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What is a Valve?
A Valve is a device comprising of an obturator or closure member that is used to control flow or pressure in a piping system.Valves are one the most important component of a piping system and specifying the correct type, design and material of valve is crucial for plant operation. The function of valves in a piping system is one of the following: Start and stop flow (On-off valves)
Control or regulate flow (Regulating valves)
Prevent back-flow (Check valves)
Divert flow (Multi-port valves)
Relieve pressure from a piping system (Pressure relief valves or Pressure-vacuum relief valves)
Types of Valves : The various types of valves commonly used in any plant facilities are:
-Gate valves
-Globe valves
-Check valves
-Ball valves
-Plug valves
-Butterfly valves
-Pinch valves
-Diaphragm valves
Valves which relieve pressure are excluded from this discussion. They are discussed in a separate article.
Classification of Valves based on Closure Element In addition to the function of the valve, valves can also be classified based on the movement of the closure element. Based on movement of Obturator or Closure Element, valves can be classified into the following broad categories.
Closing Obturator: In this type of valve the closure member i.e. disc or plug moves along the seat axis, towards or away from the direction of valve seat or port. Globe valve, Needle valve and Piston valve fall in this category of valves.
Sliding Obturator: In this type of valve, the closure member i.e. wedge gate or parallel gate moves perpendicular to the direction of flow at the valve port. Gate valves fall in this category of valves. Ball valves, Plug valves and Butterfly valves fall in this category of valves.
Rotary Obturator: In this type of valve, the closure member has a port which is turned through 90 degrees such that the port aligns with the direction of flow allowing full flow across the valve or the port is at right angles to the direction of flow shutting off the flow.
Flexible Obturator: In this type of valve, the closure member is a flexible passage which is flattened or pinched to restrict the flow and vice versa to let flow pass through the valve. Diaphragm valves and Pinch valves fall in this category of valves.
Components of a Valve
Pressure Containing Components: Valve body, bonnet or cover, disc, and body-bonnet bolting are specified as pressure-containing components of a valve as they are subject to the full design pressure of the specified valve. The pressure containing components are designed to withstand the design pressure and temperature conditions specified in the valve data sheet. Since the pressure containing components come in contact with the process fluid their material is required to be compatible with the service. In many cases for a cost effective design the pressure containing components are specified in carbon steel material with internal lining or overlay to withstand the corrosive fluid.
Non-Pressure Containing Components: Components of a valve such as valve seats, stem, yoke, packing, gland bolting, bushings, hand wheel, lever, gear operator or other valve actuators are not required to be designed for pressure containment and are considered as non-pressure containing components.
