If the storage tank already has a vent valve, why is it still necessary to install emergency pressure relief equipment?

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Article 6.2.19 of GB 50160-2008 \"Code for Fire Protection Design of Petrochemical Enterprises (2018 Edition)\": 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. Article 6.2.20: The roof of a fixed-roof tank at atmospheric pressure shall adopt a weak-roof structure or other pressure-relief measures. ” GB51428-2021 \"Code for Fire Protection Design of Coal Chemical Engineering Projects\" 7.2.18: On the vent pipes leading to the atmosphere of fixed-roof tanks containing Class A B and Class B liquids, flame arresters and breather valves shall be installed; storage tanks that are sealed with nitrogen or other inert gases shall be equipped with additional pressure relief devices. 7.2.19 Atmospheric pressure fixed-roof tanks shall be equipped with pressure relief measures, and a weak connection structure shall be used between the tank roof and the tank wall. SH/T3007-2014 \"Code for Design of Tank Areas in Petrochemical Storage and Transportation Systems\": Section 5.1.5 states that storage tanks whose protection systems use nitrogen or other inert gases shall be equipped with emergency pressure relief devices, and such devices must comply with the following requirements: a) The operating pressure of the emergency pressure relief devices shall be higher than the exhaust pressure of the breather valve, and shall be less than or equal to the design positive pressure of the storage tank ； b) The accident relief equipment shall meet the ventilation requirements to ensure the safety of the storage tank in case of failures in the nitrogen seal or other inert gas sealing pipeline systems, or in the breather valves ； c) The accident relief equipment can be directly connected to the atmosphere ； d) For accident pressure relief equipment, emergency vent manhole covers or breathing manholes with a diameter of not less than DN500 are recommended. ”Note: 5.1.5 This provision specifies the measures to ensure the safety of storage tanks in the event of a failure in the nitrogen or other inert gas sealing system or breathing valve. It can be seen here that the requirements of these three standard specifications are largely similar; the installation of emergency pressure relief devices provides a level of safety redundancy and multiple layers of protection. Is this redundant? Why are only storage tanks equipped with a nitrogen seal/inert gas seal required to have additional \"emergency pressure relief equipment,\" while ordinary tanks do not need it? We need to analyze this from the underlying logic. Atmospheric pressure tanks and low-pressure tanks are inherently weak: their design pressure is usually only +1.5 to +2 kPa; exceeding this value causes bulging, roof failure, or tearing. Nitrogen sealing brings two benefits: less oxidation and a lower fire risk, but it also introduces new risks, as the breather valve may not be able to handle the pressure. 1. Failure of the nitrogen seal valve; the pressure in the nitrogen pipeline is 0.4–0.6 MPa, which is several hundred times higher than the pressure inside the tank. Once the nitrogen seal valve gets stuck in the fully open position or starts leaking internally, high-pressure nitrogen continues to fill the tank, and the pressure can instantly cause the breather valve to fail; the capacity of such a breather valve is simply not sufficient, so it is necessary to use a pressure relief manhole with a large diameter (≥DN500) for rapid pressure release. 2. Exposure to intense sunlight, high temperatures, and high pressure: During the hot summer months, exposure to intense sunlight causes the gas inside the tank to expand and the medium to evaporate, resulting in a sharp rise in pressure. The breather valve is designed for normal breathing volume; its discharge capacity is severely insufficient, so it relies on the pressure relief manhole as a backup. The ventilation valves and emergency pressure relief devices operate according to two different principles: ventilation valves are used for normal material inflow and outflow during everyday operations, as well as to cope with temperature changes; they help maintain pressure within the range of ±0.5 to ±1.5 kPa, thus ensuring pressure balance and preventing overpressure or collapse. It is part of the nitrogen seal system for nitrogen release valves; it automatically releases nitrogen when the tank pressure exceeds +0.8 to +1.0 kPa. It is used for normal fluctuations in the nitrogen seal level, not for emergency pressure relief. It serves as a backup in case of failures in emergency pressure relief devices; when both the breathing valve and the nitrogen release valve fail and the pressure in the tank rises to +1.8–+2.5 kPa, it opens fully to relieve pressure and protect the tank.