LPG Pressure – LPG Gas Bottle Pressure – LPG Pressure Relief Valve

One of the most common questions we hear is “How much pressure in LPG cylinder?”

So, how much LPG cylinder pressure do you have and how much do you need?

First, some background regarding how much pressure in LPG cylinder:

•LPG gas pressure refers to the average force per unit of area that the LPG gas exerts on the inside walls of the LPG gas bottle.

• The higher the temperature, the higher the pressure of the LPG within the cylinder.

• The pressure of LPG (propane) goes from 0 kPa (0 PSIG or 0 bar) at -43ºC to 2482 kPa (360 PSIG or 24.8 bar) at 70ºC.

• The LPG – propane – exists as both liquid and vapour (gas) within the cylinder.

• The term “pressure” refers to the average force per unit of area that the gas exerts on the inside walls of the cylinder.

LPG (Propane) Cylinder Temperature – Pressure Chart

How much pressure in LPG cylinder?  It depends…

As temperature rises in an LPG cylinder, so does the pressure.

For example, the pressure is 0 kPa (0 PSIG or 0 bar) at -43ºC and goes up to 2482 kPa (360 PSIG or 24.8 bar) at 70ºC.

The LPG is both liquid and vapour (gas) inside the cylinder.

How much pressure in LPG cylinder?  See chart above.

Units of Measure for Pressure

There are 3 commonly used units of measure for pressure: PSIG, kPa & bar.

PSIG is pounds per square inch gauge.  PSIA is pounds per square inch absolute.

1 atm = 14.7 psia = 0 psig = 101.325 kPa

kPa, or kilopascal, is a metric unit of pressure and part of the International System of Units (SI).

The bar is also a metric unit of pressure, but is a non-SI unit.

1 bar = 100 kPa = 0.987 atm = 14.5038 psia

In What State is the LPG Gas Inside the Cylinder?

LPG as liquid & vapour

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

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

LPG cylinders are typically filled to 80%, meaning 80% liquid and 20% vapour.

This percentage changes, with the liquid LPG decreasing as you consume the gas.

LPG Pressure Relief Valve (Propane Pressure Relief Valve)

LPG pressure relief valve

If the cylinder is exposed to excessive heat, the LPG pressure relief valve allows the gas to vent and keep the pressure within safety limits.

The pressure relief valve is integrated into the main gas valve on the bottle, as shown in the accompanying picture.

It’s actually a safety valve within a valve.

This can also be known as a propane pressure relief valve.

This eliminates the possibility that the cylinder will rupture or explode.

The worst thing that can happen is the venting gas ignites and you have a plume of flame.

This will self-extinguish after the pressure drops to a safe level or when the cylinder runs out of gas.

This is why you always want to use your BBQ outdoors and away from your home or other flammable materials.

LPG Boiling Point

LPG Boiling: Vaporisation

Did you know that every time you turn on one of your gas appliances, the LPG in your gas bottles starts to boil?

This is called LPG (Propane) Vaporisation.

If you could see though the steel, you would also notice that it looks just like water boiling (see accompanying image).

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

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

The process of going from liquid to vapour (gas) is called vaporisation.

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

As a liquid, it looks a lot like water.

It is colourless and odourless in its natural state.

For more info on LPG, see LPG Gas Physical Properties

LPG-Propane Cylinder Pressure Rating – Operating Pressure

Pressure gauge in Bar

LPG cylinders and vessels are designed to handle much higher than normal operating pressures.

The typical cylinder would probably only burst with pressures over 6895 kPa or 1,000 PSIG.

That’s about 5x the normal pressure.

This would vary by the manufacturer and the cylinder itself.

LPG cylinders have pressure relief valves incorporated into the main valve, as previously discussed.

The typical pressure relief valve setting is 2585 kPa or 375 PSIG.

So, the cylinder would never actually go above this, as the valve would open and lets some gas escape, limiting the pressure inside the cylinder.

LPG cylinder pressure varies with temperature.

Even at 70ºC (158ºF), well beyond normal ambient temperatures, the pressure would only be 2482 kPa (360 PSIG).

So, not only wouldn’t it approach bursting pressure (≈ 1,000 PSIG) but, under normal circumstances, it would never even reach the 375 PSIG required to trigger the pressure relief valve.

LPG Working Pressure in Appliances

The actual working pressure of LPG in gas appliances is much less than the cylinder pressure, at 2.75 kPa.

LPG regulators are used to reduce and maintain this safe operating pressure.

It should also be noted that natural gas and LPG appliances operate at different pressures, with 1.1 kPa and 2.75 kPa, respectively.

How Much Pressure in LPG Cylinder – LPG Pressure Temperature Chart

So, we never fully answered “How much pressure in LPG cylinder?”

As previously mentioned, when LPG is stored in a gas bottle, it is under pressure.

LPG vapour pressure can vary greatly based on temperature.

LPG Pressure-Temperature Chart

The higher the temperature, the higher the pressure, as shown in the chart.

The term “pressure” refers to the average force per unit of area that the gas exerts on the inside walls of the gas bottle.

The pressure is measured in kilopascals (kPa) or pounds per square inch (PSIG).

“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.

The level of fill in the gas bottle comes into play when the LPG is being used, as it affects the rate of LPG vapourisation.

As LPG is a liquefied gas, the vapour pressure inside the cylinder will remain the same from full until the last of the liquid LPG is vaporised.

Then the pressure will fall quickly as the last of the LPG vapour is used, as well.

The use of pressure as a measurement in LPG use is very limited.

A steady pressure only shows that liquid LPG is inside the cylinder, but not how much liquid is left.

In the US, vapour pressure is measured in pounds per square inch (PSIG).

PSIG is defined as 1 pound of force applied per square inch as measured by a gauge.

As a comparison, 1 psi = 6.89476 kPa.

What is the Pressure of a 20 lb Propane Tank?  A 9kg LPG Cylinder?

BBQ gas bottles, like a 20lb propane tank or a 9kg gas bottle, have the same pressure as large gas bottles.

It all depends on the temperature.

At 100°F, a 20lb propane tank has 172 PSIG of pressure.

Similarly, a 9kg gas bottle has 1183kPa at 38°C.

Final Thoughts

How much pressure in LPG cylinder is dependent upon the temperature of the contents.

This, in turn, is affected by the ambient temperature.

The cylinder is actually capable of handling about 5x the normal pressure.

That normal pressure is far more than what is required for the working pressure of gas appliances.

To reduce the pressure, a gas regulator is used between the cylinder and the appliances.

“Safe and Simple” is the best summary of how it all works.