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The difference between flammability and combustibility

Author : Samuel Ayres, Sigma-HSE

02 September 2020

The words ‘flammable’ and ‘combustible’ are frequently used interchangeably but they are different chemical properties. There is a sense in which we suspect that they mean different things, but for the most part it seems that there is a difference without a distinction. There is a distinction, albeit subtle but it is important in terms of process safety.

Image: Shutterstock
Image: Shutterstock

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Flammable materials, whether solid, liquid or gas, can burn with a flame at ambient temperatures. One does not have to raise their temperature to a certain threshold for them to be capable of burning with a flame. Combustible materials, however, must be raised in temperature before they can burn. Consider a pool of diesel fuel; if you were to hold a match to the pool, it would not ignite. To get it to catch fire and sustain a flame, to become ‘flammable’, it must first be heated to a certain temperature that is dependent on its composition.

It is the vapour that burns. For example, diesel must be heated for it to give off sufficient vapour, which, when mixed with air, will burn. Candle wax must be heated, first to the temperature at which it will melt, and then to the point again where it will give off enough vapour to mix with the air and sustain a flame.

If a material has a flash point more than the maximum expected ambient temperatures in a geographical location, it could be considered ‘combustible’ (i.e. needs heating) rather than being ‘flammable’, which can burn spontaneously. If raised to a higher temperature, it could burn with a flame, but at ambient temperatures, this will not occur.

Therefore, it is worth checking the actual flash point of your materials. If the flash point is above, say, 32°C, then it could be classed as ‘combustible’ and if it is below 32°C, then it is a flammable liquid. These are not strict definitions, but allows you to understand the risks of handling a liquid (or solid) at a temperature above, below or around its flash point and hence whether there is a risk or not of a fire/explosion occurring, depending on the operating temperatures.

Understanding the difference between flammable and combustible when undertaking a risk assessment can save on the cost of protective systems and procedures and still be sufficient to protect your people, plants and process.

Hazardous area classification - avoiding over-zoning

In most organisations, risk assessments err on the side of safety. Whilst this should not be deemed to be an issue, it may be over cautious as it can result in large hazardous areas being designated and, in some cases, a higher than necessary classification of zoning (i.e. Zone 1/21 instead of Zone 2/22) or by blanket zoning an entire area. This can cause a large amount of financial expense in terms of purchasing and maintaining ATEX certified equipment. We have worked with many companies to help them save money by correctly zoning their factories & production facilities.

Standard IEC 60079-10-1 (International Standard: ‘Explosive Atmospheres, Part 10-1, Classification of Areas, Explosive Gas Atmospheres’, 2008 IEC) states:

“…Zone 0 or Zone 1 areas should be minimised in number and extent by design or suitable operating procedures. In other words, plants and installations should be mainly Zone 2 or non-hazardous. Where release of flammable materials is unavoidable, process equipment items should be limited to those which give secondary grade releases or, failing this (that is where primary or continuous grade releases are unavoidable), the releases should be of very limited quantity and rate. In carrying out area classification, these principles should receive prime consideration. Where necessary, the design, operation and location of process equipment should ensure that, even when it is operating abnormally, the amount of flammable material released into the atmosphere is minimised, so as to reduce the extent of the hazardous area.

Therefore, when preparing a risk assessment report, you should seek to identify the possible sources of release and their bearing on the hazardous area classification, and ensure that any Zones identified are a realistic interpretation of the actual situation, and not an over specification.

The general report format for assessing Unit Operations should include sections as follows:

1. Overview: The Unit Operation is defined and the presence of a flammable atmosphere in normal, or foreseeable abnormal, operation is considered. If it is not possible for a flammable atmosphere to occur, then the assessment stops there.

2. Presence of an Ignition Source: It is not the intention, at this stage, to determine all potential ignition sources, but just to confirm that there is at least one. This should also take into consideration the possibility of an unintentional ignition source being brought into the hazardous area (i.e. maintenance or measuring equipment).

Samuel Ayres, Sigma-HSE
Samuel Ayres, Sigma-HSE

3. Discussion of Risk: If there is a potential for a flammable atmosphere and an ignition source, then there is a discussion to determine the risk to people and whether actions are required.

4. Basis of Safety: The chosen basis of safety for the unit operation is considered along with its implications on adjacent units and their basis of safety and its physical location in the site.

5. Hazardous Area Classification (HAC): If there is a requirement in the Basis of Safety to control ignition sources, then the next section, Hazardous Area Classification (HAC), gives the relevant zones.

6. Recommendations: Finally, there are recommendations. Once the recommendations have been implemented, as far as reasonably practicable, then the Basis of Safety should be ‘sound’.

Additionally, there is Auditing of the Implementation of the Recommendations, which is usually a follow-on. It should however be undertaken on regular basis to ensure that the Basis of Safety for each operation is being maintained, or whether another assessment is required to verify the Basis of Safety or to determine whether the Basis of Safety has changed and whether it is acceptable and valid from a reasonably practicable aspect.

The correct application of Hazardous Area Classification (HAC) results in an appropriate budgetary spend on safety in the workplace. Minimising the use of expensive ‘Ex’ rated equipment and if it is required then helps to reduce the level of equipment down from potentially a Category 2 to a Category 3, reducing installation costs and replacement component stock levels.

About the author:

Samuel Ayres BEng (Hons), Business Development Executive at Sigma-HSE, studied Electronic Engineering at the University of Portsmouth between 2007-2010 and worked as an Engineering Technician for the Ministry of Defence and a Prison Officer before moving into technical sales positions. He has been with Sigma-HSE’s business development team since August 2019. His goal is to increase explosion risk awareness in the workplace and is inspired daily by his young family.

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