Considerations and Causes of Overpressure

This section discusses the principal causes of overpressure in refinery equipment and describes design procedures for minimizing the effects of these causes. Overpressure is the result of an unbalance or disruption of the normal flows of material and energy that cause material or energy, or both, to build up in some part of the system. Analysis of the causes and magnitudes of overpressure is, therefore a special and complex study of material and energy balances in a process system. 

Although efforts have been made to cover all major circumstances, the reader is cautioned not to consider the conditions described as the only causes of overpressure. Any circumstance that reasonably constitutes a hazard under the prevailing conditions for a system should be considered in the design. 

Overheating above design temperature may also result in overpressure, due to the reduction in allowable stress. A pressure relief valve cannot protect against this type of contingency. 

Likewise, if a certain emergency would involve more than one unit, then all affected units must be considered together. An example of this is the use of a stream from one unit to provide cooling in a second unit. Loss of power in the first unit would result in loss of this cooling in the second unit, and thus must be considered as part of the same contingency. 

While generally only a single contingency is considered for design purposes, there may be situations where two or more simultaneous contingencies should be taken into account e.g., if there is some remote interrelationship between them, and pressures or temperatures developed could result in catastrophic failure. Such contingencies are also considered, and the 1.5 Times Design Pressure rule may be applied in this situation. 

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