SHELL OWNS AND OPERATES A CRUDE OIL REFINERY AT ITS WILMINGTON MANUFACTURING PLANT IN CARSON, CALIFORNIA. THE COMPLEX INCLUDES A SULFUR RECOVERY PLANT WHICH CONTAINS TWO CLAUS UNITS, A SCOT UNIT AND TWO INCINERATORS, ALL OF WHICH SERVE TO RECOVER SULFUR FROM THE HYDROGEN SULFIDE PRODUCED BY THE REFINERY. PURSUANT TO A CALIFORNIA STATE REGULATION, SHELL IS REQUIRED TO TAKE ITS SCOT UNIT OFF-LINE FOR A MAINTENANCE AND SAFETY INSPECTION EVERY THREE YEARS. SUCH AN INSPECTION IS DUE BY APRIL 1988. UNLESS SHELL SHUTS DOWN ITS ENTIRE REFINERY, TAKING THE SCOT UNIT OFF-LINE WILL CAUSE A VIOLATION OF THE NEW SOURCE PERFORMANCE STAND- ARDS ( NSPS ) FOR PETROLEUM REFINERIES UNDER THE CLEAN AIR ACT.