Rate of Pressure Drop Vs Volumetric / Mass Flowrate

[Ross1985]

Hi all, I'm looking for a bit of advice.

When conducting a valve leak test, we pressure the pipework upstream of the valve, record the temperature and pressures (upstream and downstream), leave for 30 minuntes, and then record temperatures and pressures again. We then use ideal gas law (PV=ZnRT) and density at standard conditions to calculate the leak rate through the valve in kg/min. We then determine pass or fail against the performance standard.

However, we have been asked by a 3rd party consultancy to change the way we report this in terms of rate of pressure drop instead of mass (or volumetric) flowrate.

This doesn't seem right to me as a leak rate should be reported in standard conditions to allow us to compare 'apples with apples', as well as having different fixed volumes for different valves across the plant which would skew rate of pressure drop. Am i missing something?

I hope this makes sense!

 

[ashtree]

Ross1985

I agree that you are doing it the correct way but its most likely that either the consultants or their client do not understand the way you are doing it or the numbers so pressure loss is just a quick/easy/dirty way of getting something they think is important.

As you are already collectting the pressure data give it to them that way as well.Obviously it will tell them something but you might want to put some explanations around it when the numbers in different sized and rated systems dont look the same.



Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[LittleInch]

"We then determine pass or fail against the performance standard"

What does the performance standard quote the pass/fail in? my guess is a standard flow or mass flow. If so just point the consultant to the performance standard. Simply state that as the volume of gas varies across the plant or size of valve, use of pressure drop is not correct and you will continue to abide by the performance standard.


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[adammal44]

What do they mean by "pressure drop"...isn't this a leakage test for a closed valve? Unless you're doing a throttling valve or they're trying to infer leakage in a very odd way, that doesn't seem to make sense.

What application is the valve?

 

[Ross1985]

Good morning guys, thank you for your quick replies and feedback.

We are the duty holders of the platform so our technical authorities are the custodians of the performance standard. The application of the valve is a boundary emergency shutdown valve on a riser for one of the subsea tiebacks, so the test is conducted against a closed valve.

I think what the consultancy are getting at is that we conduct the test by recording pressure, so they want to see an allowable change in pressure in the performance standard which I don't think is correct, and is what I was looking for a second opinion on.

Thanks again for your feedback guys.

Ross

 

[mbt22]

With the second bit of information it makes more sense. Could it be a 'show your working' type of concern? They want the performance standard to detail what is actually measured rather than a calculated quantity.

A mid-ground might be to include how the calculation is carried out using which instruments in the standard, then you don't risk someone coming up with a 'better' way to calculate leakage rate.

Matt

 

[Ross1985]

@ MBT

I do actually save the leak off test record sheet, and the calculation together but don't tend to share it with every man and his dog. You're probably right though, if people had access to it and could see the working then it might deter them from coming up with 'better' solutions.

Thanks again for replying.

Ross

 

[BigInch]

With a continuous pipe streaming fluid flow, able to maintain a constant pressure within despite the leak, then there would be no rate of pressure drop at all. The pressure drop across the valve would presumably be constant, if inside and outside pressure did not change.

Rate of pressure drop measurement only makes any sense if it measures leakage from a closed container, however that would change as the pressure in the container dropped, so you could more easily give a Cv coefficient, such as ft3/sec lost per square root of the pressure differential across the valve.

 

[sshep]

Hey Ross,

Is it possible that they are asking for the leak rate in dP*V/time style units (mbar*l/s)?

This way to express leak rate seems a common convention, although not especially useful at an operations level. The conversion to a mass leak rate is via the system volume and ideal gas relation in the same manner that you have done.

best wishes,
sshep