Details of underslung gas tank

Squiffy

Forum Member
I'm going to mention and talk about what some may say is a controversial subject, which it really should not be and this post is purely for information and I except no responsibility if folk act on this demonstrated post.
I have recently changed my underslung 25ltr gas tank for a new one as the original was ten years old and had varying degrees of corrosion showing, not knowing how thick the walls of these tanks are I began to be concerned and so to be on the safe side I replaced it and the flexible feed pipe from the filler point back to the tank. I then proceeded to go to Portugal and back over the next 5 weeks. All went well and filled twice at various garages, apart from the filler adapter for Spain and Portugal did seem to have a connection problem.
However now we've been back for several weeks I decided to remove all the gubbins from the old tank and check out the valves and floats and what they do etc. What follows is a photo of them roughly in order of their removal.
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The first valve is the 80% float valve which operates when the float reaches a certain position and drives a mechanism which creates a pressure build up in the supply tube from the gas pump, in turn the pump feels this pressure build up and shuts off.
The second unit is the magnetic mechanism that when the float starts to rise it operates a pinion type gear which turns a shaft and at the other end revolves a disk with a magnet attached and this revolves in a circular movement which in turn through the brass plate turns a needle encased in a plastic case rather like a compass, the higher the float the further round the dial it drages the needle on the scale behind it.
The third one is the pressure relief valve and will operate under extreme stress, I'd have thought it's set to unload at just below the pressure which would rupture the integrity of the tank.
The fourth one is the supply valve that incorporates a shut off turn screw valve, the base tube I think never dips into liquid gas as it operates in the 20% zone of the tank and so supplies vapour and not liquid.
These float mechanisms are the reason these tanks have to be orientated at a precise angle of 105 degrees so that the cut off float cuts off the pumped supply gas at exactly 80%.

Now the reason I removed these valves apart from interest as to how these tanks and valves work is because I wanted to know just how thick the walls of these tanks actually are. I wanted to see just how bad the rust and corrosion had to be before the integrity of the tank would be affected. Well the rust to me seemed quite bad, hence why I replaced it after ten years, however on feeling inside with my figure and thumb trough the holes now available by the valve removal, I can say that the corrosion would have to have been far far worse than that showing on this tank, with a scraper and wire brush the depth of corrosion was insignificant to the thickness of the wall of the tank. I haven't got a calipers that enabled me to measure the exact thickness but it was significantly thick and had I known I would not have bought a new tank. Having Said that I'm happy that I have changed it as I would never have been comfortable having seen the outer corrosion of the old tank in the first place, but I'm certain there was many more years of serviceable life left in it, especially if it had been scrapped off repainted and wax oiled or equivalent.
 
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Millie Master

Forum Member
Thorough as usual Phil.

Did you by any chance fully coat the tank with either an anti stone chip or similar coating before you first fitted the tank?
 

Squiffy

Forum Member
Thorough as usual Phil.

Did you by any chance fully coat the tank with either an anti stone chip or similar coating before you first fitted the tank?
In all honesty Phil I fully intended to do so when I fitted the original but fitted it with the intention of doing it later and yes as per usual with me I never got around to doing it. At about years 6 I did scrape off flaking paint and rub down some rust spots then sprayed it with a can of red cellulose paint not really to preserve it but make it presentable for an MoT, not that it officially comes under an MoT but I thought there could be some clause in the testing manual that it would come under.
This one I've just fitted, again I was in a hurry to fit it as we were due to go on our road tour to Portugal 3 days later and just slung it up with the promise to anti rust it on our return, but now having seen what I've seen with the old tank I don't think I'll bother as I'm 73 this year and if it requires replacing in the next 12-15 years, well I'd just be glad to be able to put the whole van out to grass I'd think. I may just get a can of spray on underseal and dust it over around the bottom end to stop stone chipping but then it's all ready done 4000 miles so c'est la vie 😆. Phil
 
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Pudsey Bear

Forum Member
Would you be able to exhaust any remaining gas Phil, and cut it in half then you could measure the thickness of the steel.
 

Squiffy

Forum Member
Would you be able to exhaust any remaining gas Phil, and cut it in half then you could measure the thickness of the steel.
I have in fact emptied tha tank completely Pudsey due to taking all the valves out but it stinks of the smelly chemical that they put into propane and I remember years ago I wanted to weld some studs to the base of a Triumph petrol tank so as I could fit it to a Norton feather bed frame, I washed and washed the tank out with hot soapy water and did it again to make sure, it smelt absolutely clean and not hint of petrol but just incase i proper it on some house bricks by the corner of a wall and stood around the other corner shielded by the house wall and lit my acetylene burning torch, I reached around , placed the torch over the open filler hole and had to source another tank 😆.
The tank literally blew up and split apart it was only years later I learned the trick of welding a petrol tanks from an Indian guy who's father learned from Arabs in Egypt how to do it with impunity.
So in short I don't want to have a spark from a cutting disc or metal saw setting off what might be an explosion 😬.
One thing I would say is a little fumes in a tank is actually more dangerous than a full tank.
Not trying to teach grandma but some folk always worry about a full petrol tank when it's a small amount of petrol in a tank that's far more dangerous. So as much as I'd like to know the wall thickness I think I'll just take it down to our council tip where they take in old gas bottles. Infact yesterday the scrap metal guys came around I put it out on the road as I do when we hear them coming, he stopped asked me what it was when I told him he refused to take it as the metal merchants won't except even empty bottles. Phil
 

Pudsey Bear

Forum Member
I've seen them weld on tanks and such on YouTube Phil, what they mostly seem to do it pipe exhaust fumes in to it for a while it must do something to the fuel fumes, I never did kemystery of fizzics.
 

Squiffy

Forum Member
I've seen them weld on tanks and such on YouTube Phil, what they mostly seem to do it pipe exhaust fumes in to it for a while it must do something to the fuel fumes, I never did kemystery of fizzics.
You've got it in one, yes that's exactly what they do but I'm not sure now we use unleaded if the same chemical reaction takes place and neutralise the petrol fumes as once you've heated the tank by exhaust fumes the tank is safe to weld even days later. Don't forget that when I learned this trick U Tube wasn't even a twinkling in someone's eye 😁, so I was lucky in that one day I walked into Suresh's work shop and caught him at it, that's when he finally showed me the whole process. Phil
 

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