A PLANT MECHANIC WAS REMOVING A 3/4"" CAP OFF OF AN AMMONIA LINE TO PURGE THE SYSTEM. HE SHUT OFF THE MAIN VALVE TO THE TANK BUT NEGLECTED TO BLEED THE PRESSURE OFF THE LINE BEFORE REMOVING THE CAP. THE CAP BLEW OFF ENGULFING HIM IN A VAPOR CLOUD OF AMMONIA. THIS RESULTED IN MINOR BURNS TO HIS RIGHT HAND AND CHEEK AND INHALATION INJURIES.