SpudFiles

I have a 2kg dry powder extinguisher which has a fair amount of water in it (I planned to use it as a water cannon). I emptied it as much as I could about 2 months ago, left the valve open with it in the airing cupboard for a few days then stored it. I got it out again ready to into more permanent storage, and I've found that the small amount of water inside (when I pump it up it sprays a wet mist, is highly rust contaminated. The head doesn't seem to unscrew, so all I can do is pump it through the schrader and fill through the nozzle. Any ideas?

I wouldn't trust that extinguisher, now that it's rusty. If I were you, I'd get a whole new extinguisher that actually unscrews, so you can completely dry it out.

Well, the obvious solution is evaporate the water out.

I'm going to assume that there are seals which would be damaged by too much heat (I've never had a powder extinguisher open), so obviously shoving it in a large oven isn't really an option.
It's getting late in the year, but leaving it out on a sufficiently hot day could work.

Alternatively, if you've got access to a vacuum pump which can draw a good strong vacuum, you can evaporate the water simply by bringing the pressure down low enough.

A combination of heat and a vacuum would be a good option of course.

Alternatively, if you've got some desiccant, you could rig up it up in a bag strapped over the valve outlet, then leave it for long enough.

rag has outlined everything pretty well...

but hey!! getting the water out doesn't really solve your problems... the internal walls will rust even if you drain it every two weeks or so

you need to find a way to protect the tank from rusting... I've heard that you can use paint but that would require opening the tank... I suppose that there has to be some way to galvanise it too but so far I couldn't find much info about it and I don't know if it's practical

I am pretty interested in it too because I've been thinking of using a propane tank for an air reservoir for my fridge compressor for quite a long time

Air circulation through a valve is very poor from diffusion. Active ventilation will need to be provided.

A dry chemical extinguisher has a siphon tube to pick up powder from the bottom of the extinguisher, so just getting a tube into the neck won't provide an air exchange inside the extinguisher.

There are two options. Both require lowering the vapor pressure in the tank so the water will change to vapor and leave.

1 Dry air, nitrogen, CO2 etc purge

2 Draw a vacuum using a cold trap.

1 Will require getting a supply tube past the end of the siphon, or as an alternate repeatedly pressurizing with dry gas and venting.

2 Requires a vacuum pump that can handle the liquid that will condense in the pump, or will require a cold trap. A cold trap is a tank with a pipe in it which is filled with typically alcohol and dry ice so the water vapor in the vacuum line does not reach the vacuum pump. The water will evaporate to maintain vapor pressure and exit to the cold trap where it is frozen into ice.



The second method is used to dry high voltage electrical transformers.
http://www.scribd.com/doc/6500092/guide ... -and-above