Abstract Discussed the structural characteristics of commonly used gate valves, analyzed the cause of the accident from several aspects such as type selection, design, manufacture, installation, use, and maintenance, proposed the harm of abnormal pressure rise in the middle cavity to the valve, and summarized the protective measures.
Keywords gate valve; abnormal boost; design and manufacture; installation and commissioning; maintenance.
Gate valves overview
Gate valves are more and more widely used in various industrial systems due to their pressure resistance, temperature resistance (high and low temperature), corrosion resistance (multiple material configurations), two-way sealing function, wear resistance, and long service life. The opening and closing of the system play an important role in the stability and control of the system.
It is classified in the form of the gate plate of the common standard rising stem gate valve in China (the new structure is not listed), and it mainly includes Z40, Z41, Z42, and Z44 structures (Figure 1, Table 1). The user should select the appropriate model based on the working conditions. And standardize operations in installation, commissioning, and maintenance to avoid or eliminate accidents.
a) Z40 type wedge elastic gate valve (b) Z41 type parallel single gate valve
(c) Z41 type wedge single gate valve (d) Z42 type wedge double gate valve
(e) Z44 type parallel double gate valve
Figure 1 Common structure of gate valve
1. The valve stem is broken or deformed
Stem fracture generally occurs at the roots of the upper and lower threads, where the cross-sectional area is the smallest, and stress concentration and over-standard phenomena are prone to occur. Especially when the working conditions deviate greatly from the design parameters. For example, in a power plant, there have been many accidents where the DN175 electric gate valve was opened after the valve stem was pulled off at the trapezoidal thread undercut. The investigation found that the valve bonnet pre-tightening nut was loose, the bonnet moved up, and the stem nut was stuck. Obviously, this was an accident caused by over-adjustment of the Denso stroke and excessive protection torque.
Another type of valve stem fracture accident occurred at the moment of opening. The performance is that the gate has not been separated from the valve seat, and the valve stem is broken at the root of the upper or lower thread. The reason is usually considered to be a stuck gate, which is actually only part of the reason or a secondary reason. An important reason is the abnormal pressure increase after the valve body cavity is closed, that is, after the valve is closed, the fluid pressure of the cavity enclosed between the upstream and downstream sides of the sealing surface is much higher than the upstream pressure. There are two reasons for this phenomenon. One of the fluids in the middle cavity is heated by the upstream fluid to heat up and expand, causing the pressure to rise sharply. When the second gate is closed, the fluid is closed in the middle cavity and cannot flow out. The middle cavity space is further squeezed by the valve stem. Because the compressibility of the liquid is very limited, the pressure will also increase sharply. This phenomenon is especially likely to occur in power plants. On the Z962 type main feedwater gate valve. At the same time, its abnormal boost pressure will generally increase geometrically, far exceeding the design limit of the stem strength.
Stem bending generally occurs when the valve electric actuator is set improperly. For example, the closing torque is too large and the stroke protection is not set or the protection fails, etc., which can cause great damage to the valve.
(1) External leakage
The valve housing is generally processed by casting. Although forgings can replace castings for high-temperature and high-pressure working conditions, due to process or cost constraints, large-size shells are usually made of castings. Due to a large number of defects in castings such as porosity, slag inclusion, cracks, shrinkage, and porosity, these defects will be exposed at any time during the operation of the valve and become a hidden danger that threatens safe operation. Statistics from a power plant in 2007 showed that 37% of valve leakage accidents were caused by shell casting defects. The rest are caused by the failure of the valve stem, middle flange, and end flange seal.
Damage to the seal will also cause a large amount of system media to leak. For example, leakage at the valve stem seal is generally caused by the loose joint bolts of the packing pressure plate not being tightened in time, which often leads to major accidents in the event of occurrence. Another reason is that the surface of the valve stem is corroded by the medium due to improper treatment process or material selection, or the sealing surface of the valve stem is corroded by the unqualified packing. The up and down movement of the valve stem will bring out the packing particles from the rust on the surface, and the packing gap will leak after being repeated many times. At this time, if the maintenance is improper, the high pressure or high-speed medium will rush out the packing in a very short time, resulting in leakage. Similarly, high-pressure self-tight sealing valves with graphite stainless steel wire wound materials as seals and middle flange sealed valves with stainless steel with graphite wound gaskets should also prevent this from happening. Another reason often causes a large amount of external leakage of the medium, that is, the fitting gap between the packing gland and the valve stem is too large. During the up and down movement of the valve stem, the packing will be repeatedly rubbed due to the excessive clearance to bring out fine particles, and finally the packing compaction ratio The pressure drops, causing leakage.
There are many reasons for the internal leakage of the valve. The internal leakage and improper closing of the new valve occur soon after operation. It should be foreign matter on the sealing surface. Therefore, carefully check the newly installed valve products before putting them into operation, and carefully flush the pipes to reduce the occurrence of such accidents.
3. Gate failure
The jamming of the gate shows that the gate is stuck on the side of the guide rail of the valve body, or even detached from the valve stem. It is mainly caused by poor design and manufacturing, such as the guide rail contact width being too short or too long, and the guide rail surface being rough. When the gate is stuck in the valve seat and the valve stem is forcibly lifted, the T-slot of the gate is often broken or deformed, avoiding the “abnormal rise in pressure” factor. There are also two major causes of the temperature difference and closing force that are worth noting. In typical temperature difference conditions, such as the valve is closed in the cold state and then opened in the hot state, the gate is further compressed due to the expansion and elongation of the valve stem, which increases the closing torque of the gate and causes the gate to wedge. If the hot state is closed and the cold state is opened, the crotch width becomes larger due to the thermal deformation of the valve seats on both sides, while the cold state shrinks and becomes smaller, causing the gate to be wedged. If the above two conditions are accompanied by excessive “closing force”, that is, the closing is too tight. If the Denso drive is adjusted incorrectly or improper use of force-increasing mechanisms such as wrenches or levers, the probability of wedging of the gate will increase.
(2) Cracked gate
Gate cracking (or bottom deformation) mainly occurs in Z944/Z964 parallel plate structure products. It occurs when the bottom of the Gate directly hits the bottom of the valve body, mostly due to improper electrical installation and debugging of the electric valve, and the closing stroke is not limited or limited. It is very easy to happen when it fails.
(3) Cracks on the sealing surface
Sealing surface cracks mainly appear in valves made of alloy steel materials and are often caused by unreasonable processes. Reasonable selection of materials and reasonable control of welding process parameters can be avoided.
Analysis and measures
1. Design and Manufacturing
① There is no requirement for gate valve opening identification in the national standard. Ordinary valves can only judge the opening degree by hand, which is unreasonable on many occasions. It is recommended to design a limit device at the end of the valve stem to prevent excessive closing of the valve.
②The strength of the valve stem is weakened in many places, and the carrying capacity is reduced. Such as the root of the thread undercut, etc., these parts are often the first to break when the cavity is abnormally increased in pressure. It is recommended that the diameter of the valve stem of the electric valve above DN 350 should be appropriately enlarged, and the coaxiality of the support should be improved in the processing and assembly, and the opening and closing torque value of the electric device should be selected reasonably.
③The current national standards for gate valves do not involve the abnormal pressure rise phenomenon in the middle cavity, and do not propose the design of internal bypass or external bypass requirements, resulting in the pressure can not being relieved, and accidents often occurring during use. It is recommended that the temperature difference is large
When using a flat valve, it is advisable to use a flat valve.
④ When the valve stem nut is made of aluminum bronze, the thread surface will be polished quickly when the electric equipment rotates fast or the opening and closing is frequent, which will cause the valve to fail to move. It is recommended to appropriately increase the number of teeth in the design, effective lubrication, and regular maintenance and replacement. For important valves, a reasonable packing seal combination should be used, and important matching surface clearances (such as the matching clearance between the valve body and the bonnet of the self-tightening valve, the matching clearance between the valve stem and the packing gland, and the matching clearance between the packing gland and the bonnet Etc.) and position tolerances (such as the upper and lower coaxiality of the bonnet and the bracket, the position of the flange screw hole, etc.) should be strictly controlled.
⑤Limited to the current technological level, it is recommended that important high-temperature and high-pressure valve housings be made of full-forged steel or forged-welded structures. The parts and materials of the valve should be selected, tested, and processed in strict accordance with the relevant standards.
⑥ The edges and corners should be removed when the guide rail is processed, and sufficient joint surfaces should be reserved on both sides to ensure that the gate will not deviate from the guide rail due to serious deviation.
⑦Choose the appropriate sealing surface material and hardness matching, and ensure that the surface roughness, flatness, and coincidence meet the requirements during processing. The surface treatment of the valve stem should meet the design requirements to prevent its surface from being rusted, corroded, or the coating is peeled off early.
2. Installation and commissioning
① It is recommended to add a bypass device between the upstream and downstream of important high-pressure large-diameter valves to reduce the opening and closing pressure difference, reduce the scratches of the gate to the sealing surface at the moment of opening and closing, and at the same time help reduce the electrical torque and reduce the manufacturing cost.
②The installation of a safety valve in the middle cavity is an effective measure to control abnormal pressure increase, which can protect the valve opening and closing parts and maintain the safe operation of the system. In addition, installing an external bypass between the upstream side and the middle cavity or opening a pressure relief hole on the upstream side of the gate are both effective and feasible means.
③The pipeline should be flushed thoroughly to prevent debris (such as iron filings, welding slag, welding rod head, nuts, etc.) from damaging the sealing surface.
④It is recommended to install the gate valve stem vertically upwards. The inclined installation will increase the chance of the gate being stuck. When installed horizontally, there should be a mounting bracket to support the electric actuator, and the valve stem should be calibrated during installation.
⑤ The installation and commissioning of the electric gate valve should be carefully operated by professionals according to the requirements. The result of too large opening stroke or too high protective torque is to pull off the valve stem or jam the valve stem nut. The correct way is to adjust the opening position with the handwheel and use torque protection. Generally, the protection torque is controlled at 20% to 30%. If the closing stroke is too large and the opening and closing torque are too large, the gate will be wedged in the valve seat. If the opening and closing torque are too large, it may cause the valve stem to be pulled off, the gate T-slot is broken, and the stem nut is stuck. The correct approach is to properly lift the gate 1～3mm (mainly for high-pressure valves) after adjusting the closing position with the handwheel, so as to avoid excessive opening torque caused by an abnormal pressure rise in the middle cavity. If the closing stroke of the flat gate valve is too large, the gate will often be cracked (or deformed). The closing stroke position can be adjusted during debugging, and no excessive closing torque is required.
3. Operation and maintenance
① It is recommended that the valve with an opening and closing torque greater than 400N·m should be designed with a force-increasing mechanism, such as gear mechanism, impact handwheel, valve stem nut with bearing, etc. A 30% to 50% torque margin should be reserved for the selection of Denso. It is not recommended to use pneumatic wedge gate valves.
②The pre-tightening bolts should be tightened after the valve with self-tightening seal design runs for a period of time to prevent the bonnet from sinking when the pressure fluctuates, causing medium leakage and rushing out of the self-sealing ring. During operation, the packing compression bolts and middle flange compression bolts should be maintained normally and tightened in time. Lubricate the bearing mechanism and various threads regularly to remove dust and dirt in time. Regular action test (important valves should be opened and closed frequently to test their action performance, to prevent rust and jam, and find hidden dangers in time), regular maintenance (important valves should be regularly overhauled, and their sealing surfaces, driving mechanisms, sealing rings and other important key parts should be checked and maintained), Establish equipment maintenance record files.
The combination of design, manufacturing, model selection, installation, commissioning, and maintenance can ensure the safe use of valve products, avoid major accidents, and ensure the safe operation of products and processes, thereby prolonging the life of the equipment, extending the maintenance cycle, and saving maintenance. And production costs, improve corporate efficiency.