Contingencies and Relieving Rates

The first step in the design of protechon against overpressure is to consider ah contingencies which may cause overpressure, and to evaluate them in terms of the pressures generated and/or the rates at which fluids must be relieved. 

AH unfired pressure vessels must be protected by pressure reheving devices 

The ASTM fired pressure vessels code requires pressure-relief devices to prevent pressures from rising more than 6% above the maximum allowable working pressure. 

Design for overpressure protection in most cases consists of providing pressure-rehef devices sized to handle the calcidated relieving rates necessary to prevent emergency pressures from rising above the design pressure (plus allowable accumulation). 

As an alternative means of protection, it is economical in some cases to specify an increased equipment design pressure which will withstand the maximum pressure that can be generated, without reheving any contained fluids. Also, in some cases, the cost of the collection system can be reduced by specifying higher design pressures which will permit a higher back pressure in the collection system. 

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