Black tank problem

Groveite · 08-05-2010, 08:14 AM

The black water tank on my Cougar is leaking , only when overfilled. 2 years ago I had to buy a new one (beware of sewer suckers solving a clog) , cut out the entry hole in the right place and glued in the flange for the 3" inlet..... Worked great until a clog this winter which I was able to clear using a snake. I suspect that in my efforts I broke the glue seal at the inlet. The thought of removing and replacing the tank is most unpleasant as it took 3 people and a few choice words to get the new one in last time and I'm hoping that there is some kind of sealant that will bond both to the plastic tank and the ABS pipe as I can open up the floor of the cargo hold enough with a sabre saw to get access all around it and I had the foresight to put in a rubber sleeve to make disconnecting the toilet easy. Before replacing the tank in the past I tried polyester resin, fibreglas matting and also the super expensive tape and did not get any bonding.
Any ideas/suggestions would be gratefully received.
Ian

radams · 08-05-2010, 09:55 AM

Based on your description I suspect that your tank is made from polyethylene (the same material used for plastic milk jugs). It is probably crosslinked polyethylene, not linear, so it will not tear (that's good). Adhesives and glue will not "stick" to it. The only two ways I know to make a reliable connection to the material is either with heat, as in welding (hot air welding), or mechanically as in a tank adapter. The tank adapter is basically a pipe nipple with a flange on one end. It is inserted through the tank wall from the inside so that the flanged end with a gasket seal against the inside wall of the tank. On the outside there is a threaded nut with gasket that tighten up against the outside wall of the tank and compress the two gaskets. Then you have a male threaded nipple to connect to on the outside of the tank. The problem with hot air welding is finding someone that really knows how to do it right. The problem with the adapter is getting it inside the tank unless you have a large opening somewhere, and you probably don't.

Laramy Plastic Welders has the best info I know of on the welders and maybe they know of someone in your area that could do the work. They also have a manual available for the how to do it guy and they sell the welders and rod. Good luck.

Batman_777 · 08-05-2010, 10:01 AM

Might want to contact BP since they have been learning a lot about leaks.

Groveite · 08-05-2010, 10:35 AM

Quote:

Originally Posted by radams

Based on your description I suspect that your tank is made from polyethylene (the same material used for plastic milk jugs). It is probably crosslinked polyethylene, not linear, so it will not tear (that's good). Adhesives and glue will not "stick" to it. The only two ways I know to make a reliable connection to the material is either with heat, as in welding (hot air welding), or mechanically as in a tank adapter. The tank adapter is basically a pipe nipple with a flange on one end. It is inserted through the tank wall from the inside so that the flanged end with a gasket seal against the inside wall of the tank. On the outside there is a threaded nut with gasket that tighten up against the outside wall of the tank and compress the two gaskets. Then you have a male threaded nipple to connect to on the outside of the tank. The problem with hot air welding is finding someone that really knows how to do it right. The problem with the adapter is getting it inside the tank unless you have a large opening somewhere, and you probably don't.

Laramy Plastic Welders has the best info I know of on the welders and maybe they know of someone in your area that could do the work. They also have a manual available for the how to do it guy and they sell the welders and rod. Good luck.

I had thought about the threaded adaptor but the access to the inside is not there. I contacted the company that makes the tank when I bought it and they said to use the abs glue (yellow) and NOT to plastic weld as I had planned.... . I confirmed that with the rv parts place that sold it. The problem I suspect is that the mating surface is minimal , maybe 3/8" al lthe way around IIRC. The yellow glue did bond if that helps any "budding chemists " out there to find a solution. I hate to take it all apart or cut out the floor of the cargo area until I have a plan....
Ian

radams · 08-05-2010, 12:01 PM

Aha! That information changes the picture a bit. Apparently the tank is ABS which can be solvent cemented. That makes things a lot easier.

I agree with you that the problem was most likely the minimal mating surface area you have now. How about getting a piece of ABS sheet stock, of suitable size, attach one of those tank adapters we discussed in the center of it, and cement the whole shebang to the appropriate spot on the tank? You might need to enlarge the tank opening a smidge to clear the adapter but then you'll have a large mating surface area to cement together. Plus you'll have a sturdy fitting to connect to.

Amazon.com sells the ABS sheet if you can't find it elsewhere and the tank adapter too. BTW do not use primer on ABS. You should be able to use the same solvent cement you used before. Just be sure it's fresh.