Should the design pressure of the storage tank be based on the vent valve or the breathing manhole?

The breather valve is used for the tank to exhale and inhale during normal feeding and discharging； Also, when the temperature rises due to sunlight, the pressure inside the tank increases and needs to be vented; therefore, storage tanks with a constant-pressure fixed roof and a nitrogen seal require a breathing vent. This is to prevent overpressure in the tank in case of failures in the atmospheric pressure relief valve or breathing valve. The pressure relief capacity of the breathing vent must be greater than that of the breathing valve, as the normal pressure of the nitrogen seal is around 0.5 KPa, but this pressure can rise to 0.5 MPa if the pressure relief valve fails, which is a much higher pressure than that in a tank at normal pressure. Reference: Article 6.2.19 of the \"Code for Fire Protection Design of Petrochemical Enterprises (2018 Edition)\” (GB 50160-2008): Storage tanks for Class A B and Class B liquids that use nitrogen or other gases as a gas seal shall also be equipped with emergency pressure relief devices.

Generally, the set pressure for a breathing manhole is higher than that of a breathing valve; the design pressure must be greater than the set pressure of the breathing manhole