Fluid valve types and their respective characteristics

Valves are pipeline accessories used to open and close pipelines, control the flow direction, and regulate and control the parameters (temperature, pressure, and flow) of the transport medium. Its role plays a decisive role. Let's take a look at the types of fluid valves and their respective characteristics.

1. Ball Valves When the operating temperature is between -29 degrees and 200 degrees, most manual (open and close) ball valves are suitable for use in hydrocarbon and utility systems. Ball valves used at temperatures above 100°C should be carefully considered during application. The temperature limit of the sealing material should be above 100°C. In the case of high temperature or abrasive wear, consideration should be given to the use of metal-sealed ball valves, but it should be stated that the operating torque must be increased. There are two designs of ball valves, namely ball and trunnion ball valves. The design of the ball valve can produce high operating torque at high pressure or large diameter, but it has better sealing performance. Ball valves are not suitable for throttling, because when the valve is partially opened, its sealing surface is exposed to the treatment fluid and is damaged. For mission-critical operations, consideration should be given to the purchase of ball valves with ball seat and stem lubrication fittings, as lubrication can prevent slight leakage, reduce operating torque, and if the valve is required to have both shutdown and release functions, it should be provided Independent of the ball vent hole of the lubrication fitting.

2. The gate valve is adapted to most switching operations, vibration-free hydrocarbons, and utilities in all temperature ranges. In vibrating operation, the gate valves may open or close from their normal position. Unless the stem packing is carefully adjusted, the valve's torque characteristics are better than the ball and plug valves, but there is no 90-degree rotation operation and convenient operation characteristics. Manually operated gate valves of size DN50 (2in) or larger should be equipped with spring or expansion gates. Catch gate valves usually do not have valve chamber overpressure protection. Unprotected light rod valve is not recommended. Because the offshore environment will corrode exposed valve stems and threads, making the valve difficult to operate and easy to damage the stem packing. Gate valves with reverse acting plates are suitable for automatic shutdown system operation. A simple push-pull operator can be used for these valves, thus avoiding the complex joystick cams that are typically required for ball and plug valves. All movable parts of the gate valve with a power operator can be closed, eliminating contamination due to paint and corrosion products. Gate valves cannot be used for throttling operations. Especially for fluids containing sand, throttling can damage the sealing surface. The operating temperature of the soft seal gate valve is limited by the sealing material.

3, check valve Check valve has a variety of forms: including swing type, lift type, ball type, piston type and separate disc swing check valve. Among them, fully open swing Check Valves adapt to most non-fluctuating conditions. Swing check valves are also used in vertical pipes (flow direction upwards). At this time, there is a stop block inside the valve to prevent the flap from opening above the top dead center. Swing check valves cannot be used on vertical pipes that flow down. If used in low-fluctuation, low-flow areas, swing check valves will vibrate and eventually damage their sealing surfaces. For longer life, the disc can be coated with Stellite. In order to reduce the leakage of the seat, an elastic seal should be used. Removable valve seats should be preferred because they are easy to maintain and can easily replace the seals in the valve. Swing check valves should use a screw or bolted bonnet to check and repair discs and seats. In many cases, the high-pressure swing check valves that can be repaired on the pipeline can have a minimum size of DN65 (2.5 NPS) or DN80 (3NPS). Low profile rotary check valve (space saving), adapted to fit between two flanges. This type of check valve is normally not fully open and needs to be removed from the pipe during repair. The double-disc thin check valve occupies a small space and is lightweight, so it is usually used for offshore oil. Because fatigue can lead to spring failure, double-disc thin check valves cannot be used for pulsation operation. However, consider using a non-impact (or automatic) check valve or rotary check valve. Lifting check valves can only be used on small diameter (DN 40), high pressure pipelines to handle clean fluids. The lift check valve can be designed for use on horizontal pipes or vertical pipes, but the two cannot be interchanged. Because lift-type check valves are usually operated by gravity, they may be blocked by paraffin or debris. Ball check valves are very similar to lift check valves. Since the ball is lifted by the fluid pressure, this type of check valve does not have the tendency of the flap to close the impact. Therefore, in a pipe no larger than DN 50 (2 in), it is preferable for the cleaning fluid to frequently change the flow direction. Self-balancing, axial pistons, non-automatic check valves are recommended for use in flow fluctuating pipelines such as reciprocating compressors and pump outlets. They are not suitable for use on pipes with sand or impurities in the fluid. The piston check valve is equipped with an orifice to control piston movement. The orifice plate for the liquid is much larger than the orifice plate for the gas. Piston check valves designed for gas piping cannot be used for liquid operation unless the orifice plate in the piston is replaced.

4. Butterfly valves Common butterfly valves are suitable for rough throttling and low-pressure, hydrocarbon-free, non-hazardous operations. They are not suitable for use as important shut-off valves for vessels, tanks, etc. Wireless butterfly valves should be used at operating temperatures above 65oC or pressure levels above ASME 150 or hazardous fluids. Since only a small torque is required to open the butterfly valve, the handle should be equipped with a locking device.

5, cut-off valve cut-off valve, also known as the cut-off valve, is a mandatory sealed valve, so when the valve is closed, you must apply pressure to the valve to force the sealing surface does not leak. When the medium enters the valve from below the valve flap, the resistance to be overcome by the operating force is the friction force of the stem and the packing and the thrust generated by the pressure of the medium. The force of the closing valve is greater than the force of the opening valve, so the valve stem The diameter should be large, otherwise there will be stem bending failure. When the closing valve is opened, the opening height of the valve flap is 25%~30% of the nominal diameter. The flow has reached the maximum, indicating that the valve has reached the fully open position. Therefore, the fully open position of the stop valve should be determined by the stroke of the disc.

6. Diaphragm valve In the design of this valve, the diaphragm made of synthetic rubber is connected to the valve stem. The closing of the valve is achieved by pressing the diaphragm against a metal boss (which is part of the valve body). Diaphragm valves are used primarily in low pressure water applications up to 1400 kPag (200 psig or less) and are especially suitable for systems containing sand and other solid particles.

7, plug valve plug valve and ball valve use the same scope, and the use of temperature limits are similar. Plug valves with 90 degree rotation are lubricated and non-lubricated. Lubricated plug valves must be lubricated regularly to make them sealable and easy to operate. The number of refueling depends on the conditions of use. Its lubrication function is a solution to prevent valve jamming. In non-lubricated designs, its sealing is done with Teflon, nylon and other soft materials. They do not require regular maintenance lubrication, but when the valve is set in a position for a long time, it is more difficult to rotate. Based on these characteristics, the valve should be selected according to the specific use environment (conditions).

8, needle valve needle valve is basically a small stop valve. They are often used as shut-off valves for instruments and pressure gauges, for small flow throttling of instrument air, natural gas, and hydraulic fluids, and also for reducing pressure fluctuations in instrumentation piping. The needle valve's channel is small and it is easy to block. This should be taken into consideration when using it.