I’m trying to work out what rating Temperature wise ie T1 to T6, for a piece of equipment in a potential explosive atmosphere. This should be easy, however the substance I’m trying to work this out for is when it is in mist form. I have found some information from an Energy Institute document that states :-

“All petroleum liquids even at temperatures below that at which a flammable vapour will ordinarily form, can be ignited at temperatures well below their flash point when dispersed as a mist…”

Ok, and I know the flash point and auto ignition temp for the substance normally, but how do I find out when it is a mist. Now I know it all depends on droplet size etc, but to be honest I just want worst case. The substance is kerosene, pressurised under its own weight only, which normally has a flash point of between 38 to 72 deg C and auto ignition temperature 220 deg C. The area I feel will be Zone II and Equipment needs to be Category 3 and I feel rated IIA or IIB or IIC would be acceptable, but I became stuck on temperature.

What I’m looking for is worst case for this substance when in Mist or Aerosol form.

Could anyone point me at any guidance or know the answer (I have been looking at the HSE review “rr980” Generation of flammable mists from high flashpoint fluids. Fascinating read all 134 pages of it, but does not provide the answer.

Before anyone asks I’m considering the situation of a pin hole / crack in the container /tank that may produce the mist / aerosol.

Chris

Can't you just use ignition energies for a vapour? That has to be worst case right?

Also the following should be free access:

http://www.sciencedirect...le/pii/S0957582099707688

HYDROCARBON AEROSOL EXPLOSION HAZARDS: ICheme

No I disagree JJ, It would be telling if you told me the answer (ie the Temperature).

Have a nice weekend. I intend to do some more research Sunday when My head has cleared from the weeks chaos.

Unless you want to tell me the answer - the temperature is.....

Go on, you know you want to

:o)

Chris

Chris

I think it's BS EN 60079-14:2008 "Electrical installations design, selection and erection" ?

Clause 5.6.2 Gas or vapour which takes you to Table 4

Temp class required by area classification Ignition temp degC Allowable equip

T1 >450 T1-T6
T2 >300 T2-T6
T3 >200 T3-T6
T4 >135 T4-T6
T5 >100 T5-T6
T6 >85 T6

I work as a design engineer, often designing fuel tanks etc.

Trust me you are over complicating the problem, from how you have described your situation.

Think of the credible failure modes you have ....

Ok I feel the failure mode will be a pin hole or crack at the base of the tank or ancillary equipment. This would then lead to a jet coming out that could form a mist / aerosol etc.

From the energy institute document I quoted from above the remains of the sentence where I have the dots reads:-

when dispersed as a mist, spray or foam (eg if released under pressure from a leak).

I know the pressure at the bottom of a 6 foot deep tank will not be much, but through a crack or pin hole it could still produce (all be it localised) a mist / spray.

So back to the question which is if it will ignite at a lower temp, what would that temperature be or how may I calculate it. Would the auto ignition temperature remain the same and just the flash point lowers ?

Chris

Chris

The flash point may well be reduced due to atomised droplets but typically the ignition due to hot surfaces will be several times that of the auto ignition temperature.

Just take the temperature and gas group from EN 60079-20

for Kerosene, this is T3 and IIA.

It is as simple as that

Regards
Laz

That spoiled it!!

Of course it did :)

I must say, that I feel it is better to guide someone to their own conclusions by hinting and suggesting routes of investigation and research is a much better way of helping with problems & queries than giving answers on a plate.

I agree Paul, that way you learn more. I had got there in the last line of my post 15, I just needed a "yes" that was correct.

Again thanks for confirming my thought Lazarus.

Have a nice whatever is left of the weekend all.

Chris