Plastic Diesel Tank Drain.

x25dave

x25dave

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Hi all, I have attempted to search for this as a thread without success, so I ask the question anew.

I would like to fit a drain, into the bottom of my fuel tank. The tank is in a locker, and right up under the side deck, where the filler is connected to the top of it. The fuel line is fitted in through the top, and i guess goes down to just off the bottom. I wanted to be able to drain the stuff from the bottom of the fuel tank, is that a good idea? I will not have access through the top of the tank, and so imagine that I would be screwing and sealing something to the outside of the bottom of the tank. Does that sound feasible, to get a good, reliable seal?

Should I drain the tank and take it out completely? I imagine that this is not impossible, as it was only put in 2 years before I bought her 5 years ago.
 
The big advantage of plastic tanks is that they don't get rust and sludge like some metal tanks, although of course still susceptible to water in the fuel and the bug. Fitting a drain is not necessarily a god idea as locating and sealing it could be a challenge. Suggest you use a Pela extractor through the filler to suck up a sample from the bottom of the tank to check cleanliness of the fuel.

I have had plastic tanks for a long period - 20 years on one boat and 15 years on another with no problems, and have drained neither.
 
You can fit a drain from the outside. Have a look at how tank senders are often fitted and you will see the principle. Basically the inner flange is slotted so that it can pass through the hole into the tank and is sealed with a gasket inside and out. Once you see the principle you should be able to find a suitable fitting that will accept a valve.
 
Yes of course!! That would be a very simple solution, and maintain the original integrity of my tank.

Tunnel vision now expanded :encouragement:
 
x25dave said:
Does that sound feasible, to get a good, reliable seal?
Click to expand...

No, you won't necessarily get a good seal. Plastic tanks tend to be rotationally moulded, and the wall thickness isn't uniform. You may end up with a persistent leak which you can't fix - which is why most plastic fuel tanks have the fittings on the top. If a plastic tank needs an outlet at the bottom, it's moulded in.

If you want to remove stuff from the bottom, could you do it with a pump and a thin tube inserted through the fuel filler?
 
pvb said:
No, you won't necessarily get a good seal. Plastic tanks tend to be rotationally moulded, and the wall thickness isn't uniform. You may end up with a persistent leak which you can't fix - which is why most plastic fuel tanks have the fittings on the top. If a plastic tank needs an outlet at the bottom, it's moulded in.

If you want to remove stuff from the bottom, could you do it with a pump and a thin tube inserted through the fuel filler?
Click to expand...
Better to take the fuel level sender out and get at the bottom of the tank that way. Advantage as well is you can see the fuel and bottom of the tank thru the fuel
Stu
 
skipper_stu said:
Better to take the fuel level sender out and get at the bottom of the tank that way. Advantage as well is you can see the fuel and bottom of the tank thru the fuel
Click to expand...

As the OP said "The tank is in a locker, and right up under the side deck...", I assumed that wouldn't be possible.
 
It's an excellent idea to have a drain from the bottom of the tank. If you have enough access to do it, go for it. Gravity always works better than fiddling about with extraction devices from above.
 
NormanS said:
It's an excellent idea to have a drain from the bottom of the tank. If you have enough access to do it, go for it. Gravity always works better than fiddling about with extraction devices from above.
Click to expand...

Having deliberately made a hole in a thin-walled plastic diesel tank I then spent several years before I finally managed after umpteen attempts to properly seal in the new fitting. If it's a solid thick Tektank you should be OK, but be very wary of the thinner rotomoulded polyethylene tanks: absolutely NOTHING seems to stick to polyethylene and diesel is excellent at weeping though sealants. Every time I thought I'd got it sealed a few weeks later a weep would appear again. Having finally managed it I'm not touching the fitting again if I can help it.
 
jwilson said:
Having deliberately made a hole in a thin-walled plastic diesel tank I then spent several years before I finally managed after umpteen attempts to properly seal in the new fitting. If it's a solid thick Tektank you should be OK, but be very wary of the thinner rotomoulded polyethylene tanks: absolutely NOTHING seems to stick to polyethylene and diesel is excellent at weeping though sealants. Every time I thought I'd got it sealed a few weeks later a weep would appear again. Having finally managed it I'm not touching the fitting again if I can help it.
Click to expand...

So what kind of fitting did you use, and how did you seal it please?
Those were my original questions
 
My advise is don't put a drain in - I've had a Vetus/Plastimo plastic tank for 10 years now and never had a problem. I do regularly dose with MR16 and change filters every year. I've also fitted a screw on fuel filter to replace the original CAV filter with the glass bowl which was always a b****** to change and get leak free. The screw on filter does have a drain but never had any water come out(I check just before I change the filter). These tanks have a large, about 8" diameter access hole in the top in which all the inlets, returns, filler hole, tank sender unit etc are fitted. This could be removed for inspection if required but may be difficult refitting it all without any leaks.
 
I'd reiterate to Don't,
I have three Vetus rotationally moulded poly tank, 1 diesel and 2 water, all three leak from fittings on the top of the tank when the tanks are full, and that's with little or no head. I've tried over 4 different winter refits to seal the leaks and haven't manage to do it yet. As said above nothing seems to stick to these tanks, and I've tied O rings, rubber gaskets and cork gaskets and all sorts of sealing compounds and they have all leaked.
If anyone can suggest a foolproof way of stopping these leaks, I'd love to know.
Whilst I know that having a drain at the bottom of a fuel tank is a good idea, I wouldn't want to risk having a boat continuously smelling of diesel.
I use a fuel polisher every winter to circulate the fuel through a filter and water trap, all you need is access to insert a thin plastic or copper tube and work it down to the bottom of the tank.
 
Ian_Edwards said:
I'd reiterate to Don't,
I have three Vetus rotationally moulded poly tank, 1 diesel and 2 water, all three leak from fittings on the top of the tank when the tanks are full, and that's with little or no head. I've tried over 4 different winter refits to seal the leaks and haven't manage to do it yet. As said above nothing seems to stick to these tanks, and I've tied O rings, rubber gaskets and cork gaskets and all sorts of sealing compounds and they have all leaked.
If anyone can suggest a foolproof way of stopping these leaks, I'd love to know.
Click to expand...

When I collected my new boat 2 years ago, the first time I filled it up with diesel I found diesel spilling out of the fitting which the crass dealer had put on to the tank top to supply the Webasto. The installation instructions for the Webasto clearly say not to use the fitting on plastic tanks (for the reasons I mentioned earlier) yet the dealer ignored them. I finally managed to get it sealed, after several tries, using a nasty black sealant which I think may have been Heldite. I'd had a tube of it for ages, but I think it's only available in tins now.
 
x25dave said:
So what kind of fitting did you use, and how did you seal it please?
Those were my original questions
Click to expand...

I started with a patent made for the job "inspection hatch" with it's own gasket. Leaked immediately, and this was on the top of the tank. Changed gasket to nitrile rubber. Leaked after a while. New gasket with silicone sealant. Leaked after a very short while. Changed to cork/nitrile gasket. Leaked after a while. Gave up on patent hatch and made large alloy plate cover. Finally used cork/nitrile gasket on new alloy plate with Hylomar red under/over gasket and Hylomar Blue on fixing thread parts. So far so good. Have had similar problems with Eberspacher tank fitting - again on top of tank. Same goop/cork/nitrile fixed that too.

Having got it leak-free I'm very reluctant to disturb it and certainly won't be making any more holes, let alone in the bottom of the tank. A little diesel leak goes a long way.
 
I have managed to create diesel-tight fittings in plastic tanks using conventional plumbers tank fittings and a rubber washer/gasket.
The key is:
Getting rubber that will survive in diesel long term. VIton I think but DYOR it was a long time ago!
Finding a flat enough area of tank, or flattening the seal surface.
Being able to tighten the fitting adequately.
Not stressing the fitting afterwards.
A very light smear of suiatable sealant to take up and texture in the mating surface. IIRC one of the hermatite range is good for diesel and viton?

IMHO fittings in the 'roof' of a diesel tank need to be just as good as those in the floor.
 
Don't do it, the risk and heartache of a leaking diesel tank is not worth it. Plastic tanks flex alot, and 100l or more of fuel being thrown about in a seaway will flex even the most robust of plastic tanks - and as mentioned already, leaks occur through fittings on the top of the tank, it's even worse when the fitting is permanently under hydrostatic pressure and being pounded in a seaway.

As the top of your tank is not easily accessible, If you really want a means to access the bottom of the tank, then fit a second pipe through the same hole as the fuel pick-up, ensure it sits deeper in the tank and run it to somewhere accessible (avoiding a syphon effect). Use a pump to suck out the stuff below the fuel pick-up. As already mentioned, in a plastic tank the most likely contaminants are water and the fuel bug. Treat the bug regularly and use the second pipe to get at any water. I've had my plastic tank going for 6 years in my ownership and, touch wood, have had no fuelling problems.

My plastic water tank which sits in the bow has a moulded fitting at the bottom and despite my best efforts, it took many attempts to get a watertight seal.

There is a good reason why the Ebespächer installation instructions include a top-mounted standpipe and state:

The fuel standpipe supplied must only be fitted to the top of the fuel tank and on an even surface. It must not be fitted to the side or bottom of the tank.
Click to expand...

http://dometic3frontend.qbank.se/episerver/47cf2235760f940afff54deca3a8659d.pdf

The final very good reason for avoiding holes in the bottom of the fuel tank is that a failure of the seal or any connection to the fitting will result in bilges full of all your fuel, which is a fire risk and stinks.

Have a read of this ....

http://www.boatsafetyscheme.org/media/164488/bss guide chap2.pdf

This post is obviously my opinion and doesn't constitute "professional" advice, but I wouldn't do it if I could possibly avoid it.
 
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