The primary purpose of a pressure or vacuum relief valve is to protect life and property by venting process fluid from an over pressurized vessel or adding fluid (such as air) to prevent formation of a vacuum strong enough to cause a storage tank to collapse.   Proper sizing, selection, manufacture, assembly, testing, installation, and maintenance of a pressure relief valve are all critical for optimal protection of the vessel or system. A specific valve brand is selected, according to pressure range, temperature range, valve size, industry application and other applicable factors.