In a propane gas bottle (LPG cylinder), how does the gas in bottle go from LPG to gas?
LPG – propane is the gas in bottle (gas in a cylinder).
A propane gas bottle (LPG cylinder) works when the LPG gas in bottle, stored as a liquid under pressure, turns back from liquid LPG to gas.
This happens by releasing some of the pressure in the propane gas bottle, through use.
The LPG cylinder (propane gas bottle) works after being filled with the pressurised LPG.
The LPG gas in bottle (gas in a cylinder) is generally stored, as a liquid, in steel vessels ranging from a small BBQ propane gas bottle (LPG cylinder) to larger LPG cylinders and bulk propane tanks.
The LPG gas in bottle turns back into gas in bottles as it exits the LPG cylinder (propane gas bottle), then passing through a gas regulator.
The regulator reduces the LPG pressure to the level used by the appliances, as the pressure in the propane gas bottle is too high for most applications, at up to 2482 kPa.
Importantly, gas regulators are connected to propane gas bottle (LPG cylinder) outlet valves because they reduce the propane gas bottle pressure to the much lower 2.75 kPa working pressure.
Gas in bottle (gas in a cylinder) for propane bottles or propane tanks is LPG – propane as a liquid under pressure in LPG cylinders (propane bottles), from 0 kPa at -42°C to 1794 kPa at 54°C.
The LPG gas in bottle turns back from liquid into gas when you release some of the pressure.
The LPG exists as both liquid and gas in bottle. (See image)
The gas in bottle (gas in a cylinder) is generated when the LPG – propane liquid boils (see image) and turns from LPG to gas, when you release some of the pressure in the propane gas bottle (LPG cylinder) by using gas.
A propane gas bottle (LPG cylinder) works with some of the LPG – propane at the top of the LPG cylinders (propane tanks) as gas in bottle whilst it is liquid at the bottom. (See image above)
All LPG gases, including propane, are compressed into liquid at relatively low pressures and are also referred to as natural gas liquids – NGL.
Importantly, almost all of the uses for a propane gas bottle (LPG cylinder involves the use of the gas in bottle, as opposed to the liquefied gas.
LPG cylinders may also be called “Propane Tanks”, “Propane Bottles”, “LPG Bottles”, “LPG Gas Bottles” or just “Propane Bottle”.
Bottles of propane gas (propane bottles) work by containing both liquid and gas in bottle because LPG liquefies under relatively low pressure.
LPG cylinders (propane gas bottle) are considered low pressure versus high pressure gas cylinders, as used with CNG.
LPG cylinder sizes vary because of application and demand.
A small propane gas bottle is portable, as used in camping.
An propane gas bottle (LPG cylinder) is typically made of welded steel, aluminium or composites.
To start, steel is by far the most common material because it is the easiest to fabricate into an LPG cylinder and it is a low cost material.
Some propane bottles (propane tanks) are made of aluminium or composites since they save on weight.
Aluminium is quite common for a forklift propane gas bottle because of weight relating to safe lifting.
The use of aluminium keeps the weight down to allow for more gas in bottle since staying within the safe lifting weight limits is important.
The most recent innovation is the composite propane gas bottle (LPG cylinder).
These propane bottles (LPG cylinders) are typically fiberglass with a high impact plastic outer shell.
Some propane bottles (propane tanks) have an inner lining of HDPE whilst other have a thin steel liner because they are impermeable to the gas in bottle.
Did you know that every time you turn on one of your gas appliances, the LPG cylinder works (propane gas bottle works) as the liquid LPG starts to boil turning into gas in the propane bottles (propane tanks)?
Surprisingly, if you could see though the steel of the propane gas bottle, you would also notice that it looks just like water boiling.
The big difference is that it happens at -42°C or -44°F.
Vaporisation is LPG boiling from liquid into its gaseous state as gas in bottle – LPG cylinder.
Vaporisation is how LPG goes from liquid to gas.
Water boils at 100°C or 212°F, becoming a gas (steam).
In contrast, LPG boils at -42°C or -44°F, becoming gas vapour in the propane bottles (propane tanks).
LPG stays liquid because it is under pressure in the LPG cylinder (propane gas bottle).
As a liquid, it looks a lot like water, if you could see inside the propane gas bottle (LPG cylinder).
It becomes gas after it boils.
It is colourless and odourless in its natural state.
Now, a bit more detail on vaporisation in LPG cylinders (propane tanks).
LPG liquid boils and turns back into gas in bottle (gas in a cylinder) when you release some of the pressure in the LPG cylinders (propane tanks) by turning on your gas appliance.
Firstly, to boil, the liquid LPG draws heat from the steel walls of LPG cylinders (propane gas bottle) which, in turn, works by getting heat from the ambient air. (See image)
As with water, the more heat that is applied through the LPG cylinders (propane tanks), the more rapidly it boils, vaporising at a faster rate.
The vapour pressure in the propane gas bottles (propane tanks) also increases with temperature, as explained below.
Because the steel of the LPG cylinders (propane bottles) draw heat from the ambient air heat, cold weather will slow down the rate of vaporisation.
As a result, vaporisation also makes the propane gas bottles (propane tanks) feel colder than the ambient temperature.
Actually using the gas in bottle makes the propane gas bottle get even colder.
Under the right conditions, frost or ice can even form on the propane gas bottles (propane bottles) because you are using gas very rapidly,
From a practical point of view, it is used just like natural gas, although the LPG energy content is much higher.
As it is a portable gas in bottle, it can be used in vehicles as well as home heating, hot water and cooking.
LPG pressure refers to the average force per unit of area that the LPG gas in bottle exerts on the inside walls of propane tanks (propane bottles).
Actual propane gas bottle pressure varies with temperature.
As a result, LPG pressure in the propane gas bottle goes from 0 kPa (0 PSIG or 0 bar) at -43ºC to 2482 kPa (360 PSIG or 24.8 bar) at 70ºC.
It goes even higher with greater propane gas bottle temperatures.
The propane gas bottle pressure is reduced to at 2.75 kPa, the level used in appliances, by a gas pressure regulator. (see image)
Remember, LPG is a liquefied gas.
So, unlike pressurised gases, the pressure inside the propane gas bottle will remain the same from full until the vaporistion of the last of the liquid LPG.
Then the propane gas bottle pressure will fall, with the use of the last of the LPG vapour.
Propane gas bottle pressure measurement is in kilopascals (kPa), pounds per square inch (psi) or “Bar”.
As previously mentioned, LPG stored as gas in bottle is under pressure.
LPG cylinder pressure increases with temperature.
(LPG Pressure-Temperature Chart shown)
LPG Cylinder – Propane Gas Bottle Pressure Chart Temp Temp Pressure Pressure Pressure ºC ºF kPa PSIG Bar 70 158 2482 360 24.8 60 140 2013 292 20.1 54 130 1794 257 17.9 43 110 1358 197 13.6 38 100 1186 172 11.9 32 90 1027 149 10.3 27 80 883 128 8.8 16 60 637 92 6.4 -1 30 356 51 3.6 -18 0 152 24 1.5 -29 -20 74 11 0.7 -43 -45 0 0 0 Note: Some numbers have been rounded. Propane gas bottle pressure is measured in kilopascals (kPa) or pounds per square inch (psi).“Bar” is yet another unit of measure for pressure.
1 Bar = 100 kPa, so it is metric based but not an SI unit of measure.
Propane gas bottle pressure can vary greatly based on temperature, as shown in the chart.
The level of fill in the propane gas bottle comes into play when the LPG cylinder is being used, as it affects the rate of vaporisation.
LPG is under pressure, as a liquid, and turns back into gas in bottle when you release some pressure.
LPG cylinders work (propane tanks work) with pressure of LPG (propane) gas going from 152 kPa (24 PSIG) at 0°C to 1794 kPa (257 PSIG) at 54°C.
The LPG cylinder pressure drops to zero at -43ºC (which is just below the boiling point) and the pressure becomes greater at even higher temperatures.
The amount of gas that the appliance or appliances draw from the propane bottles (propane tanks) must be matched by the rate of vaporisation when the propane bottles work (propane tanks work).
Switching to a larger propane gas bottle can provide a higher rate of vaporisation.
Keeping the LPG cylinder full also increases vaporisation, as there is more liquid gas touching the LPG cylinder walls.
Heat is absorbed through the propane gas bottle walls and into the liquid.
The larger the propane tanks or the fuller the propane tanks, the more gas that can be vaporised at a given temperature.
Vaporisation tables are used to match the required vaporisation rates to the corresponding LPG cylinder size.
The vaporisation tables show the maximum continuous vaporisation rates, in MJ/hr, at different ambient temperatures for each available propane gas bottle size.
All LPG cylinders (propane tanks) work with some form of gas valve.
A “POL” valve is the standard type of LPG valve in Australia, for LPG cylinders (propane tanks) from 45kg to 210kg.
The POL valve connection is notable for its reverse — left-handed — thread.
A pressure relief valve is incorporated within the POL valve, for safety, in case the pressure of the gas in bottle gets too high.
Newer LCC27 valves, with added safety features, have been introduced for BBQ size propane gas bottles (propane tanks).
The gas line pigtail or POL regulator screws into the large female threaded opening on the side of the propane gas bottle valve.
It is unique in that it has a left-handed or reverse thread. So, to tighten it, you turn the connector anti-clockwise.
Tightening is achieved either with a wrench or by turning a LPG cylinder hand wheel.
You should always do a soapy water leak test after every reconnection.
LPG cylinders (propane tanks) work by turning the hand wheel anti-clockwise to start the flow of gas in bottle, but you should avoid turning it hard to the open stop.
Located on the top of the valve, the gas valve hand wheel controls the flow of gas, once the connection is secure.
Opening the LPG cylinder valve 2 or 3 turns is all you need.
You shut the gas off by turning the hand wheel clockwise, tightening firmly by hand only.
CAUTION: Never open the propane gas bottle valve when unattached
The pressure relief valve is the single most critical safety feature in how LPG cylinders work (propane tanks work).
It is incorporated within the POL or LCC27 valve and appears as the protrusion opposite the main connection.
The pressure relief valve usually incorporates some kind of plastic dust cover that should be left in place.
Pressure relief valves are designed to relieve excess pressure of the gas in bottles that might result from overfilling or exposure to excessive heat or fire.
The function of a pressure relief valve is to keep a propane gas bottle from rupturing, in the unlikely event of excessive pressure build-up.
The pressure relief valves are held in the closed position by the force of a powerful spring inside.
As long as the pressure is less than that of the spring, the valve will remain closed.
If the pressure rises beyond safe levels, the LPG cylinder pressure relief valve opens to vent the excess pressure.
If this happens, you may hear a hissing sound and see cold gas vapour being released.
Once sufficient LPG cylinder pressure is released, the valve closes.
If this ever happens, just stay clear of the area and let the gas dissipate.
Having the pressure relief valve releasing gas is a rare event. Most people will never experience an LPG cylinder venting.
You should also call your LPG supplier, from a safe location, and advise them that your LPG cylinder is venting gas.
Do not use your mobile phone, any electrical devices or other ignition sources near a venting LPG cylinder (propane gas bottle).
In conclusion, a propane gas bottle functions unnoticed after evolving over many decades into a very safe storage and gas delivery system.
LPG cylinders (propane tanks) are also easy to operate and requiring little attention until they need refilling.
For more information LPG Fittings, LPG Valves, LPG Regulators, please get in touch with us!