Exploding fire extinguisher

"Potassium acetate is the active ingredient in wet chemical fire extinguishers and it is typically used in addition to potassium bicarbonate or potassium citrate. A key function of the Class F wet chemical fire extinguisher is that when in use, it creates a mist that can cool any flames whilst preventing any oil splashing.

The active ingredients will then rest on the surface of the oil, with the potassium salts creating a saponification effect, causing a slippery, viscous foamy liquid that then envelopes the hot fat, preventing any chance of re-ignition. This foam substance will also assist in cooling the hot fat, breaking the fire triangle."

wonder what upset it?

I'd keep well out of the way of the other one! If you have a fire station nearby it might be worth telling them what happened, and also ask them what should be done with an out-of-date Wet Chemical fire extinguisher.

The potassium acetate will corrode the aluminium container (eventually)

Brian

Then you'd think it'd be varnished, or have a plastic film on the inside

Or at least a "dispose of before" date on them.

I though we were told it had one - 10 years ago

The inside is easy to see since it's now halfway inside out. The aluminium looks perfect. I looked closely along the splits, and there's no sign of any corrosion.

The bottom which is indented on the good one like on a drinks can, is bowed outwards on the exploded one. So I presume something happened inside which caused the pressure to rise.

They usually have a date by which they are no longer acceptable as safety equipment, but not one saying to trigger them and dispose of them

- many people still have an old one lying around in a garage, where it is an extra that may still work in an emergency.

Indeed. I have one lurking near the welding tackle:-( In-date extinguishers are positioned by exit doors.