EE & COWORKER WERE CHANGING PISTON & SEALS ON WATER SOFTENER. WATER WAS TURNED OFF VALVE (FEED WATER) DID NOT COMPLETE SEAL OFF RESULTING IN PRESSURE BUILDUP IN SOFTENER HEAD. FACE PLATE WAS TAKEN OFF LAST BOLT HOLDING IT BROKE, PRESSURE FORCED FACE PLATE TO BLOW OFF STRICKING EE IN FACE. IT HIT HIS SAFETY GLASSES, BUT WHEN PLATE HIT GLASSES, IT CUT HIS NOSE