Gas Stinks (Stench) – What Causes Gas to Smell – Gassy Smell: LPG Safety Check

Gas Stinks (Stench) – What Causes Gas to Smell – Gassy Smell: LPG Safety Check

Ethyl Mercaptan is what causes gas to smell (gassy smell) and why gas stinks with the distinctive gas leak smell of rotten eggs, a method of gas leak detection.

The fact that gas stinks helps with an LPG safety check.

LPG (propane) has a gassy smell (gas stinks) because an odorant  – Ethyl Mercaptan – is added (stenching) to the gas to give it that distinctive gas leak smell for gas leak detection.

Natural gas stinks with the same gas leak smell (gas smell), as the same odorant is used for the gassy smell.

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

The distinctive gassy smell that people associate with these gases is actually added to them as an LPG safety check measure (stenching).

Gas stinks because Ethyl Mercaptan odorant added to make LPG (propane) and natural gas smell like their distinctive gas leak smell (stench).

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

Let’s look at some more of the details about why gas stinks…

What Does Gas Smell Like – What Does Gas Smell Like When it is Leaking?  Propane (LPG) & Natural

What does gas smell like or what does gas smell like when it is leaking?

The answer is the same for LPG and natural gas.

For many decades, the gaseous fuels industry has added odorants (stenching) to LPG and Natural Gas so gas stinks and have a gas smell like rotten eggs or cabbage, so that people can detect gas leak smell (stench) with nothing more than their noses during an LPG safety check.

Most people think that LPG and natural gas smell like rotten cabbage or gas stinks like rotten eggs (stench).

Without stenching or the addition of a gassy smell (gas smell), leaking gas could collect without being detected.

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

The strength of the gassy smell (stench) that makes gas stink has caused some people to refer to the process of adding the gas smell as “stenching”.

What Causes Gas to Smell – What Makes Gas Smell – What Makes Gas Stink – Odor Additive (Stench)

What causes gas to smell or what makes gas stink?

Gas stinks because Ethyl Mercaptan, an odorant, is added (stenching) and makes gas smell.

However, Ethyl Mercaptan is not a gas, it is a chemical odorant (stench) that causes gas to smell.

Much research has gone into the science of the gas smell (odorant) and Ethyl Mercaptan is almost universally recognised as the best choice to cause gas to smell.

As a result, Ethyl Mercaptan is the most commonly used gassy smell (stenching agent).

Gas Stinks Because of Ethyl Mercaptan in LPG – 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 odor or gassy smell (stench).

Ethyl mercaptan is commonly used as a gas smell with natural gas and propane-LPG, as they are naturally odorless.

How & When Stench Gets Added

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

The amount of gassy smell (stench) added and the process are both carefully controlled so gas stinks in the right amount.

The terminals themselves have gas leak detector sensors that can identify gas leaks without any gassy smell (stench) having been added.

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

Ethyl mercaptan (not methyl mercaptan) is used for stenching in LPG as an odorant to generate the telltale gassy smell.

Ethyl Mercaptan (stench) 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 odorant (stench) fade MIGHT occur, reducing the overall concentration of the Ethyl Mercaptan (stenching) in both liquid and vapour LPG.

Given the detectability of the gassy smell (stench) is much less than the stenching dose used in Australia (we can detect the gas smell in parts per billion, and we stench 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 the gassy smell (stench) 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 of Gas with No Stenching or Gassy Smell

There are certain gas applications where the gassy smell (stench) is not added.

Facilities that use odorless 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 have a gassy smell (stench) of rotten cabbage!

Why do LPG (Propane) Cylinders Have Greater Gas Smell When Near Empty?

The Ethyl Mercaptan gassy smell (odorant) is slightly less volatile than the Propane in which it is dissolved.

As the Propane vapour is used by the appliance, the gassy smell (stench) is also removed in the process.

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

The gassy smell (gas smell) is noticeable at very low concentrations (by design), so this small increase in stenching 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 leak smell.

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

If you know that you can’t detect the gassy smell (gas leak smell), have problems with your sense of smell or just want an extra layer of protection, you might consider installing a gas leak detector sensor alarm.

A gas leak detector emits an audio alarm when gas leak detection occurs, 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.

Gas Smell Fade

Ethyl Mercaptan is not a perfect odorant (stenching agent).

No odorant (stench) can provide an absolute guarantee of no gassy smell (odorant) fade.

Under some circumstances, gassy smell (stench) can fade away and be replaced by a gentler smelling odor that might not be recognised as a gas leak smell.

Fade is caused by adsorption, absorption and oxidation.

Adsorption involves the gas smell (odorant) ‘sticking’ to the inside steel walls of the gas bottle.

Absorption refers to the gassy smell (stench) 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 gassy smell (stench).

Gas smell (odorant) fade is rare but it can happen.

While very few instances of gas smell (odorant) 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 gas smell (stench) 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 Gas Smell Fade

• Suppliers make sure gas bottles and tanks are properly conditioned prior to filling.
• Suppliers ensure the correct amount of odorant (stench) is added to LPG.
• Then they monitor the levels of gas smell (odorant) 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 to Prevent Stench Fade

LPG users can also assist in avoiding gas smell(stench) 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 stench 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 gas 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 stinks the way it does and why it is the ‘Gas Smell of LPG Safety’.