A sampling Engineer was working alone at workshop at evening conducting disposal of charged sampling bottle [cylinder that contain pressurized oil sample]. He left the sample plug unopened and opened the 3 valves leaving the pressurized fluid inside when he commenced dismantling the cylinder. He then hammered the piston when suddenly it blew out of the cylinder striking the engineer in both of his hands and smashing some glass in the work shop. The incident caused a fracture to his right hand index finger and left wrist