Advantages and disadvantages of several types of valves

Release Time:

2020-10-05

Gate Valve A gate valve is a valve in which the closing member (gate) moves vertically along the axis of the passage. In pipelines, it is mainly used to cut off the medium, i.e., it is used in a fully open or fully closed state. Generally, gate valves cannot be used to regulate flow. It can be used at low temperatures and pressures, as well as at high temperatures and pressures, depending on the material of the valve. However, gate valves are generally not used in pipelines that transport slurries. Advantages: ① Low fluid resistance; ② Relatively small torque required for opening and closing; ③ Can be used in ring-main pipelines where the medium flows in both directions, i.e., the flow direction of the medium is not restricted; ④ When fully open, the sealing surface is less eroded by the working medium than a globe valve; ⑤ Relatively simple structure and good manufacturability; ⑥ Short structural length. Disadvantages: ① Large external dimensions and opening height, requiring a large installation space; ② During the opening and closing process, the sealing surfaces rub against each other, resulting in greater wear and tear, and even scratches at high temperatures; ③ Generally, gate valves have two sealing surfaces, which increases the difficulty of processing, grinding, and maintenance; ④ Long opening and closing time. Butterfly Valve A butterfly valve is a valve that uses a disc-type closing member to rotate approximately 90° back and forth to open, close, and regulate the fluid passage. Advantages: ① Simple structure, small size, light weight, low material consumption, suitable for large-diameter valves; ② Quick opening and closing, low flow resistance; ③ Can be used for media with suspended solid particles, and depending on the strength of the sealing surface, it can also be used for powdery and granular media. It can be used for two-way opening and closing and regulation of ventilation and dust removal pipelines, and is widely used in metallurgical, light industry, power, and petrochemical systems for gas pipelines and waterways, etc. Disadvantages: ① The flow regulation range is not large, and when the opening reaches 30%, the flow will reach more than 95%. ② Due to the structure of the butterfly valve and the limitations of the sealing material, it is not suitable for high-temperature and high-pressure pipeline systems. The general operating temperature is below 300℃, and PN40 or less. ③ The sealing performance is relatively poor compared to ball valves and globe valves, so it is used where the sealing requirements are not very high. Ball Valve A ball valve is evolved from a plug valve. Its closing member is a sphere, which is used to achieve opening and closing by rotating the sphere 90° around the axis of the valve stem. Ball valves are mainly used in pipelines to cut off, distribute, and change the flow direction of the medium. Ball valves designed with a V-shaped opening also have good flow regulation functions. Advantages: ① Low flow resistance (actually 0); ② Because it will not get stuck during operation (without lubricant), it can be used in corrosive media and low-boiling-point liquids; ③ Complete sealing can be achieved in a wide range of pressures and temperatures; ④ Quick opening and closing can be achieved, and the opening and closing time of some structures is only 0.05~0.1s, ensuring that it can be used in the automated system of the test bench. When the quick-opening and closing valve is operated, there is no impact. ⑤ The spherical closing member can automatically position itself at the boundary position; ⑥ Double-sided sealing; ⑦ When fully open and fully closed, the sealing surfaces of the sphere and valve seat are isolated from the medium, so the medium passing through the valve at high speed will not cause erosion of the sealing surface; ⑧ Compact structure, light weight, it can be considered a reasonable valve structure for low-temperature medium systems; ⑨ The valve body is symmetrical, especially the welded valve body structure, which can well withstand the stress from the pipeline; ⑩ The closing member can withstand the high pressure difference during closing. ⑪ For fully welded ball valves, they can be directly buried underground, so that the valve components are not corroded, and the high service life can reach 30 years, which is an ideal valve for oil and natural gas pipelines. Disadvantages: ① Because the main valve seat sealing ring material of the ball valve is polytetrafluoroethylene, it is inert to almost all chemical substances, and has the comprehensive characteristics of low friction coefficient, stable performance, not easy to age, wide temperature application range, and excellent sealing performance. However, the physical properties of polytetrafluoroethylene, including its high expansion coefficient, sensitivity to cold flow, and poor thermal conductivity, require the design of the valve seat seal to be based on these properties. Therefore, when the sealing material hardens, the reliability of the seal is compromised. Moreover, the temperature resistance of polytetrafluoroethylene is relatively low, and it can only be used at temperatures below 180℃. Above this temperature, the sealing material will age. Considering long-term use, it is generally not used at 120℃. ② Its regulating performance is slightly worse than that of a globe valve, especially for pneumatic valves (or electric valves). Globe Valve A globe valve is a valve in which the closing member (valve disc) moves along the centerline of the valve seat. According to this movement of the valve disc, the change in the valve seat passage is proportional to the valve disc stroke. Because this type of valve has a relatively short valve stem opening or closing stroke and has a very reliable shut-off function, and because the change in the valve seat passage is proportional to the stroke of the valve disc, it is very suitable for flow regulation. Therefore, this type of valve is very suitable for use as a shut-off, regulating, and throttling valve. Advantages: ① During the opening and closing process, the friction between the valve disc and the valve body sealing surface is smaller than that of a gate valve, so it is more wear-resistant. ② The opening height is generally only 1/4 of the valve seat passage, so it is much smaller than a gate valve; ③ There is usually only one sealing surface on the valve body and valve disc, so the manufacturing process is better and easier to maintain. ④ Because its packing is generally a mixture of asbestos and graphite, it has a higher temperature resistance. Generally, steam valves use globe valves. Disadvantages: ① Because the flow direction of the medium through the valve changes, the low flow resistance of the globe valve is also higher than that of most other types of valves; ② Because the stroke is longer, the opening speed is slower than that of a ball valve. Diaphragm Valve A diaphragm valve is a valve in which a flexible diaphragm or a combination diaphragm is installed in the valve body and valve cover, and the closing member is a compression device connected to the diaphragm. The valve seat can be a weir shape or a straight-through flow passage wall. Advantages: ① The operating mechanism is separated from the medium passage, which not only ensures the purity of the working medium, but also prevents the possibility of the medium in the pipeline impacting the working parts of the operating mechanism. No separate sealing is required at the valve stem, unless it is used as a safety device in the control of harmful media; ② Since the working medium only contacts the diaphragm and the valve body, both of which can use a variety of different materials, this valve can ideally control a variety of working media, especially suitable for media with chemical corrosion or suspended particles. ③ Simple structure, consisting of only three components: valve body, diaphragm, and valve cover assembly. The valve is easy to quickly disassemble and maintain, and the diaphragm can be replaced on-site in a short time. Disadvantages: ① Due to the limitations of the valve body lining process and diaphragm manufacturing process, larger valve body linings and larger diaphragms are difficult to manufacture, so diaphragms are not suitable for larger pipe diameters, and are generally used in pipelines with DN≤200mm. ② Due to the limitations of the diaphragm material, diaphragm valves are suitable for low-pressure and low-temperature occasions. Generally, it does not exceed 180℃; ③ The regulating performance is relatively poor, and only regulates within a small range (generally, when it is closed to 2/3 open, it can be used for flow regulation). Supplementary System The supplementary system provides a supply on the pipeline. It can be mainly divided into rotary type (rotating according to the center of gravity) and lifting type (moving along the axis).