When absorption equipment (typically a contacting tower) is operating normally, a portion of the inlet stream passes out of the equipment with the absorbent – the process effluent flow rate is smaller than the feed flow rate by an amount equal to the absorption rate. Upon loss of absorbent flow, the effluent rate must increase to the feed rate, or an additional effluent path (i.e., a relief device) must become available.
If the feed pressure can exceed the absorber’s MAWP, the evaluation of the applicability of absorbent flow failure as a cause of overpressure becomes a question of the capacity of the effluent piping and downstream equipment relative to the feed flow rate. This effluent capacity, as well as the feed flow rate, should be evaluated at the absorber’s relief pressure; the increase in absorber pressure from operating to relief conditions can lead to both an increase in effluent capacity and a decrease in feed rate.
API Recommended Practice 521 notes that while loss of lean oil flow to a hydrocarbon absorber generally does not cause an overpressure, both acid-gas removal units removing large fractions of the inlet stream, and absorbers removing carbon-dioxide from syn-gas streams upstream of methanators often do require overpressure protection against loss of absorbent flow. In any case, each individual absorber should be evaluated for the relevance of this causes of overpressure.