adjusted K- and Cv values for glycol solution
adjusted K- and Cv values for glycol solution
(OP)
I'm writing a program to calculate pressure drop in piping systems. It takes into account % of glycol, and temperature. this all works for straight pipe.
Since I don't want to use equivalent length for elbows, valves etc. I want to sue the K- and Cv values. But those are for standard water at 60F. how do I adjust the K or Cv value for different fluids?
One (bad) idea i had is to use the ratio of how the straight pipe differed from water. for example, if in straight pipe i have 1.2 times the pressure drop compared to standard water, then I use that 1.2 factor for valves etc. but this doesn't really take into account how the fluid behaves int eh valve (which may be thinner, hence turbulent vs. laminar etc.)
Any idea? It should be some equation I can use in a software. Not just some rule of thumb number.
Since I don't want to use equivalent length for elbows, valves etc. I want to sue the K- and Cv values. But those are for standard water at 60F. how do I adjust the K or Cv value for different fluids?
One (bad) idea i had is to use the ratio of how the straight pipe differed from water. for example, if in straight pipe i have 1.2 times the pressure drop compared to standard water, then I use that 1.2 factor for valves etc. but this doesn't really take into account how the fluid behaves int eh valve (which may be thinner, hence turbulent vs. laminar etc.)
Any idea? It should be some equation I can use in a software. Not just some rule of thumb number.





RE: adjusted K- and Cv values for glycol solution
That is possible only when you are calculating pressured drop in pressure terms (for ex. bar) and not head terms (for ex. meters)
Did you check the Cv (or K) equation?
Cv = Q×(SG/ΔP)1/2
RE: adjusted K- and Cv values for glycol solution
RE: adjusted K- and Cv values for glycol solution
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RE: adjusted K- and Cv values for glycol solution
On the last page is the viscosity correction graph. Is there an equation for that graph, assuming my software is not supposed to look at a graph? Ans i assume once I have the corrected Cv value, I still use it with the density of the actual fluid (which also changes with temperature, and type of fluid)
Many of the recommendations in the thread (and elsewhere) don't take into account, that we really may have completely different flow, for example even in straight pipe i easily go to turbulent flow by adjusting temperature or pipe diameter just a bit. A valve has some thinner, curved etc. parts that we don't know (hence the manufacturer gives us the Cv value).
thanks for the help so far. this was much more than years of google :)
RE: adjusted K- and Cv values for glycol solution
Go to www.aft.com and download the Fathom or Impulse Viewer and look into the help files for the design basis.
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RE: adjusted K- and Cv values for glycol solution
Best regards
Morten
RE: adjusted K- and Cv values for glycol solution
I know how to have my software calculate Reynolds number and friction factor in straight pies for all different fluids, if that is what you meant.
RE: adjusted K- and Cv values for glycol solution
dH=f*L/D*U^2/2g (variables assumed self explanetory even though various letters are used in various text book)
and
K=fL/D
f is a funtion of abs roughness and Reynold no (Re)
Re= density*V*D/vicopise (its dimensioless)
There are numerous correlations between f and Re, Abs roughness f.eks. Churchill, S.W., 1977, "Friction factor equations spans all fluid-flow ranges.", Chem. Eng.
So instead of changing your K - that should be fixed - use the existing correlations to change your f.
Its true for a valve you dont (normally) have a friction factor - but you do have a K
Your calculation may need an adjustment of the K values for low values of Re (this is true for any type of components, valves, bends, orifices ect. One reference for these correction is "Internal flow systems# by DS Miller.
Best regards
Morten
RE: adjusted K- and Cv values for glycol solution
it looks like what you are suggesting is to determine an equivalent length and a corresponding friction factor? I'm a bit concerned since the equivalent length isn't so good - but I have to think more about if mathematically thsi isn't the same. But my fear is, your method would assume the diameter is the same as in the pipe, which isn't the case for fittings.
I'm also not sure how this method (page) 7:
ht
would be redundant or contradicting to what you say.
RE: adjusted K- and Cv values for glycol solution
RE: adjusted K- and Cv values for glycol solution
shows that the head (of that specific fluid - to get the pressure we need the density) depends on K, which has a correction factor depending on viscosity. The chart on page 7 shows that. I entered plenty of the values in Excel and tried some curve-fitting. It seems the logarithmic fit looks the best: "K-correctionfactor" = 0.075 ln(viscosity in cp)+ 1.0471. I know, it is a bit off at standard water. So we need to:
- find the correction factor for K (with the chart, or my equation)
- calculate head with K
- calculate pressure drop using density of the fluid. So we have 2 parts where the fluid properties other than water make a difference.
Does that sound reasonable?
RE: adjusted K- and Cv values for glycol solution
RE: adjusted K- and Cv values for glycol solution
the document shows the head calculation.... which is independent of the fluid. The actual pressure will be calculated separately.
So, i calculate 100 ft. If that is water, it is 100 ft water pressure. If that is glycol, it is 100 ft glycol pressure... actual pressure (in psi or Pa) will be calculated by head times gravity of the fluid. so they don't neglect the topic, they jsut fucus on head only (as opposed to pressure)