PURPOSE OF VALVES:
Valves are mechanical devices used to control the flow of system products, such as liquids and gases, in a plant piping system. An open valve permits flow through the system; closed valve shuts off the flow. Certain valves are also designed to regulate the flow of a system product through the system. This is accomplished by maintaining the valve in a partially open position. Therefore, the three valve operating positions are fully open. Throttled, and fully closed.
Valves can be classified by disc arrangement or shape, by function, by operating conditions such as flow, level, temperature, and pressure, or by the way they attach to plant system (flanging, welding, threading, etc.).
Types of Valves:
- Gate Valves
- Globe Valves
- Ball Valves
- Plug Valves
- Butterfly Valves
- Check Valves
- Relief and Safety Valves
Gate Valves are used essentially for (ON-OFF) service only, or for isolation because the disc arrangement isn't designed to control the rate of flow (no throttling).
Gate Valves most often used in liquid systems. They are always either fully open or fully closed. The following figure shows these positions.
Gate Valves aren't used in throttling positions as this causes the gate to vibrate which will lead to excessive wear then failure of the gate.
Note that this type is used in places where there only continuous flow or no flow at all.
Globe Valves are multiturn valves used to control the flow of systems products by altering the position of a disc between the fully closed positions. The Figure shows a Globe Valves and its main parts.
Ball Valves are classed as quarter-turn, straight line valves that are used to control the flow of system produced by altering the position of the hollow ball inside the valve body.
The following figure shows a Ball Valve and its main parts.The ball valve gets its name from the round shape of the "ball".
The hole in the ball through which the system products flow is called the port. The seats in the ball valve are manufactured from "Teflon". This is tough, easily machined material which has good wear resistance and high heat resistance.
Plug valves use a tapered plug to control the flow of a system product in a piping system. The plug has a tapered cast hole in the middle, called a port, through which the flow passes.
The following figure shows a Plug Valve and its main parts.Plug Valves are used for high pressure and temperature applications where other valves aren't suitable.
Plug Valves are classified as quarter-turn; straight line valves that are used in only fully opened and fully closed positions.
Butterfly Valves use flat circular disc to control the flow in a piping system.
The following figure shows a Butterfly Valve and its main parts.
Butterfly Valves are used in all three positions: Fully open, Fully closed and throttling.
When the edge of the disc faces the flow, the valve is in the fully open position. When the disc is across the flow, the valve is in the fully closed position. This is shown in the figure.
Check Valves are also known as "Sixth Valves" in markets. This type is non-return valve as it permits the flow in one direction only.
It has many types as the following:
-Plug Valves with springs
This type is used inside feed water pumps in the power plant to prevent back flow.