Control Valve Sizing


Thread Starter


In respect of Min, Normal & Max Flow thru a valve is the following correct?

MIN Flow thru the valve corresponds to the LOWEST Upstream Press and the LOWEST deltaP ??

MAX Flow thru the valve corresponds to the LARGEST Upstream Press and the LARGEST deltaP ??

please illuminate me.......thank you

That doesn't strike me as correct. If you imagine a system consisting of a pump, a control valve and a load, all of the pressure developed
by the pump is dropped through the valve and the load. To reduce the flow through the load, the valve's pressure drop must increase, so that
minimum flow would correspond to maximum valve pressure drop. With the pump working against the throttled valve, the valve's upstream pressure
would perhaps be highest at minimum flow.

If the pump speed is controlled or varied then what you describe might apply, but not if it's the valve that's controlling the flow.

Ken Irving
The subject is too large for valid help.
Think simple and right: flow depends upon DP.
Therefore, minimum DP=minimum flow.
Augment upstream pressure and manipulate some magic gadget to maintain same DP: did the flow increased ..answer = NO
Upstream/downstream pressure, solely have nothing to do with flow.
The producer is DP.
Close the valve: maximum upstream pressure, zero flow. Don't get confused: DP is not enough.
The valve is an orifice in your pipe. Big orifice for same DP will pass more flow than smaller orifice. Look at the base formula.
Flow depends upon DP and valve opening (the Cv)

It depends on your process which must be analyzed itself. Most processes will see a maximum flow at minimum delta pressures and minimum flow at maximum delta pressures.

Dr ir Gerrit M. van der Molen

A very simplistic equation to relate flow rate and pressure drop over a valve is:

W^2 = K*DP*RHO

where W = mass flow rate, DP = pressure drop, RHO = density of fluid (normally upstream of valve) and K = valve admittance (a parameter related to the Cv and valve opening, fluid viscosity and some other factors; too many to list here, even if I knew what they were!)

In this equation, the flow rate depends on the density, which will be constant for liquids but not for gases. Therefore, in the latter case
the flow rate does depend on the (upstream) pressure: if higher pressure, then higher mass flow.

Hope this helps,


PS If you use volumetric flow rate (F) instead of mass flow (W), the above equation becomes:

F^2 = K*DP/RHO

Therefore, the pressure-dependency for gas flows still holds, but if higher upstream pressure, then lower volumetric flow.

Dr ir Gerrit M. van der Molen
Advanced Control Technology (ACT) Club
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The flowrate trough a orifice all depends on the differential pressures. The lower the DP the more flow you will get and vice versa.