Abstract: Employee #1 and a coworker were beginning to tear down a gas cylinder trailer. They assumed all the cylinders on the trailer had been blown down, and proceeded to remove the hardware in preparation for hydro testing. Employee #1 was at a height of approximately 4 ft on a 9 ft stepladder, using an air wrench to remove the safety relief valves. The coworker was helping him when he heard pressurized air begin to be released from the cylinder on which Employee #1 was working. He told Employee #1 to hang on and went to the back of the trailer to open the valve and bleed off the pressure. Employee #1 did not hear him because of the pressurized air seeping out of the loosened safety valve. When it came to the end of the thread that was holding it, the safety valve, still attached to the air wrench that Employee #1 had in his hand, forcibly blew out at close to 2400 psi. The pressure pushed the air wrench toward Employee #1 and both were thrown back 31 ft, landing on the pavement. A forklift operator working about 25 ft away immediately dismounted and ran to a nearby building, where he gave instructions to call 911. Employee #1 was taken by ambulance to hospital, where he underwent surgery to repair a fractured hip. He also sustained torn muscles in his leg, but no nerve damage.