I need to size a needle valve that is placed before a flow transmitter on the lube oil inlet line to a bearing.
I know the amount of the flow thru this line (1.9 gpm) and also the model I need to choose from is Parker N series. The oil specific gravity is 0.85.
my line size is 3/4".
What is the acceptable diff. press. that I can size this needle valve for?

Two things to watch out for:  one is that many types of flow transmitters don't like to be downstream of a needle valve or any other device which produces a very non-uniform velocity profile.  You may need a long length of tubing/pipe between your valve and your flow transmitter to settle the velocity profile back down again.  Check with the flowmeter mfg to make sure, before you decide where the needle valve must go.

The other is to watch out for laminar flow.  Your flowrate is large enough that this may not be a problem, but remember that the Cv of a valve only valid in fully developed turbulent flow.  If your lube oil is viscous enough, flow through the valve trim you need may be transitional or even laminar, in which case it becomes strongly viscosity dependent.  The viscosity correction factor methods for large control valves DO NOT work well for small ones.  In laminar flow, the actual flow the valve will be capable of passing at a given pressure drop will be a great deal SMALLER than you predict.

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