Aidan wrote:

The relevant BS on pipe testing recommends that air testing should only
be used as a last resort and that, if it is necessary, the area should
be evacuated. I would strongly suggest that you pressure test with
water.

I have spent many years pressure testing pipes in aircraft with
compressed gas and I have never seen anything explode. Leak a lot, yes,
but never explode. This includes 3 to 4 inch aluminium, thin wall, pipes
pressurized to 50 PSI.

Part of the purpose of pressure testing is to cause the failure of any
weak joints.

No, it is to expose a weak joint, not to cause its failure. Proper
pressure testing is done in stages, by increasing the pressure slowly,
to establish that joints are secure. Any one in their right mind would
never apply a high pressure to an unknown pipe network without bringing
up the pressure slowly.

Dave