How Mass Flow Controllers make our gas smell

November 3, 2020 Sandra Wassink
Odorisation with ATEX mass flow controller for a gas smell

Did you know that natural gas is odourless? I didn’t… I always find it having a penetrating scent. Well, it appears that this penetrating - sulphur -  scent is added to the natural gas on purpose. Let’s see why this is.

As natural gas is combustible and odourless by nature, the government requires some safety measures here. Many countries have established similar but different safety regulations how to handle natural gas and bio methane grid-injections. In all cases gas will need to be odorised. This is mostly regulated by the Health and Safety department (HSE) of the local government.

Why does gas smell when it is odourless by nature? This is the point where gas odorisation comes in. Gas odorisation can be much more flexible and deterministic executed by dosing Mercaptans, Tetrahydrothiophene (THT) or Sulphur-free odorants into natural gas using mass flow controllers.

What about natural gas odorisation?

Odorisation of natural gas is done to act as a ‘warning agent’ in case of leakage. The idea is that people can smell the gas prematurely if it is present. Because, if there is too much gas present it can be explosive.

LEL and UEL for gas smell and safety with flow controllers
LEL (Lower Explosive Limit) and UEL (Upper Explosive Limit)

As shown in the picture, the LEL (Lower Explosive Limit) and UEL (Upper Explosive Limit) are crucial here. If the concentration of the combustible substance present in the air is too low (< LEL), then no combustion will occur. It the mixture is too rich (> UEL), there is a huge amount of gas in the air and only partial combustion will occur. Gases become dangerous in between the LEL and UEL. Therefore, it is most important for people in the surroundings to smell the gas in time, before the concentration is too high and it exceeds the LEL.

As a result, it is stated in the safety regulations that natural gas has to be detectable at a concentration level of 20% of the LEL and this is done by odorisation. Needless to say, that the odor used in the gas is not dangerous to people’s health.

When is an odour added to gas?

This depends on the type of gas line. We know distribution lines and transmission lines.
Distribution lines are local natural gas utility systems that include gas mains and service lines, such as the commercial gas used at domestic environments. All these distribution lines need to be odorised. For the transmission lines it is stated in the regulations when to odorise it.

ATEX Zone 1 Coriolis Mass Flow Meter
ATEX Zone 1 Coriolis mass flow meter

THT, Tetrahydrothiophene

For the odorisation there are many different odorants available, such as Tetrahydrothiophene (THT) and Mercaptan. Selecting the odorant depends on the properties of the gas to be odorised, pipeline layout, ambient conditions etc. Tetrahydrothiophene or THT is a well-known odor. THT is under ambient conditions a colourless volatile liquid with an unpleasant smell.

Controlled supply of THT using mass flow controllers

Bronkhorst had the pleasure of developing a solution for a Dutch customer to add THT to their biogas. Biogas was generated from anaerobic decomposition of organic matter and upgraded to natural gas quality to inject into the Dutch natural gas main. As commercial natural gas in the Netherlands has to contain at least 18mg of THT per cubic meter gas, the process of adding this to the commercial gas had to be done really accurately.

The traditional approach to add THT is using a pump with a fixed stroke volume. However, low gas flow rates using a pump for batch-wise injection may lead to liquid THT remaining in the gas lines. THT may not be mixed well with the gas and might have the wrong concentration. A homogeneous injection of THT is therefore much better. Besides this, odorants are relatively expensive which also makes an economic and accurate injection interesting.

A better solution here would be using a combination of a pump with a Coriolis mass flow controller, in our case the mini CORI-FLOW series mass flow controllers. The Coriolis instruments make it possible to dose both continuously as well as accurately.

Read more about this application in our application note 'Controlled supply of odorant to natural gas'.

Direct pump control with mass flow controller

A better solution here would be using a combination of a pump with a Coriolis mass flow controller. Due to an integrated PID controller, the mass flow controller can control the pump directly. This combination makes it possible to dose both continuously as well as accurately.

Read more about this application in our application note 'Controlled supply of odorant to natural gas'.

Hazardous areas

Something to be considered is the classification of the area. As combustion gases are explosive by nature, it is very common to classify the environment as a hazardous area. Most common classifications (in Europe) are marked as ATEX zone 1 or zone 2. Just make sure to select the right material to use.

For solutions such as THT odorisation processes, Bronkhorst can offer both ATEX/IECEx zone 1 and zone 2 solutions.

Pre-engineered Liquid Odorisation Delivery Sets are available on request for odorisation of gas flows from 10 to more than 8000 m3 gas flows feeding from 5 up to 50 mg/m3 odorant and are able to cope with virtual any national regulation.

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