How do you purge a gas storage tank so it can be cut open?

Dry ice works. Use sufficient to displace any remaining flammable gas or oxygen. The CO2 from the dry ice will not ignite.

Clean out the tank the best you can then fill it to the max with water and let it over flow. Keep it full when you cut it and make sure you never cut above the water line.

Note don't use any kind of electric cutting tool.