Propane Gas Bottle – How Do LPG Cylinders Work – Gas in Bottle

Propane Gas Bottle – How Do LPG Cylinders Work – Gas in Bottle

LPG gas is gas in bottle.

LPG cylinders work when the LPG gas in bottle, stored as a liquid under pressure, turns back into gas 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 is generally stored, as a liquid, in steel vessels ranging from small BBQ LPG cylinders to larger LPG cylinders and bulk propane tank.

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 of propane is too high for most applications, at up to 2482 kPa.

Importantly, gas regulators are connected to propane gas bottle outlet valves because they reduce the LPG gas pressure to the much lower 2.75 kPa working pressure.

Gas in Bottle Under Pressure – LPG Cylinders – Gas Cylinders

Gas in bottle (gas in gas cylinders) or LPG tanks is LPG-propane as a liquid under pressure in LPG cylinders, from 0 kPa at -42°C to 1794 kPa at 54°C.

The LPG gas in bottles turns back from liquid into gas when you release some of the pressure.

The LPG exists as both liquid and gas in bottles. (See image)

LPG Gas Cylinder Works with Liquid & Gas in Bottles

Gas in bottles is generated when the LPG-propane liquid boils (see image) and turns back into gas in bottle, when you release some of the pressure in the propane gas bottle (gas cylinders) by using gas.

An LPG cylinder works (propane gas bottle works) with some of the LPG-propane at the top of the LPG cylinder (propane tank) 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 involves the use of the gas in bottle, as opposed to the liquefied gas.

Gas Cylinders – Propane Tank – Cooking Gas Cylinders

Gas cylinders may also be called “Propane Tank”, “Propane Gas Cylinders”, “LPG Bottles”, “LPG Gas Bottles” or just “Gas Bottle”.

Propane gas cylinders (propane gas bottle) work by containing both liquid and gas in bottle because LPG liquefies under relatively low pressure.

Propane gas cylinders (propane gas bottle) are considered low pressure versus high pressure gas cylinders, as used with CNG.

Gas cylinder sizes vary because of application and demand.

A small propane gas bottle is portable, as used in camping.

What is an LPG Cylinder Made of

An 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 and it is a low cost material.

Some gas cylinders are made of aluminium or composites since they save on weight.

Aluminium is quite common for forklift gas cylinders because of weight relating to safe lifting.

The use of aluminium keeps the weight down to allow for more gas in bottles since staying within the safe lifting weight limits is important.

The most recent innovation is composite gas cylinders.

These gas cylinders are typically fiberglass with a high impact plastic outer shell.

Some have an inner lining of HDPE whilst other have a thin steel liner because they are impermeable to the gas in bottles.

How LPG (Propane) Liquid Changes to Gas – Vaporisation

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 gas cylinders?

Surprisingly, if you could see though the steel, 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.

LPG (Propane) Boiling Point

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 gas cylinders.

LPG stays liquid because it is under pressure in an LPG cylinder (propane gas bottle).

As a liquid, it looks a lot like water.

It becomes gas after it boils.

It is colourless and odourless in its natural state.

LPG (Propane) Vaporisation – How Liquid LPG Turns Into Gas in Propane Gas Bottle

Now, a bit more detail on vaporisation in gas cylinders.

LPG liquid boils and turns back into gas in bottle (gas in LPG cylinder) when you release some of the pressure in the gas cylinders (propane tank) by turning on your gas appliance.

Firstly, to boil, the liquid LPG draws heat from the steel walls of gas 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 gas cylinders, the more rapidly it boils, vaporising at a faster rate.

The vapour pressure in the propane gas bottle (propane tank) also increases with temperature, as explained below.

Because the steel of the gas cylinders 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 bottle (propane tank) 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 bottle (propane tank) 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 portable gas in bottles, it can be used in vehicles as well as home heating, hot water and cooking.

LPG Pressure and Gas Pressure Regulators

LPG pressure refers to the average force per unit of area that the LPG gas in bottle exerts on the inside walls of propane tank (propane gas bottle).

Actual LPG 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 temperatures.

The LPG 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 pressure will fall, with the use of the last of the LPG vapour.

LPG pressure measurement is in kilopascals (kPa), pounds per square inch (psi) or “Bar”.

LPG Cylinder Pressure Varies with Temperature

As previously mentioned, LPG stored as gas in bottles is under pressure.

LPG cylinder pressure increases with temperature.

(LPG Pressure-Temperature Chart shown)

“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.

LPG 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.

How Much Pressure is in an LPG Gas Cylinder?

LPG is under pressure, as a liquid, and turns back into gas in bottle when you release some pressure.

An LPG gas cylinder works (propane tank works) with pressure of LPG (propane) gas going from 152 kPa (24 PSIG) at 0°C to 1794 kPa (257 PSIG) at 54°C.

The pressure drops to zero at -43ºC (which is just below the boiling point) and the pressure becomes greater at even higher temperatures.

Vaporisation Must Match Consumption When LPG Cylinder Works (Propane Tank Works)

The amount of gas that the appliance or appliances draw from the LPG cylinder (propane tank) must be matched by the rate of vaporisation when the LPG cylinder works (propane tank works).

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 propane gas bottle walls.

Heat is absorbed through the propane gas bottle walls and into the liquid.

The larger the propane tank or the fuller the propane tank, the more gas that can be vaporised at a given temperature.

Vaporisation tables are used to match the required vaporisation rates to the corresponding propane gas bottle size.

The vaporisation tables show the maximum continuous vaporisation rates, in MJ/hr, at different ambient temperatures for each available propane gas bottle size.

LPG Cylinders Work with a Valve

All LPG cylinders work with some form of gas valve.

A “POL” valve is the standard type of LPG valve in Australia, for LPG cylinders 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 gas bottles.

Gas Line Connection – Pigtail

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 propane gas bottle hand wheel.

You should always do a soapy water leak test after every reconnection.

LPG Cylinders Work by Turning the Gas Valve Hand Wheel

LPG cylinders 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 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

Pressure Relief Valve

The pressure relief valve is the single most critical safety feature in how an LPG cylinder works (propane tank works).

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 bottle 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.

LPG Gas Cylinders (Propane Gas Bottle) Venting Gas

If the pressure rises beyond safe levels, the LPG gas cylinders 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 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 venting.

You should also call your LPG supplier, from a safe location, and advise them that your LPG gas cylinders are venting gas.

Do not use your mobile phone, any electrical devices or other ignition sources near a venting LPG cylinder.

Final Thoughts on Gas Cylinders

In conclusion, a propane gas bottle functions unnoticed after evolving over many decades into a very safe storage and gas delivery system.

Gas cylinders are also easy to operate and requiring little attention until they need refilling.