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Posted By NSO
Could someone please advise or at least point me in the general direction of where to look to find the information I need.
Put simply we fabricate and weld together mild steel to form a box, the box is then filled with compressed air to be pressure tested to check for leaks. What I need to know is what is a recommended limit (psi or bar) to safely pressure test. From somewhere I have the figure of 2 bar lodged in the old grey matter. I know we could fill the box/tank with water to reduce force should it rupture but we don't want to fill with a liquid because of rust.
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Posted By jom
I think you need to know the pressure at which the box becomes defined as a pressure vessel, which will trigger many regulatory rules.
Is it practical to plug the box and submerse it in a tank of water for a period of time, then inspect to see if water entered the box?
Another "outside the box" idea: flood the box with steam, solidly plug it and allow to cool.
A perfectly sealed box without flawed welds will collapse. If it does not collapse, it has leaks - reject it. I think that's the simplest and safest solution.
John.
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Posted By Konstanty Budkiewicz
NSO,
Elimination of a potential hazard is often a sound approach. Have you considered ultrasonic testing instead? I understand that in some circumstances it takes only 90 seconds to validate the integrity of a weld. Suggest that you search the web for details and discuss with management. No pressure hazard involved.
Kon
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Posted By Mitch
NSO,
I have been involved in fabricating diesel fule tanks from mild steel, PSI to 1.5 times head of pressure for testing with soap suds and stainless steel water treatment filter vessels, test pressure specified by client at 7 BAR, inspection cover was only fitted incorrectly once!!! These were soak tested for 4 hours minimum. I think previous poster has sound advice on what pressure constitutes a pressure vessel with all that entails if you need to work to higher pressures.
Mitch
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Posted By Jane Blunt
While it is possible to examine the welds, this will not necessarily show that the box is leak tight.
Can I suggest you get hold of the HSE publication GS04, 'Safety in Pressure Testing'. This gives quite a lot of advice and is a good starting point.
Whether or not what you propose to do is dangerous depends on many things - not just the pressure, but also the volume of the item (because the pressure and volume product gives a measure of the stored energy).
The possible failure modes of the item are also important - how would it fail if it contained a crack - would it just leak, or would it actually burst? Are there any features that could be ejected if it failed under pressure?
Jane
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Posted By NSO
Thanks to all who have replied, I've now got a copy of GS4 and also found CRR168. The box is basically a tank which will eventually contain water with a rust inhibitor as the tank forms part of a domestic heating boiler. The operating pressures are low however when the tank is filled with compressed air, I'm sure you'll appreciate that volume of air with sudden release could be a problem. We have the tanks held firmly in a cradle but there may be potential for failure somewhere in the system, so to dot i's and cross t's I wanted to be doubly sure that all was ok and that I had followed guidance if it was out there. So thanks again. Nick
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Posted By Smurfer
I love jom/john's idea! So if the box was made well, the steam test will destroy it; if it's made badly you get the chance to plug any leaks until it passes (and gets destroyed!). Now that's what I call 'outside the box'!! ;-)
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Posted By Jane Blunt
It also goes rusty ;o)
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Posted By Jay Joshi
It also depends upon the design specification and final use so called "pressure vessel" or as you have descibed it, a tank
For example, if it is not designed to withstand vaccum, it is not a good idea to fill it with steam and seal it--as steam will condense and there will be a vacuum.
Also, before you fill it with water, please check the floor/support loading/strength--as pressure vessels meant to hold gases may not have their supports designed to withstand the weight of filling it with liquids!
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Posted By Alan Nicholls
NSO
If the box is being used to contain water with a rust inhibitor; what's the problem with using water to do a leak test? All you have to do is put rust inhibitor in the test liquid! You don't need gallons of the stuff in the water for it to be effective. The pressure testing can be done without any serious consequences should the welding be less than perfect.
All air receivers built by us were tested to 16.5 BAR (Steel and tap water). All were internally coated prior to testing though. Any failure you just got wet.
Regards Alan N
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Posted By Alan Hoskins
Can you not test these using water and the rust inhibitor that will be used during service?
Alan
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Posted By Mitch
Just remember, test tank using rust inhibitor, locate leak, before re welding is inhibitor flammable? will it contaminate weld thus rendering repair useless?
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Posted By Alan Nicholls
Mitch
Good point, in my experience with inhibitors after draining down very little residual chemical is left it evaporates away to atmosphere leaving just a slight haze on the surface. unlikely to leave an explosive mixture if vented. I used to weld up motor cycle oil tanks and petrol tanks, first they were washed then filled with sand. Don't do welding any more pay other people to do it.
Regards Alan N
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Posted By Mitch
Alan,
snap, I also stopped welding car chassis a very long time ago, there's a maniac round these parts does it for a tenner an hour, I'll let him do it all day!!
Mitch
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