Fuel Hazards

[Chris_d]

Re: He\'s right, though.

Yes good one Graham! have a drink on me.

Still how do we know you're not just trying to save face?

You might think it crazy though, but I have seen the exact same thing done in motorsport. A tea-urn was used to heat petrol to up to about 80degc, gives more power when its hot apparenty. Only problem was the tea-urn was connected to an extension lead with a normal mains plug, when the petrol started to boil (failed thermostat), vapour started to overflow and creap across the floor, a mechanic saw the mist around his feet and dived for the plug and disconnected it, small spark
but a big bang! Fortunately everybody escaped with no hair and eyebrows but it was an impressive fire ball.
There is always somebody who will try these things, even quite apparently intelligent people who you think would know better.

 

[Guest]

Re: He\'s right, though.

I knew it was a wind up!

Nick

 

[peterb]

Re: Good advice, Graham

OK, I'll accept that it's a wind-up. But when you've had to investigate a fatal fire that turned out to be due to a failed thermostat (as I've done), perhaps you get a little oversensitive!

 

[ArthurWood]

It\'s all about flash points, really.

<<The vapour pressure of petrol at normal temperatures is above the flammable range>> What does that mean? If I correctly remember my phys chem and my lab experience in measuring flash points, the flash point, of petrol is around room temp. ie at room temp, petrol vapour in the presence of air and a spark will ignite immediately. Sure the liquid phase will not easily ignite, but the vapour above it does and up it goes. As for purging with nitrogen, I would suggest that this had nothing to do with cooling, but it was to remove air from the tanks so that any remaining vapour would not have an oxidising agent, ie oxygen, present to allow combustion.

 

[peterb]

Nitrogen purging

Take a tank; put in a mixture of flammable gas and air. If the amount of flammable gas is too low, then the mixture will not light. But similarly, if the amount of air is too low then the mixture won't light either. There is a range of mixtures inside which the mix will burn (and explode if in a confined space) and outside which it won't light. The mix at which it is just rich enough in fuel to light is called the Lower Explosive Limit (LEL), and the mix at which it is just too rich to burn is the Upper Explosive Limit (UEL).

Petrol and diesel both vapourise to give off flammable gas. The amount of flammable gas given off (measured, as you say, by its vapour pressure) depends on temperature. At normal room temperatures the vapour pressure of petrol is so high that the mixture in a tank containing some liquid petrol is probably going to be above the UEL, while the vapour pressure of diesel fuel is so low that the mix will be below the LEL.

So to get a petrol tank to explode you must somehow decrease the vapour pressure of the petrol, while to get a diesel tank to explode you have to increase its vapour pressure; i.e. you must cool down petrol or heat up diesel.

Flying at high altitude petrol tanks do get cooled down. Wartime bombers were being lost by bullets in their tanks while the fuel was cold, and the losses were cut down by partial purging with nitrogen. The nitrogen had nothing to do with the cooling, but it did affect the flammable limits. Interestingly, only high altitude aircraft were affected; aircraft such as ground attack fighter-bombers had no need of such systems, although they needed self-sealing tanks to prevent the burning of fuel outside the tanks.

"Flash-point" only measures one end of the range. It may be appropriate for assessing hazard from diesel fuel (normally below its flashpoint) but doesn't help in assessing explosion danger in tanks of petrol. The appropriate measure there is something called the "Upper Explosion Point"; the temperature above which the mixture becomes too rich to explode.

 

[ArthurWood]

Re: Nitrogen purging

PeterB, I bow to your greater knowledge. I was not thinking in terms of closed systems.

 

[J_Priedkalns]

Just remember; a half a cup of gasoline spilled in your bilge is equivalent to two sticks of dynamite! Ventilation is very critical in gasoline powered boats.

 

[duncan]

I am surprised that in all the comments so far no one has mentioned the tender outboard fuel........

As Nick and others state !most! fuel systems in good repair are not going to give a problem.

It's the added bits that create the risk - additional jerry cans stowed 'somewhere' - to increase range or fuel the tender outboard.

Additionally the gap between waterside petrol and supermarket prices has meant large numbers of boats being fueled in marinas etc....especially in the hot weather when they get more use.