Valves are integral components in piping systems they are the primary method of controlling the flow, pressure and direction of the fluid.  Valves may be required to operate continuously e.g. control valves, or they may be operated intermittently e.g. isolation valves, or they may be installed to operate rarely if ever e.g. safety valves.   A valve can be an extremely simple, low cost item or it may be and extremely complicated, expensive item.   In piping design the valves probably require more engineering effort than any other piping component.

 

The isolation of the downstream system from the upstream system by use of and isolation /stop valve is a critically important function..The prime requirements of this valve are tight shut off when closed and minimum restriction to flow when open.

Valves used for this function include gate valves, globe valves, ball valves, plug valves, butterfly valves, diaphragm valves and pinch valves

 

Many applications require the flow of the fluid be regulated (throttled) at some fixed or variable level between fully zero and maximum flow limits.  This is achieved by introducing resistance to flow, or by bypassing flow or by changing the direction of the flow.  An important feature for control valves is that the output variable (flow) is related to the input variable (valve position).   An ideal operating characteristic of a hand operated flow control valve is that the flow is directly proportional to the position of the handwheel

Valve types for this function include globe, needle, ball, butterfly.   Globe and needle valves are best suited for this duty but ball valves are also easily adapted to give reliable flow control..

 

In some circumstances it is important to prevent reversed fluid flow.   The type of valve for this duty is a non-return-valve (NRV) or check valve.   The important criteria when selecting these valves are, tight shut off against reverse flow, low resistance to flow for forward flow, fast response.  The valve can be operated to close by gravity, fluid flow , or spring.

Two main valve types are available for this function lift check valves or swing check valves.