FIRE SAFETY > Dangerous Substances Explosive Atmospheres

Explosive Atmospheres, ignition sources and the Fire Risk Assessor

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BLEVE:
BS EN 60079-10-1 contains methods to determine the claasification of hazardous areas due to the release and presence of flammable vapours, mists & gases.

In connection with this standard it is widely accepted that a potentially explosive atmosphere exists around any bolted flange connection located along the length of any particular pipe line (containing flammable liquid, vapour or gas). The extent of this hazrdous area is dependent on the properties of the fluid, size of pipe line/flange and pressure/temperature conditions.

The end result is that all efforts should be made to prevent any one of 14 ignition sources from being present within the hazardous area in  order to prevent ignition, explosion and secondary fires.

This situation is made more difficult as it is apparent that the extent of a hazrdous zone resulting from vapour or gas release under relatively low pressures will be smaller and confined to the vicinity of the source of release, where at greater pressure the extent of the zone will increase and exist at a greater distance fro the source.

Unfortunately, BS EN 60079 does not give consideration to jet release of flammable liquids so again, if we consider a flammeble liquid escaping from the flange scenario we will find that the jet of liquid will travel a distance in any direction (360 degrees) until reaching an obstruction and then begin to form a pool.

The pool will release vapours according to the fluid evaporation rate (depending on temperature and ventilation rate resulting in a defined explosive atmosphere above and beyound the physical boundary of the pool.

Similarly we have the potential for the formation of explosive atmospheres due to dust release or dormant layers of dusts. How do we consider this?

Interestingly or not, the ignition of an explosive atmosphere may seriously affect the general safety precautions and structural integrity of a building structure. Indeed it may also render a number MOE as unusable in a very short period of time.
In this context it is surely important that we give full and honest consideration to Schedule 1 of the RRFSO.

It was my original intention to provide a number of example calculations but i am now aware that this is not appreciated by some as it is considered as being clever. :-\

Username:
Welcome to the world of the oil industry!

Something I'm not involved with now, but I worked in the North Sea for a number of years.

As well as taking into consideration the extent of a hazardous area (around a flange for example), the other major factor is frequency of release, or failure.

This is where true Fire Engineering comes in with the development of quantatitive risk assessments, hazard and operability studies etc. Subjects which, I hasten to add, are now way beyond my level of competence!

Some good HSE guidance under COMAH here
http://www.hse.gov.uk/comah/sragtech/techmeasareaclas.htm

CivvyFSO:
If you are looking at any sort of mitigation, selection of intrinsically safe equipment etc, then that is classified by the HSE as measures to cover a process risk. Article 4 of the RRFSO is in place to clearly define the difference between process risks and general fire precautions. This points towards the DSEAR assessment being required as a separate issue to the RRFSO assessment. Owing to that I would suggest that you get the H&S bod to sort his end of the problem out, then the remaining risk should be as low as is reasonably practicable as far as the RRFSO is concerned.

BLEVE:
But how then do we explain articles 9, 12, 15,16, 19 and Schedule 1.

Again, I cannot see how anyone can complete a fire risk assessment without having sight and a good understanding of a DSEAR RA. Any fire risk assessor must be able to satify themselves that the requirements of the above articles have been demonstrably met and closed out.

Mike Buckley:
Do we have a chicken and egg situation here? It sounds to me that you cannot do a FRA without having a DSEAR RA, but then can you do a DSEAR RA without a FRA?

The other side (and correct me if I am wrong, (I know you will)) part of DSEAR looks at how likely an explosive atmosphere is to be present. It takes three stages the presence of an explosive atmosphere only rarely, the presence of an explosive atmosphere occaisionally and the presence of an explosive atmosphere virtually continuously.

So if I take the example of an hydraulic press. If one of the high pressure lines splits, it will release a mist of hydraulic oil which will produce a potentially explosive atmosphere. Should all premises, which have hydraulic presses, have a DSEAR risk assessment?

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