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I. Criteria and principles for selecting valves Valves are devices used in fluid flow or pressure systems; they are employed to regulate the flow rate or pressure of the fluid. Its functions include cutting off or connecting the medium, controlling flow rate, changing the flow rate, altering the direction of the medium flow, preventing backflow of the medium, and controlling or releasing pressure. The proper selection of valves is crucial for ensuring the safe operation of the installation, extending their service life, and enabling long-term operation of the facility. The main cause of many valve failures is the improper selection of valves; for example, cast-iron valves are used in cold regions, and the water contained in the oil accumulates within the valve body, leading to freezing in winter and subsequent cracking of the valve. In some pump outlet valves, in situations where the required flow rate is low but the pump power is high for certain reasons, gate valves with small pipes are often used to regulate the flow rate. As the gate valve disc opens partially, vibrations occur, which accelerates the wear of the sealing surface between the gate disc and the valve seat, leading easily to valve leakage. Furthermore, the quality of valves also has a significant impact on production and use. For example, if a fertilizer plant experiences a major explosion, it is found that the cause of the accident was an accident resulting from serious quality defects in the valves. The cause of the explosion was that the outlet valve in the second stage could not withstand the normal operating pressure and burst suddenly; the check valve failed, causing the semi-water gas at 0.75 MPa in the system to leak out rapidly. This gas mixed with the surrounding air to create an explosive gas mixture, leading to a chemical explosion. Tests showed that this valve fails to meet multiple specifications **of the relevant standards. Furthermore, both of these are valves that use water, steam, and oils as media; they are not designed for use with flammable gases, and the selection made by the design department was also incorrect. Therefore, selecting the right valves is extremely important for the production, safety, and long-term operation of the equipment, as well as for improving efficiency. 1. Criteria for selecting valves: The main criteria for choosing valves are their flow characteristic curves, as well as parameters such as valve coefficients. Gate valves have high resistance and a large valve coefficient, so they are suitable for regulating flow at the end of a pipeline ； Globe valves have low resistance and good shut-off performance, but they are not suitable for flow regulation ； Ball valves are suitable for rapid shutdown of systems, generally with smaller pipe diameters ； Butterfly valves are compact in size; combining the advantages of globe valves and ball valves, they offer both regulability and good shut-off performance. They also have a large diameter, which gives them a wide range of applications. This applies to general situations. Additionally, for some valves designed for single purposes—such as steam traps, pressure reducers, check valves, safety valves, breather valves, etc.—selection is relatively easy; generally, the correct valve can be chosen based on design requirements and product catalogs. 2. Principles for selecting valves: When selecting valves for petrochemical production units, the following basic principles must be satisfied: (1) Reliability. Petrochemical production requires continuous, stable operation over long periods of time. Therefore, the valves required must be highly reliable and have a large safety factor; they must not cause any major production safety incidents or casualties due to malfunctions ； It meets the requirements for long-term operation of the equipment; continuous production over a long period translates into benefits ； Furthermore, reduce or prevent \"leaks, spills, drips, and seeps\" caused by valves, create a clean and civilized factory, and implement HSE management. (2) Meeting process production requirements: The valve must meet the requirements of the operating medium, pressure and temperature, as well as its intended purpose; this is also the most fundamental requirement for selecting a valve. For example, where a valve is needed to provide overpressure protection and to discharge excess fluid, a safety valve or a relief valve should be selected ； Check valves should be used to prevent backflow of the medium during operation ； To automatically drain the continuously generated condensate, air, and other non-condensable gases from steam pipes and equipment, while preventing steam from escaping, a steam trap should be used. Furthermore, for environments with corrosive media, valve bodies made of corrosion-resistant materials should be used. (3) Easy operation, installation, inspection, and maintenance: Once the valve is properly secured, operators should be able to correctly identify the valve’s orientation, opening degree indicators, and signal indications, facilitating timely and decisive handling of various emergency malfunctions. At the same time, the structure of the selected valve type should be as simple as possible, facilitating installation and maintenance. (4) Cost-effectiveness: Pay attention to conserving investments and reducing equipment costs; today’s investments become tomorrow’s costs. For general applications, domestic production can meet the requirements; therefore, domestically produced valves should be selected ； Among the various types of valves that can meet the requirements, those that are inexpensive and have a simple structure should be chosen ； If a common material is sufficient to meet the usage requirements, there is no need to use higher-grade materials such as Cr-Mo steel, stainless steel, or babbitt alloy. II. Classification of valves 1. Classification based on the purpose and function of the valve: Shut-off valves (whose function is to open and close the flow of medium in a pipeline, such as ball valves, gate valves, globe valves, butterfly valves, and diaphragm valves) ； Regulating valves (their function is to regulate parameters such as the flow rate and pressure of the medium; examples include regulating valves, throttle valves, and pressure reducing valves, etc.) ； Check valves (whose function is to prevent the backflow of fluid in pipelines, such as check valves and foot valves) ； Shunt valves (which are used to distribute, separate, or mix fluids in pipelines, such as distribution valves and steam traps) ； Safety valves 2. Classification by actuation type: Manual valves ； Power-driven valves (such as electric valves, pneumatic valves) ； Automatic valves (valves that do not require external force to operate; they use the energy of the medium itself to move the valve, such as check valves, safety valves, self-acting pressure relief valves, and steam traps). 3. Classified by nominal pressure: they can be divided into vacuum valves (valves that operate at pressures below standard atmospheric pressure) ； Low-pressure valves (nominal pressure less than or equal to 1.6 Mp); medium-pressure valves (nominal pressure of 2.5 MPA, 4.0 MPA, 6.4 MPA) ； High-pressure valves (nominal pressure 10MPA—80MPA) ； Ultra-high pressure valves (greater than 100 MPa). 3. Classification by temperature rating: Ultra-low temperature valves (operating temperature below –80°C) ； Low-temperature valves (operating temperature between –40°C and –80°C) ； Normal temperature valves (operating temperature higher than –40°C and lower than or equal to 120°C) ； Medium-temperature valves (operating temperature above 120°C and below 450°C) ； High-temperature valves (operating at temperatures above 450°C). 4. Classified by valve body material: Non-metallic material valves (such as ceramic valves, fiberglass-reinforced plastic valves, plastic valves) ； Metal material valves (such as copper alloy valves, aluminum alloy valves, lead alloy valves, titanium alloy valves, Monel alloy valves, cast iron valves, carbon steel valves, cast steel valves, low-alloy steel valves, high-alloy steel valves) ； Valves with metal valve bodies lined: such as lead-lined valves, plastic-lined valves, and enamel-lined valves. 5. Common classification method: This method takes into account both the working principle and function as well as the valve structure; it is the commonly used classification approach in China. It can be divided into: gate valves ； Butterfly valve ； Stop valve ； check valve ； Plug valve ； Ball valve ； Clamp valve ； Diaphragm valve ； Piston valves, etc. III. Introduction to Common Valves and Selection Principles 1. Gate Valve A gate valve is a type of valve in which the closing element (gate) moves vertically along the axis of the passage; it is primarily used in pipelines to shut off the flow of medium, that is, to operate in either fully open or fully closed position. Generally, gate valves cannot be used to regulate flow. It can be used under low-temperature and low-pressure conditions as well as high-temperature and high-pressure conditions, depending on the material of the valve. However, gate valves are generally not used in pipelines transporting media such as slurry. Advantages: ① Low fluid resistance ； ②The torque required to open and close it is low ； ③It can be used in ring piping systems where the medium flows in both directions; in other words, the flow direction of the medium is not restricted ； ④When fully open, the sealing surface is subject to less erosion by the working medium compared to a globe valve ； ⑤Its structural design is relatively simple, and it is easy to manufacture ； ⑥The structural length is relatively short. Disadvantages: ① It has large external dimensions and opening height, requiring a lot of space for installation ； ②During the opening and closing process, relative friction occurs between the sealing surfaces, resulting in significant wear; in addition, scratches can easily occur at high temperatures ； ③Generally, gate valves have two sealing surfaces, which adds some difficulty to processing, grinding, and maintenance ； ④The opening and closing time is long. Selection principles for gate valves: ① For pipelines transporting oil and natural gas, single-plate or double-plate gate valves should be used. For cleaning pipelines, select a straight-stem gate valve with a single or double gate and flow guide holes. ②For the pipelines and storage equipment used for transporting finished oil, gate valves with a single or double gate and without guide holes are selected. ③For wellhead equipment used in the extraction of oil and gas, gate valves with a floating stem and valve seats equipped with flow guide holes, either single or double gate types, are commonly used; these valves generally comply with the API16A standard, while their pressure ratings range from API2000 to API10000, API15000, and API20000. ④For pipes with suspended particulate media, knife-type flat gate valves are recommended. ⑤For urban gas transmission pipelines, single-plate or double-plate soft-seal straight-through gate valves are used. ⑥For urban water supply systems, single-plate or double-plate gate valves without guides are used. 2. Butterfly valves: These are valves that use a disc-shaped closing element that rotates back and forth by about 90° to open, close, and regulate the flow of fluid. Advantages: ① Simple structure, small size, light weight, low material consumption; not suitable for large-diameter valves ； ②Rapid opening and closing, low flow resistance ； ③It can be used in media containing suspended solid particles, and depending on the strength of the sealing surface, it can also be used for powdered and granular media. It can be used for two-way opening, closing, and adjustment in ventilation and dust removal pipelines, and is widely applied to gas pipes and water pipelines in metallurgy, light industry, power, and petrochemical systems. Disadvantages: ① The flow rate adjustment range is limited; when it is set to 30%, the flow rate already exceeds 95%. ②Due to the structure of butterfly valves and the limitations of their sealing materials, they are not suitable for use in high-temperature and high-pressure pipeline systems. The normal operating temperature is below 300°C, and the pressure rating is below PN40. ③Its sealing performance is inferior to that of ball valves and globe valves, so it is used in applications where high sealing requirements are not necessary. Principles for selecting butterfly valves: ① Butterfly valves have a higher pressure loss compared to gate valves and ball valves; therefore, they are suitable for pipeline systems where strict requirements regarding pressure loss are not necessary. ②Butterfly valves can be used for flow regulation, so they are suitable for use in pipelines where such regulation is required. ③Due to the structure of butterfly valves and the limitations of their sealing materials, they are not suitable for use in high-temperature and high-pressure pipeline systems. The usual operating temperature is below 300°C, and the nominal pressure is below PN40. ④Butterfly valves have a relatively short structural length, and they can also be manufactured with large diameters; therefore, they are suitable for applications where a short structural length is required, or for valves with large diameters (such as those above DN1000). ⑤A butterfly valve can be opened or closed by rotating just 90°, so it is suitable for applications where quick opening and closing are required. 3. The ball valve evolved from the plug valve; its closing element is a sphere, and opening and closing are achieved by rotating this sphere 90° around the axis of the valve stem. Ball valves are primarily used on pipelines to shut off, distribute, and change the direction of fluid flow; those designed with a V-shaped opening also possess excellent flow regulation capabilities. Advantages: ① It has the lowest flow resistance (actually 0) ； ②Since it does not seize during operation (even without lubricant), it can be reliably used in corrosive media and liquids with low boiling points ； ③Complete sealing can be achieved within a wide range of pressures and temperatures ； ④Fast opening and closing can be achieved; the opening and closing time for certain structures is as low as 0.05–0.1 seconds, ensuring their suitability for use in automated systems on test benches. When the valve is opened and closed quickly, the operation is shock-free. ⑤The spherical closure can automatically position itself at the boundary position ； ⑥The working medium can be sealed on both sides* ； ⑦When in the fully open and fully closed positions, the sealing surfaces of the ball and the valve seat are separated from the medium, so the medium flowing through the valve at high speeds does not cause erosion of these sealing surfaces ； ⑧With its compact design and low weight, it can be considered the most suitable valve structure for low-temperature medium systems ； ⑨The valve body is symmetrical; in particular, the welded valve body structure can effectively withstand stresses from the pipes ； ⑩The closing element can withstand the high pressure difference during closure. ⑾Ball valves with fully welded valve bodies can be buried directly underground, preventing the internal components of the valve from being corroded; their maximum service life can reach 30 years, making them the ideal valves for oil and gas pipelines. Disadvantages: ① Since the main material used for the seat seal ring in ball valves is polytetrafluoroethylene, it is inert to almost all chemicals. It also boasts a low friction coefficient, stable performance, resistance to aging, a wide temperature range of operation, and excellent sealing properties. However, the physical properties of polytetrafluoroethylene, including its high coefficient of expansion, sensitivity to cold flow, and poor thermal conductivity, require that the design of the valve seat seal take these properties into account. Therefore, when the sealing material hardens, the reliability of the seal is compromised. Moreover, polytetrafluoroethylene has a low temperature resistance and can only be used at temperatures below 180°C. Above this temperature, the sealing material will age. When considering long-term use, it is generally not used at 120°C. ②Its regulating performance is somewhat inferior to that of globe valves, especially in the case of pneumatic valves (or electric valves). It has a wide range of applications and can be used for: ① Nominal diameters ranging from 8 mm to 1200 mm. ②Nominal pressure ranges from vacuum to 42 MPa. ③The operating temperature ranges from -204°C to 815°C. Selection principles: ① For main pipelines for transporting oil and natural gas, those that require cleaning and are buried underground, a full-bore, fully welded structure should be chosen ； For those buried underground, choose full-bore welding connection or flanged connection ； Branch pipes: flange-connected or welded, with full bore or reduced bore. ②For the pipelines and storage equipment used for transporting finished oil, flange connections are employed. ③For urban gas and natural gas pipelines, floating connections using flanges and internal threads are employed. ④In the oxygen pipeline systems of metallurgical plants, fixed balls with flange connections that have undergone rigorous degreasing treatment are recommended. ⑤For piping systems and equipment handling cryogenic media, cryogenic valves with valve covers are recommended. ⑥For the piping systems of the catalytic cracking unit in oil refining plants, a lift rod type can be selected. ⑦In the equipment and piping systems for corrosive media such as acids and alkalis in chemical processing systems, it is advisable to use all-stainless-steel valves made of austenitic stainless steel, with polytetrafluoroethylene used as the valve seat seal. ⑧Metal-to-metal sealing can be used in the piping systems or devices that handle high-temperature media in metallurgical systems, power systems, petrochemical plants, and urban heating systems. ⑨When flow regulation is required, a V-shaped opening regulator with worm gear transmission, pneumatic, or electric drive can be used. 4. A globe valve is a type of valve in which the closing element (valve disc) moves along the center line of the valve seat. Based on this manner of movement of the valve disc, the change in the valve seat opening is directly proportional to the stroke of the valve disc. Due to the relatively short opening and closing stroke of the valve stem in this type of valve, as well as its highly reliable shut-off function, and because the change in the valve seat orifice is proportional to the stroke of the valve disc, it is very suitable for regulating flow rates. Therefore, this type of valve is very suitable for use in cutting off or regulating flow, as well as for throttling. Advantages: ① During opening and closing, the frictional force between the valve disc and the sealing surface of the valve body is lower than that of gate valves, thus it is wear-resistant. ②The opening height is generally only 1/4 of the valve seat passage, so it is much smaller than gate valves ； ③Usually, there is only one sealing surface on the valve body and the valve disc, which makes the manufacturing process simpler and facilitates maintenance. ④Since its filler is generally a mixture of asbestos and graphite, it has a high temperature resistance rating. Gauge valves are generally used for steam valves. Disadvantages: ① Due to the change in the flow direction of the medium through the valve, the minimum flow resistance of globe valves is also higher than that of most other types of valves ； ②Due to the long travel distance, its opening speed is slower than that of a ball valve. The selection principles for globe valves are as follows: ① Globe valves are suitable for use in pipelines or devices handling high-temperature and high-pressure media. Such as in thermal power plants, nuclear power plants, and the high-temperature and high-pressure pipelines of petrochemical systems. ②In pipelines where the requirement for convective resistance is not strict. That is, in places where pressure loss is not a major concern. ③For small valves, needle valves, instrument valves, sampling valves, pressure gauge valves, etc. can be used. ④It features flow regulation or pressure regulation, but does not require high precision in regulation; it is also suitable for pipelines with a relatively small diameter, such as those with a nominal diameter of ≤50 mm. ⑤In the production of synthetic fertilizers, high-pressure angle-type stop valves or high-pressure angle-type throttle valves with a nominal pressure of PN160 (16 MPa) or PN320 (32 MPa) are suitable for use with both small and large-scale fertilizers. ⑥In the desiliconization section of the alumina Bayer process, as well as on pipelines prone to coking, straight-through globe valves or straight-through throttle valves with a separate valve body, removable valve seats, and cemented carbide sealing pairs are preferred. ⑦In water supply and heating systems used in urban construction, for pipelines with a relatively small nominal diameter, globe valves, balance valves, or plunger valves can be used, such as in pipelines with a nominal diameter of less than 150 mm. 5. A plug valve is a rotary valve whose closing element is in the form of a plunger; it operates by rotating 90° to connect or separate the passage in the plunger from the passage in the valve body, thereby enabling opening or closing. The shape of the valve plug can be cylindrical or conical. Its principle is basically similar to that of a ball valve, which was developed from a plug valve. It is primarily used in oil field extraction, as well as in the petrochemical industry. Selection principles: ① Used for distributing media and changing the flow direction of the media; with an operating temperature ≤ 300°C, a nominal pressure ≤ 1.6 MPa, and a nominal diameter ≤ 300 mm, multi-port plug valves are recommended. ② For equipment and pipelines in food industries such as milk, juice, and beer production, as well as in pharmaceutical factories, it is recommended to use austenitic stainless steel fixed-cone plug valves. ③ In oil field extraction, natural gas field extraction, pipeline branch lines, as well as in refining and cleaning equipment, where the nominal pressure class is ≤ CL300, the nominal diameter is ≤ 300 mm, and the operating temperature is ≤ 340°C, it is recommended to use oil-sealed conical plug valves. ④ In oil field extraction, natural gas field extraction, pipeline branch lines, as well as in refining and cleaning equipment, where the nominal pressure class is ≤ CL2500, the nominal diameter is ≤ 900 mm, and the operating temperature is ≤ 340°C, it is recommended to use pressure-balanced inverted cone plug valves. ⑤ In the large-scale chemical industry, in applications where fast opening and closing speeds are required for pipelines and equipment containing corrosive media, for media based on nitric acid, a conical plug valve with a PTFE sleeve made of 1Cr18Ni9 stainless steel can be used ； For acetate-based media, a polytetrafluoroethylene sleeve-sealed conical plug valve made of Cr18Ni12Mo2Ti stainless steel can be used. ⑥ In pipelines and equipment for gas, natural gas, and HVAC systems, where the nominal pressure is ≤ 1.0 MPa and the nominal diameter is ≤ 200 mm, plug-type conical stop valves are recommended. 6. A diaphragm valve is one in which a flexible diaphragm or composite diaphragm is installed within the valve body and valve cover, with the closing element being a compression device connected to the diaphragm. The valve seat can be wavy or the wall of a straight flow channel. Advantages: ① The control mechanism is separated from the medium flow path, which not only ensures the purity of the working medium but also prevents the medium in the pipeline from impacting the working components of the control mechanism. No separate sealing is required at the valve stem, unless such a measure is needed as a safety feature when dealing with harmful media ； ②Since the working medium comes into contact only with the diaphragm and the valve body, and both can be made from a variety of different materials, this valve is capable of effectively controlling a wide range of working media, especially those that are chemically corrosive or contain suspended particles. ③It has a simple structure, consisting of only three components: the valve body, the diaphragm, and the valve cover assembly. The valve is easy to disassemble and maintain quickly; replacing the diaphragm can be done on-site in a short amount of time. Disadvantages: ① Due to limitations in the lining process for valve bodies and the manufacturing process for diaphragms, it is difficult to produce large-sized valve body linings and large-sized diaphragms. Therefore, diaphragms are not suitable for use in pipes with larger diameters; they are generally applied in pipelines with a diameter of DN ≤ 200 mm. ②Due to the limitations of the diaphragm material, diaphragm valves are suitable for low-pressure and low-temperature applications. Generally not exceeding 180℃ ； ③The adjustment performance is relatively poor, allowing adjustment only within a narrow range (it can be used for flow regulation when the valve is closed to 2/3 of its opening). 7. Plunger valve: A plunger valve consists of components such as the valve body, valve cover, valve stem, plunger, hole frame, sealing ring, and handwheel (as shown in the figure). When the handwheel rotates, it drives the plunger via the valve stem to move up and down reciprocally within the port frame, thereby opening and closing the valve. In the valve, an interference fit is used between the plunger and the sealing ring; by adjusting the flange bolts on the gland, the lateral force generated by the compression of the sealing ring ensures a seal with the inner surface of the valve body and the outer surface of the plunger, thereby ensuring the valve’s sealing performance and preventing internal and external leaks. Additionally, the valve requires little torque to open, allowing it to be opened and closed quickly. This is because the surface of the plunger is machined using a high-precision external cylindrical grinder. The sealing ring is made of a new type of non-toxic sealing material with high elasticity and wear resistance, ensuring reliable sealing and long durability. This thereby increases the service life of the plunger valve. 8. Throttle valve: A throttle valve is a valve that controls the flow rate of fluid by changing the throttling cross-section or throttling length. Connecting a throttle valve and a check valve in parallel can form a one-way throttle valve. Throttle valves and check throttle valves are simple flow control valves. In hydraulic systems with positive displacement pumps, throttle valves, in combination with relief valves, can form three types of throttle speed control systems: the inlet circuit throttle speed control system, the return circuit throttle speed control system, and the bypass throttle speed control system. Throttle valves do not have a flow negative feedback function, and thus cannot compensate for speed instability caused by changes in load; they are generally used only in situations where the load changes little or where high speed stability is not required. Applications of throttle valves Since the flow rate through a throttle valve depends not only on the area of the throttle opening but also on the pressure difference before and after it, and because the valve has low stiffness, it is suitable only for applications where the load on the actuator changes little and high speed stability is not required. For throttle speed control systems where the load on the actuator varies greatly and high speed stability is required, pressure compensation for the throttle valve is necessary in order to maintain a constant pressure difference before and after the throttle valve, thereby ensuring stable flow. 9. A safety valve is a device used as an overpressure protection mechanism in pressurized containers, equipment, or pipelines. When the pressure inside a device, container, or pipeline exceeds the permissible limit, the valve automatically opens to allow for full discharge, thereby preventing any further increase in pressure within the device, container, or pipeline ； When the pressure drops to the specified value, the valve should close automatically and promptly, thereby ensuring the safe operation of the equipment, containers, or pipelines. Principles for selecting safety valves: ① For steam boiler safety valves, full-opening spring safety valves are generally chosen. ②For liquid media, micro-opening spring safety valves are generally selected. ③For air or other gaseous media, safety valves are used, with fully open spring safety valves being the common choice. ④For safety valves used in liquefied petroleum gas road tankers or railway tank cars, fully open internal safety valves are generally selected. ⑤Safety valves are used at the outlet of oil production wells (production trees), with pilot-operated safety valves being the preferred choice. ⑥For the high-pressure bypass safety valves in steam power generation equipment, pilot-operated safety valves that combine safety and control functions are generally selected. ⑦When it is required to conduct periodic opening tests on safety valves, a safety valve equipped with a lifting wrench should be selected. When the medium pressure reaches over 75% of the opening pressure, a lifting wrench can be used to slightly lift the valve disc away from the valve seat in order to check the flexibility of the safety valve’s operation⑼⑧. If the medium temperature is high, in order to reduce the temperature in the spring chamber, safety valves equipped with radiators should be used when the operating temperature for closed-type safety valves exceeds 300°C and for open-type safety valves exceeds 350°C⑼⑨. If the back pressure at the outlet of the safety valve varies, and such variations exceed 10% of the opening pressure, then bellows-type safety valves should be selected. ⑩. When the medium is corrosive, a bellows safety valve should be used to prevent critical components from failing due to corrosion by the medium. 10. Steam traps: When transporting media such as steam and compressed air, some condensate water is formed. To ensure the efficiency and safe operation of the equipment, it is necessary to remove this unnecessary and harmful fluid promptly, so as to maintain proper consumption and use of the equipment. Function: ① It can quickly remove the generated condensed water ； ②Prevent steam leakage ； ③Remove air and other non-condensable gases. Selection principle: Based on the amount of condensate generated by the steam heating equipment during normal operation, this value is multiplied by the selection correction factor k; thereafter, a steam trap with a drainage capacity that matches the established design requirements is chosen. 11. A pressure reducing valve is a valve that, through regulation, reduces the inlet pressure to a desired outlet pressure; it relies on the energy of the medium itself to automatically maintain this outlet pressure at a stable level. Principles for selecting common pressure reducing valves: 1. The fluctuation in the inlet pressure of the pressure reducing valve should be kept within 80% to 105% of the specified inlet pressure value; if it exceeds this range, the performance of the pressure reducing valve will be affected. 2. Generally, the pressure Pc behind the valve of a pressure relief valve should be less than 0.5 times the pressure before the valve, that is, Pc