Selection and Pressure Test of Various Valves

In the fluid pipeline system, the control valve is the control element, and its investment accounts for about 30% - 50% of the pipeline project cost. The main functions of the valve are opening and closing, throttling, flow regulation, isolation of equipment and piping system, prevention of media backflow, regulation and discharge of pressure, etc. The valve is also the most complex component in the pipeline, which is generally assembled from multiple parts and components and has high technical content. With the rapid development of the petrochemical industry, most of the media in petrochemical production plants are characterized by toxicity, flammability, explosiveness and strong corrosivity. The operating conditions are complex and harsh. The operating temperature and pressure are high, and the start-up period is long. Once the valve fails, some of them will lead to media leakage, which will not only pollute the environment but also cause economic losses, and some of them will lead to plant shutdown or even fatal accidents. Therefore, in the pipeline design, scientific and reasonable selection of valves can not only reduce the construction cost of the device but also ensure the safe operation of production. This paper mainly introduces the selection methods of various commonly used valves, such as gate valves, globe valves, throttle valves, plug valves, ball valves and diaphragm control valves.

Key points of valve selection
1. Making clear the purpose of the valve procurement for your equipment or device
Determine the working conditions of the valve: the nature of the applicable medium, working pressure, working temperature, operation and control mode, etc.

2. Choosing the type of valves correctly
The correct selection of valve types is based on the designer's full mastery of the whole production process and operating conditions. When selecting valve types, the designer should first master the structural characteristics and performance of each valve.

3. Determining the end connection of the valve
Among threaded connection, flange connection and welded end connection, the first two are most commonly used. Threaded valves are mainly valves with a nominal size below 50 mm. If the size is too large, it is very difficult to install and seal the connecting parts. Flanged valves are more convenient to install and disassemble, but they are heavier and more expensive than threaded valves, so they are suitable for connecting pipes of various sizes and pressures. Welding is more reliable than flange connection. However, it is difficult to disassemble and reinstall the welded valve, so its use is limited to situations where it can normally run reliably for a long time, or where the service conditions are high and the temperature is high.
 
4. Selection of the valve material
When selecting the materials of the valve shell, internals and sealing surface, we should not only consider the physical properties (temperature and pressure) and chemical properties (corrosivity) of the working medium but also master the cleanliness of the medium (whether there are solid particles or not). Besides, we should also refer to the relevant regulations of the state and user departments. Proper selection of valve material can obtain the most economical service life and the best performance of the valve. The valve body material selection sequence is cast iron-carbon steel-stainless steel, and the sealing ring material selection sequence is rubber-copper-alloy steel -F4.
 
5. Others
In addition, the flow rate and pressure grade of the fluid flowing through the valve should be determined, and the appropriate valve should be selected by using the existing data (such as valve product catalog and valve product samples, etc.).
 
 
Introduction of common valves
There are many kinds and complex varieties of valves, including gate valves, globe valves, throttle valves, butterfly valves, cock valves, ball valves, electric valves, diaphragm valves, check valves, safety valves, pressure reducing valves, steam traps and emergency cut-off valve, among which gate valves, globe valves, throttle valves, cock valves, butterfly valves, ball valves, check valves and diaphragm valves are commonly used.
 
1. Gate valve
Gate valve refers to 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 the passage of fluid. Compared with the globe valve, the gate valve has better sealing performance, less fluid resistance, less labor-saving opening and closing, and certain adjustment performance, so it is one of the most commonly used cutoff valves. Disadvantages are large size, more complex structure than the globe valve, easy wear of sealing surface and difficult maintenance, which are generally unsuitable for throttling. According to the thread position on the valve stem, the gate valve can be divided into open stem type and concealed stem type. According to the structural characteristics of the gate, it can be divided into wedge type and parallel type.
 
2. Globe valve
Globe valve is a downward closing valve, and the opening and closing part (disc) is 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 regulating performance, poor sealing performance, simple structure, convenient manufacture and maintenance, large fluid resistance and low price. It is a commonly used block valve, which is 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 round-through hole, and the ball rotates with the valve stem to realize the opening and closing of the valve. The ball valve has a simple structure, quick opening and closing, convenient operation, small volume, lightweight, few parts, small fluid resistance, good sealing performance and convenient maintenance.
 
4. Throttle valve
The throttle valve is basically the same as the globe valve except for the valve flap. Its valve flap is a throttling component, and different shapes have different characteristics. The size of the valve seat should not be too large, because its opening height is small and the velocity of the medium increases, thus accelerating the erosion of the valve flap. The throttle valve is small in size, light in weight and good in adjustment performance, but its adjustment precision is not high.
 
5. Plug valve
A plug valve is a valve that uses a plug body with a through-hole as an opening and closing part, and the plug body rotates with the valve stem to realize opening and closing. The plug valve has a simple structure, quick opening and closing, convenient operation, small fluid resistance, few components and lightweight. Plug valves are straight-through, three-way and four-way valves. Straight plug valves are used to cut off the medium, while three-way and four-way plug valves are used to change the direction of the medium or carry out medium diversion.
 
6. Butterfly valve
The butterfly valve rotates 90° around the fixed shaft in the valve body, so as to complete the opening and closing function. The butterfly valve is small in size, light in weight and simple in structure, and only consists of a few parts.

And it only needs to rotate by 90 degrees. It has a quick opening and closing and a simple operation. 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, so the pressure drop generated by the valve is very small. It has good flow control characteristics. Butterfly valves are divided into an elastic soft seal and a metal hard seal. For elastic sealing valves, the sealing ring can be embedded on the valve body or attached to the periphery of the butterfly plate, which has good sealing performance and can be used for throttling, medium vacuum pipeline and corrosive medium. Generally, the valve with a metal seal has a longer service life than the valve with an elastic seal, but it is difficult to seal completely. It is usually used in situations where the flow rate and pressure drop vary greatly and the throttling performance is good. The metal seal can adapt to higher working temperatures, while the elastic seal has the defect of being limited by temperature.

7. Check valve
A check valve is a valve that can automatically prevent fluid from flowing backward. The flap 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 at the inlet side is lower than that at the outlet side, the valve flap will automatically close under the action of fluid pressure difference, gravity and other factors to prevent the fluid from flowing backward. According to the structure, it is divided into a lifting check valve and swing check valve. The lifting type has better sealing performance and greater fluid resistance than the rotary type. For the suction inlet of the pump suction pipe, the bottom valve should be selected. Its function is: before starting the pump, pour water to make the pump inlet pipe full of water. After stopping the pump, keep the inlet pipe and pump body filled with water for restart. 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 sandwiched between the valve body and valve cover.
 
The middle protruding part of the diaphragm is fixed on the valve stem, and the valve body is lined with rubber. Because the medium does not enter the inner cavity of the valve cover, the valve stem does not need a stuffing box. A diaphragm valve has a simple structure, good sealing performance, convenient maintenance and low fluid resistance. Diaphragm valves are divided into weir type, straight-through type, right-angle type and direct-flow type.
 
Description of common valve selection
1. Description of gate valve selection
In general, gate valves should be preferred. Gate valves are not only suitable for steam, oil and other media, but also suitable for media containing granular solids and high viscosity, and valves for venting and low vacuum systems. For media with solid particles, the valve body of the gate valve shall be provided with one or two purge holes. For a low-temperature medium, a special gate valve for low temperature shall be selected.
 
2. Description of globe valve selection
Globe valves are suitable for pipelines with lax requirements on fluid resistance, i.e. pipelines or devices with low-pressure loss and high temperature and high-pressure media, and pipelines with dn < 200mm and other media. Small valves can be globe valves, such as needle valves, instrument valves, sampling valves, pressure gauge valves, etc. The globe valve has flow adjustment or pressure adjustment, but the adjustment precision is not high, and the pipe diameter is relatively small, so the globe valve or throttle valve should be selected. For highly toxic media, the globe valve sealed by bellows should be selected. However, globe valves should not be used for medium with high viscosity and medium containing particles that are easy to precipitate, nor should they be used as vent valves and valves for low vacuum systems.
 
3. Description of ball valve selection
The ball valve is suitable for medium with low temperature, high pressure and high viscosity. Most ball valves can be used in media with suspended solid particles, and can also be used in powder and granular media according to sealing material requirements. All-channel ball valve is not suitable for flow regulation but is suitable for occasions requiring quick opening and closing, so as to realize emergency cut-off. Generally, it is recommended to use ball valves in pipelines with strict sealing performance, wear, necking channel, quick opening and closing action, high-pressure cut-off (large pressure difference), low noise, gasification phenomenon, low operating torque and low fluid resistance.
 
The ball valve is suitable for light structure, low-pressure cut-off and corrosive medium. The ball valve is also the most ideal valve for low temperature and cryogenic medium, and a low-temperature ball valve with valve cover should be selected for the pipeline system and device of the cryogenic medium. When selecting a floating ball valve, its valve seat material should bear the load of the ball and working medium, and the large-diameter ball valve needs a large force when operating, DN≥ The 200mm ball valve should be driven by a worm gear. The fixed ball valve is suitable for occasions with a larger caliber and higher pressure. In addition, the ball valves used for pipelines of highly toxic materials and combustible media in the process shall have fireproof and antistatic structures.
 
4. Description of throttle valve selection
A throttle valve is suitable for places with low temperature and high pressure of the medium, places where flow and pressure need to be adjusted, and is not suitable for medium with high viscosity and containing solid particles, and is not suitable for being used as a block valve.
 
5. Description of plug valve selection
Plug valves are suitable for occasions requiring quick opening and closing, generally not suitable for steam and high-temperature media, but for low-temperature and high-viscosity media, and also suitable for media with suspended particles.
 
6. Description of butterfly valve selection
Butterfly valves are suitable for occasions with large-caliber (e.g., DN < 600 mm), short structural length requirements, and quick opening and closing requirements, and are generally used for temperature ≤ 80℃, pressure ≤ 1.0MPa medium such as water, oil and compressed air; Compared with gate valves and ball valves, butterfly valves have a large pressure loss, so they are suitable for piping systems with lax pressure loss requirements.
 
7. Check valve selection instructions
Check valves are generally suitable for clean media, but not for media containing solid particles and high viscosity. When ≤40mm, the lifting check valve should be adopted (only allowed to be installed on horizontal pipes). When dn = 50 - 400 mm, a swing check valve should be adopted (it can be installed on both horizontal and vertical pipes if installed on vertical pipes, the flow direction of the medium should be from bottom to top). When DN≥450mm, a buffer check valve should be adopted. When dn = 100 - 400 mm, the wafer check valve can also be selected. The swing check valve can be made to have a high working pressure, and the PN can reach 42MPa. It can be applied to any working medium and any working temperature range according to the different materials of the shell and seal. The medium is water, steam, gas, corrosive medium, oil products, medicines, etc. The working temperature of the medium is between-196℃ and 800℃.
 
8. Description of diaphragm valve selection
The diaphragm valve is suitable for oil products, water, acidic medium and medium containing suspended matter with a working temperature of less than 200℃ and pressure less than 1.0MPa, but not for organic solvent and strong oxidant medium. The Weir diaphragm valve should be selected for abrasive media, and its flow characteristic table should be referred to when selecting the weir diaphragm valve. A straight-through diaphragm valve should be selected for viscous fluid, cement slurry and precipitating medium. Except for specific requirements, diaphragm valves should not be used in vacuum pipelines and vacuum equipment.
 
Pressure test methods of various valves
Under normal circumstances, industrial valves are not subjected to strength tests when in use, but the repaired valve body and valve cover or corroded valve body and valve cover should be subjected to a strength test. For safety valves, their setting pressure, setting pressure and other tests shall comply with the specifications and relevant regulations. The valve installation shall be tested for strength and tightness. Common media for valve pressure tests include water, oil, air, steam, nitrogen, etc. The pressure test methods for various industrial valves including pneumatic valves are as follows:
 
1. Pressure test method of the ball valve
The strength test of the pneumatic ball valve shall be carried out with the ball half-open.

(1) Sealing test of floating ball valve: The valve is in a semi-open state, with one end introduced with test medium and the other end closed. Turn the ball several times and open the closed end for inspection when the valve is in the closed state. Check the sealing performance of the packing and gasket at the same time, without leakage. Then introduce the test medium from the other end and repeat the above test.
 
(2) Sealing test of fixed ball valve: Before the test, rotate the ball several times without load, and the fixed ball valve is in a closed state. The test medium is introduced from one end to the specified value. Check the sealing performance of the lead-in end with a pressure gauge. The accuracy of the pressure gauge is 0.5 - 1, and the measuring range is 1.5 times the test pressure. Within the specified time, no decompression phenomenon is qualified. Introduce the test medium from the other end and repeat the above test. Then, put the valve in a semi-open state, with both ends closed, and the inner cavity filled with medium. Check the packing and gasket under the test pressure, and no leakage is allowed.
 
(3) The three-way ball valve shall be tested for tightness at each position.
 
2. Pressure test method of the check valve
Test state of check valve: the disc axis of the lifting check valve is in a position vertical to the horizontal; The channel axis and disc axis of the swing check valve are approximately parallel to the horizontal line.
 
During the strength test, the test medium shall be introduced from the inlet end to the specified value, and the other end shall be closed. If there is no leakage in the valve body and valve cover, it shall be qualified.
 
The test medium is introduced from the outlet end of the leak tightness test, and it is qualified to check that there is no leakage at the sealing surface, packing and gasket at the inlet end.
 
3. Pressure test method of pressure reducing valve
(1) The strength test of the pressure reducing valve is generally assembled after the single piece test, and can also be tested after assembly. Test duration: 1 min, DN<50mm; DN65 - 150 mm is greater than 2 min; Greater than 3min for DN>150mm. After the corrugated pipe is welded with the components, the strength test shall be conducted with air, which is 1.5 times the maximum pressure after the pressure-reducing valve is applied.
 
(2) The tightness test shall be conducted according to the actual working medium. When testing with air or water, test with 1.1 times of nominal pressure; The steam test shall be conducted at the maximum allowable working pressure at the working temperature. The difference between inlet pressure and outlet pressure shall not be less than 0.2MPa. Test method: 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 range of maximum and minimum, without stagnation or blockage. For the steam pressure reducing valve, after the inlet pressure is transferred, close the block valve after the valve, and the outlet pressure is the highest and lowest. Within 2 minutes, the increase of the outlet pressure shall meet the requirements in Table 4.176-22, and the pipe volume after the valve shall meet the requirements in Table 4.18 as qualified. For the water and air pressure reducing valve, when the inlet pressure is set and the outlet pressure is zero, close the pressure reducing valve for tightness test, and it is qualified if there is no leakage within 2min.
 
4. Pressure test method of the butterfly valve.
The strength test of the pneumatic butterfly valve is the same as that of the stop valve. In the sealing performance test of the butterfly valve, the test medium should be introduced from the medium flow end, the butterfly plate should be opened and the other end closed, and the injection pressure should reach the specified value. After checking that there is no leakage in packing and other seals, close the butterfly plate and open the other end, and check that there is no leakage in the butterfly plate seal. As a butterfly valve for regulating flow, the sealing performance test is not required.
 
5. Pressure test method of plug valve
(1) when the plug valve is tested for strength, the medium is introduced from one end, the rest passages are closed, and the plug is rotated to all working positions in turn for testing, and the valve body is qualified if no leakage is found.

(2) During the sealing test, the straight-through cock should keep the pressure in the cavity equal to that in the passage, rotate the plug to the closed position, check from the other end, and then rotate the plug by 180 to repeat the above test; Three-way or four-way plug valve should keep the pressure in the cavity equal to that at one end of the passage, rotate the plug to the closed position, in turn, introduce the pressure from the right-angle end, and check at the same time from the other end.
 
Before the plug valve test, a layer of non-acidic thin lubricating oil is allowed to be coated on the sealing surface, and no leakage or enlarged water droplets are found within the specified time. The test time of the stopcock valve can be shorter, which is generally l - 3min according to the nominal diameter. The gas cock valve shall be tested for airtightness at 1.25 times of working pressure.
 
6. Pressure test method of the diaphragm valve.
In the diaphragm valve strength test, the medium is introduced from either end, the disc is opened, and the other end is closed. After the test pressure rises to the specified value, it is considered as qualified that there is no leakage in the valve body and valve cover. Then depressurize to the sealing test pressure, close the valve disc and open the other end for inspection. No leakage is considered qualified.
 
7. Pressure test method of globe valve and throttle valve.
And throttle valve strength test usually put the assembled valve in the pressure test rack, open the disc, inject the medium to the specified value, and check whether the valve body and valve cover are sweating and leaking. A strength test can also be carried out on one piece. The sealing test is only for the globe valve. During the test, the valve stem of the cut-off valve is in a vertical state, the disc is open, and the medium is introduced to the specified value from the bottom end of the disc. Check the packing and gasket. After being qualified, close the disc and open the other end to check whether there is leakage. If both the strength test and the sealing test of the valve are to be done, 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 can be inspected. Then close the disc and open the outlet end to check whether the sealing surface leaks.
 
8. Pressure test method of gate valve.
The strength test of the gate valve is the same as that of the globe valve. There are two methods for sealing tests of gate valves.

(1) the gate is opened to raise the pressure in the valve to the specified value; Then close the gate, immediately take out the gate valve, check whether there is leakage in 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, and check the seals on both sides of the gate. The above method is called the intermediate pressure test. This method is not suitable for the sealing test of gate valves with a nominal diameter less than DN32mm.

(2) Another method is to open the gate to increase the valve test pressure to the specified value; Then close the shutter, open the blind plate at one end, and check whether the sealing surface leaks. Again, do the above-repeated tests until they are qualified.
 
Sealing test at packing and gasket of pneumatic gate valve shall be carried out before sealing test of gate valve.
 
9. Pressure test method of safety valve.
(1) the strength test of the safety valve is the same as other valves. When testing the lower part of the valve body, the pressure is introduced from the inlet I=I end, and the sealing surface is closed; When testing the upper part of the valve body and the valve cover, the pressure is introduced from the El end, and the other ends are closed. The valve body and valve cover are qualified if there is no leakage within the specified time.

(2) For sealing test and constant pressure test, the commonly used medium is: saturated steam is used as the test medium for the safety valve for steam; The valve for ammonia or other gases uses air as test medium; Water and other non-corrosive liquids use water as a test medium. Nitrogen is often used as a test medium for safety valves in some important positions.
 
The seal shall be tested with the nominal pressure value as the test pressure, and the times shall not be less than twice, and no leakage within the specified time shall be deemed as qualified. There are two methods for leak detection: first, seal each joint of the safety valve, and seal the thin paper on the El flange with butter. If the thin paper bulges, it will be a leak, and if it does not bulge, it will be qualified; Second, seal the thin plastic plate or other plates on the lower part of the outlet flange with butter, seal the valve disc with water, and check that the water does not bubble. The times of constant pressure and back seat pressure test of safety valve shall not be less than 3 times, and it shall be qualified if it meets the requirements. See GB/t 12242–1989 safety valve performance test method for various performance tests of safety valves.
 
Summary of valve selection
From the above analysis, it can be seen that gate valves should be preferred in general.
 
Globe valves are suitable for pipelines with lax requirements on fluid resistance, and pipelines or devices with high temperature and high-pressure media. They are not suitable for media with high viscosity and containing particles, nor should they be used as vent valves and valves for low vacuum systems.
 
Ball valves are suitable for low temperature, high pressure and high viscosity media, and are usually used in pipelines with strict sealing performance, wear, necking, quick opening and closing action, large pressure difference, low noise, gasification, small operating torque and small fluid resistance.
 
The throttle valve is suitable for occasions where the medium temperature is low and the pressure is high and is not suitable for the medium with high viscosity and containing solid particles.
 
Plug valves are suitable for occasions requiring quick opening and closing, generally not suitable for steam and high-temperature media, but for low-temperature and high-viscosity media, and also suitable for media with suspended particles.
 
Generally, butterfly valves are used for water, oil and compressed air with temperature ≤80℃ and pressure ≤1.0MPa. Compared with gate valves and ball valves, butterfly valves have a large pressure loss, so they are suitable for piping systems with lax pressure loss requirements.
 
Check valves are generally suitable for clean media, but not for media containing solid particles and high viscosity.
The diaphragm valve is suitable for oil products, water, acidic medium and medium containing suspended matter with a working temperature of less than 200℃ and pressure less than 1.0MPa, but not for organic solvent and strong oxidant medium.
 
In the pipeline system of petroleum, chemical industry and other industries, the application, operation frequency and service of valves are ever-changing, and the most important and critical equipment is the valves to control or eliminate slight leakage. Reasonable selection of valves can reduce the construction cost and ensure the safe operation of production.