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A Little About Valves

What is Cv ?

If you are a mechanic, you would probably say that it is the joints that are on a front wheel drive vehicle, however if you work in the field of process automation you might know that it pertains to control valves.

Cv is the capacity of a control valve, or the gallons of water that will pass through an orifice with a pressure drop of 1 psi in one minute at 60 degrees Fahrenheit. This is the basic formula for liquid flow with no pipe reducers:

Where:

  • Q = Flow (gpm)
  • Gf = flowing Specific gravity                                                                                                                                                                             DP = Inlet pressure - outlet pressure in psia
  •  DP = P1 - P2

Cv allows us to compare valves of completely different geometries. A partially open 3” ball valve could have the same Cv as a fully open 2” ball valve and as a 4” globe valve. Don’t want to do the math? Many vendors have valve sizing programs available and there are also calculators on-line. In any case you still need to know what numbers to use for specific gravity and so forth and also note that Cv or the capacity is not the only consideration when choosing a control valve.

Mark Weisner
Mark Weisner
Mark Weisner is a technical specialist at ISA.

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