Specific Gravity vs Flow Rate 1

[CUBOID88]

Hi,

Just a quick question, if I change(on a test rig for valves etc) from using one oil to another oil that has a different specific gravity, and I need to supply a particular flow rate, do I need to account for the change in specific gravity in my flow rate, assuming pressure drop stays the same? I can't seem to find anywhere that suggests specific gravity has an effect on flow rate.

Thanks.

 

[ione]

You’ve already received a valid answer from BigInch (19 Apr 11 18:34)

Q = Cv * (dP / SG)0.5

You’ve changed the specific gravity and so in order to get the same pressure drop, the flow rate has to be changed. If you were in the turbulent regime flow and stay there again, the flow coefficient of your valve will be the same. In the laminar regime flow, variation of Re will affect your Cv in a non trivial way.

 

[btrueblood]

"3) Valves .. density " Was gonna post similar, but didn't want to look like I was beating dead horses.

Ione gives a very valid reply, i.e. it is good to check the simple Cv equation with actual testing. Alternatively (lacking test confirmation), it is good to tell your customer how you arrived at any test data that you are reporting, and whether any conversions/corrections have been made to account for fluid density differences. All (good) valves will have a laminar region of flow near shutoff, and if this is part of your specification, you need to worry about it.

 

[BigInch]

What's with the humanitarian concern about dead horses all of a sudden?

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[btrueblood]

Some people don't like their horsemeat tenderized. Or so I've heard.

 

[Pumpsonly]

Very interesting discussion.
If we go back to the fundamental of fluid mechanics. All losses are suppose to be expressed in Head of the fluid. We use the term pressure in practical calculation and measurement is because it is the only practical way of measuring actual conditions. You will note that in all formula using pressure as unit,the SG or density of the liquid is always present.

CUBOID88,
In answer to your repost of 21-April, if your flow meter is a differential pressure type and the read out is in direct volumetric flow, then you need to make the necessary correction with the SG and the viscosity of different fluid.
If your it is of the PD type, then you only need to consider the difference in viscosity.

If your test rig out put is a computerized one, you will definitely find those factors you need to change in the program.