LPG vs CNG - what are the risks?

[bazza40]

I have CNG on my vehicle, which gives me low-cost motoring. However, I understand CNG tanks are pressurised to 250 psi, compared with 25 psi for LPG. I assume this is because the high content of methane/ethane in CNG compared with LPG requires such pressures. Is this correct?

I have heard claims CNG has higher risk due to the higher pressure, for example in a collision. Is it possible to convert a CNG tank to LPG?

What are the statistics for deaths and injuries comparing LPG and CNG?

[kartman]

Would think you have to fit a completely new system even if your vehicle is able to convert to lpg.

[WarpSpeed]

Tanks not compatible nor is the rest of the system, but since internet sources are regarded as the the only credible sources (though this answer ironically is now on the internet) maybe best to search there as anyone's answer here is pure rubbish apparently no matter how true if not backed up by some other source anyway? Makes one wonder the useful purpose for even coming here to ask or even give advice really? Maybe it should be closed down altogether? Seems to serve no useful purpose in that case, maybe Wiki is the better source.

Additional point of info. CNG cars also have some different internals and other features like intake manifolds, so not certain what all is required for that change over in your case and model.

Edited October 3, 2014 by WarpSpeed

[bazza40]

I can accept the piping/ filling system is different. If someone can explain to me why a tank designed to handle gas pressurised to 250 psi is unable to handle a liquid pressurised to 25 psi, I'll have added to my store of knowledge.

[DGS1244]

LPG is a liquidfiable petroleum gas and therefor has a lower pressure requirement it is also heavier than air so sinks downwards. This means that when under pressure it turns to liquid in the cylinder which increases the capacity. CNG is a compressed natural gas that does not liquify at normal temperatures hence the higher pressure and heavier cylinder as the gas always remains a gas in normal conditions also it is lighter than air therefor goes upwards particularly when there is a leakage. CNG has a very limited storage capacity, even under pressure, and really is not suitable for cars unless you are always close to a refilling station or do not intend to travel far from one. If you run out of either gas remember you have to get the car to a filling station you cannot use a 'can' to top up. Fortunately LPG cars also normally keep their petrol tank so can in an emergency switch between the two. Does not work with CNG and it requires a different injection system.

[DGS1244]

I can accept the piping/ filling system is different. If someone can explain to me why a tank designed to handle gas pressurised to 250 psi is unable to handle a liquid pressurised to 25 psi, I'll have added to my store of knowledge.

The high pressure tank has a different internal piping system to the low pressure LPG tank, also the weight is very very different. CNG tanks are vastly heavier than LPG tanks.

[RBOP]

With LNG your tank will intermittently vent gas (if not driving) as the pressure will constantly increase as the temperature rises and the liquid converts to gas. If you fill your tank on LNG and then don't drive for a couple weeks, your tank will be empty with the slow venting. But when driving you can definitely go further on a tank of LNG than on a tank of CNG. There are not many LPG stations in Thailand. But we are trying to change that.

Oh sorry, I thought you meant LNG. Never mind.

Edited October 4, 2014 by RBOP

[WarpSpeed]

^Ok, was wondering and was loading both barrels.. Do take issue with not many LPG stations though, I never had any problems finding one..

[DGS1244]

With LNG your tank will intermittently vent gas (if not driving) as the pressure will constantly increase as the temperature rises and the liquid converts to gas. If you fill your tank on LNG and then don't drive for a couple weeks, your tank will be empty with the slow venting. But when driving you can definitely go further on a tank of LNG than on a tank of CNG. There are not many LPG stations in Thailand. But we are trying to change that.

Oh sorry, I thought you meant LNG. Never mind.

I think you are getting very mixed up between LPG and CNG(LNG) In either case they do not vent at any weather temperature found in Thailand unless they have faulty valves. It takes a much higher temperature to cause venting and once it starts it does not stop with human intervention. I have dealt with many LPG and CNG fires in the past and once they vent then you have a problem in extinguishment etc.

[RBOP]

LNG vents. LPG doesnt.

[RBOP]

There is a standard for LNG tanks they must hold for min. 5 days.

[Guest]

There's another issue with LPG - the government are trying to discourage its use for motor vehicles and the price has already risen slightly, and is likely to go up faster than CNG in the future.

[bazza40]

DGS 1244, I omitted to mention my vehicle is dual fuel i.e petrol and CNG. It operates seamlessly in switching over.

I would guess the composition of CNG ( mostly methane and ethane ) is less valuable as a feedstock for the chemical industry than the composition of LPG ( mostly propane and butane ), hence a rising price differential.

My OP asking for accident statistics perhaps was asking too much. Still, my fuel cost on CNG of 0.9 baht per km ( about 320 baht per fill )

is quite satisfying.