Valves 101

Valves are mechanical devices that control the flow and/ or pressure within a system or process. They are normally essential components of a piping system that conveys liquids, gases, vapors, slurries etc. Valves are the controlling element in any fluid-handling system. They should be properly selected and cared for to give the best service at the lowest cost.

Different types of valves are available: gate, globe, plug, ball, butterfly, check, diaphragm, and control valves. Each of these types has a number of models, each with different features and functional capabilities. Some valves are self-operated some manually operated or with an actuator or pneumatic or hydraulic is operated.

Functions from Valves are:

• Stopping and starting flow
• Increase or reduce a flow
• Controlling the direction of flow
• Regulating a flow or process pressure

There are many valve designs, types and models, with a wide range of industrial applications. All design satisfies one or more of the functions identified above to be useful in many application. Valves can be very expensive items, and it is important that a correct valve is specified for the work, and must be constructed of the suitable material for the process liquid.

Classification of Valves

The following are the two basic valve classifications, based on mechanical motion:

• Linear Motion Valves. The valves in which the valve closure part moves in a straight line to allow, stop, or throttle the flow. Example: gate valve, globe valve, diaphragm valves.
• Quarter Turn Valves. Some rotary motion valves require "a quarter turn", or  0° through 90° motion of the stem to go to fully open from a fully closed position (or vice-versa). Example: butterfly valves and ball valves.

In selecting the valve which will give the best service and, in turn, keep plant maintenance and operating cost at a minimum, the following things should be considered.

• Type medium to be handled: We should know in advance which liquid or gas will the valve handle. I it a clean fluid or does it contain solids? Is it corrosive or erosive?
• Pressure and temperature: These may vary throughout the handling system and should be considered in selecting the right valve materials.
• Flow considerations: You must determine if pressure drop is critical or not? Will the valve be used to be for simple shutoff or for throttling.
• Frequency of operation: We must consider that whether valve will be normally open with infrequent operation or operated frequently.