I have a 100 gal. propane tank that I removed the valves and filled with water and soap for a month. Its been sitting for two years . Is it safe to cut?

Welcome to IFI!    There was a lengthy discussion of this specific issue on this thread:

 

It was ready as soon as you removed the valves.

Flooding it didn't hurt. Once you're rid of the fuel/air mixture, it's not going to explode.

What are you cutting it with?   If you have read the threads you will be aware that cutting an empty container with a cutting torch can result in a build up of explosive gasses even if it starts just full of regular air!

Actually, if it's a clean certified & tested propane tank, as soon as you removed the valves (and the interior was in fact exposed to the outside) all the propane will be gone in minutes.

However, I have never found a old propane tank that was clean on the inside; and that's the danger. Most of the time there's at least some oil in it, which can dissolve propane and be explosive. This is why people tell you to fill it with water first. This will also remove some dissolved gas from the crud and the rust inside too.

Sometimes there's wax in there to protect the inside against corrosion. This is why people tell you to mix in soap. 

If you start mixing in acetylene bottles, the story becomes even more complex, as these bottles contains a light rock and the acetylene is dissolved in acetone. DO NEVER OPEN THOSE. 

Spent petrol or diesel tanks ... Never cut these open either. 

Bottom line: Removing valves = absolutely the most important. Filling with water is the best idea. Filling with soap water is even better. Letting is sit outside for a longer period is also good. However, i've learned never to trust gas bottles, and as such i'll never use a plasma burner or cutting torch on them, I will use cutting disks, but I never empty the bottle. I usually just cut them open when they are still half full of water.

Wayne Coe's forge building instructions have a description of how to cut a propane tank. Wayne Coe artist blacksmith is the search terms to find it. He's a member here but I don't think he's been here for a while. 

Pnut

 

When we cut the 20 pound propane tank we removed the valve and filled the tank with water and a cup of Clorox bleach. After sitting over night the smell of the Ethyl Mercaptan was gone. Then the top of the tank where the valve was located was cut out with the plasma cutter with the tank still full of water. After that was done the water was dumped and the other end was cut out. No problems encountered.

Hmmm I have a 14' long propane tank to cut...

There you have a totally different animal. I suggest letting a pro do it.

No detcord?  I was trying to talk my pastor into building a smoker from it as I was "gifted" with it.

Thomas, pull all of the fittings, fill with water and add some bleach to eliminate the mercaptan smell, let sit a day or two , dump it and go work. You will still have the “gas smell” from the mercaptan that imbedded in the steels pores and dumped on the ground when you emptied the tank. I always check mine with a CGI, but have never detected any flammable gas after flooding them. If you’re still nervous after flooding it you can purge with inert gas before cutting, or catch one of your local gas supplier employees to check it for you before cutting. 

In the meantime I have worked open +/- 6 propane tanks to entertain them into a gas forge or a melting furnace. My devise is to let all the gas that still exists to escape by removing the distribution valve (carefully unscrew), then I flush the cylinders with excess water (water flushing agent). Then I drill numerous holes in the water-filled cylinder at the location of the opening that I want to create anyway. After the holes have been made, I saw the largest possible opening with a jigsaw to allow any remaining flammable gases to escape. Only then will I process the cylinder in question with an angle grinder or my plasma cutter. So far, this cautious approach has saved me from surprises and the big bang theory.

A quick question on this subject - how do you remove the valves? On the 20 pound tanks are the valves left or right hand threaded? How much force is needed to break the thread seal i.e. 18 inch pipe wrench or a 6 foot cheater bar on a pipe wrench?

Cheers;

Don

I leave them open outside of my shop for a couple days untill I can no longer smell propane. Then I use a very large adjustable wrench to remove the valve, then I fill it with water.

If it has been filled and emptied a couple time for a couple of days, I cut them with a plasma torch.

Sometimes the valve is stubborn, but I've never broken one off.

The usual problem is holding the tank while you hunker down on a large wrench.  If there is room and a projection sometimes a pipe wrench on the tank projection will hold it while the wrench on the valve unscrews it. Also the valve stem is regular threads, so counterclockwise to unscrew.

I recently removed 5 valves from a 14' long tank and the 6' of cheater was very handy...

Just got word that a friend is giving me a bunch of old 20 lb tanks, so I suppose I'll be putting this info to the test pretty soon (at least, for those tanks that aren't refillable).

A good way to hold the tank is to use a ratchet strap and a stout something to anchor it to. I have some wide flange that works a treat. Hook the strap to to the anchor, make a couple wraps CLOCKWISE around the tank and hook it to the anchor where it can't slip off and ratchet it as tight as you can. 

When you apply big pulls on the cheater the tank WILL try to rotate with the forge but it'll tighten the strap instead of moving.

The SMELL of propane is mercaptan oil odorizer and is persistent opening the valve filling with water and draining will displace any remaining propane though the smell will probably remain. If it really bothers you add maybe 1/2 cup of chlorine bleach to the flush water and let it set over night to neutralize the mercaptan.

Frosty The Lucky.

That's how we removed the valve from the tank, strapped it to a tree and used a 4 foot cheater bar on a wrench I made to fit over the valve. You can see the wrench on the right of the forge.

I've cut open many 20lb propane bottles without issue.  With older bottles, the kind that do not have the check valve that require a fitting from a gas appliance such as a grill be screwed into the valve assembly, I open the valve all the way and turn upside down and put in the sun leaving it for a few days. The sun will warm the contents and force it out the open valve.  At night when it cools, fresh air is drawn inside which is then heated and exhausted during the day.  Repeating this process for several days typically removes the gas.  Propane is heavier than air so it will run out on its own as well.  If you are decommissioning a newer bottle, a fitting is required to be screwed in to actuate the safety valve inside.  When I am ready to cut them, I heat the outside of the bottle with a torch and then play the flame at the valve opening.  If there is flammable gas inside it will ignite but so far I have not had that happen.  Then I can cut into it with either the plasma torch, oxy acetylene or abrasive disc.

Note that when using an Oxy-Acetylene cutting torch the interior of a tank can fill with an explosive fuel/air mix from the TORCH and explode. A number of people are killed every year trying to cut clean empty sealed tanks/barrels, etc. Plasma and mechanical cutting do not have this issue. BE SAFE!