Calculating Flow Coefficient Through 2 Valves in Series 7

[zimonmayo]

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_1120.html.

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!!!

 

[BigInch]

Ya. I saw that upside down valve and couldn't believe anything on that website after that.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

 

[GHartmann]

Yea and spell "sum" as some.

What would I ever do without spellcheck?

 

[GHartmann]

OOPS I meant I had the misspelling.

 

[zdas04]

GHartmann,
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.

 

[MatthewL]

Umm, not everyone, Dave. Just got back on and saw I made a bad assumption (you know what they say about assume, don't you? Only makes an a$$ out of u and me ) Sorry for leading you around the bush, zimonmayo, and thanks to all for catching it!

Matt

Quality, quantity, cost. Pick two.

 

[zdas04]

You know that the very best thing about eng-tips.com is that if you make a mistake, someone will catch it and get the train back on the tracks. Even people as active as some of the folks in this thread (probably the three or four most active members of the Piping forum) get it wrong sometimes. When they (we) do get it wrong, someone is in the middle of our error within the hour--where else can you get that?

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.

 

[BigInch]

It was interesting thinking of valves in that way. I've never had the need to replace two valves with an equivalent valve. I have done the equivalent pipe thing, but way back before the days of the PC when every extra step was a pain. The math of finding the equivalent valve is more complex then copying and pasting a duplicate. I still can't think of why it would it be necessary to find an equivalent valve. So Zimon, what are you up to? Why?

If it ain't broke, don't fix it. If it's not safe ... make it that way.

 

[zimonmayo]

I was trying to calculate the Cv for a Double Block and Bleed Metal Seated Triple Offset Butterfly Valve which is basically 2 valves in one casting. I only have the Cv rating for a single disc / valve so this calculation should give me the Cv for the complete valve. Ok there is a section of valve body to take into account but this is minimal and is full bore so for the purposes of my application I am happy with the value obtained.

If you want to see such a valve go to the Hobbs Valve website and look for their Double Block and Bleed Valve.

 

[Latexman]

Sounds like an open/closed type valve to me; not a throttling operation. So, it makes me wonder why the velocity head method (K) is not used. Then, the K's of the two valves can be added together for the K of the assembly.

Good luck,
Latexman