There are many instuctabtles that make use of old gas cylinders.
Few go into detail on how to make them safe first.
The principal is simple use as much of the gas as possible vent to atmospheric pressure what you can't fill it completely with water then you can cut into it. Step 3 is the problem how to get the water in. What follows is how I've done this.
Step 1: Assumptions
You understand this is a hazardous procedure, you are dealing with a potentially bomb!.
You are doing it outside, away from buildings, and with no sources of ignition.
You have a propane cylinder with a functional hand wheel valve.
It is very important that between venting to atmospheric pressure and starting to fill with water and at any point you pause filling it you close the valve and treat it as a full cylinder.
This is why. Sitting with the valve open at atmospheric pressure as the cylinder warms the gas inside will expand and vent as it cools air will be sucked in potentially creating an explosive mixture.
Step 2: Measure Your Cylinder
Why? If you don't know how much your cylinder holds how do you know when it is full? (if you're lucky there will be a plate on the cylinder telling you it's water capacity 1 of mine does 2 don't). You need to know the circumference and height in cm. Using cm is makes the maths simpler you get an answer in cubic centimeters or cc, cc=ml divide by1000 and your answer is in liters If you want to use inches and do whatever math you needed to convert cubic inches to gallons you're on your own.( I relented and looked up the conversion factors)
It works out this cylinder has a capacity of 54 Liters, at atmospheric pressure the nominally empty starting point for purging it still contains about 100g of propane, small disposable cans for camping stoves are sold that hold that much
Step 3: Maybe
A couple of Instructables I've read suggest you unscrew the valve and use a hose to fill the tank. Maybe if you have the right spanner this is possible. I wouldn't be writing this if I'd managed it.
A large adjustable wasn't up to the job, and whilst I do have a large pipe wrench somewhere, with a welded on valve guard it wasn't an option. As I said it is somewhere, so even with the access a bolted on valve guard gave I couldn't try, even if I wanted to.
Having finally filled this cylinder with water I took a long handled lump hammer to the valve it took a reasonable amount of effort to knock it undone 1 turn, full of water this cylinder was the better part of 60Kg the force was turning the cylinder to start with almost more than it moved the valve. After 1 turn a large adjustable spanner was enough to finish unscrewing it. I do not consider this an option for a cylinder with gas in it. As it destroyed the valve and if it won't unscrew you have no relatively safe way to fill the cylinder with water
Step 4: Options 1
If your cylinder is small enough that you are going to be able to comfortably lift it full of water standing it in a drum taller than it is weighted down as at atmospheric pressure they float. fill the drum so the whole valve is covered open the valve and let the water slowly displace the gas. keep the drum topped up and keep track of how much water you have added so you know when it's full. This isn't so critical with this method as the cylinder only ceases to be buoyant a little before it is full. not having the means to weigh down my first cylinder I half filled it before standing it in the drum. To do this I used a large funnel that was a reasonable fit in the valve and sealed around it with soft wax from cheese rind, filled the funnel with water opened the valve and let the water slowly( there is no option it goes slowly) displace the gas, the foil tray was to check how much was leaking past the wax compared to going into the cylinder, I discovered I had to empty the tray once every 3 fills of the funnel. This gave rise to option two.
Step 5: Option 2
My second cylinder was larger would weigh almost 60kg when full of water and need a 45 gallon oil drum to stand in. When I collected this cylinder I discovered it had been standing for some time with the valve open. this means it contains a fuel air mix. As such I didn't even consider trying to unscrew the valve,and given how hard it turned out to be I'm glad I didn't.
As luck has it the threaded section of a right angle 15mm plumbing compression fitting is a sliding fit in the outlet bore of the valve. Plumbing fittings are standard right hand thread fuel gas should be a left hand thread the threads do not mesh. The gap needs filling to provide a reasonably water tight joint, and some physical support that won't allow the plumbing fitting to rotate. To do this I used a product called plumbers mait. It is a two pack putty,you cut the required amount from the roll kneed it until it is all one colour then use it sets hard in about 5 to 10 minutes even under water.(other similar products are no doubt available or the trade name varies with country) I put a bead of this around the threads of the fitting and pushed it into the valve bore smoothed it over so there were no visible gaps made sure there was clearance for the union nut on the free end (which is positioned vertical) and left it to set. 2 pack car body filler would probably do as well but it is runnier. soft wax for sealing with an alternative means of support might work. The image is after the job was done and the union was removed the putty forming the flange was originally wrapped around the threads and has been displaced as the two parts were pushed together.
I found a short section of 15mm copper pipe with an olive still attached in the scrap box. I trimmed the end of the funnel down so it was a tight push fit on the pipe, placed a union nut on the pipe and forced it into the end of the funnel then attached it to the pipe fitting in the cylinder.
Fill the funnel with water open the valve and wait as the water displaces the gas.It is a slow process the water trickles in displaces and pressurises the gas until a bubble of gas escapes past the water,let the funnel almost empty before topping it up as the gas tends to vent when there is less water in the funnel but a full funnel has the weight of the water to displace the gas, I was using 10 liter garden watering cans to keep track of how much water had gone in keeping a tally of the cans on the side of the cylinder. Make sure the water is free of debris (grass cuttings flower petals compost! etc) as the valve gap is small and easily clogged. Once the cylinder is full/ almost full of water, water has stopped going in and the volume you have put in is close to your calculated volume it is time to risk drilling a small hole in the top I used a cordless drill and a 2mm bit, once I'd had a cut into the cylinder I filled it with light oil to reduce the risk of sparks and drilled through if your calculation is out (as mine was by about 4 liters the dangers of rounding up and down an 8% error) water will now go in a lot more rapidly as the gas vents through the drill hole. as the drill hole is not at the top of the dome, once water is flowing out of the drill hole tilt the cylinder so the drilled hole is higher than the valve. There should be a short pause as the last gas vents then a flow of water again. Now is the safest time to take an angle grinder to a welded on valve guard and unscrew the valve. I note that whilst convention is that fuel gas fittings have a left hand thread the thread of the valve body to the cylinder in this case was a standard right hand thread. The brief notes I've read on other instructables suggest emptying and refilling the cylinder before cutting into it with a grinder. With my first cylinder as the project I want it for required cutting the top off just below the point the dome starts I did this with a saw whilst it was still full of water. This was messy as obviously the water started running out as soon as the wall was cut through.
Step 6: If Storing
If you are not going to use your purged cylinder immediately, make it clear it is no longer a hazardous gas cylinder paint out any warning symbols and clearly mark it as safe or purged, remove the valve, and preferably make the first large visible cut that will show it is no longer even a pressure cylinder