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
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
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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