Never connect a torch to acetylene and oxygen tanks without installing a set of flashback arrestors in the hoses at the regulators. Without these safety devices, it is possible a flash can occur within the hose, and ultimately follow back into the tanks, causing an explosion. This would be an extremely rare occurrence, but flashback arrestors will make it a "never" occurrence. Check valves are only a first line of defense and may not ultimately stop a flashback. Only the flashback arrestors will.