HOW TO CHOOSE VALVE TYPES

HOW TO CHOOSE VALVE TYPES

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In the fluid pipeline system, the regulating valve is a control component, and its investment accounts for about 30% to 50% of the pipeline engineering cost. The main functions of the valve are opening and closing, throttling, regulating flow, isolating equipment and pipeline systems, preventing medium backflow, regulating and discharging pressure etc.

The valve is also the most complex component in the pipeline. It is generally assembled from multiple parts and has high technical content.

With the rapid development of the petrochemical industry, most of the media in petrochemical production units are highly toxic, flammable, explosive, and corrosive. Operating conditions are complex and harsh, with high operating temperatures, pressures, and long start-up cycles. Once a valve fails, it may lead to media leakage, which will pollute the environment and cause economic losses. In severe cases, it may cause the device to shut down, stop production, or even cause a vicious accident.

Therefore, in pipeline design, a scientific and reasonable selection of valves can reduce the device’s construction cost and ensure safe production operation. The article mainly introduces the selection methods of various commonly used valves such as gate valves, globe valves, throttle valves, plug valves, ball valves, diaphragm regulating valves, etc.

Critical points of valve selection

  1. Clarify the purpose of the valve in the equipment or device

    Determine the working conditions of the valve: the nature of the applicable medium, working pressure, working temperature, operation control method, etc.

  2. Correctly choose the type of valve:

    The correct selection of the valve type is a prerequisite for the designer to fully understand the production process and operating conditions. When selecting the valve type, the designer should first understand each valve’s structural characteristics and performance.

  3. Determine the end connection of the valve

    The first two are the most commonly used among threaded connections, flange connections, and welded end connections. Valves with threaded connections are mainly valves with a nominal diameter below 50mm. If the diameter is too large, installing and sealing the connection will be very difficult. Flange-connected valves are easier to install and disassemble but are bulkier and more expensive than threaded valves, so they are suitable for pipe connections of various diameters and pressures. Welded connections are suitable for heavier load conditions and are more reliable than flange connections. However, it is difficult to disassemble and reinstall welded valves, so its use is limited to situations where it can usually operate reliably for a long time or where the operating conditions are severe and the temperature is high.

  4. Valve material selection

    When selecting the material of the valve shell, internal parts, and sealing surface, in addition to considering the physical properties (temperature, pressure) and chemical properties (corrosiveness) of the working medium, the cleanliness of the medium (the presence or absence of solid particles) should also be considered. In addition, you must also refer to the relevant regulations of the country and the user department. A correct and reasonable selection of valve materials can achieve the most economical service life and best valve performance. The selection order of valve body materials is cast iron-carbon steel-stainless steel, and the selection order of sealing ring materials is rubber-copper-alloy steel-F4.

  5. other

    In addition, the flow rate and pressure level of the fluid flowing through the valve should also be determined, and the appropriate valve should be selected using existing information (such as valve product catalogs, valve product samples, etc.)

Introduction to commonly used valves

There are many types of valves, including gate valves, globe valves, throttle valves, butterfly valves, plug valves, ball valves, electric valves, diaphragm valves, check valves, safety valves, pressure-reducing valves, steam traps and emergency shut-off valves, etc., among which commonly used ones are gate valves, globe valves, throttle valves, plug valves, butterfly valves, ball valves, check valves, and diaphragm valves.

  1. Gate valve

    A gate valve is a valve whose opening and closing body (valve plate) is driven by the valve stem and moves up and down along the sealing surface of the valve seat, which can connect or cut off fluid passage. Compared with stop valves, gate valves have better sealing performance, smaller fluid resistance, less effort to open and close, and have certain adjustment performance. They are one of the most commonly used cut-off valves. The disadvantages are that it is large in size and has a more complex structure than a stop valve. The sealing surface is easy to wear and difficult to maintain, and it is generally not suitable for throttling. According to the thread position on the valve stem of the gate valve, it is divided into two categories: open stem type and concealed stem type. According to the structural characteristics of the gate, it can be divided into two types: wedge type and parallel type.

  2. Shut-off valve

    The stop valve is a downward-closing valve. The opening and closing parts (valve discs) are driven by the valve stem to move up and down along the axis of the valve seat (sealing surface). Compared with the gate valve, it has good adjustment performance, poor sealing performance, simple structure, convenient manufacturing and maintenance, large fluid resistance, and low price. It is a commonly used cut-off valve, generally used for medium and small-diameter pipelines.

  3. Ball valve

    The opening and closing part of the ball valve is a ball with a circular through hole, and the ball rotates with the valve stem to open and close the valve. The ball valve has a simple structure, quick opening and closing, easy operation, small size, lightweight, few parts, small fluid resistance, good sealing, and easy maintenance.

  4. Throttle valve

    The throttle valve has basically the same structure as the globe valve except for the valve disc. The valve disc is a throttling component, and different shapes have different characteristics. The diameter of the valve seat should not be too large because the smaller the opening height, the medium flow rate increases, thus Accelerating the erosion of the valve disc. The throttle valve has small dimensions, is lightweight, and has good adjustment performance, but the adjustment accuracy is not high.

  5. Plug valve

    The plug valve uses a plug body with a through hole as the opening and closing part, and the plug body rotates with the valve stem to achieve opening and closing. The plug valve has a simple structure, quick opening and closing, easy operation, small fluid resistance, few parts, and is lightweight. Plug valves are available in straight-through, three-way, and four-way types. The straight-through plug valve is used to cut off the medium, and the three-way and four-way plug valves are used to change the direction of the medium or divert the medium.

  6. Butterfly valve

    Butterfly valve is a butterfly plate that rotates 90° around a fixed axis in the valve body to complete the opening and closing function. Butterfly valves are small, light in weight, and simple in structure, consisting of only a few parts. It only needs to be rotated 90°; it can be quickly opened and closed, and the operation is simple. When the butterfly valve is in the fully open position, the thickness of the butterfly plate is the only resistance when the medium flows through the valve body. Therefore, the pressure drop generated by the valve is tiny, so it has good flow control characteristics. Butterfly valves are divided into two sealing types: elastic soft seal and metal hard seal. For the elastic sealing valve, the sealing ring can be embedded in the valve body or attached to the periphery of the butterfly plate. It has good sealing performance and can be used for throttling medium vacuum pipelines and corrosive media. Valves with metal seals generally have a longer service life than valves with elastic seals, but it is difficult to achieve complete sealing. They are usually used in situations where flow and pressure drop changes greatly and good throttling performance is required. Metal seals can adapt to higher operating temperatures, while elastic seals have the disadvantage of being limited by temperature.

  7. Check valve

    The check valve is a valve that can automatically prevent the reverse flow of fluid. The disc of the check valve opens under the action of fluid pressure, and the fluid flows from the inlet side to the outlet side. When the pressure on the inlet side is lower than the outlet side, the valve disc automatically closes under the action of fluid pressure difference, its own gravity, and other factors to prevent fluid from flowing back. According to the structural form, it can be divided into a lift check valve and a swing check valve. The lifting type has better sealing and greater fluid resistance than the swing type. A bottom valve should be used to inlet the pump suction pipe. Its function is to fill the pump inlet pipe with water before starting the pump; after stopping the pump, keep the inlet pipe and pump body filled with water in preparation for starting again. The bottom valve is generally only installed on the vertical pipe at the pump inlet, and the medium flows from bottom to top.

  8. Diaphragm valve

    The opening and closing part of the diaphragm valve is a rubber diaphragm, which is sandwiched between the valve body and the valve cover. The middle protruding part of the diaphragm is fixed on the valve stem, and the valve body is lined with rubber. Since the medium does not enter the inner cavity of the valve cover, the valve stem does not require a stuffing box. The diaphragm valve has a simple structure, good sealing performance, easy maintenance, and low fluid resistance. Diaphragm valves are divided into weir type, straight-through type, right-angle type, and direct-flow type.

Common valve selection instructions

  1. Gate valve selection instructions

    In general, gate valves should be preferred. In addition to being suitable for steam, oil, and other media, gate valves are also suitable for media containing granular solids and high viscosity and are suitable for valves in venting and low vacuum systems. For media containing solid particles, the gate valve body should be equipped with one or two purge holes. For low-temperature media, low-temperature special gate valves should be selected.

  2. Globe valve selection instructions

    The globe valve is suitable for pipelines with lax requirements on fluid resistance; that is, pressure loss is not considered much, and pipelines or devices with high temperature and high-pressure media are not considered. It is suitable for steam and other medium pipelines with DN <200mm; small valves can use cut-off valves. Valves, such as needle valves, instrument valves, sampling valves, pressure gauge valves, etc.; stop valves have flow adjustment or pressure adjustment, but the adjustment accuracy is not required, and the pipeline diameter is relatively small, so a stop valve or throttling valve should be used Valve; for highly toxic media, a bellows-sealed stop valve should be used; however, the stop valve should not be used for media with high viscosity and media containing particles that are prone to sedimentation, nor should it be used as a vent valve and a valve in a low vacuum system.

  3. Ball valve selection instructions

    Ball valves are suitable for low-temperature, high-pressure, and high-viscosity media. Most ball valves can be used in media with suspended solid particles and can also be used in powdery and granular media according to the sealing material requirements; full-channel ball valves are not suitable for flow regulation but are suitable for occasions requiring rapid opening and closing, which is easy to implement. Emergency cut-off in accidents is usually recommended in pipelines with strict sealing performance, wear, shrinkage channels, rapid opening and closing movements, high-pressure cutoff (large pressure difference), low noise, gasification phenomenon, small operating torque, and small fluid resistance. Use a ball valve. Ball valves are suitable for light structures, low-pressure cutoffs, and corrosive media; ball valves are also the ideal valves for low-temperature and cryogenic media. For pipeline systems and devices with low-temperature media, low-temperature ball valves with valve covers should be selected; floating ball valves should be selected. The valve seat material should bear the load of the ball and the working medium. Large-diameter ball valves require greater force during operation. Ball valves with DN ≥ 200mm should use worm gear transmission; fixed ball valves are suitable for larger diameters and higher pressures. Occasions: in addition, ball valves used for processing highly toxic materials and flammable medium pipelines should have fire-proof and anti-static structures.

  4. Throttle valve selection instructions

    The throttle valve is suitable for occasions where the medium temperature is low, and the pressure is high. It is suitable for parts that need to adjust the flow rate and pressure. It is not suitable for media with high viscosity and solid particles and is not suitable for use as an isolation valve.

  5. Plug valve selection instructions

    Plug valves are suitable for occasions requiring rapid opening and closing. They are generally not suitable for steam and medium with higher temperatures. They are used for mediums with lower temperatures and high viscosity and are also ideal for mediums with suspended particles.

  6. Butterfly valve selection instructions

    Butterfly valves are suitable for situations with large diameters (such as DN﹥600mm) and short structural lengths, as well as situations where flow adjustment and fast opening and closing are required. They are generally used for water, oil, and compression products with temperatures ≤80°C and pressures ≤1.0MPa. Air and other media: because the pressure loss of butterfly valves is relatively large compared to gate valves and ball valves, butterfly valves are suitable for pipeline systems with loose pressure loss requirements.

  7. Check valve selection instructions

    Check valves are generally suitable for clean media and are not suitable for media containing solid particles and high viscosity. When ≤40mm, a lifting check valve should be used (only allowed to be installed on horizontal pipelines); when DN=50~400mm, a swing-type lifting check valve should be used (can be installed on both horizontal and vertical pipelines, such as Installed on a vertical pipeline, the medium flow direction should be from bottom to top); when DN ≥ 450mm, a buffer check valve should be used; when DN = 100 ~ 400mm, a wafer check valve can also be used; a swing check valve The valve can be made to have a very high working pressure, PN can reach 42MPa, and it can be applied to any working medium and any working temperature range depending on the materials of the shell and seals. The mediums are water, steam, gas, corrosive mediums, oil, medicine, etc. The working temperature range of the medium is between -196~800℃.

  8. Diaphragm valve selection instructions

    The diaphragm valve is suitable for oil, water, acidic media, and media containing suspended solids with an operating temperature of less than 200°C and a pressure of less than 1.0MPa. It is not suitable for organic solvents and strong oxidant media;

    Weir-type diaphragm valves should be selected for abrasive granular media. When choosing a weir-type diaphragm valve, refer to its flow characteristics table. Straight-through diaphragm valves should be selected for viscous fluids, cement slurries, and precipitating media. Diaphragm valves should not be used in vacuum pipes except for specific requirements on roads and vacuum equipment.

Various valve pressure test methods

Under normal circumstances, industrial valves do not undergo strength tests when in use, but the valve body and valve cover after repair or those damaged by corrosion should undergo strength tests. For safety valves, their set pressure, backseat pressure, and other tests should comply with the instructions and relevant regulations. The strength and tightness test should be carried out before the valve is installed. 20% of low-pressure valves shall be randomly inspected. If they are unqualified, 100% shall be inspected; medium and high-pressure valves shall be inspected 100%. Commonly used media for valve pressure testing include water, oil, air, steam, nitrogen, etc. The pressure testing methods for various industrial valves, including pneumatic valves, are as follows:

  1. Ball valve pressure test method

    The strength test of pneumatic ball valves should be carried out with the ball in a half-open state.

    Floating ball valve sealing test: Put the valve in a semi-open state, introduce the test medium at one end, and close the other end; rotate the ball several times, open the closed end for inspection when the valve is in the closed state, and check the sealing performance of the packing and gasket at the same time. There is leakage. Then, introduce the test medium from the other end and repeat the above test.

    Fixed ball valve sealing test: Before the test, rotate the ball several times without load, the fixed ball valve is in a closed state, and introduce the test medium from one end to the specified value; use a pressure gauge to check the sealing performance of the introduction end, and the accuracy of the pressure gauge is 0. Level 5~1, the measuring range is 1.5 times the test pressure. Within the specified time, if there is no pressure drop phenomenon, it is qualified; then introduce the test medium from the other end and repeat the above test. Then, put the valve in a semi-open state, seal both ends, and fill the inner cavity with the medium. Check the packing and gasket under the test pressure to see that there is no leakage.

    Three-way ball valves should be tested for tightness at various positions.

  2. Check valve pressure test method

    Check valve test status: the lift check valve disc axis is in a position perpendicular to the horizontal; the swing check valve channel axis and valve disc axis are in a position approximately parallel to the horizontal line.

    During the strength test, the test medium is introduced from the inlet end to the specified value, and the other end is closed. If there is no leakage in the valve body and valve cover, it is qualified.

    In the sealing test, the test medium is introduced from the outlet end, and the sealing surface is checked at the inlet end. If there is no leakage at the packing and gasket, it is qualified.

  3. Pressure testing method of pressure reducing valve

    The strength test of the pressure-reducing valve is generally performed as a single piece before assembly, or it can also be tested after assembly. Strength test duration: 1min for DN<50mm; more than 2min for dn65~150mm; more than 3min for dn>150mm. After the bellows and components are welded, the strength test should be conducted with air at 1.5 times the maximum pressure after the pressure-reducing valve.

    The sealing test shall be carried out using the actual working medium. When testing with air or water, the test is performed at 1.1 times the nominal pressure; when testing with steam, the test is performed at the maximum allowable working pressure at the operating temperature. The difference between the inlet pressure and the outlet pressure is required to be not less than 0.2MPa.

    The test method is as follows: after the inlet pressure is set, gradually adjust the adjusting screw of the valve so that the outlet pressure can change sensitively and continuously within the maximum and minimum range without stagnation or jamming. For steam pressure-reducing valves, after the inlet pressure is adjusted away, the valve is closed, and the valve is cut off. The outlet pressure is the highest and lowest values. Within 2 minutes, the increase in outlet pressure should comply with the requirements in Table 4.176-22. At the same time, the pipeline behind the valve The volume is qualified if it meets the requirements in Table 4.18;

    For water and air pressure-reducing valves, when the inlet pressure is adjusted and the outlet pressure is zero, close the pressure-reducing valve and conduct a sealing test. If there is no leakage within 2 minutes, it is qualified.

  4. Butterfly valve pressure test method

    The strength test of the pneumatic butterfly valve is the same as that of the stop valve. The sealing performance test of a butterfly valve should introduce the test medium from the medium flow end, the butterfly plate should be opened, the other end should be closed, and the injection pressure should reach the specified value; after checking that there is no leakage in the packing and other seals, close the butterfly plate, open the other end, and check the butterfly valve. If there is no leakage at the plate seal, it is qualified. Butterfly valves used to regulate flow do not need to undergo sealing performance testing.

  5. Pressure test method of plug valve

    When the plug valve is tested for strength, the medium is introduced from one end, the rest of the passage is closed, and the plug is rotated to the fully open working position for testing. If no leakage is found in the valve body, it is qualified.

    During the sealing test, the straight-through cock should keep the pressure in the cavity and the passage equal, rotate the plug to the closed position, check from the other end, and then rotate the plug 180° to repeat the above test;

    For three-way or four-way plug valves, the pressure in the cavity and one end of the passage should be kept equal. The plug should be rotated to the closed position in sequence. The pressure should be introduced from the right-angle end and checked at the same time from the other end.

    Before the plug valve test, a layer of thin, non-acidic lubricating oil is allowed on the sealing surface. If no leakage or expanding water droplets are found within the specified time, it is qualified. The test time of the plug valve can be shorter, generally 1 to 3 minutes, according to the nominal diameter.

    Plug valves for gas use should be tested for air tightness at 1.25 times the working pressure.

  6. Diaphragm valve pressure test method

    The diaphragm valve strength test introduces medium from either end, opens the valve disc, and closes the other end. After the test pressure rises to the specified value, the valve body and valve cover are deemed to be qualified if there is no leakage. Then, reduce the pressure to the sealing test pressure, close the valve disc, and open the other end for inspection. If there is no leakage, it is qualified.

  7. Pressure testing methods for stop valves and throttle valves

    For the strength test of stop valves and throttle valves, the assembled valve is usually placed in a pressure test stand, the valve disc is opened, the medium is injected to the specified value, and the valve body and valve cover are checked for sweating and leakage. Strength testing can also be performed on a single piece. The sealing test is only conducted on stop valves.

    During the test, the valve stem of the stop valve is in a vertical state, the valve disc is opened, and the medium is introduced from the bottom end of the valve disc to the specified value. Check the packing and gasket; after passing the test, close the valve disc and open the other end to check whether there is leakage. If both the strength and sealing tests of the valve are required, the strength test can be done first, then the pressure is reduced to the specified value of the sealing test, and the packing and gasket are inspected; then the valve disc is closed, and the outlet end is opened to check whether the sealing surface is leaking.

  8. Gate valve pressure test method

    The strength test of gate valves is the same as that of stop valves. There are two methods for gate valve sealing test.

    Open the gate to raise the pressure in the valve to the specified value; then close the gate, take out the gate valve immediately, check whether there is leakage at the seals on both sides of the gate, or directly inject the test medium into the plug on the valve cover to the specified value. Check the seals on both sides of the gate. The above method is called the intermediate pressure test. This method is not suitable for sealing tests on gate valves with nominal diameters below DN32mm.

    Another method is to open the gate to increase the valve test pressure to the specified value; then close the gate, open the blind plate at one end, and check whether the sealing surface is leaking. Turn the head back again and repeat the above test until it passes the test. The sealing test of the pneumatic gate valve packing and gasket should be carried out before the gate sealing test.

  9. Pressure test method of safety valve

    The strength test of the safety valve is the same as that of other valves using water. When testing the lower part of the valve body, the pressure is introduced from the inlet end, and the sealing surface is closed; when testing the upper part of the valve body and valve cover, the pressure is introduced from the outlet end, and the other ends are closed. The valve body and valve cover are deemed qualified if there is no leakage within the specified time.

    For sealing tests and constant pressure tests, the generally used media are safety valves for steam that use saturated steam as the test medium; valves for ammonia or other gases that use air as the test medium; valves for water and other non-corrosive liquids that use water as the test medium. Nitrogen is commonly used as the test medium for safety valves in some important locations.

    The sealing test is carried out with the nominal pressure value as the test pressure, and the number of tests is not less than twice. If there is no leakage within the specified time, it is qualified. There are two methods for leak detection: one is to seal each connection of the safety valve, and the other is to use butter to seal the tissue paper on the outlet flange. If the tissue paper bulges, it is a leak, and if it does not bulge, it is qualified;

Valve selection summary

The global valve is suitable for pipelines with loose requirements on fluid resistance, as well as pipelines or devices with high-temperature and high-pressure media. It is not suitable for media with high viscosity and particles or for use as vent valves and valves in low-vacuum systems.

Ball valves are suitable for low-temperature, high-pressure, and high-viscosity media. They are usually used in applications with strict sealing performance, wear, shrinkage channels, rapid opening and closing movements, large pressure differences, low noise, vaporization, small operating torque, and small fluid resistance in the pipeline.

The throttle valve is suitable for occasions where the medium temperature is low, and the pressure is high. It is not suitable for medium with high viscosity and solid particles and is not suitable for use as an isolation valve.

Plug valves are suitable for occasions requiring rapid opening and closing. They are generally not suitable for steam and medium with higher temperatures. They are used for mediums with lower temperatures and high viscosity and are also suitable for mediums with suspended particles.

Butterfly valves are generally used for media such as water, oil, and compressed air with temperatures ≤80°C and pressures ≤1.0MPa. Since the pressure loss of butterfly valves is relatively large compared to gate valves and ball valves, butterfly valves are suitable for pipeline systems with loose pressure loss requirements.

Check valves are generally suitable for clean media and are not suitable for media containing solid particles and high viscosity.

The diaphragm valve is suitable for oil, water, acidic media, and media containing suspended solids with an operating temperature of less than 200°C and a pressure of less than 1.0MPa. It is not suitable for organic solvents and strong oxidant media.

In pipeline systems in petroleum, chemical, and other industries, valve applications, operating frequencies, and services are ever-changing. The most important and critical equipment is the valve to control or eliminate low-level leaks. The reasonable selection of valves can reduce the device’s construction cost and ensure safe production operation.

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