How to choose valves in the industrial pipeline? (II)
Selection of valves
Valves are generally selected according to factors such as characteristics of media, operating conditions and valve requirements. The major Chinese valve manufacturer can manufacture various types of valves according to the above standards to meet the needs of refining and chemical production.
Commonly used valves are as follows:
The wedge disc of the gate valve is driven by the valve stem and moves up and down along the sealing surface of the valve seat to connect or cut off the fluid. The gate valve has small flow resistance and is easy to be opened and closed, which make the gate valve widely used for connecting or cutting off media in the pipe. When the gate valve is partially opened, eddy is generated at the back of the wedge disc, causing erosion and vibration of the wedge disc. The sealing surface of the valve seat will be easily damaged, and it’s difficult to repair it. Therefore, the gate valve is generally not used for throttling. The gate valve has advantages of small fluid resistance, no directionality, short structure length and being easily opened and closed and being used in the case of two-way flow of the medium. When the gate valve is fully opened, the sealing surface is not easily eroded. These advantages make the gate valve not only be manufactured with small diameters, but also with large diameters.
Gate valves’ stems can be divided into two kinds rising stems and non rising stems based on different threads of valve stems. The rising stem type is that the valve stem rises and falls with opening and closing of the wedge disc, while non rising stem type is that the valve stem rises and falls with opening and closing of the wedge disc without changing the position of the valve stem. Under normal circumstances, it is recommended using rising stem gate valves. When the operating space is relatively small and the rising and falling of the stem interfere with other equipment, non rising stems should be selected.
Gate valves, globe valves and check valves are divided into flange valve bonnets, pressure seal valve bonnets, thread and welded valve bonnets according to the connection between the valve bonnet and the valve body.
Flange bonnets sealed by gaskets and bolts are suitable for valves with various pressure and sizes. However, for valves with large diameters and high pressure, the weight and volume of the valve are increased due to the flange; when the valve is used at a high temperature, the bearing capacity of the bolt will be reduced due to creep relaxation.
The pressure seal valve bonnet provides tight sealing for the valve body and the valve bonnet by fluid pressure, and its weight is lighter than the flange valve bonnet. The pressure seal valve bonnet is applied to large-diameter valves with high temperature, high pressure, clean media and no corrosion.
The threaded valve bonnet is easily loosed and leakage will occur because of vibration of the threaded connection. Therefore, the threaded valve bonnet is rarely used in the refining and chemical equipment.