Specific Gravity vs Flow Rate
Specific Gravity vs Flow Rate
(OP)
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.
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.





RE: Specific Gravity vs Flow Rate
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
Chinese prisoner wins Nobel Peace Prize
RE: Specific Gravity vs Flow Rate
Sorry yes, I just realised I'm talking volumetric flow rate here so clearly it is viscosity and not specific gravity that an oil change would have on flow rate right?
RE: Specific Gravity vs Flow Rate
Density of water at 60°F is 62.4 lbm/ft^3. Density of 0.7 SG oil is 43.7 lbm/ft^3 and 0.8 SG oil is 49.9 lbm/ft^3. To determine a friction factor you need to know a Reynolds Number which is a function of density, velocity, pipe ID, and viscosity.
In addition, mostly what you can measure is volume flow rate--every liquid meter that I've ever messed with wanted a fluid density as an input (there are a couple that claim to calculate a fluid density as an output, but when you dig into the arithmetic they are making a couple of iffy assumptions).
David
RE: Specific Gravity vs Flow Rate
This statement made me think a bit. In order to land to this conclusion Reynolds number has to be the same despite the different oil used in the test rig (geometry is obviously unchanged). So the change in viscosity (dynamic viscosity) and in density (as density changes with specific gravity, as already pointed out by zds04) must compensate each other, or, that is the same, kinematic viscosity has to remain constant.
RE: Specific Gravity vs Flow Rate
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
Chinese prisoner wins Nobel Peace Prize
RE: Specific Gravity vs Flow Rate
Q = Cv * (dP / SG)0.5,
where Q is gpm, dP in psi
so there is clearly some effect when volumetric flowrate is calculated using that method. Furthermore, since Q * SG * Density water = liquid mass flowrate, it would seem that the mass flowrate is also affected by SG
Let your acquaintances be many, but your advisors one in a thousand' ... Book of Ecclesiasticus
RE: Specific Gravity vs Flow Rate
RE: Specific Gravity vs Flow Rate
Let your acquaintances be many, but your advisors one in a thousand' ... Book of Ecclesiasticus
RE: Specific Gravity vs Flow Rate
If the valves are "minor losses" in a test rig for testing those valves, the op has other problems.
RE: Specific Gravity vs Flow Rate
Let your acquaintances be many, but your advisors one in a thousand' ... Book of Ecclesiasticus
RE: Specific Gravity vs Flow Rate
But, even in the friction losses in a pipe, the rho or sp. grav. term is a direct, square-root function driver on pressure loss, whereas the viscosity comes in via a much weaker function of Re and surface roughness. Ignore it at your peril.
RE: Specific Gravity vs Flow Rate
I'm missing something here. Both density and viscosity enters pressure loss via friction factor which is related to Re. So when speaking about friction loss I can't understand how density plays a predominant role over viscosity
RE: Specific Gravity vs Flow Rate
Let your acquaintances be many, but your advisors one in a thousand' ... Book of Ecclesiasticus
RE: Specific Gravity vs Flow Rate
Yes, specific gravity and density are included in Re, which is used to determine the friction factor f, for viscous losses.
But, if you are calculating the pressure drop across a length of pipe, you use an equation something like:
dP = 0.5 * rho * V^2 * f * L/D
where dP is the pressure loss, rho is mass density, or specific gravity times rho for water, if you will, V is fluid velocity, f is a friction factor (Moody or similar), L is pipe length and D is pipe diameter.
Pressure loss is thus a direct 1:1 function of changes in specific gravity (assuming velocity V stays the same - which it won't, but that's another can of worms), plus a little bit more from the change it induces in Re. The viscosity and Re effects only come into play via the friction factor term, f. For typical piping, unless you are well down in the laminar regime, well below transition, the dependence of f is nowhere near a direct factor on dP, and in many cases (typical rough pipe surfaces) approaches a constant value with changes in Re.
My bigger point is emphasized by BigInch - viscosity has minimal effects on valve performance, much bigger effects on piping (and perhaps pumping) losses, so unless the "test rig" in the OP's shop has pressure taps say, 30 diameters upstream and downstream of his valve, the viscosity changes are unlikely to have much effect on his valve test measurements.
In the real world, where the valves are applied by you and zdas, those pipes and valves are all part of a system, and you need to keep track of both viscosity and density.
RE: Specific Gravity vs Flow Rate
Viscosity plays into the friction factor. Specific gravity plays into both friction factor and the actual flow equation. It is a much bigger deal.
David
RE: Specific Gravity vs Flow Rate
Let your acquaintances be many, but your advisors one in a thousand' ... Book of Ecclesiasticus
RE: Specific Gravity vs Flow Rate
As extreme positions:
1) If you lay in a region of the Moody chart where the friction factor is independent of Re, then viscosity will not affect pressure drop.
2) If you deal with a horizontal pipe and with a low Re, then density won't play a role, as Poiseuille-Hagen equation defines pressure drop.
RE: Specific Gravity vs Flow Rate
Let your acquaintances be many, but your advisors one in a thousand' ... Book of Ecclesiasticus
RE: Specific Gravity vs Flow Rate
Ok, thanks a lot for the extensive replies. It seems however the discussion has spiralled slightly beyond the scope of my question I think. I think my description of "I change(on a test rig for valves etc) from using one oil to another oil" was slightly misleading. What I meant by changing the testing medium was that the unit we have is supposed to be tested with a particular fluid, yet our test rig is calibrated to run on a different fluid of lower specific gravity. In other words all flow rates/pressures sensors on the rig are designed to measure for that fluid only. So, if I'm required to supply a particular flow to the inlet port of the unit (flapper valve) to achieve a certain inlet pressure, does the fact that the testing medium is different require me to adjust the original flow rate and pressure values specified?
Thanks
RE: Specific Gravity vs Flow Rate
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.
RE: Specific Gravity vs Flow Rate
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.
RE: Specific Gravity vs Flow Rate
Let your acquaintances be many, but your advisors one in a thousand' ... Book of Ecclesiasticus
RE: Specific Gravity vs Flow Rate
RE: Specific Gravity vs Flow Rate
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.