On Tue, 17 May 2005 17:48:00 +0100, "Christian McArdle"
wrote:
AFFF is far superior to dry powder and the substance in Class F (Oil
Fire) extinguishers. If you had a normal domestic spray bottle with
AFFF in it it would put out a chip pan fire.
That is untrue.
I've done it many times using exactly that - a standard domestic
trigger spray and a 10% AFFF solution.
I've also put out many fuel fires with AFFF. I've walked into a
burning pan with 5,000 gallons of flaming AVTUR carrying an AFFF
spray and walked out again after the supply is exhausted. With dry
powder you can walk in but you have to run out very quickly as the
second your run out of powder the fire flashes back.
AFFF is standard foam and should not really be used on chip
pans.
Protein Foam (derived from ox blood) is usually considered as
standard foam containing only hydrocarbon surfactants. Fluorocarbon
protein foam as used in AFFF is another thing entirely. The
difference is in the "Film Forming" part. AFFF covers a fuel surface
with a barrier preventing reignition.
For that you need special wet chemical foam for Class F fires, which
isn't the same as AFFF.
The so called "wet chemical" extinguishers use Water, Potassium
Acetate (Acetic Acid, as found dilute in Vinegar), Ethylene Gycol
(antifreeze) and CO2 (you can see why they hide the cheap ingredients
given the price charged for some). Although less effective overall
than AFFF they have the distinct advantage in commercial environments
of causing no contamination and usually requiring no cleaning
afterwards. It is this chemical mix which is sometimes referred to
as "turning oil into a soap like substance".
A proper class F extinguisher has a short lance (to keep your hand
out of the fire) and a very slow discharge rate so you don't blow the
oil out of the pan. Strictly speaking class F only refers to
commercial environments - the qualification test is something like a
30kg pan of burning oil which the extinguishing agent must put out,
keep out for somewhere between 15 and 30 mins and in doing so splash
no oil outside the container. You will find some small household
extinguishers loosely referred to as "class F" (because they contain
Acetic Acid/Glycol mix) even though they are far too small to put out
a 3kg never mind 30kg pan of oil.
When demonstrating chip pan fires the party trick was to go up to one
with a tablespoon of AFFF compound and add it to the blazing pan.
The fire went out within seconds. The only practical disadvantage of
this technique was the need to be wearing full crash gear with fire
resistant suit gauntlets and full face helmet to get the spoon over
the pan - not something most people use when cooking :-).
I agree that special wet chemical is the best type for kitchens, as it is
also effective on solid fires. However, it costs over 150 quid a pop, so is
unlikely to make inroads into the domestic market.
"Wet chemical" is cheap, Acetic Acid and Glycol are not exactly
expensive chemicals. Manufacturers don't use the correct name so they
can vastly overprice Class F extinguishers.
ABC is useful in a kitchen for non-chip pan fires, particularly dustbins and
toasters. As you say, it is very likely to make a chip pan fire worse unless
you're particularly skilled with it.
And that is why you should have no fire extinguisher in a kitchen. A
chip pan fire is spectacular and if there is an extinguisher handy
someone will try to use it with lethal results. I've seen a number
of people badly burned using extinguishers on domestic chip pan fires
and not a single case where they did any good.
Get out quickly - stay out.
--
Peter Parry.
http://www.wpp.ltd.uk/