Pneumatic pressure test at low (mbar) or higher pressure (+/- 30 barg)

[samvanwesen]

Hello All,

We we going to do some external pressure tests on a thermowell on the flange rating, but the inspector insists of an internal pneumatic test + soap detection.

We were going to put 30 barg nitrogen in the thermowell, as this is easily read on our installed manometers.

The inspector insists on pressurizing the thermowell up to a couple mbar of nitrogen MAX. He states that it will be much easier to detect a leak like this.

Does anyone know what the reasoning behind this is?

PS asking the inspector is no option, as the answer is: because that is how it is.

We would like to know, as we would think at higher pressures, there is a higher leakage rate hence more soap bubbles.

Thanks!

Sam

 

[EdStainless]

I have air tested over a range of pressures, from 1bar up to 20bar.
In practical terms there is no difference.
The air flow through any small leak will be molecular flow and fully choked.
So the pressure doesn't matter much.
I have never tested at mbar except when that was the service pressure of a line.
The test solution isn't soap, it is a surfactant solution that is rated and certified for leak testing.
And it should be Cl free, and no soap would meet that criteria.

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P.E. Metallurgy, consulting work welcomed

 

[jmec87]

What are the applicable code of construction and jurisdictional requirements for this equipment? Those may be driving the inspector's reasoning.

Not sure if it would be applicable to a thermowell, but some valves (i.e. floating plug valves) seal more tightly at high pressures than very low pressures. The higher pressure pushes the sealing members together tightly and prevents leakage past them.

The other bonus of lower pressures is that in case of ejection, the thermowell should fly a bit slower than at a higher pressure.

 

[r6155]

The inspector is right, and I agree with EdStainless.
As a guide, see ASME V.

Regards

 

[JR97]

samvanwesen

if you are doing soap bubble testing, you want the low pressure differential across the boundary. Specs I have worked with would limit the differential to about 300mBar. If the pressure differential is too great, the soap solution will "blow out" or vaporize at a leak instead of forming bubbles and the leak detection is questionable. If you are immersing the component in a water tank to detect bubbles, the higher pressure would be fine.

JR97