Overfilling Gas Bottles.

[Mark-1]

Many gas bottles are designed to be filled to no more than ~80%.

I assume the bottle pressure is limited to the saturated vapour pressure of the gas, which varies only with temperature. Obviously the bottle will be designed to cope with the saturated vapour pressure over a wide range of temperatures.

So why can't the bottle be brim full with the liquid?

 

[RichardS]

Many gas bottles are designed to be filled to no more than ~80%.

I assume the bottle pressure is limited to the saturated vapour pressure of the gas, which varies only with temperature.

So why can't it be filled brim full with gas?

Expansion space for when the bottled in stored at a higher temperature than it was filled at?

Richard

 

[garvellachs]

Because, if the bottle is warmed, the liquid gas may expand and fill the remaining space and vent though the safety valve with potentially horrible consequences?

 

[awol]

So why can't it be filled brim full with gas?

They are always brim full of gas - it's the liquid that doesn't reach the top.

 

[neilf39]

If the liquid expands with temperature then liquid will come through the regulator and that would be interesting. Also in a rolling boat it could happen if not quite full. I would guess it gives a fairly good safety margin for the angle the cylinder is resting.

 

[Sybarite]

Expansion space for when the bottled in stored at a higher temperature than it was filled at?

Richard

When we had a calor gas concession in our family business, we were told that under no circumstances should a cylinder be used other than in a vertical position. I have seen cylinders stored on their sides in boats.

 

[LittleSister]

When we had a calor gas concession in our family business, we were told that under no circumstances should a cylinder be used other than in a vertical position. I have seen cylinders stored on their sides in boats.[/
QUOTE]

They are used on their sides on fork lift trucks and the like. I don't know if these have special safety valves or other arrangements.

I recall reading years ago that gas bottles on boats should be turned off while at sea as the rolling can result in liquid getting to the regulator and piping with disastrous consequences. Can anybody confirm whether this is true?

 

[William_H]

My only experience is with liquid propane. So the bottle is filled until liquid comes out the over flow valve. Thus leaving a gas space above. This perhaps to ensure as said that only gas enters the regulator. Now vehicle gas tanks (and hot air balloons) are different in that liquid gas is drawn off not vapour gas. The liquid goes throuh a heat exchanger to provide the heat necessary for the liquid to gas conversion. The liquid is drawn off from a dip tube in the tank. Presumably having a weight on the end and a flexible coupling to fall to the bottom of the tank lying on its side. So don't compare vehicle gas sytems to domestic/ camping gas sytems. olewill

 

[DownWest]

If you filled a tank with any liquid completely it has no expansion space, so would rupture the bottle if heated. The required space over the liquid gas allows for the expansion, absorbing gas back into liquid form as the pressure in the space goes up.

 

[Steve_N]

They are used on their sides on fork lift trucks and the like. I don't know if these have special safety valves or other arrangements.

I recall reading years ago that gas bottles on boats should be turned off while at sea as the rolling can result in liquid getting to the regulator and piping with disastrous consequences. Can anybody confirm whether this is true?

Regarding fork lift truck bottles, these are of the liquid take-off type - see olewill's post.

With vapour take-off bottles I think that the danger of liquid gas getting past the regulator in a boat or motorhome application is somewhat overstated. There was someone on here a few years ago who had managed to fit a larger bottle than his locker was designed for by using it inverted with the regulator at the bottom!
I did try a test myself once outdoors using a camping cooker, a meter of flexible hose and a full (to 80%) propane bottle with cheap top-mounted regulator. The cooker flame remained steady with the bottle upside down and shaken vigorously. Nothing I could do managed to provoke a flare up to indicate that liquid had got into the hose.

 

[vyv_cox]

There has been a long history of problems in caravans and motorhomes with bulkhead regulators. Liquid carried over with the gas sits in the rubber high pressure hose between bottle and regulator and dissolves some component of the rubber. This is then carried over into the regulator, blocking its jets.

 

[jdc]

If you filled a tank with any liquid completely it has no expansion space, so would rupture the bottle if heated. The required space over the liquid gas allows for the expansion, absorbing gas back into liquid form as the pressure in the space goes up.

This is undoubtedly true, but is also true of wine bottles. So for me the real question is not "why are gas bottles not filled brim-full?", but "why are gas bottles only filled to 80% [rather than 83, or 91 or...]?".

It can't be that liquid expansion would burst the bottle if filled to more than 80%, that would only require a tiny free space as in a wine bottle, and besides the coefficient of expansion of the liquid gas and that of the metal bottle will largely cancel out.

It can't be due to vapour pressure should the temperature rise as the vapour pressure is the same whether filled to 90% or 0.9%, ie as long as there's any liquid at all.

It might be to reduce splashes of liquid entering the regulator or hose to regulator - the gas can be used in moving appliances such as boats - or maybe to give some tolerance to heel.

It might be to allow a margin of error when filling. The point being that the gas bottle is opaque, so the only way to know how much gas is in there is by weight. But the gas bottle is heavy compared to the re-fill, so one is subtracting large numbers to arrive at the 'fullness' of the cylinder, and this is renowned as a recipe for poor accuracy, especially as the scales are not the same and will have calibration errors in addition to random ones. Suppose it gives a standard deviation of 4% of fill, then to ensure that the bottle is filled to 100% in fewer than 1 in 1 million (say) cases one has to be approximately 5 sigma away. Thus the max safe nominal fill is 100% - 5 x 4% = 80%.

My bet is that it's the last reason, with a few extra margins added on for good measure.

 

[NOHOH]

Expansion space for when the bottled in stored at a higher temperature than it was filled at?

Richard

iN
In post #2 Why did you seemingly misquote the original post.....changing liquid to gas?

 

[Mark-1]

Many gas bottles are designed to be filled to no more than ~80%.

I assume the bottle pressure is limited to the saturated vapour pressure of the gas, which varies only with temperature. Obviously the bottle will be designed to cope with the saturated vapour pressure over a wide range of temperatures.

So why can't the bottle be brim full with the liquid?

Google came to the rescue. The gas pressure is indeed a constant determined by the saturated vapour pressure. However the fluid itself has a fairly high high co-efficient of expansion (0.002 - 0.003 depending on the gas) which needs room to expand.

 

[jdc]

Google came to the rescue. The gas pressure is indeed a constant determined by the saturated vapour pressure. However the fluid itself has a fairly high high co-efficient of expansion (0.002 - 0.003 depending on the gas) which needs room to expand.

This still only gives about 5%, depending on assumed max temperature allowed (and isn't that far from petrol anyway), but is undoubtedly a significant contribution.

 

[30boat]

I saw a Mith Busters show once where they were trying to make a gas bottle explode by lighting a fire underneath.They never did because they had a safety valve that vented the gas when the safe pressure was exceeded.Maybe it's only in the States but it looks like a very good safety measure.

 

[Birdseye]

If you filled a tank with any liquid completely it has no expansion space, so would rupture the bottle if heated. .

That can be quite spectacular. When I last filled my boat cylinders the neck seal on one of them stuck open so when I disconnected the hose I got a jey of liquid gas squirting out some 10m donw the house drive. Luckily no one was anywhere near smoking.

 

[srm]

I suspect that the space is required for gas to boil off the liquid. Fill the cylinder 'to the brim' with liquid and no gas can form.

 

[jdc]

...and besides the coefficient of expansion of the liquid gas and that of the metal bottle will largely cancel out.

This turns out to be bollocks - serves me right for not checking my numbers first, sorry!

 

[Mark-1]

This turns out to be bollocks - serves me right for not checking my numbers first, sorry!

Firstly, when I made my last post I somehow missed your 10am post which made my post needless.

I did ask the wrong question - as you say the question isnt why can't it be brim full but why is such a massive margin for error required. Nobody says you should fill a meths bottle to 80pc to take account of thermal expansion.

I wonder if your last reason is correct - they just can't be relied on to measure volume accurately by weight at the refill point.

Or (without working it out) if the range of temperatures they allow for is huge.