Explosion hazards in inerted tanks and how to avoid them

Hydrogen sulphide (H2S) is known for being extremely toxic, as well as highly corrosive. In an inerted tank environment, it poses an additional and serious hazard combustion which, it is suspected, has been the cause of serious explosions in the past.

Hydrogen sulphide can be present in %vol levels in “sour” oil or gas. Fuel can also be turned ‘sour’ by the action of sulphate-reducing bacteria found in sea water, often present in cargo holds of tankers. It is therefore important to continue to monitor the level of H2S, as it can change, particularly at sea. This H2S can increase the likelihood of a fire if the situation is not properly managed.

Tanks are generally lined with iron (sometimes zinc-coated). Iron rusts, creating iron oxide (FeO). In an inerted headspace of a tank, iron oxide can react with H2S to form iron sulphide (FeS). Iron sulphide is a pyrophore; which means that it can spontaneously ignite in the presence of oxygen

Excluding the elements of fire

A tank full of oil or gas is an obvious fire hazard under the right circumstances. The three elements of fire are fuel, oxygen and an ignition source. Without these three things, a fire can’t start. Air is around 21% oxygen. Therefore, a common means to control the risk of a fire in a tank is to remove as much air as possible by flushing the air out of the tank with an inert gas, such as nitrogen or carbon dioxide. During tank unloading, care is taken that fuel is replaced with inert gas rather than air. This removes the oxygen and prevents fire starting.

By definition, there is not enough oxygen in an inerted environment for a fire to start. But at some point, air will have to be let into the tank – for maintenance staff to safety enter, for example. There is now the chance for the three elements of fire coming together. How is it to be controlled?

  • Oxygen has to be allowed in
  • There may be present FeS, which the oxygen will cause to spark
  • The element that can be controlled is fuel.

If all the fuel has been removed and the combination of air and FeS causes a spark, it can’t do any harm.

Monitoring the elements

From the above, it is obvious how important it is to keep track of all the elements that could cause a fire in these fuel tanks. Oxygen and fuel can be directly monitored using an appropriate gas detector, like Gas-Pro TK. Designed for these specialist environments, Gas-Pro TK automatically copes with measuring a tank full of gas (measured in %vol) and a tank nearly empty of gas (measured in %LEL). Gas-Pro TK can tell you when oxygen levels are low enough to be safe to load fuel or high enough for staff to safely enter the tank. Another important use for Gas-Pro TK is to monitor for H2S, to allow you judge the likely presence of the pryophore, iron sulphide.

Servicing for safety… A visit to the oil refinery

Working in the office makes it easy to focus on the individual tasks and get detached from how our products are making a difference to people’s lives. One of our customers was kind enough to facilitate an onsite visit so that Andrea (our Halma Future Leader on a marketing placement) could see first-hand how our products are used and who the end users are. This meant a visit to an oil refinery to see where our Crowcon portable gas detectors are used.


“The main thing that surprised me was the sheer size of the site. The oil refinery was very spaced out and it took us 10 minutes to walk from the entrance of the site to where the Crowcon engineer’s based. The engineers and employees around different parts of the refinery wore Hi Vis jackets, big safety boots, hard hats and all appeared to have personal gas detectors. During a quick site tour, I learned the products of the oil refinery are not limited to gas or petrol, but also tar, asphalt, lubricants, washing up liquid, paraffin wax and much more.

The products are all stored in big containers with pipes all over the site. Most of the products are highly flammable which explains the big focus on safety. In the distance, there were a few dome shaped containers which are pressurised vessels. If one of them were to explode, it would have a 10 mile blast radius. Suddenly I had the urge to leave and drive about 10 miles.

Crowcon’s engineer base was full of orange T4s, Gas-Pros as well as an army of “Daleks”, I mean Detectives, awaiting calibration and service. While the harshness of this industrial environment was evident from their appearance, they were otherwise in good working order, and the service engineer worked through the devices quickly.

The end users think of them as a simple device they have to wear to do their job, and they like the simplicity and reliability of Crowcon devices. The Detectives get thrown around and Gas-Pros are almost black is comparison to the usual orange, which just showcases how important the robustness of our devices is. The dangers of this working environment are not generally a big concern to the users, this is everyday life to them. Our devices help ensure they go home after a tough shift. Ensuring the devices are functioning properly is down to the service engineers, and they need to think for the users to ensure that the devices are being used properly.

Seeing Crowcon’s devices being used and the number of times someone enquired if the devices are calibrated and ready to go back into action, highlighted just how important use of portables as part of the safety regime  is considered. “Quality” and “robust” is how users describe Crowcon products and even though they may now treat them like the life saving devices they are, the devices are regularly used and valued. They make a very flammable and dangerous environment a safer place to be.”

Facts on Fixed Detection

Optimal placement of fixed gas detectors requires thorough risk assessment. This short video highlights some of the questions you need to ask before buying or installing a fixed system at your plant or site.

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Celebrating 45 years of Gas Detection with photography!

Yes that’s right – Crowcon is another year wiser making our business 45 years old. From the humble beginning of gas engineers wanting to improve the safety of their workplace, to today, where our detectors are used in 100’s of applications across tens of thousands of sites worldwide, one thing remains; our focus on Saving Lives!

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The simple steps to looking after your gas detection equipment this winter.

Gas detectors are there to save your life, whether it is a fixed system or a portable detector, keeping them well maintained is an important part of ownership.

Our guest blogger this week, Julian, has put together simple steps to ensure your gas detector is up for the job as and when it’s required.

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