What is it the typical sketch of depressuring of main tower top vessell in case of instrument air failure ? In our case we have 2 valves that in case of instrument air failure goes to safety closed position. To open this valves by DCS both valves are provides with instrument air vessell that replace the instrument air failure.This prevent FCC reversion in case of instrument air failure. Also, there is a check valve between the safety instrument air vessell and the instrument air supply pipe. What is it the typical sketch in the FCCUs in order to prevent instrument air failure ?