If the vent system is appropriately designed and sized for fire cases, is a deluge system necessary?

No, in nearly all cases, a deluge system is not needed. The proper sizing of relief devices using documents such as CGA S1.3 will ensure that they are of sufficient size to address worst case scenarios. In addition, if the system has been sited per separation distances from exposures in accordance with documents such as NFPA 2, then there should not be a significant risk of fire
exposure. However, a deluge system to protect and cool vessels exposed to fire may be part of a system where evaluated by a system HAZOP and risk analysis. Care must be taken that the deluge system does not spray on the vent system which could lead to potential blockage. Similarly, if a system uses a thermally activated relief device (TPRD), it’s critical that the device is
not cooled by the deluge system such that it does not activate as intended. Additionally, a deluge system could extinguish the hydrogen fire. A hydrogen fire should not be extinguished unless the source of hydrogen is isolated.