Common Faults and Maintenance of Valves

By analyzing the causes of valve failures and adopting corresponding treatment and improvement methods, the failure rate of the valve can be greatly reduced, and the utilization rate of the valve can be improved, which will play a great role in stabilizing the long-term safe and stable operation of the valve, thus greatly improving the production efficiency. In addition, the fault summary of control valve has specific reference value for equipment maintenance and engineering technicians.
 
Common faults and analysis of valves
 
Common faults of valves
 
The valve plays an important role in practical use. When the fluid medium is closed and squeezed, the valve will stop running if there is a serious failure. The specific failure modes include shell parts, valve actuator parts, bellows, seals and fasteners will be damaged. If there is a small fault, the valve can still be used according to the functional requirements, but the effect will be reduced. According to the nature of valve faults, the faults are classified into damage, failure, looseness, deterioration, blockage and leakage. Through sorting out, the general faults of valves, the failure of valves to open normally or the existence of leakage are summarized, and the following faults are mainly given.
 
Internal leakage of the valve: the valve leakage is mainly due to the damage of the sealing ring, the valve body, the failure to connect the power supply, impurities between the sealing ring and the valve body, and the unsealed O-ring of the sealing seat.
 
External leakage of valves: the external leakage of valves mainly includes valve stem leakage, drain leakage and flange leakage.
 
Difficulties of switching: The main reasons for the difficulties of switching are the excessive pressure difference between upstream and downstream valves, corrosion of sealing rings and gates, and excessive pressure difference between DPE valve, valve cavity and downstream valve. The clamping and clamping deviation of the sealing ring is too large, the sealing groove is blocked, the water in the valve cavity freezes, and the torque of the rotary sealing seat is slightly larger at the end of the closing stroke of about 1/8.
 
Grease injection system failure: The main causes of grease injection channel failure are grease injection nozzle problem and internal check valve failure, which is blocked by hardened sealing grease.
 
In addition to the above-mentioned basic faults, the valve may also have faults such as broken handwheel, broken valve stem, leakage of valve body and thread, lax sealing of flange face, etc. It is only used to analyze the basic faults of the valve, so as to carry out specific maintenance of the valve. As can be seen from the above, the problems of the valve are mainly reflected in two aspects: metal parts breakage and soft sealing. In the process of maintenance, different methods should be adopted.
 
Valve failure analysis
 
According to the type of valve, perform the following types of fault analysis. The main causes of destructive failures such as fracture and wear are internal force and external force, which may be due to wear caused by self-strength and long-term use, material strength and long-term wear, aging, blocking and other dysfunctional problems. In addition, improper design and most of the reasons are erosion caused by operation, while the sealing problem of leakage is mainly caused by environmental factors such as medium impurities, medium corrosion, steam corrosion and design. Improper design will cause sealing problems and eventually lead to leakage. To sum up, apart from the material, design and process problems, the most common failures are mainly caused by corrosion, overpressure, leakage and other problems of the medium. Secondly, long-term use of the valve will cause damage to the valve under the action of internal and external forces.
 
Basic flow and tools of valve maintenance
 
Basic flow of valve maintenance
 
Domestic valve maintenance manufacturers can derive the basic process of valve maintenance from maintenance experience, such as on-site disassembly, cleaning disassembly, parts inspection, maintenance, assembly, inspection, painting, packaging and acceptance inspection. Whether it is to repair metal parts or nonmetal parts, the above basic processes should be carried out.
 
Equipment and tools for valve maintenance
 
In order to meet the requirements of valve maintenance, the valve must be equipped with corresponding equipment to facilitate its configuration according to its intended use. Special maintenance tools and common measuring instruments are used for valve maintenance. During maintenance, according to the requirements of the valve, some common tools need to be configured.
 
Preventive maintenance
 
Preventive maintenance of the valve can avoid many failures, which is the key to ensure the long-term and efficient operation of the valve. Enterprises should formulate corresponding maintenance procedures according to different types of valves, including valve installation, daily use, daily maintenance and troubleshooting. When the valve is just installed, it must be maintained at any time. After each installation, the oil and water stains on the valve parts should be wiped in time to prevent the normal operation of the valve from being damaged and shorten the service life. After the installation, the technical department shall timely evaluate the overall operating environment of the valve, reasonably estimate the service life of the valve, and replace it in advance when it expires. In addition, a reasonable valve maintenance plan should be made, and the valve should be regularly maintained to reduce the failure rate and prolong the service life of the valve.
 
Maintenance process
 
Observational check
① Check whether the valve handwheel, key, grease injection nozzle, plug, nameplate, bolts, nuts and other parts are complete, the corrosion degree of valve body and flange bolts, etc.
② Clean the valve shell to remove oil stains and sundries, and check the valve body and joints for leakage. Under pressurized and ventilated conditions, the gas leak detector should be used to check the joints, plugs, grease injectors, flange joints, etc.
③ The valve of turbine head drive should open the top cover of turbine head gearbox and turbine reducer, check whether there is water in the gearbox and reducer, whether lithium grease is deteriorated and lubricated well, and whether all metal-to-metal contacts are safely coated with lithium grease. If lithium grease needs to be replaced, the deteriorated lithium grease and foreign matter in the gear box should be cleaned and then a proper amount of grease should be added again. If the season is particularly cold, it is necessary to apply low-temperature grease.
 
Torque test
The switch of the valve is only allowed to be operated manually by one person, and it is forbidden to use the booster lever and multiple people to operate it. Before operating the valve, you should pay attention to check the opening and closing signs of the valve before operating it. If the valve with ineffective opening and excessive torque cannot be opened with a torque wrench, brute force shall not be used to forcibly open the valve, and the valve shall be cleaned and lubricated subsequently.
 
Cleaning and sewage discharge
① Determine the amount of cleaning liquid according to the size of the valve;
② Inject the cleaning liquid according to the specified amount, and carefully observe the injection pressure during the process of injecting the cleaning liquid. Confirm that the injection pressure can be established and kept within a safe range;
③ After the cleaning solution is injected, soak the valve in the cleaning solution for more than 1 hour, so that the cleaning solution has enough time to soften the deposits or dry grease between the sealing surfaces;
④ The valve cleaning solution will not damage the valve or seal, and can stay in the valve for a long time. After opening the bypass valve, fully open and close the valve for 3-5 strokes, and keep the ball valve fully open after switching;
⑤ Drain the valve cavity, and tighten the drain valve after discharging all the sundries.
 
Grease injection and anticorrosion
① Turn the valve to the fully open position, and select the appropriate amount of grease injection according to the diameter of the valve, which can exceed the amount of grease injection but is not allowed to exceed 100% of the recommended amount of grease injection;
② Clean and check the nozzle: use a wire brush or brush to remove the paint or rust of the nozzle. If the nozzle is damaged, seriously rusted or leaked, it should be replaced. Unscrew the gland of the grease injector with a wrench, that is, connect the grease injection gun to inject the sealing grease, and the joint must slide into the nozzle correctly to prevent the joint gasket from being damaged;
③ When injecting fat, pay attention to the problem of uniform fat. During normal grease injection, the grease outlet nearest to the grease injection port will be greased first, then to the low point, and finally to the high point, successively. If the grease injection pressure does not drop by helicopter, it means that the passage is blocked, and it should be cleared in time.
 
Precautions for valve lubrication
 
Too much grease will cause excess grease to accumulate on both sides of the valve, and impurities will easily adhere to the grease, which will easily damage the valve seat or valve core and waste materials; Too little grease will make the valve difficult to operate, aggravate the loss of internal parts and degrade the performance. Therefore, it is particularly important to choose high-quality grease. Enterprises should purchase grease with reliable quality in strict accordance with national and enterprise standards. During the regular maintenance of the valve, the amount of grease required by the valve components should be carefully measured. Too much or too little grease will affect the normal work of the valve. Grease injection pressure should be considered when lubricating the valve. During lubrication, always pay attention to the grease injection pressure of the valve, and the pressure should change periodically from peak to valley. Every time the pressure gauge is filled with grease, it shows that the pressure will rise, but the amplitude is not large, and it will soon fall back. Generally, the injection pressure to the valve sealing surface and valve stem does not exceed 3000PSI, and the pressure value changes slowly when the sealing system is full. The slower the change, the better the sealing effect. The pointer only rises but does not fall, indicating that the channel is blocked. The pressure gauge fluctuates greatly, indicating that the sealing system produces bubbles.
 
Maintenance of valve transmission device
 
The valve transmission device is the most important part of the valve, and it is also the most prone to failure, so the maintenance of the valve transmission device should be done first. When some valves work, they are always fully open or fully closed, and only when necessary, the single state is changed, so the transmission device is easy to be damaged and the wear speed is slow. Some valves controlling key parts need to change their opening and closing states frequently, and the transmission device will be worn during long-term operation, and the degree of wear varies according to the opening and closing times. Therefore, after the main components are installed, grease should be added to the valve to lubricate and reduce the wear of the gear box. When regularly maintaining the valve, first check the damage of the valve transmission device to determine whether it needs to be replaced. If the grease is not enough, it needs to be replenished. In addition, long-term operation will cause the temperature of the valve to rise, high temperature will accelerate the evaporation rate of internal grease, aggravate the damage rate of the valve, and necessary water bath cooling measures are also important measures to ensure the long-term operation of the valve transmission and reduce the failure frequency. If the pipeline transports highly corrosive liquid, the maintenance period of the valve transmission device should be shortened accordingly, so as to avoid the aging and corrosion of the valve transmission device caused by high corrosivity, which will lead to major failures.
 
Maintenance work
 
Replacement of valve packing
 
The stuffing box inside the valve is the material to ensure the tightness of the valve. The lack or failure of packing in the box directly affects the tightness of the valve and makes the valve lose its function. The material transported in the pipeline will also affect the aging rate of the packing in the valve. In addition, the high temperature caused by long-term operation will also accelerate the loss of the packing in the valve. During routine maintenance, check the valve packing and fill it in time to ensure that the packing in the valve has good elasticity. During the use of the valve, it is necessary to maintain the valve packing in advance, check whether there is any leakage, and tighten the bolts in time to prevent the packing from leaking. Some valve packings will add a certain amount of grease, so with the use of the valve, it can better lubricate, protect, maintain and repair the grease filling inside the packing.
 
Replacement of valve seat seals
 
Remove valve seat
① Let the valve be in the half-open position to release the possible dangerous substances in the valve cavity.
② Close the ball valve, remove the bolts and nuts of the flanges on both sides, and completely remove the valve from the pipeline.
③ Disassemble the driving device-actuator, connecting bracket, check washer, valve stem nut, butterfly spring piece, wear-resistant piece and valve stem packing in sequence.
④ Remove the connecting bolts and nuts of the body cover, separate the valve cover from the valve body, and remove the valve cover gasket.
⑤ Make sure the valve is in the "off" position, which can easily take it out of the valve body, and then remove the valve seat.
⑥ Gently push the valve stem down from the hole in the valve body until it is completely taken out, then take out the O-ring and packing ring.
Note: Please be careful to avoid scratching the valve stem surface and the packing seal of the valve.
 
Installation
① Clean and check the parts under inspection. It is recommended to replace the gasket and seal of valve seat and bonnet with spare parts package.
② Assemble in reverse order of disassembly and assembly.
③ Use torque tools according to the specified torque, cross-lock flange bolts and lock nuts.
④ After installing the actuator, rotate the valve stem to drive the valve core to rotate the corresponding input signal, so that the valve can be opened and closed.
⑤ After installing the pipeline, test the sealing pressure and performance of the valve according to the standard.
 
Conclusion
 
The common failure modes and causes of valves are introduced, which can provide information basis for failure analysis, find out weak links, and optimize and improve reliability design. Because reliability test is a simple and effective method to confirm the reliability level of products, valve maintenance technology needs to analyze the causes of control valve failures in order to continuously solve the difficulties in the subsequent maintenance process and improve the maintenance quality. Appropriate treatment and improvement methods can greatly reduce the failure rate of the control valve, improve the operation rate of the control valve, play an important role in stabilizing the process production, ensure the long-term safe and stable operation of the valve, and greatly improve the production efficiency. In addition, an overview of the failure of the control valve has specific reference value for maintenance and engineering technicians.