Valve selection: introduction to five types of common valves
1. Gate valve
The gate valve is used as a cut-off medium. When it is fully opened, the whole flow is straight. At this time, the pressure loss of the medium is minimal. The gate valve is generally applicable to the working condition where it does not need to be opened and closed frequently and the gate valve is kept fully open or fully closed. It is not suitable for regulating and throttling. For the medium with high-speed flow, when the ram is partially opened, it can cause the vibration of the valve, and the vibration may damage the sealing surface of the ram and the valve seat, while throttling will cause the ram to be eroded by the medium. From the structural form, the main difference is the form of the sealing element used. According to the form of sealing elements, gate valves are often divided into several different types, such as wedge gate valve, parallel gate valve, parallel double gate valve, wedge double gate valve, etc. The most commonly used forms are wedge gate valve and parallel gate valve.
2. Stop valve
The stem axis of the stop valve is perpendicular to the sealing surface of the valve seat. The opening or closing stroke of the valve stem is relatively short. Once the valve disc of the stop valve is in the open state, the valve seat and the sealing surface of the valve disc are no longer in contact, so the mechanical wear of the sealing surface is small. Since the valve seat and the valve disc of most stop valves are relatively easy to repair, it is not necessary to remove the whole valve from the pipeline when replacing the sealing element. This is very suitable for the occasion where the valve and the pipeline are welded together. The above features make the stop valve very suitable for cutting off or regulating the medium and throttling. When the medium passes through such valves, the flow direction changes. Therefore, the flow resistance of the stop valve is higher than that of other valves. There are several commonly used globe valves: Angle Globe Valves: in the angle globe valves, the fluid only needs to change the direction once, so that the pressure passing through the secondary valves is smaller than that of the conventional globe valves. Direct flow stop valve: in direct flow or Y-shaped stop valve, the flow of the valve body and the main flow channel form an oblique line, so the damage degree of the flow state is smaller than that of the conventional stop valve. Therefore, the pressure loss through the valve is correspondingly smaller. Plunger type stop valve: this type of stop valve is a deformation of the conventional stop valve. In this valve, the disc and seat are usually designed based on the plunger principle. The valve disc is polished into a cylindrical plug and connected with the valve stem. The sealing is realized by two elastic sealing rings on the plunger. The two elastic sealing rings are separated by a collar, and the sealing ring around the plunger is firmly pressed by the load applied on the bonnet by the bonnet nut. The elastic sealing ring can be replaced and can be made of various materials. The valve is mainly used for "opening" or "closing", but each has a special plunger or special collar, which can also be used to regulate the flow.
3. Butterfly valve
The butterfly plate of the butterfly valve is installed in the diameter direction of the pipeline. In the cylindrical channel of the butterfly valve body, the disc-shaped butterfly plate rotates around the axis. The rotation angle is between 0 and 90 degrees. When it rotates to 90 degrees, the valve is fully open. The butterfly valve is simple in structure, small in volume and light in weight. It is composed of only a few parts. Moreover, the valve can be opened and closed quickly only by rotating 90 degrees, and the operation is simple. At the same time, the valve has good fluid control characteristics. When the butterfly valve is in the fully open position, the thickness of the butterfly valve is the only resistance when the medium flows through the valve body, so the pressure drop generated by the valve is very small. The butterfly valve has two sealing forms: spring seal and metal seal. Elastic sealing valve, the sealing ring can be embedded on the valve body or around the butterfly plate. Valves with metal seals generally have a longer service life than those with elastic seals, but it is difficult to achieve complete sealing. Metal seal can adapt to higher working temperature, while elastic seal has the defect of being limited by temperature. If the butterfly valve is required to be used for flow control, the main thing is to correctly select the size and type of the valve. The structure principle of butterfly valve is especially suitable for making large-diameter valves. Butterfly valves are not only widely used in petroleum, gas, chemical industry, water treatment and other general industries, but also used in the cooling water system of thermal power stations. The commonly used butterfly valves include wafer butterfly valves or flange butterfly valves. Wafer butterfly valves connect the valves between two pipe flanges with studs. Flange type butterfly valve is provided with flange on the valve, and the flanges at both ends of the valve are connected to the pipe flange with bolts.
4. Ball valve
Ball valve is evolved from rotary valve. It has the same 90 degree rotation action, but the difference is that the plug body is a sphere, and the circular through hole or channel passes through its axis. The ratio of the spherical surface to the channel opening should be such that when the ball rotates 90 degrees, the spherical surface should be present at the inlet and outlet, so as to cut off the flow. The ball valve can be closed tightly only with 90 degree operation and small rotating force. The safe and equal valve cavity provides a small resistance and straight flow path for the medium. It is generally believed that the ball valve is most suitable for direct opening and closing, but recent development has designed the ball valve to have the function of throttling and controlling flow. The main features of the ball valve are compact structure, easy operation and maintenance. It is suitable for general working media such as water, solvent, acid and natural gas, and also suitable for media with poor working conditions, such as oxygen, hydrogen peroxide, methane and ethylene. The valve body of the ball valve can be integral or combined.
5. Check valve
The check valve only allows the medium to flow in one direction and prevents the flow in the opposite direction. Normally, the check valve works automatically. Under the pressure of fluid flowing in one direction, the valve flap opens; When the fluid flows in the opposite direction, the fluid pressure, the weight of the valve disc and the valve disc act on the valve seat, thereby cutting off the flow. Check valves include swing check valves and lift check valves. The swing check valve has a hinge mechanism, and a valve flap like a door is free to rest on the inclined valve seat surface. In order to ensure that the valve disc can reach the proper position of the valve seat surface every time, the valve disc is set in the hinge mechanism so that the valve disc has enough swing space and makes the valve disc truly and comprehensively contact with the valve seat. The valve disc can be made of metal, or inlaid with leather, rubber or synthetic cover, depending on the applicable performance requirements. When the swing check valve is fully opened, the fluid pressure is almost unimpeded, so the pressure drop through the valve will be small. The valve disc of the lift check valve is seated on the sealing surface of the valve seat on the valve body. Except that the valve disc can rise and fall freely, the rest of the valve is like a stop valve. The fluid pressure will lift the valve disc from the sealing surface of the valve seat, and the medium backflow will cause the valve disc to fall back to the valve seat and cut off the flow. According to the applicable conditions, the valve disc can be made of all metal mechanism, or the valve disc frame can be inlaid with rubber pads or rubber rings. Like the stop valve, the passage of fluid through the lift check valve is narrow, so the pressure drop through the lift check valve is larger than that of the swing check valve, and the flow of the swing check valve is limited. Select the valve according to the regulating medium parameters. In the production process, in order to make the pressure, flow and other parameters of the medium meet the requirements of the process flow, it is necessary to install an adjustment mechanism to adjust the above parameters. The main working principle of the regulating mechanism is to adjust the above parameters by changing the valve disc and the flow area between the valve disc and the valve seat. These valves are collectively referred to as control valves, of which those driven by the power of the medium itself are called self driving control valves, such as pressure reducing valves and pressure stabilizing valves. Those driven by power (such as electric power, compressed air and hydraulic power) are called other driving control valves, such as electric control valves, pneumatic control valves and hydraulic control valves.
