Flashpoints, flammable limits, auto-ignition points etc etc

toecutter · Post by **toecutter** » Fri Mar 13, 2015 10:18 am

Just started my Fire cert last week (self-study with an online provider...) and there's something that I can't quite get my head around. I understand the differences between the flashpoint, fire point, ignition point and auto-ignition point. Then along comes the Lower Flammable Limit. My course material describes it as

"the smallest concentration of flammable vapour in air that is capable of being ignited and propagating flame under test conditions"

And I understand that too. What I don't understand is the relationship between the LFL and the fire point. It seems that at a certain temperature, the LFL will be x, and that at a certain concentration the fire point will be y, so if conditions mean x=y and there is an external source of heat, we have ignition and propagation of flame (see attachment). However I am also told that the fire point doesn't vary with the fuel/air concentration. So what the blinking hell? I don't even know if I've described my problem very well. My brain's about to auto-ignite.

Any help appreciated enormously.

Post by **WillPool** » Fri Mar 13, 2015 12:12 pm

Toecutter

Welcome to the forums

I am not able to help with your query but I am sure someone will assist in due course

Good luck with your studies

Will

toecutter · Post by **toecutter** » Fri Mar 13, 2015 1:25 pm

Oops. Just noticed I got edited. It appears my language was outwith the acceptable. Sorry Mods!

Thanks Will!

Post by **Alexis** » Fri Mar 13, 2015 1:28 pm

Hi toecutter and only another warm welcome to HSfB from me too.

I do know certain people who have an answer to your query will come in with some helpful thoughts to put out your brain fire.

All the best with this one.

Post by **wendolene26** » Fri Mar 13, 2015 2:10 pm

Due to copyright laws, direct quotes from Nebosh material cannot be published on this forum. This is probably why your post was edited. You are still allowed to ask for advice or assistance with questions, but have to paraphrase the question and post a general idea of what the initial question is around rather than a direct quote. Just something to be mindful of if you're asking for help in the future. I have found them to be a rather helpful bunch on this forum

tho so please don't let this put you off asking more questions....

toecutter · Post by **toecutter** » Mon Mar 16, 2015 8:12 am

Thanks all. No, it won't put me off; my own fault because it was a mild profanity that was edited (and rightly so). In terms of my question, I've no exam date booked yet, I'm thinking it'll be June-ish so I'm not quite sitting and wringing my hands about it yet...

Post by **Alexis** » Mon Mar 16, 2015 10:27 am

toecutter wrote:Just started my Fire cert last week (self-study with an online provider...) and there's something that I can't quite get my head around. I understand the differences between the flashpoint, fire point, ignition point and auto-ignition point. Then along comes the Lower Flammable Limit. My course material describes it as

"the smallest concentration of flammable vapour in air that is capable of being ignited and propagating flame under test conditions"

And I understand that too. What I don't understand is the relationship between the LFL and the fire point. It seems that at a certain temperature, the LFL will be x, and that at a certain concentration the fire point will be y, so if conditions mean x=y and there is an external source of heat, we have ignition and propagation of flame (see attachment). However I am also told that the fire point doesn't vary with the fuel/air concentration. So what the blinking hell? I don't even know if I've described my problem very well. My brain's about to auto-ignite.

Any help appreciated enormously.

Just bringing this one back again to see if someone can help toecutter.

toecutter · Post by **toecutter** » Mon Mar 23, 2015 3:22 pm

I think I have managed to answer my own question. And I'm still not convinced it was a particularly bright question, but I've answered it none the less. I was correct in thinking that there is a range of temperatures dependant on the vapour concentration at which a vapour will ignite and propagate flame, but the Fire Point is the absolute value for the lowest temperature at which this can occur.