If you need to adjust the flow rate of media anywhere in your piping system, you’ll need a throttling valve. In addition to stopping and starting the flow of liquid or gas media, throttling valves allow system operators to reduce the volume and pressure of the media flowing through the system. In a piping and instrumentation diagram (P&ID), throttling valves are identified using the following symbol:
Throttling valve symbol in a P&ID.
Unlike a control valve, which works by partially opening or closing to change the flow rate of media, a throttling valve does not necessarily need to change position at all, since its sole purpose is to reduce media pressure and velocity in a piping system. For example, a globe valve that remains partially open throughout its entire lifespan to slow the flow of media could be considered a throttling valve.
However, since environmental factors like temperature can alter media flow rate, position control is often required to maintain a consistent flow rate as required by the piping system.
Throttling valves are often called control valves, but the terms are not always interchangeable. Control valves are valves that allow for the control of flow rate through the use of a positioning actuator, which has the ability to precisely control the position of the valve. Throttling valves, on the other hand, are used specifically to reduce media velocity or pressure in a piping system.
Put simply, nearly all control valves can be used as throttling valves, but not all throttling valves are control valves because a throttling valve does not necessarily require dynamic valve position control.
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