×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

adjusted K- and Cv values for glycol solution

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.  

RE: adjusted K- and Cv values for glycol solution

Quote:

...for example, if in straight pipe i have 1.2 times the pressure drop compared to standard water

That is possible only when you are calculating pressured drop in pressure terms (for ex. bar) and not head terms (for ex. meters)

Quote:

Any idea? It should be some equation I can use in a software.

Did you check the Cv (or K) equation?

Cv = Q×(SG/ΔP)1/2

 

RE: adjusted K- and Cv values for glycol solution

Just ignore my first comment. That is plain stupidity.

RE: adjusted K- and Cv values for glycol solution

This topic is not well covered in the general literature. It has come up here before and a search for terms like "Cv" "Re" and "Reynolds" will get you a few hits. See for example thread378-258214: Flow Coefficient for fluid other than water
 

Katmar Software - Engineering & Risk Analysis Software
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

RE: adjusted K- and Cv values for glycol solution

(OP)
I followed that other thread and the best I came up with was using this procedure: http://www.irmdiesel.com/AmotDocsFR/Vanne%20thermostatique%203%20voies%20R%20en%20acierRvalve%2001.pdf

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

You dont adjust the K and Cv values you adjust the friction factor based on Reynold no.

Best regards

Morten

RE: adjusted K- and Cv values for glycol solution

(OP)
Morten: i don't understand why i would adjust friction factor. with a valve that has a Cv value, I don't really have a friction factor. since the geometry inside the valve is unknown, I also don't know the Reynolds number. Reynolds number and friction factor I only have in relatively straight pipe (at least in a way that i can calculate it)

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

Because thats how Darcys (or Fanning) law works - K is a dimensionless number where:

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

(OP)
MortenA:
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:
http://www.irmdiesel.com/AmotDocsFR/Vanne%20thermostatique%203%20voies%20R%20en%20acierRvalve%2001.pdf

would be redundant or contradicting to what you say.
 

RE: adjusted K- and Cv values for glycol solution

A valve is not a pipe.  Viscosity has little influence on the flow of what is, essentially, a variable-area orifice plate (at least, within the range of variation of water and glycol mixes at temperature up to near boiling).  Specific gravity, or density, however, directly affects the flow rate of fluid through an orifice plate.  Katmar pointed the OP to another recent thread, where all of this was hashed out in excruciating detail.  Adjust the specific gravity for temperature and glycol percentage, and the Cv law will compute the correct flow for the valve (well, assuming you have a valid Cv number in the first place).  

RE: adjusted K- and Cv values for glycol solution

(OP)
btrueblood: I think you are right. but besides density viscosity also needs to be accounted for. This document on page 7 http://www.irmdiesel.com/AmotDocsFR/Vanne%20thermostatique%203%20voies%20R%20en%20acierRvalve%2001.pdf
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

Certainly.  Though I'd be a bit nervous that the document you show has little information on correcting for sp. gravity, and instead ONLY publishes a correction curve based on viscosity.  Hopefully they did their maths correctly.

RE: adjusted K- and Cv values for glycol solution

(OP)
actually my equation should be viscosity in centistokes...

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)

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources