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.

 

[IRstuff]

Doesn't that depend on whether your flow rate is specified in volume or mass?

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[CUBOID88]

IRStuff,

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?

 

[zdas04]

Wrong. While the density of an oil doesn't change materially with moderate changes in pressure and temperature, it does change dramatically with changes in molecular weight (i.e., changes in specific gravity).

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

 

[ione]

“Assuming pressure drop stays the same”.

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.

 

[IRstuff]

I think that you need to change the pressure to accommodate a different viscosity to maintain the same volumetric flow rate. Just compare the difficulty in pumping water with pumping gear oil.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[BigInch]

Valve flowrate in terms of the valve's Cv number is
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

 

[ione]

I can speak for my self, even if zds04’s post and mine seem to be on the same wavelength. My reply was mainly focused on friction pressure drop, anyway if we want to take into account minor losses too (i.e. valves, fittings, etc.) viscosity still keeps playing a role. The flow coefficient of a valve Cv is viscosity dependent throughout the Reynolds number, and it is particularly sensitive in the laminar region (check 2-K Hooper method and 3-K Darby method). Changing the fluid characteristics, that is passing from an oil to another, produces effects on pressure drop, and so I firmly believe that the assumption of a constant pressure drop is an illusion.

 

[BigInch]

Sure there are also effects from viscosity that can be included in the standard Cv equation when necessary, however, since the OP was asking about effects only from changes in specific gravity, I used the simple form of the Cv equation, which kind of ignores that because it assumes cool water with a viscosity of 1 cP. Should there be significant changes in viscosity, you would have to use a more complicated form of the Cv equation.

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

 

[btrueblood]

The effect of viscosity changes on flow rate through a typical control valve are about an order of magnitude lower than the effect of specific gravity changes.

If the valves are "minor losses" in a test rig for testing those valves, the op has other problems.

 

[BigInch]

minor losses, in this case amounts to only a figure of speach meaning losses other than frictional losses, which are usually "relatively minor" except in these cases of partially closed valves and/or w/ short pipes.

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

 

[btrueblood]

Understood and can definitely see where prior posts are coming from, and even agree with them - because in the real world the sp. g just doesn't vary that much from 1.0 (oil, alchohol, water, glycol - until you start pumping liquid metals that is), while viscosity change change several orders of magnitudes between those liquids, and even change by an order of magnitude or so across the useful temperature ranges of the liquids.

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.

 

[ione]

btrueblood,

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

 

[BigInch]

Yes that's true, but viscosity friction loss also requires a rather great length of pipe to add up to anything close to what you get from a pinched valve, so the distinction between "minor" and "major" is still very much relative to your system. Example: When you have small flowrates, even an enormously high viscosity won't add up to much pressure drop, even if your pipe is long, so its all relative what you can ignore between any two or more given systems anyway.

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

 

[btrueblood]

ione,

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.

 

[zdas04]

I recently did a comparison on a flowing gas stream. Varying viscosity by 50% (from half the value I got from NIST to twice that value) with all other things being equal, changed my flow rate (using the isothermal equation and a direct FIND for friction factor in MathCAD) by 4% from lowest value (1.3% less than the NIST viscosity) to the highest (2.7% over the value from the NIST viscosity). With liquid the spread would have been wider, but still not very wide.

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

 

[BigInch]

Nailed!

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

 

[ione]

I see your point guys.... anyway to establish whether density contribution on pressure drop prevails over viscosity contribution or vice versa it is necessary to define the flow regime.
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.

 

[BigInch]

3) Valves .. density

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

 

[CUBOID88]

Hi guys,

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