Valves

The variety of valves available for use in piping systems is extensive. This is due to the range of functions that valves perform, the diversity of fluids carried, and the varying conditions under which valves must perform these tasks. Valves can be examined under the following headings:
● Basic parts
● Functions performed by valves
● Valve types
● Installation of valves
● Specification of valves.
The main structure of the valve is the body, which contains – or to which is attached – the other parts of the valve. The main structure must possess sufficient mechanical strength and sufficient resistance to corrosion, erosion and high temperature to meet service conditions. The material from which the valve body is made is important and common materials in use include carbon steel, low-alloy steel, bronze, brass, stainless steel. The operator is the method of actuating the valve. Valves may be operated manually: by the use of handwheels, levers and chains, by geared handwheels on larger valves or by powered operation employing electric, pneumatic or hydraulic actuators. Powered actuators are normally used when:
● Rapid opening or closing is required
● The valve is operated very frequently
● Access to the valve is difficult
● The operation of the valve requires great effort
● Valve operation presents a safety hazard.
Functions performed by valves
Valves perform the following basic functions.
● They shut off the supply in a pipeline or they enable a piece of the pipeline to be isolated so that repairs to piping or equipment can be carried out faulty or damaged items can be replaced, etc. This is shut-off or stops valve.
● The throttle, regulate or restrict the flow passing along a pipeline by partially closing the area of flow through the valve.
● They redirect the flow at a branch line by changing the path along which the flow occurs.
● They protect a system against excessive pressure or sudden increases in pressure. These are safety valves or relief valves. When the pressure in a line reaches a pre-set high pressure, the valve opens and allows the pressure to escape either to the atmosphere or to another part of the system. Safety valves are
the ones that are usually used for steam, air or other gases. Relief valves are usually used for liquids.

● They enable one part of a continuous system of piping to operate at a different pressure from another part. These are pressure-reducing valves (also known as pressure regulators) and are often used in air piping to reduce the compressor or mainline pressure down to a low value for operation of low- pressure equipment.
● They prevent flow in one direction along a pipe or they allow flow in one direction only. This valve is referred to as a non-return, or check or reflux valve.