Not a good idea as air seems to always come with some water content, which eventually drops out and collects in places like dead ends. Air (N2+O2+H2O, there's that O again) usually leads to some amount of corrosion. No better place for that to occur then in a dead end where any deposited water tends to stick around.
In addition to the above, I'd consider the diameter of the pipe: If you have a 2" pipe and a 24" deadleg - that's bad. If you have a 24" pipe with a 24" deadleg that might not be as bad. Also - what is the orientation of the deadleg - up, down, or horizontal? And is the deadleg off the run or branch of a tee?
I would think that the worst case would be a 2" pipe with the flow coming vertically down to a tee with the deadleg on the run (i.e. down) and the flow going horizontal. You'd get a cyclonic type separation of any water which would accumulate. Best case would be either a 24" with the opposite flow: Vertical up with the deadleg on top, flow turning horizontal - or maybe a horizontal run with the deadleg on the branch of the tee going up (not likely to ever see liquid).
I am sorry for not saying the orientation of the deadleg. The deadleg is horizontal. And the fluid is tranferred at 145 degree centigrade and 1.3Mpa. So it may mean that there is only steam in the pipe and the fluid may not be corrosive to the pipe?
