Emergency valves are required by API on storage tanks in order to protect the tank against excessive pressure caused by external fire exposure or flashes within the tank. The excessive pressure caused by an external fire is generally because an adjacent tank is on fire or some other structure in proximity is on fire. Flashes are generally caused by a chemical reaction in the tank. Regardless of the cause of the excessive pressure, an opening larger than the normal pressure/ vacuum valve is necessary in order to carry off the additional volume resulting from the fire exposure the tank is experiencing. API 2000 states emergency venting may be accomplished by the use of:
1. Larger or additional open vents
2. Larger or additional pressure/vacuum valves or pressure relief valves
3. A gauge hatch which permits the cover to lift under abnormal internal pressure
4. A manhole cover which permits the cover to lift under abnormal internal pressure
5. A connection between the roof and shell which is weaker than the weakest
