Absolute inerting

Absolutely inerted mixture is one which does not form a flammable atmosphere when mixed with air in any proportion because the ratio of inert to fuel is sufficiently high

Absolute inerting is established when inert gas is added to the explosible mixture to such an extent, that by addition of any amount of air or fuel the explosible range can no longer be reached. The release of such an inerted atmosphere to the ambient air would thus not result in the formation of an explosive atmosphere.

Any mixture with less than 5 % propane will always be non-explosible when mixed with air, and therefore is absolutely inert.

If inerted systems operate with an overpressure and are located in confined spaces or small rooms, leaks from the inerted system will release the inerted atmosphere to the surrounding area. The oxygen concentration in the confined space will be reduced, especially if gases with high density are released (e.g. carbon dioxide or argon). Therefore inerted systems should be designed as gas tight as possible. Additional measures might be required, such as ventilation or air analysis to detect leakage. Also, leakage may cause the formation of an explosive atmosphere external to the system if absolute inerting has not been achieved.

Inert gases

Inerting may be achieved by using a non-flammable gas which will neither react with a given fuel nor with oxygen. This has to be considered carefully. Some material may react with steam, carbon dioxide or even nitrogen under some conditions. For example, molten lithium metal reacts with nitrogen. The most commonly used inert gases are:

a) Nitrogen

Nitrogen may either be received from a commercial supplier with an appropriate purity or may be generated from ambient air at technical quality by on-site facilities.

b) Carbon dioxide

Carbon dioxide may be received from a commercial supplier at an appropriate purity.

c) Steam

Steam with pressures over 3 bar might be used as an inert gas, as its oxygen content is usually negligible. Condensation has to be taken into account and might lead to a pressure drop which supports air ingress into the plant or create a vacuum. When using steam for fire fighting in dust plants the condensation can be an advantage as the dust becomes wet, preventing a dust dispersion and extinguishing smoulders.

However, there can be a risk of increased mass, chemical reaction due to the water, or microbial activity.

d) Flue gases

Flue gases from combustion can be used if the oxygen concentration can be controlled sufficiently. Fluctuations in oxygen concentration have to be taken into account, and appropriate measures to minimise fluctuations have to be taken (e.g. gas buffer storage). Flue gases shall be assumed to be similar to nitrogen when defining the limiting oxygen concentration.

e) Noble gases

Argon or other noble gas may be received from a commercial supplier at an appropriate purity. Their use will be limited due to economic reasons to applications where no other inert gas can be identified. Helium may be advantageous as an inerting medium where hydrogen is used, as the molecular size of helium approaches that of the hydrogen, so leaks may be more readily detected.

Methods of Inerting – General

There are four recognised methods of inerting a system, which are detailed below – These are:

a) Pressure swing inerting

This method pressurises the system with inert gas and vents down to atmospheric pressure. The cycle is repeated until the required oxygen concentration is reached. It is only suitable for a system which can be pressurised.

b) Vacuum swing inerting

This is similar to pressure swing inerting, but evacuates the system and releases the vacuum with inert gas. This method is suitable where a system can withstand vacuum but not pressure, such as glass vessels.

c) Flow through inerting

This method feeds inert gas at one point and simultaneously vents gas at another point remote from the feed point. This method is suitable for a system that cannot withstand either internal or external pressure.

Also, in a long thin vessel or pipeline, pressure or vacuum swing inerting may be ineffective due to poor mixing if the gas is fed and removed from the same end, so the flow through method would be applicable

d) Displacement inerting

This method relies on a large density difference between the inert gas and the air being removed. It is usually only suitable for specialised situations where there is a large density difference and mixing is likely to be poor.

Absolute inerting is a nice approach to avoid Ex installation. Still we have to deal with Ex here. Ex safety critical item we call it.

Reliability – Demands for safety critical equipment

The definition of the demands for safety critical equipment involves the following steps:

 definition of basis of safety for the inerted equipment. This may involve the use of inerting to modify the probability of the occurrence of flammable atmospheres;

 identification of safety critical equipment distinguished from process control equipment as defined in IEC 61508-1 to IEC 61508-3;

 the safety critical equipment should comply with requirements of European Directive 2014/34/EU and should be covered by a conformity assessment;

 a risk assessment shall be carried out in accordance with IEC 61508-1 to IEC 61508-3 or with an equivalent or higher safety standard, and safety critical equipment shall comply with IEC 61511-1 to IEC 61511-3 or with an equivalent or higher safety standard.

So only the usage of an Ex gas detection system shall not solve the case unless they are safety critical…