Is LPG Liquid or Gas? LPG Liquid vs LPG Vapour

Is LPG Liquid or Gas? LPG Liquid vs LPG Vapour

LPG exists in two different forms, LPG liquid and gas (vapour).

As a LPG liquid, it is stored in a pressurised vessel.

It is typically used as a gas (vapour) but there are LPG liquid applications, as well.

The pressure and temperature at which it is stored determines which kind you have.

Is Propane (LPG) a Gas or a Liquid?

The answer to this is “it depends’.

Propane is a liquid when it is under pressure or below -42°C.

Propane is a gas when the pressure is relieved and the temperature is over -42°C.

Liquid Gas?

LPG – propane – cannot be both LPG liquid and a gas at the same time.

The change from LPG liquid to gas is known as vaporisation.

How Does LPG Work?

LPG – Liquid Petroleum Gas – is stored under pressure, as a LPG liquid, in a gas bottle.

It turns back into gas vapour when you release some of the pressure in the gas bottle by turning on your gas appliance.

The LPG gas vapour is held in the top of the bottle and the LPG liquid at the bottom, as shown in the accompanying image.

Almost all of the uses for LPG involve the use of the gas vapour, not the liquefied gas.

LPG Vapour vs Gas

First, let’s sort out some of the terminology.

The two terms, vapour and gas, are used interchangeably by most people in reference to LPG.

Vapour (or vapor in American spelling) is the more technically correct term for LPG, as it is in gaseous and liquid equilibrium at room temperature.

It can be turned back into LPG liquid by increasing the pressure on it without reducing the temperature.

A gas has one defined state at room temperature.

So, vapours are gases however not all gases are vapours.

Same but Different

In Australia, LPG is Propane.

Propane is a flammable hydrocarbon gas with 3 carbon and 8 hydrogen atoms in a propane molecule.

The chemical formula for propane is C₃H₈.  (Propane molecule model shown)

LPG liquid and vapour characteristics have some similarities to water and steam.

Liquid LPG is colder, denser and heavier.

LPG vapour is warmer and much lighter.

Liquid LPG and vapour LPG both have applications but are not interchangeable when used.

The choice depends on the purpose.

In most applications, such as home appliances, LPG is used as vapour (gas). 

However, some LPG fuelled products require it to be in LPG liquid form, such as with autogas LPG liquid injection systems.

Different Cylinders

LPG is liquid in a cylinder with an area at the top of the cylinder where it turns into vapour.

However, the cylinders come in two configurations, Liquid Withdrawal and Vapour Withdrawal.

The difference depends on from where the LPG is extracted.

If drawn from the bottom of the cylinder, it comes out as LPG liquid.

If drawn from the top of the cylinder, it exits as vapour.

The cylinders are clearly marked as either “Vapour Withdrawal” or “Liquid Withdrawal”.

In Australia, LPG liquid withdrawal cylinders are also painted with a blue top, to make them instantly recognisable.

(See blue top image below)

Using the wrong type of cylinder for an application could pose a serious safety hazard.

LPG as a Liquid

Water boils at 100°C, becoming a gas (steam).

In contrast, LPG liquid boils at -42°C becoming gas vapour.

It stays LPG liquid because it is under pressure in a gas cylinder.

As LPG liquid, it looks a lot like water.

It is colourless and odourless in its natural state.

Unlike water, 1 kilogram of LPG liquid does NOT equal 1 litre of liquid LPG.

LPG density or specific gravity is about half that of water, at 0.51.

In Australia, where LPG is propane, 1kg of LPG has a volume of 1.96L.

Conversely, 1L of LPG weighs 0.51kg.

The distinctive smell of LPG comes from an odorant that is added to LPG, for safety and leak detection reasons.

Caution should always be used to avoid direct exposure, as liquid LPG is cold enough to cause severe cold burns on exposed skin.

LPG as a Vapour or Gas

LPG liquid becomes vapour at temperatures of -42°C and above.

LPG expands to 270 times the volume when it goes from liquid to gas.

So, 1L of LPG liquid equals 270L of gaseous LPG.

As there are 1000L in a cubic meter (M3), 1L of LPG liquid expands to 0.27M3.

The lower and upper limits of flammability are the percentages of LPG that must be present in an LPG/air mixture.

This means that between 2.15% and 9.6% of the total LPG/air mixture must be LPG in order for it to be combustible.

LPG gas vapour is heavier than air and will sink to and collect at the lowest point.

If LPG is vented to the outside air, it will quickly dissipate with the slightest movement of air.

Conversely, if LPG is vented into a sealed structure, with no air movement, the LPG gas will collect on the floor and rise vertically as more LPG is vented into the structure.

Obviously, this would create an extremely hazardous situation.

Are Liquefied Gas and Compressed Gas the Same?

No.

As already discussed, some gases, like LPG – propane and butane – become liquid under pressure.

Other gases, like LNG – Liquefied Natural Gas (methane) – only become liquid when cooled cryogenically.

Still others just become compressed at high pressure but never become liquid as a result of pressurisation.

Know the Difference for Safety

As previously mentioned, most LPG applications use vapour.

Appliances such as water heaters, room heaters and cookers all use vapour.

If these appliances were to have LPG liquid flow to their burners, the result could possibly be a fire or similar safety hazard.

This is why you must use the right type of cylinders and LPG cylinders should always be kept upright, so that only vapour is released.

An upside-down vapour cylinder, or even one laying on its side, could release the LPG as a liquid.