AIChE/DIERS · API RP 520/521
Detailed coverage of runaway reaction relief, DIERS methodology, two-phase venting, and API 520 Part II standards.
DIERS (Design Institute for Emergency Relief Systems) methodology calculates relief requirements for runaway exothermic reactions. It determines required vent area, self-heat rate, and time to maximum rate (TMR) for tempered, hybrid, and gassy systems per API 520 Part II.
Tempered systems produce vapor-only relief where boiling absorbs the reaction heat, and standard API 520 sizing applies. Gassy systems produce non-condensable gas requiring two-phase venting, which uses the DIERS Homogeneous Equilibrium Model (HEM) for sizing.
Self-heat rate is the temperature rise per unit time (°F/min) caused by an exothermic reaction under adiabatic conditions. It is calculated from the heat generation rate divided by the product of reacting mass and specific heat capacity.
API 520 specifies Kd = 0.975 for conventional PSVs and pilot-operated valves, Kd = 0.85 for balanced bellows valves, and Kd = 0.62 for rupture disks used alone.