Selection of the right type :

Valve may be defined as a machine that controls the flow of liquid or gas in a piping system. Control includes fully stopping or opening, regulating or letting the flow in one direction while restricting the other. Selection of a valve type with best performance depends on how you want to control the flow.

Functions of a valve:

*Fully open or fully close the flow
*Regulate the flow
*Restricting the flow in a direction that is not desired
*Regulate the pressure by partially permiting the flow.

After selecting the appropriate type, the most economical selection will involve a valve with least operation and maintenance cost. Minimizing these costs means choosing the suitable material and optimizing the wall thickness of this material in the construction. The material and the wall thickness should be selected according to the following characteristics of the fluent:

1-Chemical characteristics of the fluid (corrosiveness etc.)
2-Phase of the fluid (liquid or gas)
3-Pressure and temperature of the fluid at operating conditions
4-Frequency of operation of the valve.

Main types of valves :

Gate Valves:

Gate valves are used in flows where no or minimum pressure drop is desired. This type of valve fully opens or closes the flow. If used partially open, the flowing fluid will vibrate the gate and this will lead to erosion and mechanical failures. Throttling or regulating the flow is not possible with gate valves.

Globe Valves:

As can be seen in the figures below, globe valves are used to regulate or to throttle the flow. Flow control in these valves can be made by small increments. Pressure drop obtained by throttling should not go over 20% because it can lead to noisy vibrations and to consequent mechanical failures in the valve and piping system.

Check Valves:

Check valves prevent flow in one direction and fully close in case the direction of the flow reverses for any reason. The pressure difference in the desired direction open the disc (or clapet), which lets the fluid to flow with very little friction loss. These valves can be used in both horizontal and vertical lines. They are especially suitable for low flow rates.