Needle valve sizing 1

[neweng2008]

Hi all,

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?
I appreciate your help.
Regards,

 

[JLSeagull]

I have not checked the Parker differential pressure capability. However, consider selecting a model that requires tools for adjustment. This could prevent someone from unintentionally closing the wrong valve.

 

[neweng2008]

Thanks for your comment. Yes this model has a set screw that locks the valve on desired settings.

 

[moltenmetal]

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.

 

[neweng2008]

Thanks for the info, moltenmetal. I have a DP type flow transmitter and I have checked with the manufacturer about the distance required to put the needle valve upstream of the FT. (around 20")
As for the laminar/turbulent flow, I should check it out. I think the flow is turbulant where I am intending to place the valve.
If I get the turbulent flow, the question will be what size?
with the model I have chosen, for the 1/2" valve I will have 3.22 psid pressure drop across the valve and for the 3/4" valve, I will have 0.50 psid press. drop.
Which one should I choose for this application?

 

[moltenmetal]

Those are pretty low pressure drops, indicating a comparatively large valve in both cases. Unless your oil is VERY viscous, you very likely don't have to worry about laminar flow.

Which one to use? I don't know anything about your application other than what you've told me, so can't advise you. What are you trying to do? The rule of thumb is that a control valve (manual or automatic) should represent at least 30% of the frictional pressure drop in the piping system such that it's the controlling resistance even when open. Like all rules of thumb, it can be broken successfully by those skilled in the art when necessary.

 

[neweng2008]

moltenmetal, thank you for all the information. very helpful.