Tank filling, butane, 15kg to 3.9kg.

[Pinger]

A friend with a motorhome has been told the 3.9kg butane tanks he uses for cooking are to be discontinued and he has asked me about filling them from a 15kg tank. After a trawl of the 'net I'm more confused than when I started. There's a couple of videos (American and Australian) that show it being done but in both cases it is with a bleed screw to release the pressure on the smaller tank. My friend's tanks do not have a bleed screw (but there is a small fitting with what appears to be a tamperproof plastic cap). Another video shows transfer being achieved without any bleeding (but does mention the benefit of having the donor tank warmer than the receiving tank). This is what my friend would like to do but to my mind without a temperature differential pressure between the two tanks no transfer is possible. Can someone please confirm that assumption is correct? Does anyone know what lies behind the tamperproof plastic cap? A bleed screw?

If there is a bleed screw there, is it a case of juggling its opening so that it's open enough to lower pressure but not so open as to permit lowering to atmospheric pressure allowing the incoming liquid to vaporise and escape without any real filling occurring?

The option of a permanently plumbed in cylindrical tank with multi valve and filling on an LPG forecourt is a possibility as is some form of pump to enable transfer but both are massively complicated compared to what is desired.

My friend has seen on ebay a suitable connecting hose and is convinced that connecting the two tanks with it will enable transfer as is the case with say, oxygen welding gas. My suspicion is our old friend saturation pressure denies this.

Any advice welcome!
TIA.

[LPGC]

It may be possible to somehow fit a bleed screw to a butane bottle which might allow filling it from another butane bottle (due to the lower pressure in the receiving bottle) but I've never heard of that being done and without modifying the tank itself I can't see how it could work (I can't see how having a bleed screw on the valve itself would work as that would just allow gas to straight from the output tank out of the vent so would lower the pressure of gas trying to push it's way into the receiving tank). Even if it could work you'd need to have the sending tank upside down so that liquid gas would come out of it into the other tank instead of vapour.

There are 'bleed screws' on some of the American RV gas storage tanks but they're not there to be used as bleed screws... they are tell-tale ports. During filling the tell-tale port is left open, gas vapour comes out continually during filling until the tank is 80% full at which point liquid gas (instead of vapour) will come out of the tell-tale, the person doing the filling knows the tank is now full so stops the filling and closes the tell-tale. The tell-tale is an indicator that the tank has reached the 80% level, it's not there to speed up filling, the gas is transferred into the tank via a pump which pumps at higher than tank pressure.

You won't be able to transfer much gas from one tank to another without a pump. Butane and propane are stored as a liquid in the tank and as long as there's any liquid in both tanks and both tanks are at around the same temperature the pressure in both tanks is around the same... the amount of gas in each tank (or percentage of total fill volume in the tanks) doesn't effect pressure by much and unless there is a pressure difference there's nothing to push gas from one tank to another.

You haven't asked about this but if you fill a butane bottle from an LPG forecourt in the UK (or from a propane bottle) you'll be filling the butane bottle with propane (not butane). Regulators may be designed for use with butane or propane but not usually both, they are designed for different input pressure and output pressures and in the case of direct fit (regulator directly fitted to the bottle) the design of the interface/connection between the bottle and the regulator differs... so you can't connect a butane regulator straight onto a propane bottle or vice/versa.

He may be able to buy a propane bottle of similar dimensions to his existing bottle (would work the same as butane but would just need to change the regulator). It is possible to refill standard camping type propane bottles at forecourts but normal bottles don't have the built in 80% fill limiter (can be unsafe to overfill) and most forecourts won't allow you to refill them (they'll stop you if they realise what you're doing). You can buy a SafeFill / GasIt type purpose designed forecourt refillable bottle but not all forecourts will even allow you to refill those.

[Pinger]

Thanks LPGC.
The conversation began with me being asked the 'fill limit' so there is already an awareness of the safety aspect there and I'd made him aware of the need to invert the donor tank. I see the purpose of the tell-tale port now.
Good point (I'd overlooked) re propane at LPG forecourts. I had made him aware it would need a permanently installed tank to be allowed forecourt filling (there are prohibition signs at the forecourts here regarding 'loose' tanks) but moot unless adapting the stove to propane. Maybe though the adaptation (regulator) would be acceptable if a similarly sized propane bottle was an option.

The options are reducing to relying on a bleed valve but like you I can see the incoming LPG vaporisng and heading straight out - unless carefully manipulated (still curious what the blanked fitting on the 3.9kg tank is for and whether it can be repurposed as a bleed valve - while exercising diligence re fill limit). The use of a pump - and having seen previous posts on this topic - a tricky proposition. Or, trying to use a temperature gradient between the two tanks to create enough of a pressure differential to enable transfer between the two tanks - but what method of heating the donor tank? Given that two 3.9kg tanks last him the entire summer season, partial fills would probably be acceptable - and avoid overfilling.

What I can tell him so far though is that his hoped for solution of just connecting two tanks with a hose and opening the valves isn't going to work - which is pretty much what I suspected but was thrown by the fact that in essence that is how a refillable gas (cigarette) lighter is refilled. I assume it works because the lighter's tank is reduced to atmospheric pressure prior to refilling - back to a non-existent bleed valve/tell-tale port then!

[Pinger]

Update. Just had a chat with said friend. A propane tank the same size is the same as a butane one - £100 each, non-returnable. There is a plan though. He's going to contrive a tell-tale port (at appropriate height) and use it to release pressure just enough to allow LPG in from (inverted) donor tank as he's seen being done on YouTube with the option of creating a temperature gradient by cooling the small tank as much as possible and heating the donor tank as much as practicable (this is also known to work). Either of these methods or a combination should allow filling to an acceptable level.

So, unless anyone has a better idea, that's probably the question(s) answered. Thanks again - for confirming the mere interconnection isn't sufficient and for the points on propane (which focused minds toward the above solution(s)).

[LPGC]

The problem with the temperature gradient method is that if you pump warm gas from a full tank into a cold empty tank it doesn't take long before you have two half full tanks of gas at roughly the same temperature and pressure.

[Pinger]

I'm going to press for the temp gradient method - and suggest at least a second attempt (re-heat) to enable a reasonable filling of the second cylinder. Half full is pretty close to the 80% limit anyway and probably acceptable.

[LPGC]

My half full was just an example figure, I'm not sure how much gas you'll manage to transfer in a single go using the heat gradient method.

I regularly connect the outlet of a partly full 70L autogas tank (containing propane) to the inlet of a new 90L empty autogas tank to transfer a couple of litres.

At first it transfers very quickly... there's no pressure in the receiving tank to start with so the incoming gas evaporates inside it and cools it (especially in winter, ice forms on the outside areas of the receiving tank that are 'splashed' on the inside by the incoming liquid gas and at the bottom where the cold liquid gas has settled). Initially I have to 'throttle' the speed of transfer by only partially opening the sending tank's manual outlet valve, otherwise the sending tank's excess flow prevention system will kick in and stop the flow of gas.

Very soon the speed of transfer slows from that initial rush (I expect this will be when the receiving tank has some pressure because incoming gas has boiled inside it) but gas continues to flow from tank to tank and around a litre of gas transfers in 20 seconds from the initial opening of the valve. I expect this is due to the difference in temperature between the sending and receiving tank (since the receiving tank has been cooled by incoming evaporating gas). At this point I can open the sending tank's outlet valve fully but the excess flow prevention valve doesn't kick in.

I stop the transfer soon after this point because I don't want to transfer much more gas than one or two litres. But such is the rate of the slowing in transfer (I can hear and feel the speed of flow in the transfer pipe) that I expect if I left the connection open it would only transfer around 5 or 10 litres of gas in total even if I left the connection open for (say) a week. I also expect that if I transferred just that one or two litres of gas, then stopped the transfer, allowed both tanks to get to the same temperature and then tried to restart the transfer I wouldn't be able to transfer any gas at that point because there'd be no temperature gradient (the receiving tank would have warmed to the same temp as the sending tank) and there wouldn't be much difference in pressure between the 2 tanks at the same temperature. An 80% full tank of gas does have more pressure than a 10% full tank of gas at the same temperature but temperature makes more difference in pressure than percentage of full level. A very small difference in pressure wouldn't be enough to see any transfer of gas between these types of tank because it will take a small pressure difference to even open the one-way fill valve on the receiving tank.

I expect there'd be some interesting relationships between speed of transfer, temperature gradient, the capacity of both the sending and the receiving tank and the initial percentage of full of both tanksif we were to experiment or read up on it... The percentage of full level will change at a different rate for both tanks if they're different capacities and/or depending on how full the tanks are at the start of the transfer.

Yes I think you could use multiple sessions of the heat gradient method (re-cool the receiving tank, reheat the sending tank then restart the transfer) but it seems it could be a bit of a time consuming way of doing it compared to using a pump.

[Gilbertd]

I'm surprised he's using Butane as it has a boiling point of 0.1C so in cold weather it will stay a liquid and not a vapour. That's why your Butane fag lighter doesn't work in cold weather.....

[Pinger]

Multiple attempt with temp gradient will be tried first I think. My friend is keen on installing a tell-tale port but I've cooled on that idea on account of it possibly creating a weakness in the tank that could cause a rupture should the tank ever become over-pressurised (while we can avoid that, there's no knowing who's hands the tanks will eventually end up in or how aware they will be of the danger of over-filling).

If there was a viable pump I'd recommend that route but there's none that I'm aware of and given it would likely be electrically powered, sparks from a motor don't appeal. Presumably, it wouldn't have to be pumping liquid - if it ingested the LPG in vapour form it could compress it to liquid. An AC or refrigeration pump comes to mind.

The two tanks he has (one full, the other 3/4 full) will last the summer but he's on the look-out for a spare tank to start transfer with. If one is found, we'll have a better idea of how effective temp gradient is. Still no final decision on how to heat the donor tank - space heater has been suggested - but I'm sure there are magnetic electric heating elements designed for affixing to steel oil drums to facilitate the pouring of heavy viscous oils in cold environments. Maybe worth a search for them.

[Pinger]

I'm surprised he's using Butane as it has a boiling point of 0.1C so in cold weather it will stay a liquid and not a vapour. That's why your Butane fag lighter doesn't work in cold weather.....

Me too. Presumably he's just running with what's there. The motorhome is summer use only though.

[LPGC]

We've been talking pumps, heating tanks, magnetic heaters, modifying tanks, heat gradients, etc.... But why go to any of the bother when he reckons 2 small tanks will last him all summer anyway?

But if a tank is modified there's no need to ever give/sell it on to anyone else?

[Pinger]

We've been talking pumps, heating tanks, magnetic heaters, modifying tanks, heat gradients, etc.... But why go to any of the bother when he reckons 2 small tanks will last him all summer anyway?

Because, he's planning for next year.

But if a tank is modified there's no need to ever give/sell it on to anyone else?

How to put this without being overly morbid? The tanks will outlast him. Who's hands they fall into after that is anyone's guess. He'll leave a will - but not instructions on tank filling.

[LPGC]

I see the point of transferring gas from the big bottle to the smaller one... I expect there isn't space for a 7kg bottle so he wants to keep using the 3.7kg bottle that's been discontinued(?), the big bottles are better value for money than buying gas in smaller bottles especially if he's already got a full 15kg bottle.

But the small bottles that are still available from other suppliers are worth a mention e.g. the 2.72kg 907 bottle... More cost to set up (buy the bottle, change the regulator) and higher cost per kg for the gas every year but such small bottle should fit in the same position as the 3.7 and no need to transfer bottle to bottle (and none of the setup cost for transferring bottle to bottle).

Your thread though! We can forget other tank options and concentrate on how to transfer gas if that's the way you want it to go. But I'd shy away from making a separate hole in the 3.7kg bottle.

[Pinger]

I see the point of transferring gas from the big bottle to the smaller one... I expect there isn't space for a 7kg bottle so he wants to keep using the 3.7kg bottle that's been discontinued(?), the big bottles are better value for money than buying gas in smaller bottles especially if he's already got a full 15kg bottle.

Correct - it is all about available space.

But the small bottles that are still available from other suppliers are worth a mention e.g. the 2.72kg 907 bottle... More cost to set up (buy the bottle, change the regulator) and higher cost per kg for the gas every year but such small bottle should fit in the same position as the 3.7 and no need to transfer bottle to bottle (and none of the setup cost for transferring bottle to bottle).

He is adamant that his options in that regards reduce to having to spend £100 on each bottle to stick with bottles that fit. And equally adamant that he's not paying £100/bottle! I will though, mention the 2.72kg 907 bottle.

Your thread though! We can forget other tank options and concentrate on how to transfer gas if that's the way you want it to go. But I'd shy away from making a separate hole in the 3.7kg bottle.

Hopefully, if the transfer can be done sufficiently well with temp gradient alone, then the idea of drilling for a tell-tale/bleed port will vanish. If not, I'll try and talk him out of it anyway.

[tom-madbiker]

This is what I use for transfering from a bottle to a tank or similar I keep it in the car on long journeys as an emergency measure too it will fill a 40 litre tank in about 5 mins obviously use at own risk as it's not rated for lpg I have very long wires on mine so I can connect to the vehicle battery to fill also I have a filter in line OSIAS Universal Inline Extern Fuel Pump Fits 0580254910 https://amzn.eu/d/ewv1GQA

[Gilbertd]

That's the same as the Lucas and Bosch petrol pumps used on no end of 70s, 80 and early 90s cars. The only thing you need to be aware of is that the petrol lubricates them so I suspect it won't last long with LPG going through it.

[tom-madbiker]

To be fair I don't use it often but it's very handy in an emergency especially as my Dutton phaeton and my Volvo amazon are both lpg only

[Pinger]

Thanks for the pump suggestion - and the comment re lack of lubrication. Limited use so viable.

What engine is in your Dutton TMB?

[tom-madbiker]

1.6 Mazda MX-5 with a secu3 programnable ECU for the sparks

[Pinger]

1.6 Mazda MX-5 with a secu3 programnable ECU for the sparks

Significantly better than the 1600 Ford X-flow in mine or the subsequent 1800 LET Opel Manta unit that ate headgaskets.