What Purpose is That Clamp?

I am in process of fixing a poor waste plumbing job done on my Coachmen. I noted in an earlier edition of FMC Magazine, in your House Calls column, you had a picture of a gray water holding tank exit with a reducer installed. I also noted the hose clamp around the ABS exit pipe. I have a similar hose clamp on my RV and I am wondering what purpose is served by the clamp. Clearly clamping ABS together cannot be an acceptable practice. Roger M. 

Great question Roger! You’ll see that set-up often, the clamp (and sealant), whenever the holding tank is made out of HDPE; high-density polyethylene. All waste plumbing including the outlet pipe is made of ABS. Unfortunately, you cannot cement, bond or weld dissimilar thermoplastics. The actual outlet fitting on an HDPE tank is tapered a little and some technicians could not understand that the tighter you tightened the clamp, the more it tried to squeeze the ABS pipe out of the hole. It is necessary to cut the ABS outlet pipe to just the right length so that it wouldn’t slip out and such that it would not protrude too far into the tank outlet, trapping debris under the outer circumference of the pipe sticking inside the tank. It wasn’t necessarily a great design but when coupled in tandem to the other tank outlet via the whole termination assembly, it sufficed. Today you’ll mostly see spun-on or welded threaded fittings to make that connection. Pictured here is the sealant I use. Just slop it on the outside of the pipe and tighten the clamp just enough to keep it from leaking. It sill might be possible to manually rotate the pipe in the tank outlet, but once you reconnect the entire waste manifold, all will stay in place.