Preface
The 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. With the loss of medium fluid, the consumption of power plant increases, the cost rises and the economic benefit drops. If the medium fluid is toxic, flammable, explosive, corrosive and other leakage, it is easy to cause poisoning, fire, explosion and other casualties, accelerate the corrosion rate of plant equipment, shorten its service life, seriously pollute the surrounding environment, damage power production, damage people's health. The existence of leakage is a serious threat to the safety of production, and makes the unplanned shutdown accidents increase. The following introduces some valve leakage reasons and plugging methods, as well as the valve repair and maintenance methods, for reference.
Forms and factors of leakage of Ø 2 valve
2.1 leakage and causes of valve packing
During the operation of the valve, there is relative movement between the valve stem and 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 the influence of temperature, pressure and the characteristics of fluid medium, the valve packing is the most prone to leakage. It is due to the gradual weakening of packing contact pressure, aging of packing itself, loss of elasticity and other reasons. At this time, the pressure medium will leak out along the contact gap between the packing and the valve stem, which will blow away part of the packing and wash the valve stem out of the groove for a long time, so as to enlarge the leakage.
2.2 flange leakage
The flange seal of the valve mainly relies on the pre tightening force of the connecting bolt to achieve sufficient sealing specific pressure through the gasket to prevent the leakage of the fluid medium under the sealing pressure. There are many reasons for the leakage, such as the insufficient pressing force of the sealing gasket, the roughness of the joint surface does not meet the requirements, the gasket deformation and mechanical vibration will cause the leakage of the sealing gasket and flange joint surface. In addition, bolt deformation or elongation, gasket aging, resilience decline, cracking, etc. will also cause the flange surface seal is not tight and leakage. There are also human factors that can not be ignored for flange leakage, such as the partial sealing pressure is insufficient due to the eccentric installation of sealing gasket, the tightening force is excessive, which exceeds the design limit of sealing gasket, the uneven force in the process of flange tightening or the offset of the center line of two flanges, which causes false tightening phenomenon, and so on.
2.3 leakage and causes 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, air holes, cracks, etc. the erosion and cavitation of fluid medium are also common factors causing the leakage of valve body.
Principle and advantages of Ø 3 plugging under pressure
3.1 principle of plugging under pressure
The principle of plugging under pressure is based on the sealing mechanism of solid sealing material under dynamic condition of liquid medium. The method is to install special equipment in the leakage part, use the cavity formed between the sealing part and the special equipment, use the special high-pressure glue injection tool to inject the sealant into the cavity, and fill the whole cavity space, so as to balance the extrusion pressure of the sealant and the pressure of the leakage medium, and establish a new sealing structure to block the channels of the leakage hole and block the leakage of the medium.
3.2 advantages of 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) The social and economic losses are reduced.
Introduction of sealing methods with pressure for several common leakage points
According to the specific situation of the production site, the general leakage can be eliminated by replacing the valve, valve packing, flange gasket or welding holes. However, for the valves in production and operation, corresponding technical measures must be taken to eliminate the leakage when they cannot be isolated, so as to ensure the normal operation of safe production of the unit.
4.1 sealing treatment under pressure for valve packing chamber leakage in power plant
It is a safe and reliable technical means to use the injection type plugging technology with pressure at present. It uses special fixtures and hydraulic injection tools to inject the sealant into the sealing cavity formed by the fixture and the outer surface of the leakage part, so as to quickly make up for various complex leakage defects. Under the condition that the injection pressure is greater than the leakage medium pressure, the leakage is stopped by force, and the injection itself changes from plastic body to elastic body 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, there are two kinds of sealants produced and used at home and abroad: one is thermosetting sealants, which can change from plastic body to elastomer only when it reaches a certain temperature, and it is solid at room temperature; the other is non thermosetting sealants, which is suitable for dynamic sealing operation at normal temperature, low temperature and high temperature. This kind of sealants are mostly made into rod-shaped sealants It has good injection processability and filling property under certain pressure, and does not lose the function of valve opening and closing. Here are two common methods:
(1) When the wall thickness of the valve stuffing box is greater than 8 mm, the injection type plugging under pressure can be used to eliminate the defects under dynamic conditions. The injection hole can be directly opened on the wall of the valve stuffing box. The sealing cavity is the stuffing box itself of the valve, and the effect of the sealing agent injected into the valve stuffing box is the same as that of the packing. First of all, use a 10.5mm or 8.7mm diameter drill bit to open a hole at an appropriate position on the outer wall of the valve stuffing box. If the hole can not be drilled through, leave about 1-3mm. Withdraw the drill bit and tap with a M12 or M10 tap. After tapping, screw on the special plug valve for injection and make it in the open position. Use a 3mm diameter long rod drill bit to drill through the remaining valve stuffing box wall. At this time, the leakage medium will flow along the valve In order to prevent the high temperature, high pressure, toxic or corrosive medium from shooting out when drilling, a baffle can be used before drilling a small hole. First, drill a hole with a diameter of 5mm on the baffle with a drill, so that it can cover the overlong drill. If the remaining wall thickness of the baffle drill is added, there will be no danger. After drilling through the small hole, take out the drill bit, screw the special plug valve for injection to the closed position, cut off the medium and connect the high-pressure injection gun for injection sealing injection. If the medium pressure in the valve stuffing box is low, a 3mm diameter long drill bit can be used to drill through the small hole directly, and then the sealing agent injection operation can be carried out.
In June 2003, the self sealing gland leakage of the electric main steam valve of No.3 unit in Panzhihua Iron and Steel Co., Ltd. was solved by using this kind of plugging technology, which avoided the problem that can only be solved once the unit has to be shut down.
(2) For the valve with thin stuffing box wall, auxiliary fixture can be used for dynamic sealing operation. The auxiliary clamp is just to make up for the insufficient wall thickness of the valve stuffing box, which 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 get the ideal local bonding surface by the fixture machining method. If the conditions permit, the outer wall of the valve stuffing box can be properly repaired to make it better fit with the auxiliary fixture. If the shape of the packing outer wall of the leakage valve is complex or the trimming conditions do not permit, an asbestos rubber plate or rubber plate can be padded at the bottom of the auxiliary clamp, and the clamp bolts can be tightened, so that the rubber plate padded below can well plug the gap of the joint surface. The auxiliary fixture should have a thread block matching with the injection plug valve, and the next operation is consistent with the stuffing box with wall thickness. After the whole sealing operation is completed, do not open and close the valve immediately, and can be put into normal use only after the sealing agent is solidified.
In the middle of November 2002, the flange of the balance valve of the inlet valve of the high pressure heater in the power plant of Panzhihua Iron and Steel Co., Ltd. leaked. Because the stuffing box wall of the valve was thin, the auxiliary fixture was used for dynamic sealing operation, and the leakage problem of the valve was solved.
4.2 sealing treatment of valve flange leakage under pressure in power plant
4.2.1 copper wire containment method
It is suitable for plugging with pressure where the gap between two flanges is small, the gap is uniform, and the pressure of leakage medium is low. It can directly place special injection joints for bolts 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. It is not allowed to loosen the nut of the required joint at the same time, so as to avoid the obvious decrease of the sealing pressure on the gasket, the increase of leakage, and even the leakage medium will blow the gasket away, resulting in irreparable consequences. If the original leakage is large, the G-shaped clamp can be used to maintain the balance of the sealing pressure. After installing the injection bolt, insert the copper wire whose diameter is slightly smaller than the leakage flange gap into the flange gap with tools, and punch the outer edge of the flange 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 to carry out dynamic sealing operation. The injection direction should be from the opposite of the leakage point, and the end point should be near the leakage point.
In June 2003, the vertical flange surface leakage of low-pressure connecting pipe of unit 1 in Panzhihua Iron and Steel Co., Ltd. was solved by using this plugging technology, which avoided the occurrence of a shutdown accident.
4.2.2 steel belt containment method
When the gap between the two flanges is a little larger and no more than 8mm, and the medium pressure is less than 2.5MPa, the steel belt containment method can be used for sealing operation. It has a high requirement for the coaxiality of two flanges, and a low requirement for the uniformity of flange clearance. Generally, the thickness of steel strip is 1.5-3.0 mm, and the width is 20-30 mm. Welding or riveting can be used in the production. Transition gasket shall be added under the two joints, and the number of injection joints shall be determined according to the flange size. When installing the steel strip, the steel strip should be placed on the gap between the two flanges, the connecting bolts should be screwed a few times, and then the two transition gaskets should be added to cover the flange gap, and the bolts should be tightened continuously to form a complete sealing cavity. At this time, the dynamic sealing operation can be carried out.
4.2.3 convex clamp plugging method for flange
When the leakage flange clearance is greater than 8mm and the medium pressure is greater than 2.5MPa, from the safety and reliability considerations, we should design and manufacture a flange fixture with accurate processing size, good overall sealing performance and high pressure resistance, which has a high success rate of dynamic sealing operation and is a widely used sealing technology. Before operation, the plug valve should be installed on the fixture of the operator, and the plug valve should be in the open state. The operator should stand at the upper air outlet. 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 can be connected on the fixture, so that the operator can avoid and less contact with the leakage medium. During installation, 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 leakage. Do not align the injection hole with the bolt, so as not to increase the resistance of injection. After the clamp bolt is tightened, the maximum contact gap between the clamp and the flange shall not exceed 0.5mm, and the injection shall 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 plugging under pressure.
This plugging method is widely used in the routine maintenance of No.1, No.2 and No.3 units in power plant of Panzhihua Iron and Steel Co., Ltd. It is a plugging method that must be mastered by pipe valve maintenance specialty. For example, in 2003, the flange leakage of the primary and secondary valves of the feed pump positive heating and return water drainage of units 1 and 2; in February 2003, the flange leakage of 70% electric isolation valve from the auxiliary steam of unit 2 to the deaerator drainage; in March 2003, the flange leakage of the inlet flange of the high pressure heater of unit 1, etc.
4.3 sealing treatment of valve body leakage under pressure in power plant
The leakage treatment of valve body can be applied to the leakage treatment of pipeline,
4.3.1 bonding method
It is a method of plugging under pressure by using the special properties of adhesive. For pressure medium and sand hole with small leakage, first polish the metal luster around the leakage point, then align the leakage point with taper pin, and drive in with appropriate force, so as to significantly reduce the leakage or temporarily plug. Using the characteristics of fast curing speed of adhesive, the pin is timely coated with adhesive, and a new solid sealing structure is established, so as to achieve the purpose of leakage prevention. For the defects of high medium pressure and large leakage, the jacking tool can be used for sealing operation. During the operation, the jacking mechanism is fixed on one side of the valve, and the high-speed jacking screw is used to make the axial direction of the jacking screw face the leakage point. The jacking screw is rotated, and the rivet at the end of the jacking screw is tightly pressed on the leakage part to force the leakage to stop. If the top of the rivet is smaller than the area of the leakage point, a soft metal sheet can be padded under the rivet. After the leakage stops, the metal surface around the leakage point shall be cleaned in time to remove rust and oil, and the prepared adhesive shall be applied around. After the adhesive is fully cured, remove the fixing screws of the jacking screw and rivet, and take down the jacking mechanism. In order to ensure its pressure resistance effect, glass cloth can be used to reinforce the treated leakage part.
4.3.2 welding method
a) In the case of low pressure and small leakage of the leakage medium in the valve body, a nut with an inner diameter more than one time larger than the leakage point can be used to let the leakage medium flow out from the nut, the nut is welded on the valve body, a bolt with the same specification as the nut is matched, an rubber pad or asbestos pad is placed at the bottom of the nut, and the top of the bolt is twisted into the nut by wrapping the raw tape, so as to prevent the leakage Yes. For the valve body with high leakage medium pressure and large leakage, the 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 iron plate, open the isolation valve, align the center hole of the iron plate with the leakage point and fit it on the valve body, so that the leakage medium can flow out through the center hole of the iron plate and the isolation valve. If the joint surface is not good, rubber or asbestos pad can be placed on the joint surface, and then the iron plate around the valve body is welded well, and then the isolation valve can be closed to achieve the purpose of re sealing.
b) When the leakage medium of the valve body is high temperature and high pressure, but the valve has large size and small leakage, the welding method can also be used. First of all, the gaps related to the leakage point on the valve body are directly welded (no leakage point is welded), and then a section of pipe meeting the working conditions (working temperature and pressure) is used. The length of the pipe depends on the site environment (generally about 200 mm), and its diameter should be less than 1 mm If the leakage is larger than the leakage point, a valve corresponding to the pipe shall be welded at one end of the pipe, and the valve shall be fully opened, and then the other end of the pipe shall be aligned with the leakage point. After welding, the valve shall be closed to stop the leakage
4.4 universal plugging method
No matter which part of the valve leaks, if the above methods are difficult to operate, you can also use the "wrapping method": use the plate (pipe) that meets the working conditions to make a box that can wrap the whole valve or the valve body with leakage points, and weld it on the valve so that it can wrap the leakage points. If it is difficult to weld the box, you can open a hole on the box until the last use Method B) of welding method: the method of adding exhaust valve can be easily sealed and welded. This welding method has been used many times in the drainage system of main steam passage of unit 1, bypass secondary valve of main steam passage of unit 3 and high pressure heater of unit 1, 2 and 3 in power plant of Panzhihua Iron and Steel Co., Ltd. This kind of plugging method is the most widely used and the most effective one in the routine maintenance of No.1, No.2 and No.3 units in power plant of Panzhihua Iron and Steel Co., Ltd., and it is also a plugging method that must be mastered by the pipe valve maintenance specialty.
Conclusion
In the production site, there are other components of plugging under pressure. If we can master some basic plugging knowledge, it will be of great help to improve the economy of the power plant. If only one 100MW unit is started and stopped once, the direct economic loss will reach 300000 yuan