Polyethylene fuel tank sealing

[jwilson]

For nearly three years I've had a problem re-sealing an existing fuel return access fitting in the top of a polyethylene diesel tank. It was removed to drain the tank after much water got in via a hosepipe and the deck filler. (Don't ask, wasn't me!) S/S tank fittings are held in place with self-tappers into top of tank. Originally there was a rubber gasket, removed by engineer who first re-sealed tank after draining with some blank gunk. Didn'y work, leaked. same engineer re-sealed, slightly better, but still a weep. Since then a different engineer has had several more goes at resealing, both with and without gaskets, and with various sealants.

I spoke to Tek-tanks and they said that if a nitrile rubber washer didn't work to try an adhesive called INNOTEC: I used this just before launch this year after ultra-cleaning the joint faces, and left a thick bead round teh joint face and into the screw holes to semi-harden before tightening down the screws a day later, and then glooping more of the sealant over the entire top of the screws and fittings, as one of the four screws was now failing to grip fully.

Leak gone from a teaspoon in a week to a teacup. I have ordered some fresh nitrile rubber sheet 1.5mm thick, and am thinking of enlarging the screw holes in the metal return fitting to use bigger self-tappers. You cannot get inside the tank to put nuts in, without cutting another hole which you will then have to seal!

Any other suggestions?

 

[Modulation]

I had exactly the same problem and went through the same processes - with precisely the same results. I came to the conclusion that it had to be the self tappers not being able to grip sufficiently to ensure a tight seal. Bolts and nuts would sort the problem but how do you get inside the tank, as you say!!
I made up a thick nitrile washer - overlapping the hole by 20mm all around -and cut 6 holes in it to take 6 no 25mm long bolts . Next I made matching holes in the tank where the old screw holes were, first cutting away ridges in the underside of the screw fixings (presumably there to give the screws more to bite onto). You have to avoid letting stuff fall into the tank. I then put the bolts into the nitrile washer and put a nut tightly on each . You now have a big nitrile washer with 6 bolts on it and with a bit of dexterity and a lot of patience you can feed this through the original hole and into the tank and manipulate the bolts up through the holes in it. I attached a bit of strong thread to retrieve the washer if it fell into the tank during this process - it didnt. I used a sealant as a precaution as well. You now effectively have 6 studs protruding from the tank and bolting onto them is not a problem as they wont turn - fixing them to the nitrile washer first prevents that .
Hard to describe this fully but I've had no leaks since - 4 years ago.
Hope this helps.

 

[jwilson]

I had exactly the same problem and went through the same processes - with precisely the same results. I came to the conclusion that it had to be the self tappers not being able to grip sufficiently to ensure a tight seal. Bolts and nuts would sort the problem but how do you get inside the tank, as you say!!
I made up a thick nitrile washer - overlapping the hole by 20mm all around -and cut 6 holes in it to take 6 no 25mm long bolts . Next I made matching holes in the tank where the old screw holes were, first cutting away ridges in the underside of the screw fixings (presumably there to give the screws more to bite onto). You have to avoid letting stuff fall into the tank. I then put the bolts into the nitrile washer and put a nut tightly on each . You now have a big nitrile washer with 6 bolts on it and with a bit of dexterity and a lot of patience you can feed this through the original hole and into the tank and manipulate the bolts up through the holes in it. I attached a bit of strong thread to retrieve the washer if it fell into the tank during this process - it didnt. I used a sealant as a precaution as well. You now effectively have 6 studs protruding from the tank and bolting onto them is not a problem as they wont turn - fixing them to the nitrile washer first prevents that .
Hard to describe this fully but I've had no leaks since - 4 years ago.
Hope this helps.

Thanks for the suggestion, and I can see how it would work. But I'm guessing you are starting with a bigger hole, mine is well under an inch diameter hole with four self-tappers around it holding down the return pipe attachment. Not sure how I'd get in to cut away any ridges in the underside of the screw fixings, or stop drillings getting into the tank with just a tiny hole to work through.

 

[RobinBirch]

I had a similar problem with a Tek Tank. Overfillin the tank would result in the surplus diesel weeping out past the joint on the plate that takes the fuel filler pipe. Not pleasant!

Taking it apart, good clean and a liberal dose of hylomar universal blue did the trick - absolutely fine 2 years on.

Have to say that the use of self tappers into the soft moulded plastic of the Tek Tank didn't strike me as being too reliable. The nitril washer would adequately seal the aperture if only the purchase of the screws had been adequate.

 

[Modulation]

Thanks for the suggestion, and I can see how it would work. But I'm guessing you are starting with a bigger hole, mine is well under an inch diameter hole with four self-tappers around it holding down the return pipe attachment. Not sure how I'd get in to cut away any ridges in the underside of the screw fixings, or stop drillings getting into the tank with just a tiny hole to work through.

Well you could bite the bullet and accept that you will have to make as econd access panel - maybe 50 mm in dia close to the original hole. That will give you access to the underside of the original which you can then deal with. you could then seal the new hole the way I've mentioned.
Still I vcan sympathise with not wanting any more holes to seal! Best of luck