Chemical Engineering April 2019 - 39

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important to consider alternate
solutions to address safety and
production challenges.
Relieving at the source
As a typical example, an overpressure
protection system (OPS)
(Figure 1) can be used to protect
a reactor by shutting off the
pressure source where it reaches
the critical vessel. One approach
shuts down the system when it
reaches 116% of the rated pressure,
and before the PRV is set to
open. This is a basic safety instrumented
function (SIF) and should
be part of the larger safety instrumented
system (SIS) protecting
the process unit or larger plant.
If designed and applied properly,
the OPS may be able to
mitigate the need for an additional
pressure-relief system.
This is particularly critical when
the pressures involved make a
PRV impractical, or the systems
designed to handle a product
release might be inadequate for
the potential volume. Such situations
can emerge when a plant
is changed to increase production
or a process unit is modified.
The capital expenditure costs of
enlarging a flare system can be
much higher than adding an OPS.
There are situations where an OPS cannot
be used to replace a conventional pressurerelief
system, such as compressed air or
steam. In such situations, it becomes necessary
to have the two systems work together,
with the PRV serving as a backup.
The design and implementation of an OPS
must be approached just like any other SIF
following the procedures outlined under
the International Electrotechnical Commission's
(IEC) 61511 standard (ISA-84). If this
standard is not familiar, there are many resources
available covering the topic in far
greater depth. IEC 61511 provides a complete
safety lifecycle approach, covering
everything from conceptual process design
with hazard analysis and risk assessment,
all the way to operation, evaluation and
management of change. Any OPS should
be evaluated and designed by experts with
competency in safety standards and process
design within this framework. For purposes
of this discussion, we will concentrate
on the middle steps of the process covering
FIGURE 2. Safety-certified
smart pressure transmitters
have internal capabilities to
perform continuous loop diagnostics
to spot problems such
as clogged impulse lines
detailed design and installation, but all the
other elements need to be included.
Looking at the individual parts
Returning to the earlier example, let's focus
on the OPS function itself. It is a basic SIF
designed to shut down flow in the event of
an excessive pressure incident. Since it is
a SIF, it has three basic parts: a sensor, a
logic solver and a final element. These can
be simple or highly sophisticated, but any
components used must be certified to operate
in a SIS context, with any uncertified
components justified as proven-in-use.
A SIF must be able to perform its function
independently without depending on any
larger automation system, or even plant utilities.
Should there be larger failures in the unit,
it must be able to shut off the flow at least
once and remain closed. A bare-bones system
could use a pressure switch connected
to a simple logic solver and a shut-off valve
designed to fail closed in the event of a loss
of electric power or compressed air. This can
probably do the job, but will be difficult to test,
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Chemical Engineering April 2019

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