Individual relief valves are sized for the highest relieving rate required by any of the applicable causes of overpressure for the equipment being protected. The relief system branch and main headers and the flare tip are sized for the worst case simultaneous relief rate caused by a single relieving contingency. The most common overpressure contingencies causing simultaneous releases from a number of relief valves are: fire, power failure, cooling water failure and utility failure.
In addition to releases from relief valves there may be reliefs from pressure control valves and automatic de-pressuring systems. Sizing of the total relief system has to take into consideration these additional loads as well.