Fuel tank feeds...?

[Gordonmc]

I am getting round to changing the mild steel diesel tanks and am looking for some pointers on the pros and cons... and even legal/safety requirements for the outlets.
Two tanks of approx. 60 litres are enclosed either side of the cockpit with pipes joined by a junction upstream of the only seperator/filter. Both outlets are at the bottom of the tank. Each tank also has a sludge trap and drain on the floor.
All of the off-the-shelf plastic tanks I have seen so far have all the outlets, breathers and returns going through the top with no drain or sludge trap.
Is there a logic to this?
I can't see how it helps avoid a spill if there is a leak in the pipework as the fuel will syphon.
I am trying to avoid extensive re-plumbing but it looks as though the only way to keep the existing set up is to have stainless tanks made bespoke and use the existing fittings.
Any thoughts?

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[Evadne]

The outlet pipe in from the top doesn't reach the bottom of the tank, so the whole tank is a sludge trap. I'd have still thought you need a drain though, to get rid of the water and solids.

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[William_H]

The tank should have a definite low point for water to settle which has a drain (preferably) or provision for a pump (known as a thief pump in fuel storage business) to take off water. In underground storage tanks they tilt the tank, in aircraft they use a dish shape in the bottom of the tank. The standard for light aircraft is 1/2% of total tank capacity which can not be sucked up by engine where water can lie and which can be drained. So the engine pick up should not reach to the lowest point but a little short of it. For an outlet pipe at the bottom, you risk a leak draining fuel into the bilge. For a pipr through the top you need the engine to suck the fuel and any leak will allow air into the system. I would go for bottom pipe unless anyone else has good reason to go top. All just theory I'm afraid. regards will

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[snowleopard]

don't know if you will find off-the-shelf tanks with sludge trap, certainly i've never seen them advertised. the way you get round the water/sludge problem is by fitting an agglomerator. this traps the gunge so you can drain it off.

maine prices are of course horrendous but the identical component is fitted to the land rover discovery so should be cheaper from a motor factor. there is also the benefit that they have a tap for draining whereas the marine ones have a plug requiring a spanner.

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