Pressure Drop Comparision

[macmet]

I am trying to compare the pressure drops of two valves in one line. The line is the cold water return to a hot water boiler.

The two valves are as follows,

#1 -> 6" globe. Cv = 440.

#2 -> 8" globe. Kvs = 765.

But, I found a conversion of "K" to Cv, which converts to a Cv of ~70. So valve #2 becomes,

#2 -> 8". Cv = 70.

This shows a significantly higher pressure drop in the 6" valve (IMO). I expected a higher pressure drop, but not one of that magnitude.

So, my questions are,

Is my opinion, that this is a significantly larger pressure drop, fair or reasonable?

And, I seem to recall that Cv values are unitless, in which case, shouldn't the Kvs be unitless? The spec sheet says Kvs is measured in m3/hr.

The conversion factor I used converted "K" to Cv, not "Kv" to "Cv". Is this the same thing or do I need a completely different conversion equation?

Thanks for the help.

 

[MikeHalloran]

Cv is NOT unitless, and the usual definition leads you to assume the WRONG units.

The real units are GPM / [ SQRT (psid) ]



i.e., saying that "Cv is GPM at 1.0 psid" is correct but misleading, and a STUPID way of defining anything, because 1.0 happens to be its own square root.


Mike Halloran
Pembroke Pines, FL, USA

 

[DLite30]

I found that Kvs=1.17*Cv

 

[DLite30]

Ooops...sorry...that is:

Cv=1.17*Kvs

 

[macmet]

Ok, well all these comments have made me realize I don't know enough about Cv. So, I am going to read up on it.

I thought Cv was proportional to pressure drop, but it looks like that is a wrong assumption.

I have some work to do.

 

[ione]

By definition of flow coefficient:

Cv, used with US customary units, is the flow capacity (USGPM) of water at defined temperature that will create a pressure drop of 1 lb/in2 (psi).

Kv is the flow capacity (m3/h) of water at a defined temperature that will create a pressure drop of 1 bar.

Conversion factor

Cv = 1.16*Kv

Note: Kvs is the Kv of a control valve fully open.

 

[macmet]

Alright, well after doing some ( long overdue) reading on the subject, my pressure drops seem more in line with what I was expecting.

However, the Kv value I have is Kvs. How would I convert that to Kv? Is it as simple as saying the pressure drop at 50% flow is 50% of wide open pressure drop?

 

[ione]

Suggest you take a read to the thread below

http://www.eng-tips.com/viewthread.cfm?qid=280761

 

[macmet]

Should I not be concerned that that is for ball valves and not globe valves?

One of the links in that thread showed Cv = Kv/14.28 which is quite a bit different than the equations I've been given so far.

 

[ione]

I can reassure that the correlation is:

Cv = 1.16*Kv


The second correlation:
Kv = 14.28*Cv

Stands when flow is expressed as l/min instead of m3/h.

Further to type of valve, you are right the attached paper was for ball valves.

For globe valves we have linear globe, where the flow is directly proportional to the valve lift, and fast opening which will give a large change in flow for a small valve lift from the closed position.
Suggest to consult your supplier which should have Cv vs % lift diagram for your specific valve.

 

[macmet]

I just wanted to close this thread up for anyone who searches for similar information in the future.

I ended up contacting the vendor, to get the Kv values at the various open/close conditions, and while doing so asked for the Cv values. The information he provided confirmed that Cv = 1.16*Kv.

Thanks everyone for your contributions, it was very helpful.

Cheers.

 

[sheiko]

Hi macmet,

Beware that "K" may stands for resistance coefficient (ie coefficients used to calculate pressure drops of various fittings a
d valves, as shown in various resources) and an equation do exist to convert any "valve flow coeff Cv" to a "resistance coeff K" in order to facilitate the calculation of the pressure drop through this valve (at 100% opening).
The above mentionned "K" is unitless and has indeed nothing to do with the SI units version of "Cv" ie Kv = 0.86*Cv (or Cv = 1.16*Kv).
Of course, as for "Cv", "Kv" can be transformed to a "resistance coeff K" as well.

"We don't believe things because they are true, things are true because we believe them."