EMPLOYEE WAS KNEELING ON #186 SHIELD, 3012 LONGWALL SET UP FACE HOLDING ONTO A SHUT OFF VALVE TO BLEED AIR FROM A HIGH PRESSURE SHIELD TO SHIELD HOSE. AS HE WAS HOLDING THE VALVE IN HIS LEFT HAND AND TURNING VALVE ON WITH HIS RIGHT HAND, THE VALVE BLEW OUT OF HIS GRASP AND STRUCK HIM ON THE TOP OF THE LEFT HAND