10. Gas for Model Steam Boilers
10. Gas for Model Steam Boilers
There are three types of gas commonly used in firing model steam boilers. All may be labelled as
LPG (Liquid Petroleum Gas) but for reasons set out below it is important to use their correct names.
They are Propane, Butane and Isobutane. In Australia Propane is commonly labelled as LPG when
used in motor vehicles, barbecues and the like. The tanks are always constructed with heavy duty
sheet steel and have a POL connection.
Each gas is stored in liquid form which vaporises in the tank as gas is drawn off and should burn off
in its gaseous form. The liquid to gas conversion occurs at or above specific temperatures for each
gas. Propane vaporises at -42°C, Isobutane at -11.75°C and Butane at -0.4°C. They are all colourless
and odourless in their raw state and denser than air– i.e. They will sink to the lowest solid level
possible when air is present. This is the reason it is recommended that refillable gas tanks working
inside the hull of a boat be removed from the boat and refilled in open air.
The following table is presented courtesy of ELGAS Ltd. (www.elgas.com.au)
Recommended Gas Mixture:
Our standard recommendation is to use a gas with a maximum of 30% propane and the balance either
butane or isobutane or a mixture of both. The propane acts as a propellant as well as being
useful for burning as the tank empties. We have had reports that the low temperature performance
of isobutane drops of more quickly than an isobutane/propane mix. We have not substantiated this
but consider, if correct, that it is caused by the iso-butane alone has a faster decline of pressure as
the tank pressure drops than where propane is present. Also we have advice that in the USA there
is 80% butane/ 20% isobutane product that is probably less expensive and has better cold vapor
pressure than 70-30 Butane/Propane.
Pure butane is satisfactory at ambient temperatures of about +5°C and above and pure iso-butane
down to about –9°C. These temperatures are “about” and higher than the defined “boiling point” because
there is a gradual decay in the rate at which the liquid vaporises as the temperature approaches
the “boiling point”.
Apart from the ambient temperature there are two other factors that influence the temperature of the
liquid gases in the tank.
The first is that when refilling a refillable tank, the temperature of both the master tank and the tank
being refilled falls. The refillable tank should at least be felt warm before refitting it to the steam
plant. If available a hair dryer can rapidly warm the cold or frosted tank. Also a short dunk in warm
water can be helpful. Re-connecting a tank that has frost on it is asking for a short run!
Similarly, as the gases from the refillable tank burn off, the remaining liquid in the tank cools and can
progressively reach a temperature where gas generation declines to a point where the boiler stops
generating steam. This is most likely to occur in cool climates when running with pure butane
Identifying the Gas in Retail Outlets 70/30 Butane/Propane
Unfortunately the part of the gas industry dealing with retailing these gases in small tanks has not established
standards for presentation of the labelling of the contents of the tanks involved.
Also in an extensive search we could find no specification for the type of container that should be
used for storing and selling retail quantities of the various gases used for model steam boilers, with
one exception. Propane (LPG) is extensively sold in heavy steel tanks for use in domestic situations
for outdoor barbecues, heaters etc. These are too heavy to be used in model marine situations and
are excluded in the following discussion.
Given the wide range of brands available on the market we have found that a good way of identifying
a 70/30 gas tank, after giving up on trying to understand the label, is inspecting the fitting on the top
of the tank.
Refilling From a Master Gas Tank: (Usually Disposable)
Miniature Steam refillable gas tanks are fitted with an industry standard Ronson refill valve that work in conjunction with a short or long Refillable Gas Tank Adaptor shown below, and illustrated below in use on a small refillable gas tank. Refillable gas tanks are necessary in model steam marine situations because of limited space in most boats hulls.
Where there is space to locate a refilling master tank in the hull with the steam plant, the Control Valve, P/N 4357, shown above may be used. This has a gas stop valve that is screwed onto the top of the master gas tank and controls the Lindal EN417 gas valve. The threaded pipe is connected directly to the burner gas line. This Control Valve may also be fitted in static boiler installations where space is not a problem to directly connect larger master tank.
Refilling a refillable gas tank from a master tank at first, can be confusing - because:
- the first attempt may not present the shaft on the master tank adapter correctly aligned with the centreline of the receiving Ronson valve, leading to lots of gas spluttering out of the misaligned surfaces.
- the refillable tank will not be set on a stable, firm and horizontal surface (OUT OF THE BOAT OR STEAM PLANT!) leaving the operator unaware that he can and should apply more pressure to the operation to stop leaking as well as correcting the misalignment.
- the gas pipe connection to the valve on the refillable tank has not been disconnected so that liquid
- gas can’t be seen when the tank is full. No liquid showing - the tank is not full!
- there can be an expectation of rapid refilling! Depending on the size of the receiving tank, and the amount of gas in the master tank, it can take up to TWO minutes! Just keep going until liquid gas appears at the open gas valve.
- frosting occurs on the tank - this normal see Guide # 2 Gas for Model Steam Boilers.
This is a technically correct term for the gas pressure in the tank shown as “psig” or “pounds per square inch gauge” as displayed by a standard imperial measure pressure gauge, or “bar” in metric measurements. (1 bar = 14.5 psig). The table following is being referred to in the following text.
The table shows how the gas pressure in a tank, decays as the temperature of a gas in it, decreases.
The yellow highlight section is the one of most interest here in that we generally recommend the
70/30 butane/propane mix, but have commented on the 100% butane as being acceptable in temperate climates.
There is a qualifier to that earlier statement.
Irrespective of the ambient conditions, if the rate of consumption of gas from a tank of butane is very rapid, the tank will cool quickly, and may well get down the temperature scale where the evaporating (boiling) of the liquid gas slows and may eventually cause the boiler to stop for lack of heating.
We believe that a gas pressure of about 20psi is required to produce a minimum heat for a boiler, “more pressure is better” if you can achieve it. Then you can control the engine speed by adjustments to the gas cock, varying the rate of steam production.
Isobutane is also included in the relevant gases that are commercially available. We could not find an equivalent set of data as in the table, but one reliable reference stated: “Isobutane has about 64% less vapour pressure than propane but about 44% more than butane (at 21ºC)” . We think this is an endorsement for its use in cool gas situations , whether environmental, or induced by the operating conditions.