Evaluating Impact of Increasing Unit Throughput: Is It Just a Linear Scale-up of Required Relief Rates?

Rate-dependent scenarios refer to overpressure scenarios for which the applicability and/or the relief requirement for such scenarios are dependent on the design throughput of a process system or unit. Except for the external fire scenario, typical overpressure scenarios for individual equipment might have the potential to be rate dependent.

It is difficult to generalize whether a non-fire scenario would be rate-dependent or not without reviewing the underlying analysis. For example, if the relief rate for a blocked outlet scenario were based on the normal flow rate through an equipment, then this scenario could be rate dependent. However, if such relief rate were based on the maximum capacity of an upstream pump, the same scenario might no longer be rate dependent. Moreover, if credit were then taken for normal outflow through another open path to reduce such relief rate, the same scenario might still be rate-dependent.

The controlling scenario for sizing the pressure relief system for an individual system might be rate-dependent. This might also be true for a common disposal system such as flare. The adequacy of such systems against any proposed increase in unit or facility throughput should be evaluated carefully.

The main objectives of this paper are to provide process engineers general guidance on:

• how to identify rate-dependent scenarios using some examples,
• how to evaluate and document such scenarios, as well as
• how to evaluate potential impacts on such scenarios due to a change (such as increase) in the throughput for a unit or facility.