Advantages and disadvantages of valves

Thread Content

Advantages and disadvantages of valves: Gate valves are valves in which the closing element (gate) moves vertically along the axis of the passage; they are primarily used in pipelines to shut off the flow of medium, that is, to operate in either fully open or fully closed positions. Generally, gate valves cannot be used to regulate flow. It can be used in low-temperature and low-pressure conditions as well as in high-temperature and high-pressure conditions, and it can be adapted to different valve materials. However, gate valves are generally not used in pipelines for transporting media such as slurry. Advantages: ① Low fluid resistance ； ②The torque required to open and close is low ； ③It can be used in ring piping systems where the medium flows in both directions; in other words, the flow direction of the medium is not restricted ； ④When fully open, the sealing surface is subject to less erosion by the working medium compared to a globe valve ； ⑤Its structural design is relatively simple, and it is easy to manufacture ； ⑥The structural length is relatively short. Disadvantages: ① It has large external dimensions and opening height, requiring more space for installation ； ②During the opening and closing process, there is relative friction between the sealing surfaces, resulting in significant wear; in addition, high temperatures can easily cause scuffing ； ③Generally, gate valves have two sealing surfaces, which adds some difficulty to processing, grinding, and maintenance ； ④The opening and closing time is long. Butterfly valve: A butterfly valve is a type of valve that uses a disc-shaped closing element to rotate back and forth by about 90° in order to open, close, and regulate the flow of fluid. Advantages: ① Simple structure, small size, light weight, low material consumption; not suitable for large-diameter valves ； ②Fast opening and closing, low flow resistance ； ③It can be used in media containing suspended solid particles, and depending on the strength of the sealing surface, it can also be used for powdered and granular media. It can be used for two-way opening, closing, and adjustment in ventilation and dust removal pipelines, and is widely applied to gas pipelines and water conduits in metallurgy, light industry, power, and petrochemical systems. Disadvantages: ① The flow rate adjustment range is limited; when it is set to 30%, the flow rate already exceeds 95%. ②Due to the structure of butterfly valves and the limitations of their sealing materials, they are not suitable for use in high-temperature and high-pressure pipeline systems. The normal operating temperature is below 300°C, and the pressure rating is below PN40. ③Its sealing performance is inferior to that of ball valves and globe valves, so it is used in applications where high sealing requirements are not necessary. Ball valve: It evolved from the plug valve; its closing element is a sphere, and opening and closing are achieved by rotating this sphere 90° around the axis of the valve stem. Ball valves are primarily used on pipelines to shut off, distribute, and change the direction of fluid flow; those designed with a V-shaped opening also possess excellent flow regulation capabilities. Advantages: ① It has the lowest flow resistance (actually 0) ； ②It does not get stuck during operation (without lubricant), allowing it to be reliably used in corrosive media and low-boiling-point liquids ； ③Complete sealing can be achieved over a wide range of pressures and temperatures ； ④Fast opening and closing can be achieved; the opening and closing time for certain structures is as low as 0.05 to 0.1 seconds, ensuring their suitability for use in automated systems on test benches. When the valve is opened and closed quickly, the operation is shock-free. ⑤The spherical closure element can automatically position itself at the boundary position ； ⑥The working medium is reliably sealed on both sides ； ⑦When in the fully open and fully closed positions, the sealing surfaces of the ball and the valve seat are separated from the medium, so the medium flowing through the valve at high speeds does not cause erosion of these sealing surfaces ； ⑧With its compact design and low weight, it can be considered the most suitable valve structure for low-temperature medium systems ； ⑨The valve body is symmetrical; especially in the case of welded valve bodies, it can effectively withstand the stresses exerted by the pipes ； ⑩The closing element can withstand the high pressure difference during closing. ⑾Ball valves with fully welded valve bodies can be buried directly underground, preventing the internal components of the valve from being corroded; their maximum service life can reach 30 years, making them the ideal valves for oil and gas pipelines. Disadvantages: ① Since the main material used for the seat seal ring in ball valves is polytetrafluoroethylene, it is inert to almost all chemicals. It also boasts a low friction coefficient, stable performance, resistance to aging, a wide temperature range of operation, and excellent sealing properties. However, the physical properties of polytetrafluoroethylene, including its high coefficient of expansion, sensitivity to cold flow, and poor thermal conductivity, require that the design of the valve seat seal take these properties into account. Therefore, when the sealing material hardens, the reliability of the seal is compromised. Furthermore, polytetrafluoroethylene has a low temperature resistance and can only be used at temperatures below 180°C. Above this temperature, the sealing material will age. When considering long-term use, it is generally not used at 120°C. ②Its regulating performance is somewhat inferior to that of globe valves, especially pneumatic valves (or electric valves). Globe valve: It is a valve in which the closing element (valve disc) moves along the center line of the valve seat. Based on this type of movement of the valve disc, the change in the valve seat opening is directly proportional to the stroke of the valve disc. Due to the relatively short opening and closing stroke of the valve stem in this type of valve, as well as its highly reliable shut-off function, and because the change in the valve seat orifice is proportional to the stroke of the valve disc, it is very suitable for regulating flow rate. Therefore, this type of valve is very suitable for use in cutting off or regulating flow, as well as for throttling. Advantages: ① During opening and closing, the frictional force between the valve disc and the sealing surface of the valve body is lower than that of gate valves, thus it is wear-resistant. ②The opening height is generally only 1/4 of the valve seat passage, so it is much smaller than gate valves ； ③Usually, there is only one sealing surface on the valve body and the valve disc, which makes the manufacturing process simpler and facilitates maintenance. ④Due to its filler, which is generally a mixture of asbestos and graphite, it has a high temperature resistance rating. Gauge valves are generally used for steam valves. Disadvantages: ① Due to the change in the flow direction of the medium through the valve, the minimum flow resistance of globe valves is also higher than that of most other types of valves ； ②Due to the long travel distance, its opening speed is slower than that of a ball valve. Plug valve: It is a rotary valve whose closing element is in the form of a plunger; by rotating it 90°, the passage opening on the plunger is brought into communication with or separated from the passage opening on the valve body, thereby enabling the valve to be opened or closed. The shape of the valve plug can be cylindrical or conical. Its principle is basically similar to that of a ball valve, which was developed from a plug valve. It is primarily used in oil field extraction, as well as in the petrochemical industry. Diaphragm valve: It refers to a valve in which a flexible diaphragm or composite diaphragm is installed within the valve body and cover, with the closing element being a compression device connected to the diaphragm. The valve seat can be wavy or the wall of a straight flow channel. Advantages: ① The control mechanism is separated from the medium flow path, which not only ensures the purity of the working medium but also prevents the medium in the pipeline from impacting the working components of the control mechanism. No separate sealing is required at the valve stem, unless such a safety measure is needed when dealing with harmful media ； ②Since the working medium comes into contact only with the diaphragm and the valve body, both of which can be made from a variety of different materials, this valve is capable of effectively controlling a wide range of working media, especially those that are chemically corrosive or contain suspended particles. ③It has a simple structure, consisting of only three components: the valve body, the diaphragm, and the valve cover assembly. This valve is easy to disassemble and repair quickly, and the diaphragm can be replaced on-site in a short time. Disadvantages: ① Due to the limitations imposed by the valve body lining process and diaphragm manufacturing process, it is difficult to produce large valve body linings and large diaphragms; therefore, diaphragms are not suitable for larger pipe diameters and are generally used in pipelines with a diameter of DN≤200mm. ②Due to the limitations of the diaphragm material, diaphragm valves are suitable for low-pressure and low-temperature applications. Generally not exceeding 180℃ ； ③The adjustment performance is relatively poor, allowing adjustments only within a narrow range (it can be used for flow regulation when the valve is closed to 2/3 of its open position). Safety valve: It refers to a device used as an overpressure protection mechanism in pressurized containers, equipment, or pipelines. When the pressure inside a device, container, or pipeline rises above the allowable level, the valve opens automatically, allowing for complete discharge in order to prevent further increases in pressure within that device, container, or pipeline ； When the pressure drops to the specified value, the valve should close automatically and promptly, thereby ensuring the safe operation of the equipment, containers, or pipelines. Steam traps: When transporting media such as steam and compressed air, some condensate water is formed. To ensure the efficient operation and safe functioning of the equipment, it is necessary to remove this unnecessary and harmful fluid in a timely manner, so as to maintain optimal performance of the equipment. It has the following functions: ① It can quickly remove the condensed water that is generated ； ②Prevent steam leakage ； ③Remove air and other non-condensable gases. Pressure reducing valve: A valve that reduces the inlet pressure to a desired outlet pressure through regulation, and relies on the energy of the medium itself to maintain the outlet pressure stable automatically. Check valve: also known as a backflow valve, non-return valve, backpressure valve, and one-way valve. These valves open and close automatically due to the force generated by the flow of the medium itself within the pipeline; they are a type of automatic valve. Check valves are used in piping systems; their main functions are to prevent the backflow of fluid, to stop pumps and driving motors from rotating in reverse, and to allow the release of fluid from containers. Check valves can also be used in pipelines that supply auxiliary systems whose pressure may rise above the system pressure. They can mainly be divided into swing-type (rotating around the center of gravity) and lifting-type (moving along the axis). Definition of flow coefficient: 1. C: The volume of water at a temperature of 5°40’C that flows through the valve per hour (expressed in m3), when the valve is fully open and the pressure difference across it is 1 kgf/cm2. C is the general symbol for the flow coefficient, which has been widely used in China for a long time. 2. Kv: The volume of water at a temperature of 540°C that flows through the valve per hour, in cubic meters, when the valve is fully open and the pressure difference across it is 100 KPA. Kv=1.01C, which is currently recommended for use in our country. 3. Cv: The volume of water flowing through the valve per hour (in m3), at a temperature of 15°C (60°F) and with a pressure difference of 1 lb/in2 across the valve when it is fully open. Cv=1.167C – Introduction to the flow coefficient and cavitation coefficient of valves. The flow coefficient and cavitation coefficient of valves are important parameters; these values are generally provided in the data related to valves used in advanced industrial production, and they are even printed in the product catalogs. There is basically no such information available for the valves produced in our country, as obtaining such data requires conducting experiments; this is one of the key factors indicating the gap between our country’s valves and those at the world’s advanced level. The flow coefficient and cavitation coefficient of a valve are important parameters; these values are generally provided in the data sheets for valves used in advanced industrial production, and are even printed on the sample sheets. There is basically no such information available for the valves produced in our country, as obtaining such data requires conducting experiments; this is one of the key indicators of the gap between our country’s valves and those at the world’s advanced level