Connecting a third LPG tank

Jamie Taylor
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Joined: Sun May 29, 2022 5:58 pm

Re: Connecting a third LPG tank

#21 Post by Jamie Taylor »


Well I plumbed them all in, but disconnected the solenoids for the other two. I set it up with 2 T-valves so, unless I cocked it up, the gas would just go down the fuel line to the front.

There was quite a bit of difference in pressure between the gas in the two types of tanks, so I am confident it is a vapour take off. There are a couple of tanks going for about £40/50 around here so will get one of those, but will make sure it is a liquid take off first :)

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Re: Connecting a third LPG tank

#22 Post by LPGC »

Gas solenoids (on tank valves) will allow gas to flow the opposite of usual flow direction even if the solenoid is closed. This allows backflow from parts of the system that don't have any headroom which could otherwise burst if temperature increases... But on a multiple tank setup it also allows gas from a tank at higher pressure to flow into the lower pressure tank even when the lower pressure tank outlet solenoid is closed, unless using a one-way T (2 tanks), 2 one-way T's (3 tanks), etc, to prevent backflow.

Still seems most likely it's a vapour take-off but the above was worth a mention. Even if it is a vapour take-off tank, you said the valve on it looks similar to the other (liquid take off) tank multivalves.. so instead of changing the tank you could just fit a liquid take-off multivalve on the existing tank.

None of the following will be relevant in your situation...

On multiple tank setups if all solenoids are open there can be a situation where if some tanks are full and some tanks are empty, drawing gas from the full tank can see temperature and hence pressure of the full tank(s) become lower than the empty tank(s), in which case vapour from the empty tank may enter the feed line to the engine and when such vapour lock reaches the reducer it starves the reducer of liquid gas so the reducer can only flow a small fraction of it's rated output capacity until the vapour lock has cleared.. but clearing the vapour lock can be a catch22 situation because the reducer has to have gas thoughput to clear the vapour lock but the electronics will see low pressure and switch back to petrol due to low pressure from the reducer not being able to flow enough gas.

Similar can happen on vehicles fitted with more than one reducer, even if only 1 tank is fitted. The vehicle runs out of gas, the driver refuels and sets off again. At light engine loads the reducer set to the highest pressure output supplies all the gas that the engine calls for so the other reducer doesn't have any throughout of gas (it's just sat there doing nothing really) and there's vapour in it's feed pipe from when the engine ran out of gas. Now the driver uses more engine power which requires more gas than the reducer that is being fed with liquid gas can supply without it's pressure falling, so the other reducer with the vapour lock needs to come in to play but that reducer can't supply it's full rating's worth of gas because it's currently supplied with vapour instead of liquid. At this point the system may 'beep back to petrol' due to low gas pressure. If the driver presses the switch button to cycle back to gas again the problem will definitely clear after several attempts of using high power and may even have cleared after the first 'beep'. The likelihood of this happening depend on factors such as tank outlet flow rate, engine power, difference in pipe lengths to multiple reducers and which reducer (shortest or longest pipe length) is set to the highest pressure. It can be overcome entirely by fitting solenoids to all reducers, not using high engine power until reducer feeds are bled of vapour by running on each reducer in turn until that reducer's feed is bled.
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Jamie Taylor
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Posts: 13
Joined: Sun May 29, 2022 5:58 pm

Re: Connecting a third LPG tank

#23 Post by Jamie Taylor »

Hi Simon,
Sorry I have only just seen your reply.

In the end I bought two more cylinders and plumbed those in using one-way T-valves. Because the solenoids seem to draw to much power I have put the 4th cylinder on a separate circuit as a back up.

Thanks for all the info!


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