Service of Dry Risers

[nearlythere]

Has anyone ever considered a vacuum or dry pressure test of a dry riser to check for leaks rather than carrying out a wet test?
Would this be considered a suitable alternative?

[SandDancer]

It may be an option, but care must be took with the construction material of the riser making sure it can  withstand a vacuum pressure rather than a straight pressure test as you may collapse the pipework!

Also something at the back of my addled brain is telling me that water needs to be used,  if not, joints can dry out and fail 

[AnthonyB]

I have heard in the past of testing under gas pressure, but I'm not sure if it was air or a special mixture - never seen this first hand though, only water testing.

[SeaBass]

Air pressure can be used for static testing but not dynamic flow rates, which are just as important. If air pressure testing is used, pressure relief valves need to be included in the pressurisation equipment to prevent any risks arising from over pressurisation of the riser pipe work and valves. Most risers also have AAVs at the head of the pipe work which are designed to vent air from the system. This complicates air pressure testing.
I think that the joints that SandDancer is referring to are Vitriolic joints which are found in wet risers and sprinkler systems rather than dry risers.