The Selection of Common Valves

1. In the selection of gate valves for chemical production facilities, gate valves are a common type of valve that will not change the direction of liquid flow. When the gate valve is fully open, the resistance coefficient is relatively small, and the range of diameters, pressures, and temperatures used is relatively wide. In the case of the same caliber, compared with globe valves, it can save installation space, so its selection and use range are more extensive. However, gate valves also have some drawbacks, such as high height and long opening and closing time. 

During the opening and closing process, there is a possibility of erosion affecting the sealing surface. There are certain standard requirements for the cleanliness of the medium. It is difficult to manufacture using non-metallic corrosion-resistant materials in production. In the open state of the gate valve, located at the back of the gate plate, the medium can form eddy currents, causing erosion and vibration of the gate, and the sealing surface of the valve seat is more prone to corresponding damage.

Therefore, when it is necessary to adjust the flow rate, gate valves are not suitable. When effectively controlling the opening and closing of fluids in fully open and trapped states, gate valves are suitable for use.

According to the thread position of the valve stem, gate valves are divided into two forms: visible and hidden stem. In corrosive media environments, rising stem gate valves are preferred, while hidden stem gate valves can be used when facing low-pressure and non-corrosive media environments.
 
According to the gate structure, gate valves can be divided into wedge and parallel gate valves. Wedge gate valves can be divided into two types: single and double gate valves. Parallel gate valves are mainly used in oil and gas transportation systems, but are relatively less commonly used in chemical plants. Regarding the application of gate valves, if the medium is steam, oil and gas, or high-temperature oil products, or if the switch position is frequent, gate valves can be reasonably selected. However, they are not suitable for situations where the medium is prone to coking. Under conditions of high temperature media that are prone to coking, wedge type single plate gate valves can be reasonably selected, while wedge type double gate valves can be reasonably selected for media such as steam and oil.

2. Ball valves can be selected reasonably for low temperature, high temperature, and high viscosity media. In the presence of suspended solid particles in the medium, most ball valves exhibit good applicability. Using sealing materials as the standard, ball valves can also be reasonably selected in granular, powdery and other media. For full channel ball valves, they are less commonly used in flow regulation and more commonly used in rapid opening and closing situations, which can quickly complete emergency shutdown operations in case of accidents. 

In chemical pipelines, ball valves can be reasonably used if there are strict standards for sealing, wear, etc., or if high-pressure cut-off, rapid opening and closing actions, and contraction channels are required. In terms of media, such as corrosiveness, lightweight structure, and low-pressure shut-off, ball valves are also applicable. 

Ball valves can also play an important role in low-temperature and cryogenic media. In the pipeline system and equipment of low-temperature media, low-temperature ball valves should be used and equipped with corresponding valve covers. The use of floating ball valves has corresponding standard requirements for valve seat materials, which need to be able to effectively bear the load generated by the ball and working medium. When using large-diameter ball valves, due to the high force required for operation, turbine transmission is the main form when DN is more than 200mm. 

When the caliber and pressure standards are both large, a fixed ball valve should be used. In addition, ball valves in pipelines containing flammable and toxic media should be equipped with corresponding fire and anti-static structures to fully ensure safety.

3. When selecting check valves for cleaning media conditions, check valves should be used more frequently. Check valves have certain requirements for the cleanliness of the medium. When DN is less than 40mm, it is suitable to use a lift check valve, which should be located in a horizontal pipeline for reasonable design and installation. When the DN is between 50 to 400mm, it is suitable to use a rotary lift check valve. Both horizontal and vertical pipelines can be designed and installed reasonably.

If it is located in a vertical pipeline, it is necessary to ensure that the flow direction of the medium is from bottom to top. When DN is more than 450mm, a buffer check valve is suitable for use. When the DN is between 100 and 400mm, clamp-type check valves can also be selected. The use of swing check valves can withstand high working pressures, with a PN standard of up to 42MPa. Based on the differences in materials of each part, check valves can be considered for use in various working media and temperature ranges, such as steam, corrosive media, and oil products, with temperatures ranging from -196 to 800 ℃.

When there is no high standard requirement for fluid resistance during the selection of globe valves in chemical design, globe valves can be considered. In the face of high-temperature and high-pressure media, globe valves also exhibit a relatively wide range of applications. If the DN is less than 200mm, a shut-off valve can be used in the case of a vaporization pipeline.

In specific practical aspects, small valves such as sampling valves, needle valves, and instrument valves, or valves used in low vacuum systems and exhaust valves, can also use globe valves. As for globe valves, their function is to regulate and control pressure and flow rate. Therefore, there is no strict standard requirement for regulating accuracy. When the pipeline diameter is not large, globe valves can be preferred. When the medium is toxic, in order to fully ensure the sealing performance, it is necessary to use a bellows globe valve. The cleanliness of the medium is poor, and it is not suitable for globe valves when there are large particle precipitates or moderate viscosity. 

During the chemical design phase, when there are strict requirements for sealing performance standards, priority should be given to using globe valve design. Due to the presence of sealing rings, good sealing performance can be fully guaranteed, and gasket replacement is relatively easy. Only special attention needs to be paid to the installation of globe valves, and the valve direction should be kept the same as the flow direction of the medium.

5. The selection of throttle valves is based on the analysis of structural form. Throttle valves and globe valves have obvious similarities, but the difference is that the throttle valves do not have corresponding throttle components. Considering the overall situation of the valve market, throttle valves are relatively small in size but have shown significant advantages in safety supervision and energy conservation. However, throttle valves also have certain shortcomings, such as the need to improve the accuracy of regulation and control. Therefore, during the chemical design phase, throttling valves can be designed for media with low temperature and high pressure. Throttle valves can also be used to regulate flow, pressure, and other requirements. In situations where the cleanliness of the medium is poor or the viscosity is high, it is not suitable to use a throttle valve.

6.​ The selection of butterfly valves: Butterfly valves have relatively low fluid resistance, quick opening and closing, and significant advantages in weight, structure, and size, which can effectively save material costs. In the design of cutting and throttling, choosing butterfly valves is more suitable for the demand, but in high flow regulation, due to the influence of sealing materials, there are also certain limitations in the range of units of use.

Butterfly valves can also be selected in the design of liquid media containing suspended solids and large-diameter pipelines. Due to continuous optimization and improvement in materials, design, and processes, the performance of butterfly valves has also undergone corresponding changes, and their applicability has become more extensive. Butterfly valves, with their economy and flow regulation performance, can be given priority consideration when applicable to butterfly valves. 

Designing butterfly valves is also very beneficial for situations where the design pressure is not high, the pipeline diameter is large, and it opens and closes quickly. Butterfly valves can be divided into soft and hard seal types, and the specific applicable type needs to be selected based on the fluid medium temperature. It should be noted that the soft seal performance is significantly better than the hard seal. Among commonly used butterfly valves, the most common is the large-diameter valve, which is mainly used for media such as crude oil, oil products, water, etc. The temperature is within 300 ℃ and the pressure is within 1.0MPa.

7. The selection of plug valves: Plug valves are the earliest valve components designed and used, with good sealing performance, capable of achieving bidirectional sealing. It is easy to open and close, and it has a long service life. It can be used with emphasis in material systems with high harm, but the opening and closing torque is relatively large, resulting in relatively high price and cost. There will be no accumulation of liquid in the chamber of the plug valve, and it will not have a polluting effect on the materials of the intermittent device. In chemical pipeline design, the use of plug valves can complete cutting and distribution operations, and can change the flow direction of the medium. 

Plug valves are also quite common in multi-channel structural design. Valves can connect multiple flow channels, simplifying pipeline system design and avoiding the use of too many valves and connecting accessories. Plug valves are divided into two types, namely non-lubricated and lubricated. Under the action of forced lubrication, the oil sealed plug valve can form an oil film, which not only has higher sealing performance, but also relatively effortless opening and closing, and reduces damage to the sealing surface. However, it is important to focus on the issue of material contamination and maintain it regularly. In general, non-lubricating use is relatively common. For plug valves, they are more suitable for operating environments such as low medium temperature and high viscosity, and fast switching. However, plug valves are usually not used for media such as steam and high temperature.

8. The selection of diaphragm valves. Diaphragm valves have obvious structural characteristics, and effective control of fluid motion is achieved through rubber film, plastic film, etc. For diaphragm valves, the fluid resistance is relatively small and can achieve bidirectional sealing. It can be used in media such as low pressure, suspended viscous fluids, and corrosive slurries. The operating mechanism maintains a safe separation from the medium channel and effectively cuts off the fluid through an elastic diaphragm. 

Diaphragm valves are widely used in the design of industrial production systems such as medicine, health, and food. The temperature resistance of the diaphragm material itself determines the suitable temperature environment for the diaphragm valve. Based on the structure, it can be divided into straight through type and weir type. Diaphragm valves are suitable for operating environments mainly composed of suspended solids, oil products, and acidic media, with temperatures not exceeding 200 ℃ and pressures not exceeding 1.0MPa. However, diaphragm valves are usually not used in environments with strong oxidants, organic solvents, and other media.