Valve selection skills
1、 Key points of valve selection
A. Specify the purpose of the valve in the equipment or device
Determine the working conditions of the valve: the nature of the applicable medium, working pressure, working temperature, operation etc.
B. Correctly choose the valve type
The correct selection of valve type is based on the designer's full mastery of the whole production process and operating conditions. When choosing valve type, the designer should first master the structural characteristics and performance of each valve.
C. Confirm that the end connection of the valve
In threaded connection, flange connection and welded end connection, and the first two are most commonly used. Threaded valves are mainly valves with nominal diameter less than 50mm. If the diameter is too large, it is very difficult to install and seal the connecting part. The installation and disassembly of flange connected valves are more convenient, but they are bulkier and more expensive than threaded valves, so they are suitable for pipeline connection of various sizes and pressures. Welded connection is applicable to the condition of load cutting, which is more reliable than flange connection. However, it is difficult to disassemble and reinstall the welded valve, so its use is limited to the occasions where it can normally operate reliably for a long time, or where the service conditions are engraved and the temperature is high.
D. Selection of valve material
Select the materials of the shell, internals and sealing surface of the valve. In addition to considering the physical properties (temperature, pressure) and chemical properties (corrosivity) of the working medium, the cleanliness of the medium (whether there are solid particles) shall also be mastered. In addition, refer to the relevant provisions of the state and the user department. Correct and reasonable selection of valve material can obtain the most economical service life and best service performance of the valve. The material selection sequence of valve body is nodular iron - carbon steel - stainless steel, and the material selection sequence of sealing ring is rubber - Copper - alloy steel - F4.
2、 Introduction to common valves
A. Butterfly valve
Butterfly valve is that the butterfly plate rotates 90 degrees around the fixed shaft in the valve body to complete the opening and closing function. The butterfly valve has the advantages of small volume, light weight and simple structure. It is only composed of a few parts.
And only rotate 90 °; It can be opened and closed quickly and the operation is simple. When the butterfly valve is in the fully open position, the thickness of the butterfly plate is the only resistance when the medium flows through the valve body. Therefore, the pressure drop generated through the valve is very small, so it has good flow control characteristics. Butterfly valve is divided into elastic soft seal and metal hard seal. For elastic sealing valve, the sealing ring can be embedded on the valve body or attached around the butterfly plate, with good sealing performance. It can be used not only for throttling, but also for medium vacuum pipeline and corrosive medium. The valve with metal seal generally has a longer service life than that with elastic seal, but it is difficult to achieve complete sealing. It is usually used in occasions with large changes in flow and pressure drop and good throttling performance. Metal seal can adapt to higher working temperature, while elastic seal has the defect limited by temperature.
B. Gate valve
Gate valve refers to the valve whose opening and closing body (valve plate) is driven by the valve stem and moves up and down along the sealing surface of the valve seat, which can connect or cut off the fluid channel. Gate valve has better sealing performance than stop valve, small fluid resistance, labor-saving opening and closing, and has certain regulation performance. It is one of the most commonly used block valves. The disadvantage is that the size is large, the structure is more complex than the stop valve, the sealing surface is easy to wear and difficult to maintain, and it is generally not suitable for throttling. According to the thread position on the valve stem, the gate valve can be divided into exposed rod type and concealed rod type. According to the structural characteristics of the ram, it can be divided into wedge type and parallel type.
C. Check valve
The check valve is a valve that can automatically prevent the backflow of fluid. The valve disc of the check valve is opened under the action of fluid pressure, and the fluid flows from the inlet side to the outlet side. When the pressure at the inlet side is lower than that at the outlet side, the valve disc will automatically close under the action of fluid pressure difference, its own gravity and other factors to prevent fluid backflow. According to the structural form, it is divided into lifting check valve and swing check valve. The lifting type has better sealing performance and large fluid resistance than the swing type. For the suction inlet of the pump suction pipe, the bottom valve should be selected. Its function is to fill the inlet pipe of the pump with water before starting the pump; After stopping the pump, keep the inlet pipe and pump body full of water for restart. The bottom valve is generally only installed on the vertical pipe at the pump inlet, and the medium flows from bottom to top.
D. Ball valve
The opening and closing part of the ball valve is a ball with a circular through hole. The ball rotates with the valve stem to open and close the valve. The ball valve has the advantages of simple structure, fast switching, convenient operation, small volume, light weight, few parts, small fluid resistance, good sealing and convenient maintenance.
E Globe valve
The globe valve is a downward closed valve, and the opening and closing part (valve disc) is driven by the valve stem to move up and down along the axis of the valve seat (sealing surface). Compared with gate valve, it has good regulation performance, poor sealing performance, simple structure, convenient manufacture and maintenance, large fluid resistance and low price. It is a commonly used block valve, which is generally used for medium and small diameter pipelines.
