I want to Cut amnd welding on a butane tank, what is the mothod to vent the tank before I cut in to it with a tourch?
You will see three fittings on butane tanks. Filler/vent, safety popoff valve, and a gas outlet. Canadian tanks have a high-pressure popoff, but in my view, they don't work.
1. Have the fuel company evacuate the tank for you: they have the training and procedure to assure a complete job.
2. Note that the tank is specially annealed steel. It is soft on purpose, so that it will dent instead of break. Once the tank is evacuated and purged, that soft steel is still probably unsafe to cut with an oxyacetalene rig: cut it with a reciprocating saw and have water flowing over the cut the whole way to avoid igniting the little bit of butane/propane that may have seped into the steel.
3. Let the tank air out throughly for at least two days in toe open air to make sure that you don't have a nasty surprise waiting for you.
4. DO NOT put that re-welded tank back into service storing butane! They make great harbor floats and such, but be sure to paint it Coast Guard required fluorescent orange before you put it on the chain and mark out safe passage in your harbor.