A 4"" WATER PUMP INTAKE APPARENTLY BECAME PLUGGED& AS THE PUMP RAN DRY HEATED THE REMAINING WATERIN THE HOUSING; EMPLOYEE WHEN INSPECTING PUMP NOTICED THE CAP ON THE PUMP HOUSING WAS LOOSE & LEAKING; HE TRIED TO TIGHTEN THE CAP WHILE THE PUMP WAS RUNNING. CAP POPPED OPEN IN HIS HAND, SPLASHING STEAM & HOT WATER OVER HIS LEFT ARM, CHEST AND LEFT SIDE OF HIS FACE.