1 Valve selection key points
1.1 Clearly define the purpose of the valve in the equipment or device.
Determine the operating conditions of the valve, including the nature of the applicable medium, working pressure, working temperature, and control method.
1.2 Correctly select the type of valve.
The correct selection of valve types is a prerequisite for designers to fully grasp the entire production process and operating conditions. When selecting valve types, designers should first understand the structural characteristics and performance of each type of valve.
1.3 Determine the end connection of the valve.
Among threaded connections, flange connections, and welded end connections, the first two are the most commonly used. Threaded connections are mainly used for valves with nominal diameters below 50mm. If the diameter is too large, the installation and sealing of the connection part can be very difficult. Flange-connected valves are more convenient to install and disassemble, but they are bulkier and more expensive than threaded valves. Therefore, they are suitable for pipe connections of various diameters and pressures. Welded connections are suitable for conditions with higher loads and are more reliable than flange connections. However, the disassembly and reinstallation of welded valves are more difficult, so their use is limited to situations where long-term reliable operation is possible or conditions of high load and temperature.
1.4 Valve material selection
When selecting the material of the valve body, internal components, and sealing surfaces, consider not only the physical properties (temperature, pressure) and chemical properties (corrosiveness) of the working medium but also the cleanliness of the medium (presence of solid particles). In addition, refer to relevant regulations from national and regulatory authorities. The correct and rational selection of valve materials can ensure the most economical service life and optimal performance of the valve. The order of valve body materials is: cast iron – carbon steel – stainless steel, and the order of sealing ring materials is: rubber – copper – alloy steel – F4.
1.5 Others
In addition, determine the flow rate and pressure level of the fluid flowing through the valve. Use existing data, such as valve product catalogs and samples, to select the appropriate valve.
2 Introduction to Commonly Used Valves
There are many types and varieties 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 trap valves, and emergency shut-off valves, among which the commonly used ones are gate valves, globe valves, throttle valves, plug valves, butterfly valves, ball valves, check valves, and diaphragm valves.
2.1 Gate Valve
A gate valve refers to a valve with a closing element (valve disc) driven by a valve stem to move up and down along the sealing surface of the valve seat, allowing or cutting off the fluid passage. Gate valves have better sealing performance and lower fluid resistance than globe valves. They are easy to open and close, and have certain regulating capabilities, making them one of the most commonly used shut-off valves. The disadvantages include large size, complex structure compared to globe valves, susceptible to wear on the sealing surface, and difficult maintenance. Gate valves are classified into rising stem and non-rising stem types based on the thread position on the valve stem. They can also be classified into wedge and parallel types based on the structure of the gate.
2.2 Globe Valve
A globe valve is a downward-closing valve where the closing element (valve disc) is driven by the valve stem to move up and down along the axis of the valve seat (sealing surface). Compared to gate valves, globe valves have good regulating performance, poor sealing performance, simple structure, convenient manufacturing and maintenance, higher fluid resistance, and lower cost. They are commonly used in medium to small diameter pipelines.
2.3 Ball Valve
The closing element of a ball valve is a ball with a circular through hole. The ball rotates with the valve stem to achieve the opening and closing of the valve. Ball valves have a simple structure, quick operation, easy operation, small size, light weight, few components, low fluid resistance, good sealing performance, and easy maintenance.
2.4 Throttle Valve
The throttle valve has a structure similar to the globe valve, with the throttle element as the valve disc. Different shapes have different characteristics, and the diameter of the valve seat should not be too large. Throttle valves have small external dimensions, light weight, good regulating performance, but low adjustment accuracy.
2.5 Plug Valve
The plug valve uses a plug with a through hole as the closing element. The plug rotates with the valve stem to achieve the opening and closing of the valve. Plug valves have a simple structure, quick operation, easy operation, low fluid resistance, few components, and light weight. Plug valves can be straight-through, three-way, or four-way. Straight-through plug valves are used for cutting off the medium, while three-way and four-way plug valves are used for changing the direction of the medium or diverting the medium.
2.6 Butterfly Valve
A butterfly valve has a disc that rotates 90° within the valve body to achieve opening and closing. Butterfly valves are compact, lightweight, and have a simple structure consisting of only a few components. They can be quickly opened or closed with a 90° rotation, making them easy to operate. When fully open, the thickness of the butterfly disc is the only resistance to the flow of the medium, resulting in a small pressure drop. Butterfly valves are available in two sealing types: resilient-seated and metal-seated. Resilient-seated valves, with the seal ring embedded in the valve body or attached to the periphery of the butterfly disc, have good sealing performance and can be used for throttling as well as in medium vacuum pipelines and corrosive media. Valves with metal seals generally have a longer lifespan than those with resilient seals but are difficult to achieve complete sealing. They are typically used in situations with large variations in flow and pressure drop, where high throttling performance is required. Metal-sealed valves can withstand higher working temperatures, while resilient-sealed valves have temperature limitations.
2.7 Check Valve
A check valve is a valve that automatically prevents fluid backflow. The valve disc of a check valve opens under the pressure of the fluid, allowing the fluid to flow from the inlet side to the outlet side. When the pressure on the inlet side is lower than the outlet side, the valve disc, under the influence of fluid pressure difference, gravity, and other factors, automatically closes to prevent fluid backflow. Check valves can be classified into lift-type and swing-type based on the structural form. Lift-type check valves have better sealing performance but greater fluid resistance. For the suction pipe of a pump, it is advisable to use a foot valve, which serves to prime the pump by filling the inlet pipe with water before starting the pump and maintains the inlet pipe and pump body filled with water after the pump is stopped. Foot valves are generally only installed on vertical pipes at the pump inlet and the medium flows from bottom to top.
2.8 Diaphragm Valve
The closing element of a diaphragm valve is a rubber diaphragm 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 cavity inside the valve cover, the valve stem does not require a packing box. Diaphragm valves have a simple structure, good sealing performance, low fluid resistance, and are easy to maintain. Diaphragm valves can be classified into weir-type, straight-through, right-angle, and straight-flow types.
3 Common Valve Selection Guidelines
3.1 Gate Valve Selection Guidelines
In general, gate valves should be the preferred choice. Gate valves are suitable for media such as steam, oil, and those containing granular solids and high viscosity. They are also suitable for valves in venting and low vacuum systems. For media with solid particles, gate valves should have one or two blowdown holes on the valve body. For low-temperature media, special low-temperature gate valves should be selected.
3.2 Globe Valve Selection Guidelines
Globe valves are suitable for pipelines where fluid resistance requirements are not strict, considering pressure loss is not significant. They are also suitable for high-temperature and high-pressure media pipelines or devices, especially for steam pipelines with DN < 200mm. Small valves such as globe valves, needle valves, instrument valves, sampling valves, and pressure gauge valves can be selected. Globe valves can be used for flow control or pressure regulation, but with low precision requirements and small pipe diameters. They are not suitable for toxic media, viscous media, and media prone to sedimentation. Globe valves are not recommended for use as vent valves or in low vacuum systems.
3.3 Ball Valve Selection Guidelines
Ball valves are suitable for low-temperature, high-pressure, and highly viscous media. Most ball valves can be used for media with suspended solid particles, and depending on the sealing material requirements, they can also be used for powdery and granular media. Full-bore ball valves are not suitable for flow control but are suitable for quick opening and closing in emergency shutdown situations. Ball valves are recommended for pipelines with strict sealing performance, wear, reduced throat passages, rapid opening and closing actions, high-pressure cutoff (large pressure difference), low noise, gasification phenomena, low operating torque, and low fluid resistance. Ball valves are suitable for light structures, low-pressure cutoff, and corrosive media. For low-temperature and cryogenic media systems, low-temperature ball valves with added valve covers are recommended. When using floating ball ball valves, the seat material should bear the load of the ball and the working medium. For large-diameter ball valves, a worm gear transmission form should be selected for operation with DN ≥ 200mm. Fixed ball ball valves are suitable for larger diameters and higher pressure conditions. Additionally, ball valves used in pipelines for processing toxic materials and combustible media should have fire and antistatic structures.
3.4 Throttle Valve Selection Guidelines
Throttle valves are suitable for occasions where the medium temperature is low and the pressure is high, and they are suitable for parts where flow and pressure need to be adjusted. Throttle valves are not suitable for high-viscosity media and media containing solid particles, and they should not be used as isolation valves.
3.5 Plug Valve Selection Guidelines
Plug valves are suitable for occasions that require quick opening and closing, generally not suitable for steam and high-temperature media. They are suitable for low-temperature and highly viscous media and can also be used for media with suspended particles.
3.6 Butterfly Valve Selection Guidelines
Butterfly valves are suitable for large diameters (e.g., DN > 600mm), short structural lengths, and occasions where flow control and quick opening and closing are required. They are generally used for water, oil, and compressed air media with temperatures ≤ 80°C and pressures ≤ 1.0MPa. Due to the relatively large pressure loss compared to gate and ball valves, butterfly valves are suitable for systems with less stringent pressure loss requirements.
3.7 Check Valve Selection Guidelines
Check valves are generally suitable for clean media and should not be used for media containing solid particles and high viscosity. For DN ≤ 40mm, lift check valves (only allowed to be installed on horizontal pipelines) are suitable. For DN = 50-400mm, swing check valves with lifting mechanism are recommended (can be installed on both horizontal and vertical pipelines, with medium flow from bottom to top when installed on vertical pipelines). For DN ≥ 450mm, buffer-type check valves are recommended. For DN = 100-400mm, dual-plate check valves can also be considered. Swing check valves can be designed for high working pressures, with a PN of up to 42MPa. Depending on the material of the body and sealing components, swing check valves can be applied to any working medium and temperature range between -196 and 800°C. Media may include water, steam, gas, corrosive media, oil, pharmaceuticals, etc.
3.8 Diaphragm Valve Selection Guidelines
Diaphragm valves are suitable for working temperatures below 200°C and pressures below 1.0MPa for oil, water, acidic media, and media with suspended solids. They are not suitable for organic solvents and strong oxidizing agents. For abrasive particulate media, weir-type diaphragm valves are preferred, and when selecting them, refer to their flow characteristics table. For viscous fluids, cement slurry, and precipitating media, straight-through diaphragm valves are suitable. Except for specific requirements, diaphragm valves should not be used on vacuum pipelines and vacuum equipment.