LPG tanks question

[unclebill]

OK oh learned ones, LPG tanks, possibility that I may buy a rangy with LPG, problem is that it is a silly p*ddl*ng 50 litre tank, on a 4litre engine the proverbial ashtray on a motorbike springs to mind.

Question, if you fit extra tanks, is it just a matter of teeing the things in on the output side and possibly teing them on the filling side or is there more to it than that.

I know about that they have to be pressure vessels and tested etc., am just interested in the schematics of an installation.Cheers

[rtbarton]

AFAIK they are Tee'd on the output side. Each tank should have its own shut-off cock on its output and I guess a means of bleeding the air out.

One filler will fill the whole lot, once the air is replaced by gas it will liquifgy in all the tanks under pressure from the pump on the forecourt.

[Fatboy]

Hi,

The tanks need to be tee'd into both sides. The filler side is usually LPG proof hose which is a much larger ID than the discharge side which is insulated copper pipe (like brake pipe).

On the tanks, the discharge goes through solenoid controlled shut-off valves that are linked to the control system. When the engine is off or the ignition off, the valves are closed.

Roger - You don't need to bleed off the air when you first fill the tanks. When they are first filled, I assume it is compressed and remains as gas at the top of the tanks. The discharge pipe goes close to the bottom of the tank so stays in liquid PG.

[rtbarton]

Roger - You don't need to bleed off the air when you first fill the tanks. When they are first filled, I assume it is compressed and remains as gas at the top of the tanks. The discharge pipe goes close to the bottom of the tank so stays in liquid PG.

That seems a waste of space. My tank is fitted with a valve on the top which is arranged so that as soon as the tank is 80% full liquid comes out. When first filling most of the air will be expelled through this valve first.

[Fatboy]

Roger,

I'm not honestly sore how it works.

The pickup has to go to the bottom to ensure it is in the liquid in the tank. Air would not liquify at those low pressures so while the LPG is liquid, the air remains gas.

As the tank fills, the liquid will be at the bottom therefore the air at the top. There would therefore have to be some nifty system on the pickup to let the air out but then close to draw liquid LPG...

[rtbarton]

I've been thinking about this a bit. Normally you only fill tanks with liquid upto 80%. Perhaps it works out that the compressed air accounts for the 20%, thus you don't need to expel it.

As you say, the outlet pipe is usually about 8mm dia, this would make filling the secondary tanks very slow so it makes sense to connect inlet & outlet.

[simonr]

The tank is filled to a pressure of about 10 bar - so the air in the tank will occupy about 1/10th the original space.

Some air will disolve in the LPG and so, over time, the amount of air in the tank will decreasen over time.

I would however think that the tank ought to be vented or flushed through with gas otherwise you have an explosive mixture sitting above the liquid - which could be bad.

Si

[rtbarton]

explosive mixture sitting above the liquid

But at that pressure all the LPG will be liquid, so you'd have air over liquid. You'll end up with a mixture though when the tank is "empty", but you get this in petrol tanks too, in fact it is worse in petrol tanks because you will have a vapour mixture, and you have the electrics of the fuel sender and sometimes a pump.

[Guest WALFY]

Roger

My first gas system had that valve setup aswell. The bloke at the filling station didn't like it at all. IIRC it's called an ullage valve. An early form of pressure release. Later tanks went to a different system for shutting off the gas flow into the tank.

[Fatboy]

I would however think that the tank ought to be vented or flushed through with gas otherwise you have an explosive mixture sitting above the liquid - which could be bad.

With a check valve on the fill side and deep pickup on the discharge side, I can't see how you could flush all of the air out?

Its an interesting idea though.. The fillup pump shuts off on pressure (LPG liquid with air as gas). If you could flush out all of the air, in theory you should be able to get liquid to the top therefore more mileage out of a tank ?

[Niall_CSK]

You can't flush the air out completely, it mixes, disolves as already mentioned. The older LPG tanks often had a vapour take off on the top as well as the liquid pickup. In fact I was told by a very long time LPG installer that originally the tanks were supplied as vapour tanks without liquid pickups and these were made by the installers to suit the installation, i.e. they sometimes put the tanks at angles etc.

You won't get any more range out as the air simply compresses, when you get down to vapour it runs like a bag of nails even on LPG vapour anyway, so even if it did "sit in the top of the tank" you would never be able to run the engine long enough to get to the air

Back to the original question.

On the outlet I would advise putting a non return check valve (they do have a small slight spring in them as well), any decent installer will be able to get them from the importers as they are standard items.

Reason for this is that, depending on your tank layout you can get liquid rushing between tanks under certain cornering conditions. I had a twin undersill setup that had them installed from new, one of the valves stuck and the screaching noise when it went from one tank to the other made me think I had blown a tyre and was on the rim, yes it's that noisey, particularly when the tank is near empty it acts like an amplifier

Filling side has check valve built in to the tank inlet, it should come with the tank.

An alternative would be to fit individual switches for each tank solenoid and use up one tank at a time, must be through the system though so that it fails safe and are closed with the ignition off.