Class IV Control Valve Leakage Rates 1

[BillyBadFace]

"Should the test pressure i.e. 50 psi be taken into consideration during leakage testing of a class IV control valve, to identify the "Rated Valve Capacity".

 

[btrueblood]

No, "rated valve capacity" is the maximum flow rate the valve is rated to pass, i.e. wide open Cv at maximum rated differential pressure.

 

[JimCasey]

I approach this problem differently:
Class 4 leakage is defined as 0.01% of rated valve capacity. NOBODY rates their valve capacity in flow units: they rate the capacity in Cv.

Example: Typical 4" globe control valve with a Cv of 200. Class IV shutoff Cv would be 0.01% of 200, or 0.02.
Testing the leakrate on 50 psi water, 0.08 gpm would be allowable.

 

[JimCasey]

Thinking about it more: why don't they just define Class IV as 3.89 x 10-4 gpm per rated Cv, tested on water at 50 psi.

Still, and important, and universally not recognized in the world: Leakage at actual process conditions may or may not be related to the test leakage. (unless the actual process condition is 50 psi water at 70F)

 

[NGiLuzzu]

I usually consider the the "Rated Valve Capacity" as the maximum flow rate through the valve at its "rated travel" (which usually means completely open) and at the relevant test conditions.
So, I would simply input the appropriate fluid, pressure and temperature data into a Sizing Program or equivalent formulas and would divide the resulting flow rate by 10000 in order to obtain the Class IV maximum allowable.

I agree with JimCasey when he says "leakage at actual process conditions may or may not be related to the test"; in addition, I recall about a similar statement written in a reference standard (I don't remember if FCI-70-2-2003, IEC 60534-4 or both).

There are a lot of other threads about leakage classes within this forum, maybe you'll find interesting to find out some...

Hope this helps, 'NGL

 

[BillyBadFace]

Gents,

Thank you very much for your replies they have been very helpful. I think I will use this site more often it seems to hsve good people subscribing to it.
Cheers
BBF

 

[btrueblood]

"Thinking about it more: why don't they just define Class IV as 3.89 x 10-4 gpm per rated Cv, tested on water at 50 psi."

Because that's not how the spec. (ANSI/FCI 70-2) is written. Max. rated flow is the max flow that the valve can pass, which occurs at the max. rated operating differential. Testing leakage at 1000 psi is expensive, so the spec. is written at a more comfortable 50 psi. Correlating the Class IV leak rate to operating conditions is "an excersize left to the reader", and yes, the spec. states you shouldn't try. If leak rate at a given shutoff pressure/temp./fluid type is of drastic importance, specify it on the purchase order.

 

[JimCasey]

Btrueblood,
I can understand your logic as to how you arrived at the opinion: "Max. rated flow is the max flow that the valve can pass, which occurs at the max. rated operating differential. " But I don't think that's what the writers of the standard meant.

FCI70.2 which has been essentially unchanged for years and was known before as ANSI B16.014 says 0.01% of rated valve capacity, which apprently means something different to you than it does to me.

I have always interpreted "rated valve capacity" as "Published Cv". I Believe that was the intent of the writers of the spec. However, a National Standard should not be open to interpretation so it is valuable that you express your differing opinion so that ultimately the standard can be clarified.

The reason I feel that "Rated Cv" is what they meant is one of practicality. Cv IS an index of valve capacity. What is the rated FLOW? It is not uncommon for the exact same valve to be offered in ASME Class 150, 300, and 600. Same trim, same flow path dimensions, just different flanges. Does the leakage increase when the rating changes to 300 from 150? The theoretical maximum Dp goes from 285psi at 100F to 740psi at 100F. The Cv does not change. If you run the temperature up to 500 degrees and the pressure rating decreases,thereby decreasing the allowable Dp and presumably the max flowrate, is the allowable leakage correspondingly decreased? Does the allowable leakrate change more if the valve is made of WCB or CF8M?

Or, is the valve's max flow governed by velocity? Enough flow to make 740 psid drop in a typical wide-open valve would cause such a high velocity that the valve/system would be eroded even if it was not experiencing cavitation (assuming water service). So would you rate the flow at (picking an arbitrary but high number for liquid) 50 fps velocity? Then take 0.01% of the 50 ft/second flowrate for the Class IV leakage rate? Then would you decrease the allowable leak rate if the piping supporting the valve was sch 80 and not sch 40? After all, Sch 80 pipe has a smaller id., and 50fps would occur at a significantly smaller flowrate.

My point is: Calculating the allowable leakage based on Rated flow is WAY too complicated, but leakage based on Rated Cv is simple, easily interpreted, and comparable from one valve to the next. FCI 70.2 is really only an indication of fit and finish of the valve components anyway (except possibly class V).