ThreeSheds Posted October 10, 2008 Share Posted October 10, 2008 Just a quick question - when an LPG tank is filled for the first time (or after a long period vented like mine) what happens to the air that is already in there? Will it eventually dissolve in the liquid and get carried through the system? Cheers, Rog Quote Link to comment Share on other sites More sharing options...
mickeyw Posted October 10, 2008 Share Posted October 10, 2008 Just a quick question - when an LPG tank is filled for the first time (or after a long period vented like mine) what happens to the air that is already in there? Will it eventually dissolve in the liquid and get carried through the system?Cheers, Rog The air gets compessed as you fill the tank, which is why the tank can only be filled to 80% of its volume. For example, my 80 litre tanks can only hold 64 litres of LPG. (There is an automatic shut off valve that controls this.) It is this pressure that then forces the LPG out of the tank again. Also the fact that the LPG is pressurised is what keeps it is liquid form. At atmospheric pressure LPG changes to its gaseous form which is how it is used in the engine. If your tank has 'vented' you have a leak and there won't be any LPG left in there. I suggest you check all your seals and connections before refilling. Michael Quote Link to comment Share on other sites More sharing options...
ThreeSheds Posted October 10, 2008 Author Share Posted October 10, 2008 If your tank has 'vented' you have a leak and there won't be any LPG left in there. I suggest you check all your seals and connections before refilling.Michael Thanks for that. My tank has been 'vented' because I have had the pipes off it for a while (like about 3 years!) I always thought that the liquid was forced through by it's own vapour pressure - it's obvious really that it's the compressed air like you said. Cheers, Roger Quote Link to comment Share on other sites More sharing options...
Landowner Posted October 10, 2008 Share Posted October 10, 2008 Just a quick question - when an LPG tank is filled for the first time (or after a long period vented like mine) what happens to the air that is already in there? Will it eventually dissolve in the liquid and get carried through the system?Cheers, Rog What an excellent question B) Quote Link to comment Share on other sites More sharing options...
pinny Posted October 10, 2008 Share Posted October 10, 2008 What an excellent question B) i always thought if air is present in a gas cylinder there is a risk of explosion when we work on a live gas connection/main if air enters a main all breaks loose pilot lights in houses are turned off as there is a risk of explosion if air enters a gas main is this true for cylinders. Quote Link to comment Share on other sites More sharing options...
Landowner Posted October 10, 2008 Share Posted October 10, 2008 i always thought if air is present in a gas cylinder there is a risk of explosion when we work on a live gas connection/main if air enters a main all breaks loose pilot lights in houses are turned off as there is a risk of explosion if air enters a gas main is this true for cylinders. I would say not and venture the following. The air will not mix with the gas as the gas is in liquid form in the cylinder. Am I right? Quote Link to comment Share on other sites More sharing options...
ThreeSheds Posted October 11, 2008 Author Share Posted October 11, 2008 I would say not and venture the following.The air will not mix with the gas as the gas is in liquid form in the cylinder. Am I right? Oh I think that there will be a good mix of air and gas (evaporated liquid) above the liquid... I don't think, however, that there will be any risk of explosion since there can be no ignition source that I can think of. I hope ! I am still wondering if the air will eventually dissolve in the liquid and get replaced by gas evaporated from the LPG... Just found this at http://www.geocities.com/lpgmanuk/cop11.html: (I added the bold) It is good practice to reduce the oxygen content of the tank(s) to 10% or less by volume before introducing LPG. This can be achieved by introducing an inert gas eg, nitrogen or carbon dioxide dilution of the air in the tank(s) until the required end point is reached. 'good practice' - hmm so I would think that it's ok to have air in there then? But since I am planning to get the system checked out before filling I guess the man will know what to do... Interesting that the high-tech leak detection equipment advised is soap solution (although the article does appear to have been written in the mid nineties so there may be improvements on this in use by now) Rog Quote Link to comment Share on other sites More sharing options...
mrFrog Posted October 11, 2008 Share Posted October 11, 2008 i always thought if air is present in a gas cylinder there is a risk of explosion when we work on a live gas connection/main if air enters a main all breaks loose pilot lights in houses are turned off as there is a risk of explosion if air enters a gas main is this true for cylinders. I guess the reason there is a risk in gas mains and houses, is that anything connected to the mains (oven, rings, boiler) that is using gas, e.g. turned on (cooking, or heater pilot light on). Any air trapped in the gas line will result in the pilot light / oven / ring to go off, and it won't re-ignite on its own, so gas will freely leak into the house unburnt, hence the risk of explosion. In an lpg tank, as said above, the liquid will flow out first and won't mix with air (mixing gases is actually pretty hard, and usually results in them separating again if left for too long, a bit like oil and water). Either way, engines have constant sparks (except diesel), so if a couple of engine revs are run on air rather than LPG, you'd notice a cough/splutter, and then the engine would start again as soon as LPG is re-introduced,unlike a gas ring which doesn't spark 3000 times a minute. Hope this makes sense. Quote Link to comment Share on other sites More sharing options...
mickeyw Posted October 14, 2008 Share Posted October 14, 2008 Liquid is lighter than gas (in this case anyway). Autogas systems pick up from the bottom of the tank, (unlike domestic heating LPG systems that take vapour from the top of the tank), so until you run out of LPG all you will get is liquid. Therefore you should not have any problems with engine spluttering due to air getting into the pipes. The only scenario when you could pick up air is if you are running very low on fuel and throw your vehicle into a hard corner or roundabout, thereby causing all the liquid to go to one end of the tank, away from the fuel pick up pipe. Just realised this could also apply for side slopes when off road, but the same can happen to a petrol or diesel engine. Solution - don't run so low on fuel! As you have said, the fuel starvation would only be momentary, so momentum would keep the engine running. Michael Quote Link to comment Share on other sites More sharing options...
Escape Posted October 14, 2008 Share Posted October 14, 2008 Liquid is lighter than gas (in this case anyway).Michael Liquid is heavier than gas (for any given substance). But I'm sure that's what you ment to say, and I agree with your reasoning. Quote Link to comment Share on other sites More sharing options...
hattymender Posted October 14, 2008 Share Posted October 14, 2008 Been running on LPG for years and the question never occured to me. But if you didn't have some air in the tank what would you have when the fuel runs out? Quote Link to comment Share on other sites More sharing options...
Bowie69 Posted October 15, 2008 Share Posted October 15, 2008 Been running on LPG for years and the question never occured to me. But if you didn't have some air in the tank what would you have when the fuel runs out? LPG vapour and the original air mix, as the pressure drops it will slowly evaporate. Quote Link to comment Share on other sites More sharing options...
Roverdrive Posted October 19, 2008 Share Posted October 19, 2008 FWIW I was told that the air should be vented off as you will not get anything like the correct volume of LPG in the tank. When boh my systems were fitted, a small amount of LPG was put into the empty ( air filled ) tank. The valves were then shut and the tank inverted so that the dip tube for the LPG liquid was in the gas space at the top. The outlet valve was then opened to force the air out as it is both lighter than LPG gas and liquid. The 80% fill is to allow for expansion of the liquid if for example you fill your tank at night and park it in the sun the next day. An air mix shouldn't be explosive in the tank as it SHOULD be over rich for ignition. Wouldn't like to try it on an empty LPG tank as it could then be coming in to the exposive range. I think the fuel starvation issue spoken of is more a case of LPG gas being drawn from the tank rather than liquid at low level. I don't know what the expansion rate is for LPG from Liquid to gas, but I would bet that the 10mm pipe cannot flow sufficient LPG in gaseous form from the tank to maintain the correct mixture leading to weak mixtures and misfires. Mike Quote Link to comment Share on other sites More sharing options...
sean f Posted October 19, 2008 Share Posted October 19, 2008 When a flamable gas (LPG in this case) is mixed with air there is a range at which the mixure becomes explosive, to much gas and there will not be enough oxygen present to little and there won't be enough fuel. In an engine this is controlled so the mixure in the cylinders is in this explosive range when the spark from the plug reaches it hence making the engine work. In a tank the mixture should always be in the to rich range, this is why a nearly empty tank is the most dangerous situation (same is true on a petrol tank with the vapours being the gas). The risk in a household situation is that if enough air enters the gas main then the entire sytem might drop to the maximum explosive limit in which case it is possible for the flame to go back up the pipe and cause an explosion, this is why when a gas main catches fire it is often safer to leave it burning until the gas can be turned off and the concentration reduced in a safe and controlled manner. (This is the reason for blow back preventors on oxy-acetyline gear, the same effect is seen if the gas is switched off at the bottle, the pipe just contains less volume than a househole gas main) The upper explosive limit of methane can be demonstrated with a coffee tin with a few holes and a gas meter, I am not going to explain how in case some one tries and manages to blow them selves up but any one with a knowledge of gas should be able to work it out, the clue is methane is lighter then air. Quote Link to comment Share on other sites More sharing options...
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