bsafe-
if my memory serves correctly, the threaded ends of the pipe are 50% thinner than the pipe itself due to the thread cutting. so by joining them with a coupler, you now have two threaded ends in very close proximity to each other. this creates a very weak joint.
call up a fencing company. they sell 20 or 21 foot chunks of galvo pipe for pretty cheap. maybe look for three inch, and you'll sleep a lot better!
if your ears are open, you'll find someone that can weld for you at some point.
adam