1 Key points of valve selection

1.1 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 and operation control method, etc.;

1.2 Correctly select the type of valve

The correct selection of valve type is based on the designer’s full grasp of the entire production process and operating conditions. When selecting the valve type, the designer should first master the structural characteristics and performance of each valve;

1.3 Determine the end connection of the valve

Among threaded connection, flange connection and welding end connection, the first two are most commonly used. Threaded valves are mainly valves with a nominal diameter of less than 50mm. If the diameter size is too large, the installation and sealing of the connection are very difficult. Flange-connected valves are more convenient to install and disassemble, but they are heavier and more expensive than threaded valves, so they are suitable for pipe connections of various diameters and pressures. Welding connections are suitable for heavy load conditions and are more reliable than flange connections. However, it is difficult to disassemble and reinstall the valves connected by welding, so its use is limited to the occasions where it can usually operate reliably for a long time, or the use conditions are harsh and the temperature is high;

1.4 Selection of valve materials

In addition to considering the physical properties (temperature, pressure) and chemical properties (corrosiveness) of the working medium, the cleanliness of the medium (whether there are solid particles) should be mastered when selecting the materials of the valve shell, internal parts and sealing surface. In addition, the relevant regulations of the state and the user department should be referred to. Correct and reasonable selection of valve materials can obtain the most economical service life and the best performance of the valve. 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;

1.5 Others

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

2 Introduction to Common Valves

There are many types of valves, and the varieties are complex. The main types are gate valves, stop 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, among which the commonly used ones are gate valves, stop valves, throttle valves, plug valves, butterfly valves, ball valves, check valves, and diaphragm valves.

2.1 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 the passage of the fluid. Compared with the stop valve, the gate valve has better sealing performance, less fluid resistance, less effort in opening and closing, and has certain adjustment performance. It is one of the most commonly used shut-off valves. The disadvantages are large size, more complex structure than the stop valve, easy wear of the sealing surface, and difficult maintenance. It is generally not suitable for throttling. According to the thread position on the gate valve stem, it can be divided into two types: rising stem type and concealed stem type. According to the structural characteristics of the gate plate, it can be divided into two types: wedge type and parallel type.

2.2 Stop valve

The stop valve is a downward closing valve, in which the opening and closing parts (valve disc) 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.

2.3 Ball valve

The opening and closing parts of the ball valve are spheres with circular through holes, and the sphere rotates with the valve stem to realize the opening and closing of the valve. The ball valve has a simple structure, fast switching, convenient operation, small size, light weight, few parts, small fluid resistance, good sealing, and easy maintenance.

2.4 Throttle valve

Except for the valve disc, the throttle valve has basically the same structure as the stop valve. Its valve disc is a throttling component, and different shapes have different characteristics. The diameter of the valve seat should not be too large, because its opening height is small and the medium flow rate increases, thereby accelerating the erosion of the valve disc. The throttle valve has small dimensions, light weight, and good adjustment performance, but the adjustment accuracy is not high.

2.5 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, fast opening and closing, easy operation, small fluid resistance, few parts, and light weight. Plug valves are available in straight-through, three-way, and four-way types. Straight-through plug valves are used to cut off the medium, and three-way and four-way plug valves are used to change the direction of the medium or divert the medium.

2.6 Butterfly valve

The butterfly valve is a butterfly plate that rotates 90° around a fixed axis in the valve body to complete the opening and closing function. The butterfly valve is small in size, light in weight, simple in structure, and consists of only a few parts.

And it can be quickly opened and closed by rotating 90°, and it is easy to operate. 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 very small, so it has good flow control characteristics. Butterfly valves are divided into two types of sealing: elastic soft seal and metal hard seal. For elastic seal valves, 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, as well as for 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 occasions where flow and pressure drop vary greatly and good throttling performance is required. Metal seals can adapt to higher operating temperatures, while elastic seals have the defect of being limited by temperature.

2.7 Check valve

A check valve is a valve that can automatically prevent fluid backflow. The valve 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 that on the outlet side, the valve disc automatically closes under the action of factors such as fluid pressure difference and its own gravity to prevent fluid backflow. According to the structural form, it is divided into lift check valve and swing check valve. The lift check valve has better sealing than the swing check valve and greater fluid resistance. For the suction port of the pump suction pipe, a foot valve should be selected. Its function is: to fill the pump inlet pipe with water before starting the pump; to keep the inlet pipe and pump body full of water after stopping the pump in preparation for restarting. The foot valve is generally only installed on the vertical pipe at the pump inlet, and the medium flows from bottom to top.

2.8 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 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 need 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 current type.

3 Common valve selection instructions

3.1 Gate valve selection instructions

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

3.2 Stop valve selection instructions

The stop valve is suitable for pipelines with low requirements for fluid resistance, that is, the pressure loss is not considered much, as well as pipelines or devices with high-temperature and high-pressure media. It is suitable for steam and other media pipelines with DN < 200mm; small valves can use stop valves, such as needle valves, instrument valves, sampling valves, pressure gauge valves, etc.; stop valves have flow regulation or pressure regulation, but the regulation accuracy is not high, and the pipeline diameter is relatively small, so stop valves or throttle valves should be selected; for highly toxic media, bellows-sealed stop valves should be selected; but stop valves should not be used for media with high viscosity and media containing particles that are easy to precipitate, nor should they be used as vent valves and valves for low vacuum systems.

3.3 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 for powdered and granular media according to the material requirements of the seal; full-channel ball valves are not suitable for flow regulation, but are suitable for occasions requiring rapid opening and closing, which is convenient for emergency cut-off in accidents; ball valves are usually recommended for pipelines with strict sealing performance, wear, shrinkage channels, rapid opening and closing, high-pressure cut-off (large pressure difference), low noise, gasification phenomenon, small operating torque, and small fluid resistance; ball valves are suitable for light structures, low-pressure cut-off, and corrosive media; ball valves are also the most ideal valves for low-temperature and deep-cold media. For pipeline systems and devices for low-temperature media, low-temperature ball valves with valve covers should be selected; when using floating ball valves, the valve seat material should bear the load of the ball and the working medium. Large-diameter ball valves require greater force during operation, and DN≥200mm ball valves should use worm gear transmission; fixed ball valves are suitable for occasions with larger diameters and higher pressures; in addition, ball valves used for pipelines of highly toxic process materials and flammable media should have fireproof and anti-static structures.

3.4 Selection Instructions for Throttle Valve

Throttle valves are suitable for occasions with low medium temperature and high pressure, and are suitable for parts that need to adjust flow and pressure. They are not suitable for media with high viscosity and containing solid particles, and are not suitable for isolation valves.

3.5 Selection Instructions for Plug Valve

Plug valves are suitable for occasions that require fast opening and closing. They are generally not suitable for steam and high-temperature media. They are used for media with low temperature and high viscosity, and are also suitable for media with suspended particles.

3.6 Selection Instructions for Butterfly Valve

Butterfly valves are suitable for occasions with large diameters (such as DN﹥600mm) and short structural length requirements, as well as occasions that require flow regulation and fast opening and closing. They are generally used for media such as water, oil and compressed air with temperatures ≤80℃ and pressures ≤1.0MPa; since butterfly valves have a relatively large pressure loss compared to gate valves and ball valves, butterfly valves are suitable for pipeline systems with lax pressure loss requirements.

3.7 Selection Instructions for Check Valve

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

3.8 Diaphragm valve selection instructions

Diaphragm valves are suitable for oil, water, acidic media and media containing suspended matter with working temperature less than 200℃ and pressure less than 1.0MPa, but not for organic solvents and strong oxidants. Weir-type diaphragm valves are suitable for abrasive granular media. The flow characteristic table should be used for the selection of weir-type diaphragm valves. Straight-through diaphragm valves are suitable for viscous fluids, cement slurries and sedimentary media. Except for specific requirements, diaphragm valves should not be used on vacuum pipelines and vacuum equipment.