Valves are devices used to control the flow of a fluid stream. There are different kinds of valves used in plants based on the needs.
Basically, Valves can be grouped into two main types:
Shut off valves: They are ON/OFF valves either allow full flow or shut off completely. Shut off valves can be operated by hand or automatically. They are not used for controlling the flow. eg: Gate valves
Control valves: These valves act as the correcting element of a control loop. Control valves are normally operated automatically, there are also hand operated valves. There are many types control valves
eg: Globe valves, Butterfly valves, Ball and Plug valves, Pinch valves, Needle valves, Check valves,
The Gate Valves:
Gate valves are shut off valves. There are only two positions for the on/off, simple blocking system
A globe valve controls the flow adjusting the gap between the Globe shaped plug and the seat ring.
There are two types of globe valves used for industrial purpose ‘shaped plug’ and ‘plug and cage’
Shaped plug valves are used for low-pressure flow control. They are also available in two types single plug and double plug valves.
A Single plug valve cannot withstand high pressure because the line pressure pushes the plug directly so that it is used for low-pressure flow.
This line pressure problem can overcome in double plug valve system. The line pressure is applied to both the plugs. So that the line pressure on the upper plug is balanced by the pressure on the bottom plug. Less force is needed to move the valve stem to control high pressure.
Plug and cage:
Just as in the single plug valve system there is a plug which seats in the seat ring to control the flow. And there is an extra hole in the plug to control the flow. The opening of the valve is depended on the position of the plug and hole position.
Butterfly valve is a simple system which has a rotating thin disc rotating across the flow
A butterfly valve can be operated manually and automatically. Valve is actuator driven. The effect on the fluid by the closing and opening of the disc is shown below.
Ball and Plug Valve :
Ball and plug type are similar to butterfly valves, the only difference is the shape of the rotating plugs. The ball valves are a sphere with a hole drilled in it. The plug valves are a tapered cylinder with a hole in it. These valves are also used as shut off valves.The full flow and shut off positions are shown in below.
These valves also can be used for providing three/four-way connection. By adjusting the plug position one can the path of the flow as shown figure below.
Usual valves cannot be used for the flow control of corrosive liquids like acids because the valve part may get corrode easily. This can be avoided by using diaphragm covering the actuator stem at both the side of the valve wall. The pipe used also must be made of a material which does not corrode easily.
Very low pressure can be controlled needle valves.
The flow is controlled by the needle plug which fits into a small hole in the seat. These valves can have manual or automatic actuators.
Needle type valve work same as the single plug valves works but the needle valve is small in size.
The check valves are non-return valves. These valves alow the fluid to flow only in one direction.
Check valve will be open only when the fluid is flowing in the allowed direction. If the fluid flows in the opposite direction, the pressure in the opposite direction closes the valve automatically.
A check valve will not control the flow of the fluid stream but control the direction or prevents the fluid to flow in opposite direction.