Chemical Engineering July 2022 - 25

However, the focus of this article is on the direct discharge
of atmospheric relief devices through the tail
pipe (as opposed to venting through a broken bellows).
To evaluate whether the discharge of an atmospheric
relief device occurs to a " safe location, " the engineers and
designs should follow two steps:
Step 1. Determine the possible hazards of discharged
fluids that are expected to release from the relief devices.
This includes whether the fluid is flammable/
combustible, toxic, or neither. The potential fluid discharges
should be based on a thorough pressure-relief
system analysis. Users are cautioned that relief devices
located on equipment primarily containing no hazardous
fluid might still have the potential for hazardous
fluid discharge.
Step 2. Depending on the hazardous fluid type, the engineers/designers
can apply the appropriate screening
methods, as listed in the following sections.
Non-flammable, non-toxic fluid
Usually, these are the safest systems to discharge to atmosphere,
since they rarely result in large-scale harm.
When analyzing the location of the discharge, consider
the following:
Effects on personnel. The effluent from a vent cannot
be universally determined " safe " just because it doesn't
burn and is non-toxic. If the effluent stream could come
into contact with personnel or equipment, the following
criteria should be considered:
* Impingement. The effluent stream contacting individuals
or equipment could cause injury or damage.
High-velocity fluid also has the potential to injure
individuals who may be in the discharge path.
When analyzing the impingement of an effluent on
personnel, the engineer should analyze the immediate
disposition of the fluid, as well as the subsequent
movement of the fluid. The analysis should
include nearby work areas, walkways, ladders and
other areas designed for occupancy. Direct contact
of a hot fluid, such as steam or water, could cause
burns. Vapor liquid mixtures or those which may
condense (for example, saturated steam) may result
in the liquids " raining " down, even if the discharge is
directed upwards.
* Oxygen-deficient atmosphere. A large release of a
simple asphyxiant into a building or confined area
could dilute the oxygen in the atmosphere and thus
make the area uninhabitable. Common industrial
gases, like nitrogen, helium and argon, are classified
as simple asphyxiants.
* Emergency operations. The ability
of operators to
safely perform emergency procedures during a release
should be considered. If a nearby isolation
valve requires manual operation during a release, the
designer needs to consider the effect of the release
on the operator.
* Noise. Depending on the characteristics of the vent and
effluent, the noise generated from a release may be
loud enough that plant personnel, even with hearing
protection, may not be able to work in the area.
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Chemical Engineering July 2022

Table of Contents for the Digital Edition of Chemical Engineering July 2022
