兰泄漏等等。
4.3 Treatment of under-pressure plugging for valve body leakage in power plants
The leakage treatment of the valve body can be applied to the leakage treatment of the pipeline. There are the following two methods:
4.3.1 Bonding method
It is a method to use the special properties of adhesives to plug leaks under pressure.For pressure medium and blisters with small leakage, first grind out the metal luster around the leakage point, then use a tapered pin to align the leakage point, and drive in with appropriate strength to significantly reduce the leakage or temporarily block it.Take advantage of the fast curing speed of the adhesive, fill the pin with adhesive in time to form a new solid sealing structure to achieve the purpose of preventing leakage.For the defects of high medium pressure and large leakage, the top pressure tool can be used for sealing operation. During operation, the top pressure mechanism is fixed on one side of the valve, and the high-speed top pressure screw is rotated so that the axial direction of the top pressure screw is facing the leakage point. The top pressure screw uses the rivet at the end of the top pressure screw to tightly press on the leaking part to force the leak to stop.
If the area of the top of the rivet is smaller than the area of the leakage point, a piece of soft metal can be placed under the rivet. After the leakage stops, clean the metal surface around the leakage point in time to remove rust and oil stains, and apply the prepared adhesive around it.After the adhesive is fully cured, remove the fixing screws of the top pressure screw and the rivet, and remove the top pressure mechanism. In order to ensure its pressure resistance effect, the leaked parts can be reinforced with glass cloth or the like.
4.3.2 Welding method
a) When the leakage medium pressure of the valve body is low and the leakage is small, 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, and the nut is welded to the valve body. Fit a bolt with the same specification as the nut, place a rubber pad or asbestos pad at the bottom of the nut, and screw the top of the bolt into the nut with raw tape to prevent leakage.For the valve body with high leakage medium pressure and large leakage, drainage welding method can be used.First, use an iron plate with a round hole in the middle, weld an isolation valve with the same diameter as the round hole on the round hole of the iron plate, open the isolation valve, and align the center hole of the iron plate with the leakage point and attach it to the valve body. Let the leakage medium flow out through the center hole of the iron plate and the isolation valve.If the bonding surface is not good, you can place rubber or asbestos pads on the bonding surface, then weld the iron plate around the valve body, and then close the isolation valve to achieve the purpose of resealing.
b) The leakage medium of the valve body is high temperature and high pressure, but the valve has a larger external size and the leakage is not large. The welding method can also be used. First, directly weld all the gaps related to the leakage point on the valve body (not Welding leakage point), and then use a length of pipe that meets the working conditions (working temperature, pressure) depending on the site environment (usually about 200mm), its diameter should be larger than the leakage point, and a valve that is compatible with the pipe Weld on one end of the pipe and fully open the valve, then take the other end of the pipe to the leakage point and weld it, then close the valve to stop the leakage.
4 Universal plugging method
No matter which part of the valve is leaking, if the above methods are difficult to operate, the "wrapping method" can also be used: use a plate (pipe) that meets the working conditions to make a box that can wrap the entire valve or the valve body with leaks, and It is welded on the valve so that it can cover the leaking point. If it is difficult to weld the box body, you can open a hole on the box body until the end and use the method b) in the welding method: the method of adding an exhaust valve is It can be easily sealed and welded.This welding method has been used many times in the drainage system of the main steam passage of the #1 unit of Panzhihua Iron and Steel Power Plant, the bypass secondary door of the main steam passage of the #3 unit, and the drainage system of the high pressure heater of the 1#, 2# and 3# units , Have achieved satisfactory success.This leakage plugging method is the most effective method used in the daily maintenance of Pangang Power Plant #1, #2, #3 units, and it is also a leakage plugging method that must be mastered by pipe valve maintenance professionals.
5 Conclusion
There are pressure plugging of other parts at the power plant production site. If we can master some basic plugging knowledge, it will be of great help to the economic improvement of the power plant.Just start and stop one 100MW unit in our plant, the direct economic loss amounts to more than 30 yuan.Therefore, we can successfully plug leaks under pressure at the on-site leak points and reduce the number of unplanned shutdowns. The benefits are significant.In short:
(1) Leakage plugging under pressure is a kind of emergency repair work.The leakage point of the leak plugging under pressure is a temporary treatment measure, which has certain limitations and timeliness.Under conditions, it is still necessary to conduct a thorough inspection of the leaking part.The fundamental way to eliminate the phenomenon of "running, leaking, dripping and leaking" on site and improve the health of equipment operation is to rely on the rationality of planned maintenance and the improvement of equipment maintenance technology.
(2) The working environment with pressure plugging is harsh, the working time is long, the labor intensity is high, there are many uncertain factors in the operation, and the operation risk is high.Safety preparations before work are very important. Risk analysis before work must be adequate, and safety measures must be in place.
(3) Leakage plugging under pressure is a highly professional technology, which requires operators' on-site resilience, mastery of mechanical expertise and the use of special tools for plugging under pressure.Due to the high requirements for operators and special equipment, the promotion of this technology in thermal power plants is somewhat difficult.At present, the on-site leak plugging work under pressure is done by some professional companies.
(4) Leakage plugging under pressure is a new technology, it also has a process of continuous improvement and perfection, and it has its own limitations and scope of application.Leakage plugging under pressure cannot solve all leakage problems, and it is still in the stage of exploration and improvement.
