Experts teach you how to choose valves correctly

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Experts show you how to choose valves correctly. Gate valves are used to stop the flow of a medium; when fully open, the flow path is completely unobstructed, resulting in the lowest pressure loss for the medium flowing through it. Gate valves are typically suitable for applications where frequent opening and closing are not required, and where the gate plate needs to remain fully open or fully closed. Not suitable for use as a regulator or throttle. For media flowing at high speeds, partial opening of the gate plate can cause vibration in the gate, and this vibration may damage the sealing surfaces of the gate plate and the valve seat; moreover, throttling exposes the gate plate to erosion by the medium. In terms of structural form, the main difference lies in the type of sealing element used. Based on the design of the sealing element, gate valves are often classified into several different types, such as wedge gate valves, parallel gate valves, parallel double-plate gate valves, and wedge double-plate gate valves. The most commonly used types are wedge gate valves and parallel gate valves. Stop valve: The axis of the stem of a stop valve is perpendicular to the sealing surface of the valve seat. The valve stem has a relatively short opening or closing stroke, and it provides a very reliable shut-off action, which makes this type of valve highly suitable for shutting off, regulating, or throttling fluids. Once the disc of a globe valve is in the open position, there is no longer any contact between its seat and the sealing surface of the disc, resulting in a very reliable shut-off action; this makes such valves highly suitable for shutting off, regulating, or throttling fluids. Once a globe valve is in the open position, there is no longer any contact between its seat and the sealing surface of the disc; as a result, mechanical wear on these sealing surfaces is minimal. Moreover, since the seat and disc of most globe valves can be easily repaired or replaced without having to remove the entire valve from the pipeline, this makes them particularly suitable for situations where the valve is welded to the pipeline. The flow direction of the medium changes as it passes through such valves; therefore, the flow resistance of globe valves is higher than that of other valves. The commonly used globe valves include the following: 1) Angle-type globe valve ； In angular stop valves, the fluid only needs to change direction once, resulting in a lower pressure drop across the valve compared to stop valves with conventional designs. 2) DC globe valve ； In straight-through or Y-type globe valves, the flow channel of the valve body forms an angle with the main flow path; as a result, the degree of disruption to the flow pattern is less than that in conventional globe valves, and consequently the pressure loss across the valve is also lower. 3) Plunger-type globe valve: This type of globe valve is a variant of the conventional globe valve. In such valves, the valve disc and seat are typically designed based on the plunger principle. The valve disc is polished to form a plunger that connects with the valve stem, and sealing is achieved through two elastic sealing rings fitted over the plunger. The two elastic sealing rings are separated by a collar, and the sealing rings around the plunger are pressed firmly in place by the load applied to the valve cover by the valve cover nut. The elastic sealing ring can be replaced and is made from a variety of materials. This valve is primarily used for opening or closing, but it also features a special type of plunger or special rings that allow it to be used for regulating flow. Butterfly valve: The butterfly disc of a butterfly valve is installed in the diameter direction of the pipeline. Within the cylindrical channel of the butterfly valve body, the disc-shaped butterfly plate rotates around its axis by an angle ranging from 0° to 90°; when it reaches 90°, the valve is in its fully open position. Butterfly valves have a simple structure, are small in size and light in weight, consisting of only a few components. Moreover, it can be quickly opened and closed by simply rotating 90°, making the operation simple; at the same time, this valve boasts excellent fluid control properties. When the butterfly valve is in the fully open position, the thickness of the butterfly disc is the only resistance to the flow of the medium through the valve body; as a result, the pressure drop across this valve is very small, giving it excellent flow control properties. Butterfly valves come in two types of sealing: elastic sealing and metal sealing. Elastic-sealing valves, where the sealing ring can be embedded in the valve body or attached around the butterfly disc. Valves with metal seals generally have a longer lifespan than those with elastic seals, but it is difficult to achieve a complete seal. Metal seals can withstand higher operating temperatures, while elastic seals have the drawback of being limited by temperature. If a butterfly valve is to be used for flow control, the key is to correctly select its size and type. The structural principle of butterfly valves makes them particularly suitable for manufacturing large-diameter valves. Butterfly valves are widely used not only in general industries such as petroleum, gas, chemicals, and water treatment, but also in the cooling water systems of thermal power plants. The commonly used butterfly valves are wafer-type butterfly valves and flange-type butterfly valves. A wafer butterfly valve is connected between two pipe flanges using bolts, while a flanged butterfly valve has flanges on it, with the flanges at both ends of the valve being secured to the pipe flanges using bolts. Ball valve: The ball valve evolved from the plug valve. It has the same 90-degree rotation lift action; the difference is that the plug body is spherical, with a circular hole or passage running through its axis. The ratio between the sphere and the opening should be such that when the sphere rotates 90 degrees, the entire area at the inlet and outlet should be covered by the sphere, thereby blocking the flow. A ball valve can be closed tightly with just a 90-degree rotation and a very small torque. The completely equal inner cavity of the valve provides a flow path with very low resistance and a direct route for the medium. It is generally believed that ball valves are most suitable for direct on-off operation, but recent developments have designed them to also serve the purpose of throttling and controlling flow rates. The main advantages of ball valves are their compact structure, ease of operation and maintenance. They are suitable for use with common working media such as water, solvents, acids, and natural gas, as well as for media under harsh working conditions, such as oxygen, hydrogen peroxide, methane, and ethylene. The ball valve body can be integral or modular. Valves are selected to prevent backflow of the medium. The function of this type of valve is to allow the medium to flow in only one direction and to prevent it from flowing in the opposite direction. Typically, such valves operate automatically; the valve disc opens under the pressure of fluid flowing in one direction ； When the fluid flows in the opposite direction, the fluid pressure together with the weight of the valve disc causes the valve disc to act on the valve seat, thereby stopping the flow. Check valves belong to this type of valve, and they include swing check valves and lift check valves. A swing check valve has a hinge mechanism, as well as a valve disc that resembles a door and rests freely on an inclined valve seat surface. To ensure that the valve disc always reaches the appropriate position on the valve seat surface, it is designed with a hinge mechanism, allowing sufficient rotation space for the valve disc and enabling it to make true, full contact with the valve seat. The valve disc can be made entirely of metal, or leather, rubber, or synthetic covers can be embedded in metal, depending on the requirements of its performance. When a swing check valve is fully open, the fluid pressure encounters little resistance; therefore, the pressure drop across the valve is relatively small. The disc of the lift-type check valve is seated on the seat sealing surface of the valve body. Apart from the valve disc, which can move up and down freely, this valve operates in a similar manner to a globe valve: fluid pressure lifts the valve disc off the sealing surface of the valve seat, while backflow causes the valve disc to return to the valve seat, thereby stopping the flow. Depending on the operating conditions, the valve disc can be of a fully metallic structure, or it can consist of a frame with rubber gaskets or rings embedded in it. Like globe valves, the flow passage through lift check valves is also narrow; therefore, the pressure drop across lift check valves is greater than that across swing check valves, and the flow rate of swing check valves is subject to fewer restrictions. Select valves based on the parameters of the medium. During the production process, in order to ensure that parameters such as the pressure and flow rate of the medium meet the requirements of the manufacturing process, control mechanisms need to be installed to adjust these parameters. The main working principle of the regulating mechanism is to adjust the aforementioned parameters by changing the flow area between the valve disc and the valve seat. Valves of this type are collectively referred to as control valves. Those that are driven by the power of the medium itself are known as self-acting control valves, such as pressure reducing valves and pressure stabilizing valves. Those that are driven by external power sources (such as electricity, compressed air, or hydraulic power) are called externally driven control valves, such as electric control valves, pneumatic control valves, and hydraulic control valves. Reposted from Shanghai Hengxing Pump and Valve Manufacturing Co., Ltd.