Cv and acceptable leakage of air

[simar187]

We have a butterfly style damper for air service on an industrial system that was certified by the manufacturer as ANSI/FCI70-2 Class IV, which allows for a leakage rate of 0.01% of damper capacity. The Cv of the damper is 32042 and if we were to do a water test, at 1 psi differential, we would expect the acceptance criteria to be 0.01% x 32042 = 3.2 USGPM. However, we will be doing our test with air and are somewhat confused as to how to determine the acceptance criteria in SCFM of air. It has been debated that Cv is unitless so as long as we are considering standard conditions that the same logic applies as shown with water earlier. That is, 0.01% x 32042 = 3.2 SCFM. When we test this logic using a Cv calculator online, with 32042 as an air flow input value at 1 psi differential, at standard conditions the resultant Cv is not 32042. We are trying to do the test and write our test procedure and any help to clarify our confusion would be appreciated.

 

[MikeHalloran]

Cv for water is not unitless.

Its real units are GPM/sqrt(PSId).

I'd like to, er, throttle, the savant who came up with the usual statement of "GPM at 1.0 PSID", which, because 1 is its own square root, hides the nature of the number.

More detail here:

http://www.eng-tips.com/viewthread.cfm?qid=124819&page=6

I don't remember off the top of my head how to correlate it to air flow.



Mike Halloran
Pembroke Pines, FL, USA

 

[simar187]

Many thanks for your reply Mike. Having checked out the thread you mentioned, we are still at a loss as to how we establish the acceptable air leakage in scfm that qualifies the damper to Class IV? And if we take a different tack, how would a manufacturer calculate the Cv of a given valve without a test - that is based on free flow area, rated velocity and 1.0 psid. Would they have to calculate based on air to use the resultant Cv or would it have to be based on water and then use the resultant Cv and how would the two values differ given they would both be based on standard conditions? Note that the ANSI/FCI standard allows for the leakage test to be by air or water.

 

[JimCasey]

FCI70.2 specifies the test to be nominally at 50 psid.
Measuring the leak rate at other differential pressures may be useful, but you can't extrapolate that to "Class IV". Conversely, leak rates determined in class IV testing cannot reliably be extrapolated to other conditions, as in: I have a class IV valve, how much 1000 psi steam will it leak?

With this valve Your Cv at allowable leakrate would be 3.2.
The allowable flow with 50 psi air to atmophere would be 6251 SCFH

 

[simar187]

Hi Jim,

First, this is a damper not a valve and ANSI/FCI says the test medium pressure will be as you said, 45-60 psi but it also says OR within +-5% of the maximum operating differential which would be 0.75 psi in our case at hand. So, if we were testing at 1.0 psi with air then it would appear that you would agree the allowable leakage would be 3.2 scfm (192 scfh)which would require some correction because the actual test would be at 0.75 psi which would be?

Many thanks for your help

 

[JimCasey]

In the Flowserve sizing program I get 12.047 SCFM with 1 psig air dropping to atmosphere across a Cv of 3.2.

Damper/valve distinction: Apply the duck test.
Besides: Class IV is class IV.

Gas sizing equation is Cv=Q/(1360 P1 Y sqrt(xt/(GTZ)))
Q is in SCFM
P1 is PSIA inlet pressure
Y is expansion factor =0.96 in this case
X is pressure drop ratio DP/P1=0.068
G=specific gravity =1
T=460R
Z=compressibility=1

When I run the equation manually I get a leakage flowrate allowable of 25 scfm. The computer does more correction for temp and other thermodynamic parameters, so I trust it better, but the flowrates are at least the same order of magnitude.