filling gas tanks on roundhouse locos

[steveh99]

At the end of a day of intensive running at an exhibition, I found it increasingly difficult to fill the gas tank fully with ever decreasing run times as a result. Is there a temperature problem? Any ideas?

[Big Jim]

Cold can be a problem with butane. Probably not at an exhibition though.

[Gralyn]

I seem to remember reading that it is something to do with temperature.
Let the gas tank cool down after a run by cycling locos. I bet that better brains than mine can come up with a technical explanation.

[GTB]

At the end of a day of intensive running at an exhibition, I found it increasingly difficult to fill the gas tank fully with ever decreasing run times as a result.  Is there a temperature problem? Any ideas?

As Graylyn suggests, the problem is gas tank temperature.

During a run, if the tank is located in the cab it absorbs heat from the boiler. Which is why you often need to turn down the gas during a run, as the gas pressure increases with tank temperature.

If you don't leave the tank to cool down to ambient temperature after a run, the residual heat in the tank will be absorbed by the liquid butane as it enters the tank. The liquid butane will start to boil and the tank pressure will then increase rapidly. Once it increases to the same pressure as the butane can, no more liquid butane can transfer and the tank will appear to be full, even though there may be only a little liquid gas in it.

The tank on the loco has to have cooled to the same temperature as the butane can before it can be completely refilled. The greater the temperature difference between the can and the tank, the less liquid will be transferred.

In cold conditions the outcome is the same, but for the opposite reason. If the butane can is too cold, there isn't enough pressure in the can to force the liquid into the loco tank.

Regards,
Graeme

[steveh99]

Thanks Guys

I am trying out a new regime whereby at the end of a run, I immediately refill the boiler with cold water to help cool the loco as well as pre warm the water, but I don't refill with gas until I am ready to reuse the loco, this can be up to 1 hour later. Previously I was trying to refill with gas immediately after running the loco

Rgds

[steveh99]

Has anyone trying insulating the gas tank to stop it getting too hot. I was thinking of a pad underneath it to stop it touching the footplate, something like a disc cut from a polystyrene coffee cup. Might also be able to make a jacket from the same stuff to wrap round the sides of the tank??

[Gralyn]

I have used two thicknesses of mounting card between the boiler and gas tank on my Accucraft 0-4-0 Decauville as I was having to wait a long time to re-gas the loco. This did cut down the wait time.

[GTB]

Has anyone trying insulating the gas tank to stop it getting too hot.

you haven't told us what loco this is, which makes any definitive answer difficult.

I have one loco where space in the cab is very tight and I had to fit a piece of 0.8mm ply as a spacer between the gas tank and the boiler to ensure they could never physically touch. It doesn't make the gas tank noticeably cooler at the end of a run than other members of the fleet though.

I've also got an Aster loco where there is a metal bracket connecting the gas tank and boiler to keep the tank warm enough to keep gas pressure up.

It's a double edged sword........

The gas tank needs to absorb some heat from the boiler during the run to turn the liquid butane into gas, otherwise as gas is drawn off the tank cools until the pressure drops too low to keep the burner lit, even though there is still butane in the tank.

If you check a gas fired tender loco where the gas tank is in the tender you'll find it is usually in a water bath to supply the heat needed to maintain gas pressure.

I doubt there is enough space in the average cab to completely insulate a gas tank, but insulation might cause problems keeping the burner lit in cold weather.

If you want to try insulation, use plywood, card, or wool felt. Boilers at 40psi are hot enough to soften most plastics, especially foams. Also make sure you can easily remove it if it does affect gas pressure in cold weather.

I got exhibitions out of my system decades ago, but when running a new loco in on the bench, I've found I have no trouble refuelling it if I wander off to do something else for an hour or so before the next run.

I haven't done any testing but suspect that an empty copper boiler open to air cools quicker than one filled with water, as the specific heat of water is about 10 times that of copper.

Regards,
Graeme

[JMORG]

Be careful with filling the boiler up with cold water if it's still hot, the thermal shock could cause damage.

[steveh99]

Has anyone trying insulating the gas tank to stop it getting too hot.

you haven't told us what loco this is, which makes any definitive answer difficult.

I have one loco where space in the cab is very tight and I had to fit a piece of 0.8mm ply as a spacer between the gas tank and the boiler to ensure they could never physically touch. It doesn't make the gas tank noticeably cooler at the end of a run than other members of the fleet though.

I've also got an Aster loco where there is a metal bracket connecting the gas tank and boiler to keep the tank warm enough to keep gas pressure up.

It's a double edged sword........

The gas tank needs to absorb some heat from the boiler during the run to turn the liquid butane into gas, otherwise as gas is drawn off the tank cools until the pressure drops too low to keep the burner lit, even though there is still butane in the tank.

If you check a gas fired tender loco where the gas tank is in the tender you'll find it is usually in a water bath to supply the heat needed to maintain gas pressure.

I doubt there is enough space in the average cab to completely insulate a gas tank, but insulation might cause problems keeping the burner lit in cold weather.

If you want to try insulation, use plywood, card, or wool felt. Boilers at 40psi are hot enough to soften most plastics, especially foams. Also make sure you can easily remove it if it does affect gas pressure in cold weather.

I got exhibitions out of my system decades ago, but when running a new loco in on the bench, I've found I have no trouble refuelling it if I wander off to do something else for an hour or so before the next run.

I haven't done any testing but suspect that an empty copper boiler open to air cools quicker than one filled with water, as the specific heat of water is about 10 times that of copper.

Regards,
Graeme

They are all Roundhouse (Anne, Jack, Katie) The tanks appear to be fixed to the main frame with a right angle bracket so fitting anything much under the tank is going to be unlikely. The summary seems to be leave them for a while before refilling and relighting. I might need to buy another loco to share out the roster a bit more to allow more cooling time.

[GTB]

They are all Roundhouse (Anne, Jack, Katie)

OK. They all have the Roundhouse type A gas tank. There is a bit of slack in the mounting holes, but you'd be lucky to get more than a sheet of paper under the tank.

Jack and Katie are more or less twins, with the gas tank tucked in between the boiler and the cab side with about 1mm between them. The boiler on a Lady Anne doesn't project as far into the cab, have you noticed any difference in filling the gas tank after extended running?

 I might need to buy another loco to share out the roster a bit more to allow more cooling time.

Seems as good an excuse as any for adding a loco. to the roster. It would also mean more reliable running, as failure of one loco would be less of an issue.

Regards,
Graeme