Review unit-wide impacts on closed-drain drums
API 521 standard helps decipher the correct operating pressure for this system
Closed-drain drums are generally intended to receive hydrocarbon-drained liquids from various upstream sources. These drains may be maintenance
drains, continuous-process drains and open-hazardous drains from drip pans. The vent line of this vessel type is normally routed to the low-pressure (LP) cold vent headerultimately culminating into a LP cold vent tip. Liquid from the drum is removed periodically. The drum inbreathes during pumping out and out breathes during liquid inflow through the vent line on top.
The question is what should the design pressure be without any liquid seal at the vapor outlet to prevent air ingress? To a process designer, a simple design is very straight forward. The process designer will consider gas blow-by scenarios from upstream contributory inlet streams and design the vent line of the closed-drain drum, sufficiently large to cater for the largest inflow rate either from non-continuous maintenance drains or the liquid rate from continuous drains coming through restriction orifices (provided for preventing blow-by scenarios). This would prevent the overpressure generation by allowing adequate out breathing facility in case of large inlet flow to the drum. Based on this simplistic concept, the design pressure of an open-to-atmosphere drum system is set at 3.5 barg = 50 psig, as per standard practice.
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