FURNACE AT PREPARATION PLANT WAS SHUT DOWN AND GAS VALVES ISOLATED AT 10:30 AM IN PREPARATION FOR REMOVING POWER. POWER WAS RESTORED AT 2:00 PM AND THE GAS VALVES WERE OPENED, VAPORIZER TURNED ON & FURNACE LIT. FURNACE WAS SHUT DOWN A 2:30PMBECAUSE GAS PRESSURES WERE NOT ***. AT 5:30PM, ATTEMPT TO RESTART - FURNACE WENT THRU CYCLE NORMALLY. GAS PRESSURE WENT UP 6 TO 16 BEFORE IGNIT