Valve knowledge

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Chapter 4: Installation, Maintenance, and Operation of Valves Once the valve has been properly selected, it must also be installed, maintained, and operated correctly in order to maximize its efficiency. Section 1: Installation – The quality of valve installation has a direct impact on its performance, so it must be given careful attention. (1) Direction and Position: Many valves are directional, such as globe valves, throttle valves, pressure relief valves, check valves, etc. If installed in the wrong direction, it will affect their performance and lifespan (as in the case of throttle valves), or they may not function at all (as in the case of pressure relief valves), or it could even pose a danger (as in the case of check valves). For ordinary valves, there are direction indicators on the valve body ； In the event that it is not available, it should be correctly identified based on the working principle of the valve. The valve chamber of the stop valve is asymmetrical on the left and right; the fluid must flow through the valve opening from bottom to top, which results in low fluid resistance (due to the shape of the valve). It is easier to open the valve as the medium pressure acts upward, and once closed, the medium does not exert pressure on the packing, facilitating maintenance. This is why stop valves cannot be installed in reverse. Other valves also have their own characteristics. The valve must be installed in a location that facilitates operation ； Even if installation is temporarily difficult, it is necessary to consider the operators’ long-term work. It is best for the valve handwheel to be at chest level (usually 1.2 meters above the operating floor), as this makes it easier to open and close the valve. The handwheel of the floor valve should face upward and not be tilted, to avoid awkward operation. For wall-mounted units, space must also be left next to the device’s valves for the operator to stand. It is necessary to avoid working with the head raised, especially when dealing with acids, bases, toxic substances, etc., as it is very unsafe. The gate valve should not be installed in an inverted position (with the handwheel facing downward), as this will cause the medium to remain in the space above the valve cover for an extended period, leading to corrosion of the valve stem; it is also prohibited by certain process requirements. It is also very inconvenient to replace the packing at the same time. Straight-through gate valves should not be installed underground, as moisture can cause corrosion of the exposed valve stem. For lift-type check valves, it is necessary to ensure that their valve discs are vertical during installation, so as to allow for smooth lifting and lowering. For swing check valves, it is necessary to ensure that their pin shafts are horizontal during installation, so as to allow for smooth swinging. The pressure relief valve must be installed vertically on a horizontal pipe, without being tilted in any direction. (II) Construction operations: Care must be taken during installation to avoid hitting valves made of brittle materials. Before installation, the valve should be inspected to verify its specifications and model, and to check for any damage, especially to the valve stem. It needs to be turned a few more times to check if it is skewed, as the valve stem is most likely to get damaged during transportation. The debris inside the valve also needs to be removed. When lifting the valve, do not tie the rope to the handwheel or valve stem, as this may damage those components; instead, it should be tied to the flange. The pipelines connected to the valve must be thoroughly cleaned. Compressed air can be used to blow away iron oxide filings, sediment, slag, and other debris. These debris not only easily scratch the sealing surfaces of the valves, but large particles of debris (such as welding slag) can also block small valves, rendering them ineffective. When installing threaded valves, the sealing filler (flax mixed with lead grease or PTFE tape) should be wrapped around the pipe threads, avoiding contact with the inside of the valve, so as to prevent accumulation there and disrupt the flow of the medium. When installing flanged valves, be careful to tighten the bolts symmetrically and evenly. The valve flange and the pipe flange must be parallel with an appropriate gap, to prevent excessive pressure from being generated on the valve, which could even lead to cracking. Special attention should be paid to brittle materials and valves with low strength. For valves that need to be welded to pipes, spot welding should be performed first, followed by fully opening the closing element, and then performing a full weld. (III) Protection facilities: Some valves also require external protection, namely insulation and heat retention. Heating steam pipelines are sometimes also added inside the insulation layer. It depends on the production requirements to determine which types of valves should be insulated or kept cool. In principle, whenever the temperature of the medium inside the valve drops too much, it can affect production efficiency or cause the valve to freeze; in such cases, insulation, or even heat tracing, is required ； When valves are exposed, which is detrimental to production or can cause problems such as frosting, insulation is required. Insulation materials include asbestos, slag wool, glass wool, perlite, diatomaceous earth, vermiculite, etc ； Insulation materials include cork, perlite, foam, plastic, etc. Water and steam valves that have not been used for a long time must have any accumulated water drained. (IV) Bypasses and instruments: Some valves, in addition to the necessary protective devices, also require bypasses and instruments. A bypass has been installed to facilitate the maintenance of the steam trap. Other valves also have bypasses for installation. Whether to install a bypass depends on the condition of the valve, its importance, and the requirements of production. (5) Packing replacement: Among the valves in stock, some have packing that is no longer functional, and some use packing that is not suitable for the medium being handled; therefore, it is necessary to replace the packing. Valve manufacturers cannot take into account the wide variety of different media used by end-users; ordinary packing is always installed in the stuffing box, but when in use, it is necessary to ensure that the packing is suitable for the specific medium. When replacing the packing, press it in circle by circle. The seams around each circle should be at a 45-degree angle, with the seams between circles offset by 180 degrees. The packing height must take into account the amount of further compression that the gland can provide; at the same time, it is necessary to allow the lower part of the gland to compress the packing chamber to an appropriate depth, which is generally 10-20% of the total depth of the packing chamber. For high-demand valves, the seam angle is 30 degrees. The joints between the circles are offset by 120 degrees. In addition to the fillers mentioned above, shaped fillers such as rubber O-rings (natural rubber is resistant to weak alkalis at temperatures below 60 degrees Celsius, nitrile rubber is resistant to oils at temperatures below 80 degrees Celsius, and fluororubber is resistant to various corrosive agents at temperatures below 150 degrees Celsius), triple-layered polytetrafluoroethylene rings (resistant to highly corrosive agents at temperatures below 200 degrees Celsius), and nylon bowl-shaped rings (resistant to ammonia and alkalis at temperatures below 120 degrees Celsius) can also be used depending on the specific circumstances. Wrapping a layer of PTFE tape around the ordinary asbestos packing can improve the sealing effect and reduce the electrochemical corrosion of the valve stem. When compressing the packing, turn the valve stem at the same time to ensure even compression throughout and to prevent it from being too tight; apply even force when tightening the gland, without tilting it. Section 2: Maintenance. The maintenance of valves can be divided into two scenarios ； One is storage and maintenance, and the other is usage and maintenance. (1) Storage and maintenance: The purpose of storage and maintenance is to prevent the valves from being damaged or having their quality reduced during storage. In fact, improper storage is one of the main causes of valve damage. Valves should be stored in an orderly manner; small valves should be placed on shelves, while larger valves can be arranged neatly on the floor of the storage area. They must not be piled up haphazardly, and the flange surfaces must not come into contact with the ground. This is not only for aesthetic reasons, but mainly to protect the valve from being damaged. Unnecessary losses such as broken handwheels due to improper storage and handling, misaligned valve stems, and loose or lost fixing nuts between the handwheel and the valve stem should be avoided. For valves that will not be used in the short term, the asbestos packing should be removed to prevent electrochemical corrosion and damage to the valve stem. Valves that have just arrived in the warehouse need to be inspected; if rainwater or dirt has gotten inside them during transportation, they must be wiped clean before being stored. The inlet and outlet of the valve should be sealed with wax paper or plastic sheets to prevent dirt from getting in. The machined surfaces of valves that are prone to rusting in the atmosphere should be coated with anti-rust oil for protection. Valves placed outdoors must be covered with materials such as tarps or sheets to protect them from rain and dust. The warehouse where valves are stored should be kept clean and dry. (II) Use and Maintenance The purpose of use and maintenance is to extend the lifespan of the valve and ensure reliable operation. The valve stem threads, which are in constant friction with the valve stem nut, should be coated with a little yellow grease, molybdenum disulfide, or graphite powder to provide lubrication. Valves that are not opened and closed frequently should still have their handwheels turned regularly, and lubricant should be applied to the valve stem threads to prevent seizure. For outdoor valves, a protective cover should be installed on the valve stem to prevent rain, snow, dust, and rust from causing damage. If the valve mechanism is of the mechanical type, lubricant should be added to the gearbox on a regular basis. Keep the valves clean at all times. Regularly check and maintain the integrity of the valve’s components. If the fixing nut of the handwheel comes loose, it must be replaced with a proper one; makeshift solutions should not be used, as this will wear out the square section at the upper part of the valve stem, gradually reducing its fit reliability and eventually preventing the valve from being operated. Do not rely on the valve to support other heavy objects, and do not stand on the valve. The valve stem, especially the threaded portion, should be wiped regularly, and any lubricant that has become dirty due to dust should be replaced with fresh lubricant, as dust contains hard particles that can easily wear down the threads and the surface of the valve stem, thereby reducing its service life. Section 3: Operation For valves, it is necessary not only to know how to install and maintain them, but also how to operate them. (1) Opening and closing of manual valves: Manual valves are the most widely used type of valves. Their handwheels or handles are designed to be operated using ordinary human strength, taking into account the strength of the sealing surfaces as well as the necessary force required for closing. Therefore, it cannot be turned using a long lever or long wrench. Some people* are used to using wrenches, and they must be careful not to apply excessive force, as this can easily damage the sealing surfaces or break the wrench, the wheel, or the handle. When opening and closing the valve, apply force smoothly without any impact. The various components of certain high-pressure valves that are operated by impact have taken into account this impact force, which is different from that of ordinary valves. For steam valves, they should be pre-heated and of the condensed water removed before being opened. The opening process should be carried out as slowly as possible to avoid water hammer. After the valve is fully open, turn the handwheel slightly in the opposite direction to ensure a tight fit between the threads, thereby preventing loosening and damage. For straight-stem valves, remember the position of the stem when the valve is fully open and fully closed, to avoid hitting the dead center when it is fully open. It also facilitates checking whether it is functioning properly when fully closed. If the valve seat comes loose, or large debris gets trapped between the valve core seals, the position of the valve stem when the valve is fully closed will change. When the pipeline is used for the first time, it contains a lot of dirt inside. The valve can be slightly opened to allow the high-speed flow of the medium to carry away this dirt, after which it should be closed gently (not quickly or forcefully, to prevent residual impurities from damaging the sealing surfaces). The valve should then be opened again, and this process repeated several times until all the dirt is removed, after which normal operation can resume. For normally open valves, dirt may adhere to the sealing surface; when closing them, it is necessary to clean them using the aforementioned method before closing them properly. If the handwheel or handle is damaged or lost, it must be replaced immediately; adjustable wrenches cannot be used as a substitute, as this may damage the square shape of the valve stem, resulting in poor operation of the valve and potential accidents during production. In some media, cooling after the valve is closed causes the valve components to contract; therefore, the operator should close it again at an appropriate time to ensure that no gaps remain in the sealing surfaces. Otherwise, the medium will flow rapidly through these gaps, easily eroding the sealing surfaces. During operation, if it is found that the task is too difficult to carry out, the reasons should be analyzed. If the packing is too tight, it can be loosened slightly; if the valve stem is skewed, personnel should be notified for repair. In some valves, when they are closed, the closing element expands due to heat, making it difficult to open them ； If it must be opened at this time, loosen the valve cover threads by half a turn to one full turn to relieve stress on the valve stem, and then turn the handwheel. (II) Precautions: 1. For valves operating at temperatures above 200°C, since they are installed at room temperature, the temperature rises during normal use, causing the bolts to expand due to heat and increasing the gaps. Therefore, it is necessary to tighten them again; this process is known as \"thermal tightening\". Operators must pay attention to this step, as failure to do so can lead to leaks. 2. When the weather is cold and the water valve remains closed for an extended period, the water accumulated behind the valve should be drained. After the steam valve stops supplying steam, the condensate also needs to be drained. There is a plug at the bottom of the valve, which can be opened to drain water. 3. Non-metallic valves: some are hard and brittle, while others have lower strength. When operating them, the force used for opening and closing should not be too great, especially forceful actions should be avoided. Be careful to prevent the object from being bumped. 4. When using a new valve, do not tighten the packing too much; ensure there is no leakage, as this will prevent excessive pressure on the valve stem, which could accelerate wear and make it difficult to open and close the valve. Chapter 5: Inspection, Repair, and Lifespan of Valves Whether using new valves or repaired ones, pressure testing for leaks must be carried out before installation. Section 1: Pressure Testing and Leak Testing. Pressure testing refers to the strength test of the valve body. Leak testing refers to the inspection of the tightness of the sealing surfaces; these two tests are used to evaluate the main performance of valves. The test medium is usually plain water at room temperature; kerosene can be used for important valves. For the pressure setting test of safety valves, nitrogen, which is a relatively stable gas, can be used; steam or air can also be used as alternatives. For diaphragm valves, air is used for testing. (1) Test pressure: The strength test pressure for valves is related to the nominal pressure as follows: Nominal pressure (Mpa) – Strength test pressure (Mpa): 0.1, 0.2, 0.25, 0.4, 0.4, 0.6, 0.6, 0.9, 1.0, 1.5, 1.6, 2.4, 2.5, 3.8, 4.0, 6.0, 6.4, 9.6, 10.0, 15.0, 16.0, 24.0, 20.0, 30.0, 25.0, 38.0, 32.0, 48.0. As can be seen from the above, for these commonly used valves with a nominal pressure ranging from 0.4 Mpa to 32 Mpa, their strength test pressure is 1.5 times their nominal pressure. The valve sealing test pressure is equal to the nominal pressure. (II) Test methods: Pressure testing and leak testing are carried out on a test bench. The structure of the test bench is shown in Figure 5-1; it has a compression component on top and a pipeline connected to the pressure testing pump at the bottom. After the valve is tightened, the pressure testing pump starts to operate, and the value of the pressure applied to the valve can be read from the pressure gauge on the pump. When filling the pressure test valve with water, all air inside the valve must be removed. The upper pressure plate on the test bench has exhaust holes, which are opened and closed using small valves. The sign that the air has been completely drained is when everything coming out of the exhaust hole is water. After closing the exhaust vent, start pressurizing. The pressure increase should be gradual, not sudden. After reaching the specified pressure, hold it for 3 minutes; if the pressure remains unchanged, it is considered qualified. Figure 5-1 Valve testing bench. The pressure testing and leak detection procedure can be divided into three steps: 1. Open the valve passage, fill the valve chamber with water (or kerosene), and increase the pressure to the level required for strength testing; then check whether there are any leaks in the valve body, valve cover, gaskets, or packing. 2. Close the valve completely, apply pressure to one side of the valve to the nominal pressure, and check for leaks from the other side. 3. Reverse the valve and test the opposite side. Figure 5-2: Valve pressure testing – Part 1; Figure 5-3: Valve pressure testing – Part 2. Section 2: General procedures for maintenance. When removing a valve, mark it and the flanges connected to it with identification numbers for maintenance purposes. Also record the valve’s working medium, operating pressure, and operating temperature, so that the appropriate materials can be selected for repairs. When servicing valves, it is required to do so in a clean environment. First, clean the outer surface of the valve, either by blowing it with compressed air or by cleaning it with kerosene. But remember the nameplate and other markings. Inspect for external damage and make a record. Next, disassemble the various components of the valve, clean them with kerosene (do not use gasoline to avoid fires), check for any damage to the components, and make a record of it. Conduct strength tests on the valve body and valve cover. If it is a high-pressure valve, non-destructive testing is also required, such as ultrasonic testing and X-ray testing. The sealing ring can be inspected using red lead powder to check the fit between the valve seat and the gate (valve disc). Check whether the valve stem is bent, if it is corroded, and what the condition of its threads is. Check the degree of wear on the valve stem nut. Address the issues that were detected. Defects in the valve body due to welding repairs. Weld on or replace the sealing ring. Straighten or replace the valve stem. Repair all components that need to be repaired ； Replace those that cannot be repaired. Reassemble the valve. During assembly, all gaskets and packing must be replaced. Conduct strength tests and sealing tests. Section 3: Common Faults and Prevention (I) General Valves 1. Packing box leakage This is one of the main causes of leaks, and it is a common issue in factories. The reasons for packing box leakage are as follows: 1. The packing is not compatible with the corrosiveness, temperature, and pressure of the working medium ； 2. The filling method is incorrect; in particular, inserting the entire filler rod at once makes leaks most likely to occur ； 3. The machining accuracy or surface finish of the valve stem is insufficient; it may be elliptical or have scratches ； 4. The valve stem has suffered pitting, or has rusted due to lack of protection in an outdoor environment ； 5. Valve stem bending ； 6. The filler has aged after being used for too long. 7. The operation is too forceful. 2. Leakage from the closing element. Figure 5-4 shows a schematic of leakage at the sealing surface; Figure 5-5 shows a schematic of leakage at the root of the sealing ring. Leaks in the stuffing box are generally referred to as external leaks, while leaks from the closing element are called internal leaks. Leaks from the closing element occur within the valve and are not easy to detect. Leakage of the closing element can be divided into two categories ； One type is seal surface leakage (as shown in Figure 5-4) ； Another type is leakage at the root of the seal (as shown in Figure 5-5). The reasons for this leakage include: 1> Poor grinding of the sealing surfaces ； 2> The sealing ring does not fit tightly with the valve seat and valve stem ； 3> The connection between the valve body and the valve stem is not secure ； 4> The valve stem is bent and twisted, causing the upper and lower closing elements to be out of alignment ； 5> The shutdown is too rapid, resulting in poor contact between the sealing surfaces or damage to those surfaces ； 6> Improper material selection, unable to withstand corrosion by the medium ； 7> Using stop valves and gate valves for regulating purposes; their sealing surfaces cannot withstand the impact of high-speed flowing media ； 8> In some media, gradual cooling after the valve is closed can cause fine cracks to appear in the sealing surfaces, which can also lead to erosion ； 9> Certain sealing rings are connected to the valve seat and valve stem via threads, which can easily lead to the formation of oxygen concentration cell effects, resulting in corrosion and loosening ； 10> The valve cannot be closed properly due to the penetration of impurities such as slag, rust, and dust, or because mechanical parts in the production system have fallen off and blocked the valve core. 3. Failure in the lifting of the valve stem. The reasons are: 1> Excessive force applied, causing damage to the threads ； 2> Lack of lubricant or failed lubricant ； 3>, Valve stem bending and torsion ； 4> The surface finish is not good enough ； 5> The fit tolerance is inaccurate, resulting in excessive tightness ； 6> The valve stem nut is tilted; 7> Improper material selection ； For example, when the valve stem and the valve stem nut are made of the same material, it is easy for them to stick together ； 8> Threaded wave medium corrosion (referring to plug valves or valves with the valve stem located at the lower part) ； 9> The outdoor valves lack protection; the valve stem threads are covered with dust and sand, or they become corroded due to rain, dew, frost, etc. 4 Other: Valve body cracking: usually caused by freezing. In cold weather, the valves must be equipped with insulation and heating measures; otherwise, after shutdown, the water in the valves and connected pipelines should be drained completely (if there are plug valves at the bottom, these can be opened to drain the water). Damaged handwheel: caused by impact or forceful operation with a long lever. It can be avoided as long as the operators or other relevant personnel pay attention. Filler gland fracture: Uneven force is applied when compressing the filler, or the gland is defective. To compress the filler, turn the screw symmetrically without any deviation. During manufacturing, attention should be paid not only to large and critical components but also to minor parts such as gland seals, otherwise it will affect performance. Failure in the connection between the valve stem and the gate: Gate valves often use a rectangular-shaped end on the valve stem to connect with the T-shaped groove of the gate; sometimes, this T-shaped groove is not machined, which leads to faster wear of the rectangular end of the valve stem. It is mainly addressed from the manufacturing perspective. However, the user can also perform additional machining on the T-groove to give it a certain level of smoothness. The gate plates of double-gate valves cannot be pressed tightly against the sealing surface: The tension on the double gate plates is generated by the wedges; in some gate valves, these wedges are made of poor-quality material (low-grade cast iron), and they wear out or break after short use. The top wedge is a small component that replaces the original cast-iron part. (II) Automatic valves 1. Spring-loaded safety valves: One of the faults is leakage at the sealing surface. The reasons are: ① Debris is trapped between the sealing surfaces ； ②The sealing surface is damaged. Such failures need to be prevented through regular maintenance. The second fault is low sensitivity. The reasons are: ① Spring fatigue ； ②The spring was used improperly. The spring is fatigued and should undoubtedly be replaced. Improper use of the spring is due to the user’s failure to pay attention to a spring-loaded safety valve with a specified pressure rating; it has several pressure ranges, and each range corresponds to a specific spring. For a safety valve with a nominal pressure of 16 kg/cm2, if it is used in a pressure range of 2.5–4 kg/cm2 and equipped with a spring rated for 10–16 kg/cm2, it can still open, but its response is inconsistent and not very sensitive. 2. Common faults of check valves include: ① Damaged valve disc ； ②Medium backflow. The reason for the valve disc breaking is that the pressures of the fluid on both sides of the check valve are in equilibrium and exert opposing forces on each other; as a result, the valve disc constantly impacts against the valve seat, and valve discs made of brittle materials such as cast iron or brass get broken. The preventive measure is to use check valves with valve bodies made of ductile materials. The reasons for medium backflow are: ① Damage to the sealing surface ； ②Impurities get trapped in it. Repairing the sealing surface and removing impurities can prevent backflow. The above description of common faults and preventive methods is only intended to provide guidance; in actual use, other faults may arise. To proactively and flexibly prevent valve failures, the most fundamental thing is to be familiar with its structure, materials, and operating principles. Section 4: Service Life of Valves. Due to the wide range of applications for valves, the variety of media used in their operation, and the different levels of corrosion they experience, there are no regulations regarding their service life, neither in standards nor in industry standards. In 1980, the Ministry of Machinery Industry formulated the “Regulations on Quality Classification for Products in the Valve Industry” in order to improve the quality of machinery industry products and to support national evaluations of high-quality products; these regulations set specific requirements for life testing for first-class and top-grade gate valves and globe valves ； For high and medium pressure valves with Dn≤200–400, the number of scratch resistance cycles is 2000 for first-class products, and 4000 for top-quality products. Due to the special nature of nuclear power plants, various countries have established regulations regarding the service life of nuclear power valves ； Countries such as the United States and France stipulate 40 years, while Japan sets it at 30 to 40 years. Chapter 6: The Development of Valve Technology. The advancement of valve technology depends on the progress in materials science and structural engineering. In recent years, great progress has been made in China in the development of synthetic materials, new alloy steels, and new technologies, as well as in the use of emerging metal materials. All these have contributed new elements to the advancement of valve technology. Take the development of titanium metal as an example: due to its extremely high affinity for oxygen, titanium can form a strong oxide protective layer, which enables it to remain in a passive state in many highly corrosive environments and thus exhibits excellent corrosion resistance. It can be widely used in acids, alkalis, and organic substances. While stainless steel valves only last for one or two weeks in certain mixed organic acids, titanium valves can be used for a long time. Additionally, the metal zirconium is another metal with excellent corrosion resistance. Synthetic materials are also emerging in increasing numbers; materials such as polypropylene, polycarbonate, and nylon are used to manufacture valves, which are already employed in the chemical industry. New types of ceramic valves are also being gradually introduced into use. New structures are also being introduced continuously; for example, the introduction of hard-sealed butterfly valves has been going on in China for over a decade now. Development and application of high-temperature and high-pressure power station valves. New sealing material ； Applications of flexible graphite. It involves immersing a certain liquid into the lattice of graphite and then forcing it to vaporize, thereby changing the orientation of the lattice structure and reducing the brittleness of graphite so that it becomes a softer material. It can be used as gaskets and packing; due to its high purity, it can resist almost all types of chemicals. If nickel wires are added to it to enhance its strength, it can be utilized as packing or gaskets for electric control valves operating under high temperature and pressure conditions. To reduce costs and enhance competitiveness, countries around the world are moving toward larger-scale complete sets of equipment ； The largest power generation units currently in use have a capacity of 1.3 million kilowatts (manufactured in the United States and Switzerland); compared to one 600,000-kilowatt generator unit along with three 200,000-kilowatt units, the cost per unit of power for such equipment is reduced by 10–20%. The costs associated with building power plants – including costs per unit of power, manufacturing time, and steel consumption – are also reduced by 20%. For a refinery with an annual processing capacity of 6 million tons, the investment required is only 69% that of two refineries with a capacity of 3 million tons each; steel consumption is 53% less, land usage is 54% less, and production costs are 75% lower. Meanwhile, labor productivity increases by 170%. The largest refineries abroad currently have an annual processing capacity of 36.4 million tons, and there are already over 20 refineries with a capacity of 20 million tons or more per year. With the development of the refining industry, oil pipelines have also seen rapid growth. The longest oil pipeline built to date is 4,830 kilometers long (Iran). The longest gas pipeline is 7,680 kilometers long (running from Alaska through Canada to the United States), with maximum diameters of 1,420 mm and 1,620 mm. The maximum pipeline pressure ranges from 9 to 12 MPa; these pipelines impose strict requirements on valves, and all of them are operated remotely. Thermal power generation also sees its operating parameters increase as units become larger, in order to improve efficiency ； Specifications: Temperature, Pressure. Subcritical temperature: 566°C, 16.9 MPa; Critical temperature: 583°C, 24.6 MPa; Supercritical temperature: 650°C, 35 MPa. In summary, the increase in the size of equipment imposes stricter requirements on valve materials, structure, and reliability.