Pressure test passed but vacuum test failed

[seagal3232]

Hello everyone,

My company is working on a vessel that can work under pressure (around 3-5barg) and under vacuum (-0.5barg),

So far the positive pressure tests were successful with no leaks or pressure drops, but during the vacuum test, the pressure rises really quickly,

Is this normal? My only explanation is that some of the fittings may not work under vacuum, or something similar,

Any ideas of what could be happening are appreciated,

Regards

 

[3DDave]

Also can be temperature change.

 

[seagal3232]

It's all under ambient temperature, and you can hear the leak whistle.

I should add that the vessel is weighted so it's all connected with stainless steel hoses, 60 degree cone fittings

 

[LittleInch]

You've answered your own question.

Many seals etc are designed only to resist internal pressure, not vacuum. Chevron seals or lip seals don't work well in the "reverse" direction.

What sort of "fittings" do you have?
Check the data sheet and vendor data to see if they are qualified for vacuum / negative pressure.

You could also be getting collapse of hoses?

We don't have a lot to go on here.

"Vessel is weighted" ?? Eh
Hoses or flexibles?


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

 

[seagal3232]

Sorry, I mean flexibles yes, stainless steel, 60 degree cone fittings. I'll check with my supplier, thank tou

 

[Heaviside1925]

OP,
A few thoughts.
To 3DDaveus point, just as heat of compression, pulling a vacuum will have the opposite effect. Much more noticeable, any condensables could be boiling off, further reducing the temperature. Steady state will only be reached once the remaining volume heats back up with ambient warming, in turn, a pressure rise.

Consider a pin hole leak. It's an orifice, so it has choked flow, at a certain differential pressure, no more flow can be realized. This choked flow may be small compared to your pressure test, so even though the internal pressure may be high, only the flow dictated by the choked flow differential pressure will be allowed. Whereas with the vacuum, since the differential pressure is much small, the leak may be much more noticeable.

Also consider the pin hole again. During a pressure test, the flow will be from the inside of the vessel out. So, any particulate matter, fluids, corrosion may block the hole. During a vacuum test, flow is in the opposite direction, so clean, dry air is flowing through the same hole unrestricted.

Could it be a fitting? Yes, could it be the vessel? Yes. You are just going to have to find the source of the leak.

 

[SnTMan]

...you can hear the leak whistle.

Is the vessel generally accessible? Can you go round it with a mechanics stethoscope?

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[r6155]

Please more information: diameter x length and shell thickness.
RT exam extension? and PT/MT?

Regards

 

[r6155]

Perform a minimum air pressure to detect leaks.
I guess some leaks at the welds.

Regards