How inert gas systems work

Malcolm Latarche
Malcolm Latarche

10 September 2017

Inert gas systems are essential on ships that frequently carry hazardous cargoes with some of the most dangerous being those that are explosive or flammable. One of the ways to reduce the hazard is to blanket the cargoes (both wet and dry) with an inert gas – usually nitrogen but occasionally other gases including exhaust gas.

Use of inert gas systems in dry cargo vessels is voluntary but for oil and chemical tankers it is compulsory under SOLAS and more specifically chapter 15 of the Fire Safety Code.

The latest changes to the rules on inert gas systems became effective in January 2016 and cover measures to prevent explosions on oil and chemical tankers transporting low-flashpoint (less than 60°C) cargoes. The development of the amendments followed many years of work aimed at preventing explosions on oil and chemical tankers that transport low-flashpoint cargoes and follow recommendations made by an Inter-Industry Working Group (IIWG) that was established to study incidents of fires and explosions on chemical and product tankers.

The IIWG included the European Chemical Industry Council (CEFIC), the International Association of Classification Societies (IACS), the International Association of Ports and Harbours (IAPH), the International Chamber of Shipping Limited (ICS), the International Association of Independent Tanker Owners (INTERTANKO), the International Parcel Tankers Association (IPTA), the Oil Companies International Marine Forum (OCIMF) and the International Group of P&I Clubs.

SOLAS regulations II-2/4.5.5 and II 2/16.3.3 require an inert gas system to be fitted on all new oil and chemical tankers of 8,000dwt and above, (when transporting low-flash point cargoes of <60°C). Oil tankers above 20,000dwt were already required to install such systems.

For tankers fitted with exhaust gas inerting systems, the application of inert gas must be carried out during loading, on passage, unloading, tank cleaning and purging prior to gas freeing. However, for chemical tankers, the application of inert gas may take place after the cargo tank has been loaded, but before the commencement of unloading (only if nitrogen is used as the inerting medium) and shall continue to be applied until that cargo tank has been purged of all flammable vapours before gas freeing.

The oxygen limit (all tankers) for inert gas supplied to cargo tanks has also been lowered from 8% to 5% for new systems. The exemption for existing chemical tankers with tank volumes not exceeding 3,000m3, fitted with tank cleaning machines with a throughput not exceeding 17.5m3/h (per nozzle) and a total combined throughput not exceeding 110m3/h, does not apply to chemical tankers delivered after 1st January 2016.

All inert gas systems work on the principal of reducing the oxygen content below that which supports combustion. There are different ways of doing this as previously mentioned.

A flue gas system with a scrubber cools and clean flue gas available from the ship’s boilers and distribute the gas to the cargo tanks, typically during cargo discharge. As an exhaust gas, flue gas already contains less than 5% oxygen, which is the IMO requirement for inert gas and further treatment is therefore not necessary although control and monitoring systems are obviously essential elements of the overall system.

Where flue gas from boilers is not available, an inert gas generator can be used to burn MDO or HFO to produce inert gas with an oxygen content of 2 – 4%. The gas is treated in the same way as flue gas to cool and clean it. Some ships have system which are a combination of both mentioned types and which are used as a boiler when needed and as a gas generator at other times as necessary.

As an alternative to flue gas systems, nitrogen can be used as the inert gas. Nitrogen can be produced in a nitrogen generator or taken onboard as a ready produced gas. A nitrogen generator operates using a flow of compressed air which is separated using a membrane into nitrogen and oxygen the two main components of air. In a generator, the oxygen is vented to the atmosphere and the nitrogen is stored in a buffer tank. Nitrogen generators are equipped with sensors that continually monitors the oxygen content in the nitrogen output.

If the level of oxygen rises above 1% of the design value, then an alarm is activated. If the level of oxygen rises further a high alarm operates and redirects the flow to atmosphere and closes the discharge line to the buffer tank. Obviously, the exhaust from a nitrogen generator consists of almost pure oxygen and this can be a problem in itself making for a local atmosphere that is oxygen rich and which could be a danger in itself.

As well as for blanketing cargoes, inert gases do have other uses on board ships. Although not commonplace, adding inert gas to ballast water can help prevent corrosion in the tanks and be an effective re-growth inhibitor for organisms that may be taken on in the ballast.