Thread Content
Pressure-containing leak sealing technology for valves 1: Valve leaks often occur at the packing, flange seals, and valve body. Prolonged leakage can cause erosion of the valve stem and flange seal surfaces, ultimately leading to the failure of the valve. In addition, the loss of fluid medium increases the costs for power plants, raising expenses and reducing economic efficiency. If the medium fluid is toxic, flammable, explosive, corrosive, or otherwise prone to leakage, it can lead to accidents such as poisoning, fires, and explosions. It can also accelerate the corrosion of plant equipment, reducing its service life; in severe cases, it can pollute the surrounding environment, disrupt electricity production, and harm people’s health. The presence of leaks poses a serious threat to safe production, leading to an increase in unplanned shutdowns. The following outlines some causes of valve leakage and methods for sealing leaks, as well as methods for repairing and maintaining valves, for reference. 2 Forms and factors of leakage outside the valve 2.1 Leakage of the valve packing and its causes During the operation of a valve, there is relative motion between the valve stem and the packing, which includes rotation and axial movement. As the number of switches increases, the number of relative movements also increases; in addition, factors such as temperature, pressure, and the properties of the fluid medium play a role as well. The valve packing is the part most prone to leakage. It is caused by the gradual decrease in the contact pressure of the filler, the aging of the filler itself, and the loss of its elasticity. At this point, the pressure medium will leak outward through the gap between the packing and the valve stem; over time, this can blow away part of the packing and erode grooves in the valve stem, thereby increasing the leakage. 2.2 Flange leakage: The sealing of a valve’s flange relies primarily on the pre-tightening force of the connecting bolts; gaskets are used to achieve a sufficient sealing pressure in order to prevent the leakage of the pressurized fluid being sealed. There are various reasons for leakage: insufficient compression force on the gasket, roughness of the mating surfaces that does not meet requirements, deformation of the gasket, and mechanical vibrations – all of these can lead to poor sealing between the gasket and the flange surfaces, resulting in leakage. Additionally, bolt deformation or elongation, gasket aging, reduced resilience, and cracking can also lead to poor sealing of the flange surfaces and resulting leaks. There are also human factors that cannot be ignored in flange leaks; for example, improper installation of the gaskets can result in insufficient local sealing pressure or excessive tension, exceeding the design limits of the gaskets. Additionally, uneven force application during flange tightening or an offset between the centers of the two flanges can lead to a false sense of tightness, all of which can cause leaks. 2.3 External leakage of the valve body and its causes The main reason for external leakage of the valve body is defects arising during the manufacturing process, such as sand holes, pores, cracks, etc. Additionally, the erosion caused by the fluid medium and cavitation are also common factors that lead to leakage in the valve body. 3 Principles and Advantages of Plugging Leaks Under Pressure 3.1 Principle of Plugging Leaks Under Pressure The principle of plugging leaks under pressure is based on the sealing mechanism of solid sealing materials, with liquid media operating under dynamic conditions. The method involves installing specialized equipment at the leakage site; by utilizing the chamber formed between the sealed area and this specialized equipment, a dedicated high-pressure glue injection tool is used to inject sealant into the chamber, filling it entirely. This allows the compressive force of the sealant to balance the pressure of the leaking medium, thereby creating a new sealing structure that blocks all the pathways of the leakage and prevents the medium from escaping. 3.2 Advantages of plugging leaks under pressure (1) No shutdown or isolation of the system is required. (2) There is no need to depressurize the system. (3) Saves a large amount of energy and labor. (4)**Reduces power loss caused by equipment isolation or shutdown. (5) Reduced social and economic losses. 4 Introduction to pressure-based methods for sealing several common leakage points: Depending on the specific conditions at the factory’s production site, common leaks can be resolved by replacing valves, changing valve gaskets, replacing flange gaskets, or welding the holes shut. However, for valves in operation, when it is not possible to isolate them, appropriate technical measures must be taken to eliminate leaks in order to ensure the safe and normal operation of the unit. 4.1 Pressure-based sealing to address leaks in the valve packing chambers of power plants: The injection-type pressure-based sealing technique is a relatively safe and reliable method at present. It utilizes special clamps and hydraulic injection tools to inject sealant into the sealing cavity formed between the clamps and the outer surface of the leaking area, thereby quickly addressing various complex leakage issues. When the pressure of the injected fluid is greater than the pressure of the leaking medium, the leakage is forcibly stopped; the injected fluid itself changes from a plastic state to an elastic state in a short time, forming an elastic sealing structure that can maintain a certain working sealing pressure, thereby achieving re-sealing. Currently, sealing injections produced and used domestically and internationally can be roughly divided into two categories: one is thermosetting sealing injections, which can transform from a plastic state to an elastic state only when they reach a certain temperature; at room temperature, they remain in a solid state ； Another category consists of non-thermally curing sealing compounds, which are suitable for dynamic sealing applications in normal temperature, low-temperature, and high-temperature environments. These sealing compounds are usually available in the form of rod-shaped solids or two-component paste-like materials; when placed in a high-pressure injection gun, they exhibit good injectability and filling properties under certain pressures, without compromising the valve’s ability to open and close. The following introduces two common methods: (1) When the wall thickness of the valve stuffing box is around 8 mm or more, to eliminate defects under dynamic conditions using pressure-based sealing agents, it is possible to create injection holes directly on the wall surface of the valve’s stuffing box. The sealing chamber in this case is the valve’s stuffing box itself, and the function of the sealing agent injected into it is similar to that of the packing. First, make holes at appropriate positions on the outer wall of the valve stuffing box using drills with a diameter of 10.5 mm or 8.7 mm; the holes should not be drilled all the way through – leave about 1–3 mm remaining. Remove the drill and then use taps of M12 or M10 to tap the holes. After tapping is complete, attach the special plug valve for injecting fluid and set it in the open position. Use a long drill bit with a diameter of 3 mm to drill through the remaining wall of the valve stuffing box; at this point, the leaking fluid will be forced out in the direction of the drill bit’s movement. To prevent injuries caused by high temperatures, high pressures, toxic, or highly corrosive fluids during drilling, a shield can be used beforehand. Drill a hole with a diameter of 5 mm in this shield so that it can cover the long drill bit, and drilling through the remaining wall thickness in this way will be safe. After drilling through the small hole, remove the drill bit, turn the special stopcock for injection to the closed position to cut off the medium connection, and then use the high-pressure injection gun to carry out the operation of injecting the sealing agent. If the pressure of the medium inside the valve stuffing box is low, a long drill bit with a diameter of 3 mm can also be used to drill a hole directly, followed by the sealing injection process. In June 2003, the self-sealing gland leakage of the electric main steam valve No. 3 at Panzhihua Iron and Steel Plant was resolved using this leak-sealing technique, preventing a situation where shutdown would have been necessary to fix the issue. (2) For valves with thin stuffing box walls, auxiliary clamps can be used for dynamic sealing operations. The auxiliary clamp is intended solely to compensate for the insufficient wall thickness of the valve stuffing box; it functions as a special connector fixed to the outer wall of the valve stuffing box, used to connect the high-pressure injection gun. The mechanical machining method for fixtures makes it difficult to achieve an ideal local fit surface; where conditions permit, the outer wall of the valve stuffing box can be appropriately repaired to ensure a better fit with the auxiliary fixture. If the outer shape of the packing of the leakage valve is complex or the conditions for trimming it do not permit it, an asbestos rubber sheet or a rubber sheet can be placed at the bottom of the auxiliary clamp; by tightening the clamp bolts, the rubber sheet placed there can effectively fill any gaps in the contact surface. The auxiliary fixture should be equipped with a threaded patch that matches the injection stopcock. Subsequently, the same procedures as those used for stuffing boxes of the same wall thickness should be followed. After the entire sealing process is completed, do not open or close the valve immediately; wait until the sealing material has hardened before using it normally. In mid-November 2002, the flange of the balance valve at the water inlet door of the high-pressure heater in the Panzhihua Iron and Steel Plant leaked. Due to the thin walls of the packing box of this valve, auxiliary fixtures were used to carry out dynamic sealing operations, which resolved the leakage issue. 4.2 Pressure-containing leak sealing for valve flange leaks in power plants 4.2.1 Copper wire sealing method: This method is suitable for pressure-containing leak sealing in situations where the gap between the two flanges is small and uniform, and the pressure of the leaking fluid is low. Special bolt injection connectors can be installed directly on the removed bolts, with at least two such connectors being used. When installing an injection fitting, one nut should be loosened. After the fitting is installed, the nut should be tightened quickly again, and then another fitting can be installed. It is not permissible to loosen the nuts of multiple fittings at the same time, as this can lead to a significant decrease in the sealing pressure on the gasket, an increase in leakage, and in severe cases, the leaking fluid may even blow away the gasket, resulting in irreparable damage. If the original leakage was severe, a G-clamp can be used to maintain equilibrium in the sealing pressure. After installing the injection bolts, a wire with a diameter equal to or slightly smaller than the gap between the flanges should be inserted into that gap using appropriate tools; at the same time, the outer edge of the flange should be shaped to form a lip, so that the wire remains fixed within the flange gap. This creates a new sealing chamber. Then, a high-pressure injection gun can be connected for dynamic sealing operations; the injection should be carried out sequentially from the direction opposite to the leak site, with the end point being near the leak location. In June 2003, the leakage at the vertical flange surface of the low-pressure connection pipe of Unit #1 at Panzhihua Iron and Steel Plant was resolved using this leak-sealing technique, thereby preventing a shutdown incident. 4.2.2 Steel strip sealing method: When the gap between two flanges is slightly large but does not exceed 8 mm, and the medium pressure is less than 2.5 MPa, the steel strip sealing method can be used for dynamic sealing. It has high requirements for the coaxiality of the two flanges, but lower requirements for the uniformity of the flange gap. The thickness of the steel strip is generally selected to be 1.5–3.0 mm, with a width of 20–30 mm being sufficient. Welding or riveting can be used for fabrication; transition gaskets must be placed beneath both joints. The number of mounting joints to be used is determined based on the dimensions of the flanges. When installing the steel strip, it should be placed in the gap between the two flanges. The connection bolts should be tightened a few turns, after which two transition gaskets are added to cover the entire gap between the flanges. The bolts are then tightened further to create a complete sealed cavity, at which point dynamic sealing can be carried out. 4.2.3 Convex clamp sealing method for flanges: When the leakage gap in the flange is greater than 8 mm and the medium pressure is above 2.5 MPa, for reasons of safety and reliability, it is necessary to design and manufacture flange clamps with precise dimensions, excellent overall sealing performance, and high pressure resistance. This method offers a high success rate for dynamic sealing, and it is a widely used sealing technique. Before starting the operation, a stopcock should be installed on the fixture used for working on the female part, with the stopcock in the open position. The operator should stand upwind. If the leakage pressure or flow rate is high, compressed air can be used to blow the leaking substance to one side, or a long rod can be attached to the fixture so as to allow the operator to avoid or minimize contact with the leaking substance. During installation, the injection hole of the fixture should be located between the bolts connecting the two flanges, and it is necessary to ensure that there is an injection hole near the leak site; the injection hole should not be aligned with the bolts to avoid increasing the resistance to the entry of fluid. After the fixture bolts are tightened, the maximum gap between the fixture and the flange should not exceed 0.5 mm. The injection should start from the point farthest from the leakage site and gradually move closer to the leak until it stops. This method can also be used for plugging leaks in pipes under pressure. This leak-sealing method is widely used in the routine maintenance of Units #1, #2, and #3 at Panzhihua Iron and Steel Plant, and it is a technique that professionals involved in pipe and valve maintenance must master. For example, in 2003, there were leaks at the flanges of the primary and secondary drain valves for the feed pump hot return water in Units #1 and #2 ； In February 2003, leakage of auxiliary steam from Unit #2 to the flange in front of the 70% electric isolation valve of the deaerator drain ； In March 2003, there were leaks at the inlet flange of the high-pressure heater of Unit #1, among other issues. 4.3 Pressure-based plugging to stop leaks in power plant valve bodies The methods for dealing with leaks in valve bodies can also be applied to pipeline leaks, and there are two such methods: 4.3.1 Adhesive method This is a method that makes use of the special properties of adhesives to stop leaks under pressure. For areas with pressure media and minor leakage due to sand holes, the area around the leak point can first be polished until it gains a metallic shine; thereafter, a tapered pin can be used to target the leak point and driven in with appropriate force, thereby significantly reducing the leakage or sealing it temporarily. Taking advantage of the fast curing speed of the adhesive, the area around the pin is promptly coated with adhesive to form a new solid sealing structure, thereby achieving leak prevention. For defects characterized by high medium pressure and large leakage amounts, a pressing tool can be used for sealing purposes. During operation, the pressing mechanism is fixed to one side of the valve, and a high-speed pressing screw is used so that its axis is aligned with the leakage point. By rotating this pressing screw, the rivets at its end press firmly against the leaking area, thereby stopping the leakage. If the tip of the rivet is smaller in area than the leak site, a piece of soft metal can be placed under the rivet. Once the leakage stops, clean the metal surface around the leak site promptly to remove rust and oil, then apply the prepared adhesive to that area. Once the adhesive has fully cured, remove the fixing screws of the compression screw and rivets, and take off the compression mechanism. To ensure its pressure resistance, the repaired leaky area can be reinforced using materials such as glass cloth. 4.3.2 Welding method a) When the pressure of the leaking medium in the valve body is low and the leakage rate is small, a nut with an inner diameter more than twice that of the leakage point can be used to allow the leaking medium to flow out through it. The nut is then welded to the valve body, and a bolt of the same specifications as the nut is used. A rubber pad or asbestos pad is placed at the bottom of the nut, and electrical tape is wrapped around the top of the bolt before it is screwed into the nut, thereby preventing leakage. For valve bodies with high pressure of the leaking medium and large leakage amounts, the drainage welding method can be used. First, use an iron plate with a circular hole in its center; weld a isolation valve with a diameter matching that of the hole onto this circular opening in the iron plate. Open the isolation valve, align the central hole of the iron plate with the leakage point, and attach it to the valve body, so that the leaking fluid can flow out through the central hole in the iron plate and the isolation valve. If the fitting surface is not good, a rubber or asbestos pad can be placed on it; then the area around the iron plate can be welded to the valve body, and closing the isolation valve will achieve re-sealing. b) In cases where the leaking medium is at high temperature and pressure inside the valve body, but the valve has large dimensions and the leakage amount is not significant, welding can also be used. First, seal all gaps on the valve body related to the leak site (without sealing the actual leak point itself). Then, use a section of pipe with a length suitable for the operating conditions (temperature and pressure), with the exact length depending on the site conditions (usually around 200 mm is sufficient); the diameter of this pipe should be larger than that of the leak point. Weld a valve compatible with this pipe to one end of the pipe, open the valve fully, align the other end of the pipe with the leak point and weld it in place, after which closing the valve will stop the leakage. 4.4 Universal Leak Sealing Method: If it is difficult to apply the methods mentioned above due to leaks in any part of the valve, the “encapsulation method” can be used: create a casing made from material suitable for the operating conditions, either in the form of sheet metal or pipe, that can enclose the entire valve or the valve body at the location of the leak. Weld this casing to the valve so that it covers the leak point. If welding the casing proves difficult, holes can be made in the casing to be sealed later using method b) from the welding techniques: adding a vent valve can facilitate proper sealing through welding. This welding method has been used on multiple occasions in the drain systems of the main steam ducts of Unit #1 at Panzhihua Iron and Steel Plant, as well as in the secondary valves of the main steam duct bypasses of Unit #3, and in the drain systems of the high-pressure heaters of Units #1, #2, and #3, achieving satisfactory results each time. This leak sealing method is the most commonly used and effective one in the routine maintenance of Units #1, #2, and #3 at Panzhihua Iron and Steel Plant; it is also a leak sealing technique that professionals involved in pipe and valve maintenance must master. 5 Conclusion: There are other components at the production site that require plugging under pressure. If we can master some basic knowledge of leak plugging, it will be of great help in improving the economic efficiency of power plants. A single start-up or shutdown of a 100MW unit results in direct economic losses of over 300,000 yuan. Therefore, we were able to successfully seal the leak at the site under pressure, reducing the number of unplanned shutdowns, and the benefits of this are significant. In summary: (1) Plugging leaks under pressure is a type of emergency repair work. Plugging leaks under pressure is a temporary solution with certain limitations and a limited duration of effectiveness. If conditions permit, the leaking area still needs to be thoroughly inspected and repaired. The fundamental way to eliminate problems such as leaks, spills, and other inefficiencies at the site and to improve the operational efficiency of equipment is to ensure the rationality of planned maintenance activities and to enhance the techniques used for equipment maintenance. (2) The working environment for plugging leaks under pressure is harsh, the operation time is long, the labor intensity is high, there are many uncertainties during the process, and the risks associated with such operations are significant. Safety preparations before starting work are extremely important; a thorough risk analysis must be conducted prior to any operation, and safety measures must be properly implemented. (3) Plugging leaks under pressure is a highly specialized technique that demands high levels of on-site adaptability from the operators, a solid understanding of mechanical knowledge, as well as proficiency in using specialized tools for such tasks. Due to the high requirements for operators and specialized equipment, the adoption of this technology in thermal power plants presents certain difficulties. Currently, the work of plugging leaks under pressure at the site is carried out by specialized companies. (4) Plugging leaks under pressure is a new technology that is still in the process of continuous improvement and refinement; it has its own limitations and scope of application. Plugging leaks under pressure cannot solve all leakage problems; it is still in the stage of exploration and improvement. Basic principle of plugging leaks under pressure: When leaks occur in flanges, pipes, valves, and other components of equipment operating under normal production conditions, and the leaking fluid is ejected outward while still hot and under pressure, it is possible to select or create appropriate clamps at the location of the leak. These clamps can utilize their existing sealed cavities, or a new sealing cavity can be created at that location. Sealing gel with plasticity and the ability to harden, resistant to the leaking fluid and high temperatures, is then injected into this sealing cavity. This increases the pressure inside the cavity above that of the system, and under certain conditions the sealing gel quickly hardens, thereby creating a new, stable sealing structure that eliminates the leak. Technical features of plugging leaks under pressure: 1. The economic benefits of plugging leaks under pressure are significant: when stopping a leak, operations can be carried out while the system is still in operation at normal temperature and pressure, thus ensuring uninterrupted production and avoiding economic losses resulting from shutdowns for repairs. 2. Safe and reliable: When eliminating leaks in flammable and explosive areas, the entire process ensures no sparks are generated, no welding is required, thus guaranteeing safety. 3. Wide range of applications: This technology can be used to eliminate leaks in all types of fluid media. 4. Strong applicability: No special treatment is required at the leak site; sealing can be carried out under pressure. It enables efficient, flexible, safe, and quick elimination of leaks. 5. Good detachability: Without damaging the original structure of the equipment or pipes, the new sealing mechanism can be easily removed, facilitating future maintenance of the equipment. 6. Low price. Application scope of pressure-based leak sealing: 1. Leakage locations: holes, cracks, welding defects on flanges, equipment, and pipelines, as well as leaks at threaded connections and packing boxes. 2. Leaking media: This technology can be used to eliminate leaks from almost all types of media, including various waters, water vapor, air, oxygen, nitrogen, hydrogen, gas, ammonia, liquefied gas, gasoline, diesel, heavy oil, lubricating oils, acids, bases, esters, alcohols, benzene derivatives, various heat carriers, various hydrocarbons, and various chemical gases and liquids. 3. Temperature of the medium being leaked: -195°C–900°C. 4. Medium pressure under leakage: Vacuum 0–32 MPa (320 Kg/cm2). Economic and social benefits of plugging leaks under pressure: 1. The benefit obtained from applying this technology is the avoidance of losses caused by unplanned shutdowns. 2. It prevents significant energy losses; among the eliminated leakage points, the majority (80%) were steam leaks. Apart from some major leaks that affected production, most were minor leaks. 3. It reduces the loss of certain raw materials, supplies, and products. 4. It eliminates leaks of flammable and explosive substances, prevents fire and explosion accidents in the equipment, and ensures safe production. 5. It eliminates noise and reduces environmental pollution. 6. It is easy and fast to operate, reduces maintenance time, and lowers repair costs. The cost of sealing under pressure can be recovered in the short term from the energy and materials saved by eliminating leaks. Pressure-sealing methods: Different sealing methods are used depending on the location of the leak. 1. There are three sealing methods for flange leaks: (1) Fixed flange clamps (Figure 1) – the clamps are designed based on the pressure and temperature of the leaking system, as well as the dimensions of the leakage site. An injection valve should be pre-installed on the fixture and in an open state; after the fixture is installed, it must be ensured that there is an injection valve at the leakage point. Adjust the gap on the fixture to meet the requirements, and then the sealant can be injected. (2) Steel strip bundling method: A stainless steel strip with a thickness of 1–2 mm is used to wrap around the outer circumference of the flange at the leakage site, thereby serving as a substitute for flange clamps. It is only suitable for leakage points with a pressure of 2 MPa or less. The installation steps are as follows: A. Remove a nut near the leakage point, install a threaded insert joint, and then tighten the nut. To ensure safety, use a clamp to secure the area before removing the nut; repeat the same steps on the corresponding nut. (Figure 2) B: Select an asbestos gasket that is slightly larger than the flange gap, and gently tap it into the flange gap using a hammer so that it overlaps by about 5 mm. C. Use a tensioner to tighten the steel strip. (Figure 3) D. Attach the injection gun at the joint to inject the sealant. (3) Soft brass wire containment method: A. Follow the construction steps of the steel strip fastening method (Figure 2) to install the screw hole injection joint. B. Use a flat chisel attached to a pneumatic hammer to insert the copper wire into the flange gap (Figure 4). C. Seal the gap with a round-bladed chisel attached to a pneumatic hammer to prevent the copper wire from coming out (Figure 5). D. Install the injection gun to inject the sealant. (4) The injection of sealant is carried out following a specific procedure: generally, injection starts from the point farthest from the leak, moving alternately from both sides toward the leak site, and finally sealant is injected at the leak point until the leak is completely eliminated. 2. The sealing method for leaks in pipe elbows, tees, and straight pipes typically involves using appropriate box-type clamps to completely enclose the leaking area, after which a sealant is injected into the enclosed space to eliminate the leak. The method of injecting sealant is the same as that used for flange sealing. It is also possible to use seal ring grooves designed according to the applicable temperature, pressure, and medium; asbestos packing is placed in these grooves, the clamp is applied to the leaking area, and then sealant is injected into the asbestos packing in the grooves to compress it against all the connection points, thereby eliminating the leak. (Figure 6) 3. Sealing methods for valve stuffing boxes (1) Karan method: Fix the Karan element on the outer surface of the middle and lower parts of the stuffing box; use a long drill bit with a diameter of 3–4 mm to penetrate the wall of the stuffing box through the inner hole of the Karan set screw. Then attach a syringe to the set screw and inject sealant, which quickly eliminates leaks. (Figure 7) (2) Threaded injection valve: When the size of the large valve’s packing box is relatively large, a blind hole is drilled on the outer surface of the lower part of the packing box; threads are created using a tap, and an injection valve is installed. The plug is then opened, and a drill bit with a diameter of about 3–4 is used to drill through the wall of the packing box. An injection gun is then attached to the injection valve, and sealing material is injected, which quickly eliminates any leaks. (Figure 8) 4. Sealing method for leaks at threaded connections: Place a clamp on the outer surface of the area where there is a thread leak, secure it with a set screw. Through the hole in the set screw, use a long drill bit with a diameter of about 3–4 mm to drill through the wall of the female thread. Then, use an injection gun to insert a small amount of sealant, which will quickly eliminate the leak. (Figure 9) Precautions for plugging leaks under pressure: 1. Select the appropriate sealant based on the conditions of the leak site (temperature, pressure, medium). 2. A correct analysis of the causes of leakage is necessary; for example, leakage resulting from corrosion or erosion of the equipment or pipes by the medium, which leads to thin walls, should be approached with caution. This technology primarily addresses sealing, rather than strengthening the equipment. 3. For leaks caused by cracks in the weld or base material, this technique should not be used when it is impossible to control the further propagation of those cracks. 4. Operators working at the site where leaks occur must strictly follow the relevant safety and technical procedures; work must be carried out in a safe manner, and reckless actions are not permitted.