Pn. Lodal et al., REVISED EMERGENCY VACUUM RELIEF DEVICE SIZING FOR ATMOSPHERIC DISTILLATION SYSTEMS, Journal of loss prevention in the process industries, 8(6), 1995, pp. 331-341
Conventional vacuum relief methodologies are usually protective respon
ses; that is, they accomplish their purpose by substitution of an iner
t gas (usually nitrogen) for the process gases removed by an external
vacuum source, or for condensable vapour collapsed by an internal proc
ess mechanism (e.g. condensation). While this approach is theoreticall
y possible for all potential vacuum scenarios, it becomes practically
impossible to implement for installations where a rapid phase change c
an impart near-instantaneous system pressure reductions. The procedure
outlined in this paper takes a preventive approach: eliminate the sou
rce of vacuum generation before the safe lower system pressure limit i
s reached. For distillation and other refluxing systems, this vacuum s
ource is usually the main overhead condenser, which is designed to col
lapse large volumes of condensable vapour. To eliminate the vacuum sou
rce requires elimination of the system's ability to rapidly condense v
apour. This goal is accomplished by introduction of inert gas directly
into the condensing system to 'blanket' the heat transfer surface and
stop condensation. The procedure determines the rate, amount and loca
tion for introduction of inert gas. The required design data include:
(i) system starting pressure, (ii) maximum allowable system vacuum, (i
ii) volume of the condensing system, and (iv) normal system condensing
rate. By determining the rate at which the condenser removes vapour v
olume from the system, and designing an inert gas delivery system to m
eet or exceed this rate, the vacuum generation potential of the system
is effectively eliminated using a much smaller quantity of inert gas
than with the more traditional volume substitution methods.