Calculating Flow Coefficient Through 2 Valves in Series
Calculating Flow Coefficient Through 2 Valves in Series
(OP)
Hi Guys,
I have been looking for a formula which can be used to calculate the overall Cv values for 2 valves installed in series and came across the following from http://www.engineeringtoolbox.com/adding-kv-cv-d_1.... The formula given reads as follows (I have substituted Kv for Cv):
1 / (Cvt)^2 = (1 / (Cv1)^2) + (1 / (Cv2)^2)
Which can be re-expressed as the following:
Cvt = (Cv1 x Cv2) / (Cv1^2) + Cv2^2)^0.5
So based on the above, for the valves I am dealing with (Cv of single valve @ 90˚ = 9482):
Cvt = (9482 x 9482) / (9482^2 + 9482^2)^0.5
Cvt = 89,908,324 / (89,908,324 + 89,908,324)^0.5
Cvt = 89,908,324 / (179,816,648)^0.5
Cvt = 89,908,324 / 13409.573
Cvt = 6704.79
So based on the above the calculated total Cv for the 2 valves (ignoring pipework etc.) would be 6705 (rounded up).
Does the above appear correct? The answer does seem like it would make sense!!!
I have been looking for a formula which can be used to calculate the overall Cv values for 2 valves installed in series and came across the following from http://www.engineeringtoolbox.com/adding-kv-cv-d_1.... The formula given reads as follows (I have substituted Kv for Cv):
1 / (Cvt)^2 = (1 / (Cv1)^2) + (1 / (Cv2)^2)
Which can be re-expressed as the following:
Cvt = (Cv1 x Cv2) / (Cv1^2) + Cv2^2)^0.5
So based on the above, for the valves I am dealing with (Cv of single valve @ 90˚ = 9482):
Cvt = (9482 x 9482) / (9482^2 + 9482^2)^0.5
Cvt = 89,908,324 / (89,908,324 + 89,908,324)^0.5
Cvt = 89,908,324 / (179,816,648)^0.5
Cvt = 89,908,324 / 13409.573
Cvt = 6704.79
So based on the above the calculated total Cv for the 2 valves (ignoring pipework etc.) would be 6705 (rounded up).
Does the above appear correct? The answer does seem like it would make sense!!!





RE: Calculating Flow Coefficient Through 2 Valves in Series
They have a hard time distinguishing between series and parallel.
Cv for series assemblies add directly.
If it ain't broke, don't fix it. If it's not safe ... make it that way.
RE: Calculating Flow Coefficient Through 2 Valves in Series
I'm not sure what your point is. The image you have embedded shows 2 valves installed in parallel, not in series. If you scroll down the web page I linked in my earlier post it shows and image of the valves installed in series, along with the calculation for this. Cv for parallel installations is a case of adding the 2 Cv values together, but not for series (nor would I expect this to be the case)
RE: Calculating Flow Coefficient Through 2 Valves in Series
They have a hard time distinguishing between series and parallel.
Cv for series assemblies add directly.
If it ain't broke, don't fix it. If it's not safe ... make it that way.
RE: Calculating Flow Coefficient Through 2 Valves in Series
What do you mean by 'Cv for series assemblies add directly' - are you able to give us an example?
If I was installing the valve I mention above in PARALLEL then the calculation would would be:
Cvt = Cv1 + Cv2
Cvt = 9482 + 9482
Cvt = 18,964
Giving an overall Cv of 18,964.
RE: Calculating Flow Coefficient Through 2 Valves in Series
Matt
Quality, quantity, cost. Pick two.
RE: Calculating Flow Coefficient Through 2 Valves in Series
Zimon. NEVER BLINDLY USE FORMULAS, ESPECIALLY THOSE YOU GET OFF THE INTERNET!
If it ain't broke, don't fix it. If it's not safe ... make it that way.
RE: Calculating Flow Coefficient Through 2 Valves in Series
I would say that in SERIES the valves are installed on the same line, one after ther other as follows:
--->--->----
But in PARALLEL, the line / flow is split so there are 2 PARALLEL lines running alongside each other:
--->---
--->---
This is how it is depicted on the web page I linked to which I believe is correct.
Are you trying to tell me that the pressure drop through 2 valves on the same line is less than it would be through one valve? That wouldn't make any sense!
RE: Calculating Flow Coefficient Through 2 Valves in Series
For the same overall differential pressure, you'll flow less through two series valves, and more through parralel set up, so your Cv for parrallel operation is greater than the Cv for series.
RE: Calculating Flow Coefficient Through 2 Valves in Series
Good luck,
Latexman
RE: Calculating Flow Coefficient Through 2 Valves in Series
The equation above (except for an extra closing parenthesis) is a valid restatement of the initial equation (and I got the same answer as the OP). Logically, if I get a dP in the first valve then the second valve has a lower dP available so the answer should be less than either valve's Cv.
David Simpson, PE
MuleShoe Engineering
"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
RE: Calculating Flow Coefficient Through 2 Valves in Series
Good luck,
Latexman
RE: Calculating Flow Coefficient Through 2 Valves in Series
<Quote>
Control Valves in Parallel
The resulting Kv or Cv if two valves are installed in parallel can be calculated as
Kvt = Kv1 + Kv2 (1)
where
Kvt = resulting Kv
Kv1 = Kv valve 1
Kv2 = Kv valve 2
Control Valves in Series
The resulting Kv or Cv if two valves are installed in series can be calculated as
1 / (Kvt)2 = 1 / (Kv1)2 + 1 / (Kv2)2 (2)
<End Quote>
The formulas are reversed! Series valves are ADDED together, just like any other fitting. If I have two normal valves on a pipe run, in series, and I am looking to find out the losses, I ADD them together, along with the pipe lengths, fittings, etc. that constitute the piping structure. If the valves are in PARALLEL, the losses are resolved by the second equation, which they have mistakenly put under series valves. Flow resistance is handled just like any other resistance (electrical, heat, mass, etc.), if the resistance is in series, the terms are added
Rt = R1 + R2 + R3 + ...
if in parallel, they are handled as
1/Rt = 1/R1 + 1/R2 + 1/R3 + ...
Matt
Quality, quantity, cost. Pick two.
RE: Calculating Flow Coefficient Through 2 Valves in Series
Now, we know that for pipe fittings in series we can add K values. We all do this every day. Cv and K are related by Cv = 29.9 x d2/√K (Ref Crane 410 Eq 3-16)
To simplify the math assume constant d and we can make 29.9 x d2 = J and now
Cv = J/√K or K = (J/Cv)2
The total resistance = KT = K1 + K1 = (J/Cv1)2 + (J/Cv2)2
The combined Cv = J/√KT
And with a bit of manipulation you have the exact formula you quoted.
There is a bit of confusion with people quoting the engineeringtoolbox formula. The Kv that they have used is the European version of Cv and not the resistance factor K. As I stated above, for resistance factors in series you can add the K values, but not the Kv values. It looks like zimonmayo understood this from the beginning, but many people (particularly those in the USA who don't often come across the European Kv) do get confused by the similarity.
Katmar Software - Uconeer 3.0
http://katmarsoftware.com
"An undefined problem has an infinite number of solutions"
RE: Calculating Flow Coefficient Through 2 Valves in Series
So as Katmar says, it is a conductive factor which means that parallel valves should be a direct add while series valves would be a cumulative effect to lower the Cv below the lowest value.
David Simpson, PE
MuleShoe Engineering
"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
RE: Calculating Flow Coefficient Through 2 Valves in Series
Cheers
RE: Calculating Flow Coefficient Through 2 Valves in Series
------------------------------------
OK, so this is the same equivalent pipe for a looped pipeline problem, but with valves. I agree that with the same differential pressure, there's more flow with two parallel valves, just as with 2 parallel pipes.
---------------------------------------------------
Zimon, sorry I got off on the wrong perspective.
I must apologize to the Toolbox too.
To make amends, a spreadsheet.
If it ain't broke, don't fix it. If it's not safe ... make it that way.
RE: Calculating Flow Coefficient Through 2 Valves in Series
If it ain't broke, don't fix it. If it's not safe ... make it that way.
RE: Calculating Flow Coefficient Through 2 Valves in Series
RE: Calculating Flow Coefficient Through 2 Valves in Series
The Cv of two valves in series is NOT the some of the two Cvs.
Someone is getting Cv confused with "K" pipe resistence factors.
The original analysis is in factor correct.
RE: Calculating Flow Coefficient Through 2 Valves in Series
If it ain't broke, don't fix it. If it's not safe ... make it that way.
RE: Calculating Flow Coefficient Through 2 Valves in Series
What would I ever do without spellcheck?
RE: Calculating Flow Coefficient Through 2 Valves in Series
RE: Calculating Flow Coefficient Through 2 Valves in Series
Your spelling is absolutely the smallest of your problems. By the time you posted, everyone who had been confused had admitted their mistake and everyone was on the same page. Your chastising the people who made errors was kind of beating a dead horse.
David Simpson, PE
MuleShoe Engineering
"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
RE: Calculating Flow Coefficient Through 2 Valves in Series
Matt
Quality, quantity, cost. Pick two.
RE: Calculating Flow Coefficient Through 2 Valves in Series
David Simpson, PE
MuleShoe Engineering
"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
RE: Calculating Flow Coefficient Through 2 Valves in Series
If it ain't broke, don't fix it. If it's not safe ... make it that way.
RE: Calculating Flow Coefficient Through 2 Valves in Series
If you want to see such a valve go to the Hobbs Valve website and look for their Double Block and Bleed Valve.
RE: Calculating Flow Coefficient Through 2 Valves in Series
Good luck,
Latexman