LPG – Propane Gas Smell: Ethyl Mercaptan – What Does Natural Gas Smells Like

LPG (propane) smells because an odourant – Ethyl Mercaptan – is added to the gas to make it easier to detect a leak.

Natural gas smells like the same aroma, as the same odourant is used.

In their natural state, LPG (Propane and Butane) and Natural Gas (Methane) smell like nothing, as they are all odourless gases and are also referred to as natural gas liquids – NGL.

The distinctive LPG smell that people associate with these gases is actually added to them as a safety measure.

Ethyl Mercaptan is the odourant added to make LPG (propane) and natural gas smell like their distinctive aroma.

• This odourant is added to the gas as it leaves the main storage terminals.
• Most people describe the smell as either rotten eggs or rotten cabbage.
• There are certain end products that require odourant free LPG, such as hair spray or deodorant that use LPG as a propellant.
• Some people cannot smell the odourant and must rely on electronic leak sensors.

Let’s look at some more of the details…

What Does Propane (LPG) & Natural Gas Smell Like?

For many decades, the gaseous fuels industry has added odourants to make LPG and Natural Gas smell like rotten cabbage so that people can detect gas leaks with nothing more than their noses.

Most people think that LPG and natural gas smell like either rotten cabbage or rotten eggs.

Without the addition of an odourant, leaking gas could collect without being detected.

This would create a dangerous condition that could lead to an explosion or fire.

The strength of the odourant has caused some people to refer to the process of adding the odourant as “stenching”.

Which Gas is Mixed with LPG to Detect Smell? Odour Additive

Ethyl Mercaptan is the odourant added to make LPG (propane) and natural gas smell.

However, Ethyl Mercaptan is not a gas, it is a chemical oudourant.

Much research has gone into the science of odourants and Ethyl Mercaptan is almost universally recognised as the best choice.

As a result, it is the most commonly used odourising agent.

Ethyl Mercaptan in Propane

Ethyl mercaptan in propane is also known as Ethanethiol.

It is a sulfur compound with the chemical formula CH₃CH₂SH that is a clear liquid with a very strong and distinctive odour.

Ethyl mercaptan is commonly used as an odourant with natural gas and propane-LPG, as they are naturally odourless.

How & When it Gets Added

In the case of LPG, the Ethyl Mercaptan is added to the gas as it leaves the main storage terminals.

The amount added and the process are both carefully controlled.

The terminals themselves have gas detectors that can identify gas leaks without any odourant having been added.

Ethyl Mercaptan is Used in LPG Because of its Stability Over Time

Ethyl mercaptan (not methyl mercaptan) is used in LPG as an odourant to generate the telltale smell.

Ethyl Mercaptan will maintain the chemical equilibrium across the liquid and vapour space.

However, if the tank liquid level is low, and much of the tank surface inside is exposed, then a small amount of odourant fade MIGHT occur, reducing the overall concentration of the Ethyl Mercaptan in both liquid and vapour LPG.

Given the detectability of the odourant is much less than the dosing used in Australia (we can detect parts per billion, and we dose in parts per million), the small amount of fade should not be an issue.

Methyl Mercaptan Used in LPG

Methyl mercaptan is not used in LPG.

People get confused between Ethyl Mercaptan, which is used as an odourant in LPG, and Methyl Mercaptan, which is not used in LPG.

Methyl mercaptan reportedly used as a dietary supplement for animals.

How to Test for Leaks

Special Cases with No Odourant

There are certain applications where the odourant is not added.

Facilities that use odourless gas must have the same gas detection equipment as the gas terminals.

For example, Butane is commonly used as an aerosol propellant.

Needless to say, we wouldn’t want things like hair spray and deodorant to smell like rotten cabbage!

Why do LPG (Propane) Cylinders Smell More When Near Empty?

The Ethyl Mercaptan odourant is slightly less volatile than the Propane in which it is dissolved.

As the Propane vapour is used by the appliance, the odourant is also removed in the process.

Due to its lower volatility, there is a slightly higher concentration of odourant as the last of the Propane liquid is converted to vapour and used.

The odourant is noticeable at very low concentrations (by design), so this small increase in concentration can be very apparent.

Some People Can’t Detect the LPG or Natural Gas Smell

Some people cannot detect the LPG or natural gas smell.

This can be due to illness or if they have been continuously exposed to the smell.

If you know that you can’t smell gas, have problems with your sense of smell or just want an extra layer of protection, you might consider installing a gas detector.

Gas detectors emit an audio alarm when gas is detected, similar to a smoke alarm.

Ethyl Mercaptan Trivia

Whilst some people cannot smell Ethyl Mercaptan, evidently FLYS can!

We are told by Mackay Queensland gas fitter Josh Dalton:

“FLYS love the smell of ethyl mercaptan. If you see a bunch of fly’s buzzing around your Meyer, cylinder, pipe work, regulators, fittings or appliances, there may be good chance there is a leak.
It’s one of the first things I look for automatically.”

Thanks for the tip, Josh.

To explain, some species of flies are attracted to the Sulphur in the Ethyl Mercaptan and will buzz around like bees.

The amount of Ethyl Mercaptan required is very small, so even a very small leak will attract some flies in the vicinity.

Odourant Fade

Ethyl Mercaptan is not a perfect odourant.

No odourant can provide an absolute guarantee of no odourant fade.

Under some circumstances, it can fade away and be replaced by a gentler smelling odour that might not be recognised as a gas leak.

Fade is caused by adsorption, absorption and oxidation.

Adsorption involves the odour ‘sticking’ to the inside steel walls of the gas bottle.

Absorption refers to the odour being absorbed by another substance such as water.

Oxidation occurs when the inside of a gas bottle is exposed to air, causing a chemical reaction which can result in LPG losing its smell.

Odourant fade is rare but it can happen.

While very few instances of odourant fade have been recorded in Australia, it has happened in other countries.

The presence of rust or moisture within an LPG tank could cause this fade.

To prevent this, new cylinders are filled with dry and inert nitrogen gas, to prevent both rust and eliminate the presence of moisture.

Once filled with LPG, the risk is virtually eliminated.

What Suppliers do to Prevent Fade

• Suppliers make sure gas bottles and tanks are properly conditioned prior to filling.
• Suppliers ensure the correct amount of odorant is added to LPG.
• Then they monitor the levels of odourant at all points of the supply chain.
• They make sure gas storage tanks and gas bottles are clean inside and out.
• Gas bottles and tanks are purged of air, water and other substances prior to use.

What You Can Do

LPG users can also assist in avoiding odourant fade by making sure that all disconnected gas cylinders have their valves closed, even when completely empty, to stop air (oxygen) and moisture from getting inside the cylinder.

This helps prevent the possibility of internal rusting and fade by oxidation.

Rust and moisture are also one of the things that are looked for when gas cylinders are periodically re-inspected.

The presence of either is cause for condemnation of the cylinder.

You can also follow the these tips:

• Do not store your gas bottles indoors or in an enclosed space, in case of a leak.
• Do not use any gas bottle, regulator, hose or fitting that is damaged.
• Only buy gas bottles, regulators, hoses and fittings from reputable suppliers.

So, now you know why gas smells the way it does and why it is the ‘Smell of Safety’.