Chemical Engineering March 2019 - 50

Feature Report
The Value of Safety
Instrumented Systems
Mike Klein
Emerson
T
here are four key areas of
concern for chemical producers
that are always top
of mind in daily operations.
These are, in no specific order, ensuring
safety, optimizing production
efficiencies,
minimizing
emissions
and improving reliability. In many
ways, these concerns are interconnected
and are governed by many of
the same standards.
To meet the stringent standards
that govern this industry, safety instrumented
systems (SIS) are one of the
solutions that make the difference,
precisely because they not only impact
worker safety, but also environmental
and safety issues around the
plant. Not only do these risks impact
workers and the environment, they
also impact the financial health of a
company. Any time something happens
that violates codes, standards
and regulations, it can have a significant
impact on the balance sheet.
Though engineers can create systems
that monitor and manage the
safety and operations of a plant
down to the smallest detail, it doesn't
always mean that is what's called
for. If a shutdown is triggered over a
deviation from standard that would
ordinarily be within acceptable tolerances,
it can mean extra costs, a recalibration
or repair of critical equipment
that might have been impacted
by the shutdown.
A safety system is separate from
a control system and is focused on
the prevention of accidents with the
understanding that it will shut down
the plant should an accident occur.
A safety system is the final layer of
protection a plant has to ensure the
50
safety of its people
and the environment.
Of the many standards
that govern the
design and implementation
of safety
systems, the most
notable and commonly
applied one is
IEC 61511, established
by the International
Electrotechnical
Commission (Geneva,
Switzerland; www.
Part 2
A safety instrumented system (SIS) is important to ensure the safe operation of a plant. The SIS
requires up-to-date sensors - recent developments in vortex flowmeters are presented here
Emerson
FIGURE 1. The quad vortex sensor has four independent transmitters
and sensors, a configuration that is recommended for SIL 3 applications
iec.ch), which applies to the chemical
process industries (CPI), including
the chemical, petroleum refining,
petrochemical, pharmaceutical, pulp
and paper, and power sectors.
Sensors are the key enabler
The primary enabling component of
a quality safety system is the field
instrument, or sensor. Top-quality
sensors are key to helping engineers
design and maintain the established
protocols that help a chemical plant
meet regulatory requirements. These
sensors are typically the first devices
to be exposed to the unsafe condition
that will ultimately trip the safety
shutdown process. The challenge is
to have sensors that work together to
accurately assess the process to detect
an unsafe condition, but that do
not report a false reading that might
lead to an unnecessary shutdown.
Aspects of the design process engineers
need to consider when developing
an effective loop design are, that it
must meet the risk-reduction requirement
levels and establish the correct
probability of failure. Once those variables
and parameters are known, an
engineer can calculate the safety integrity
level (SIL) requirements.
The results from those calculations,
and using the standards set
in IEC 61511, will determine sensor
choices. In addition, a history of use
and failure rate of previously used
sensors within the user's own facility
will be a factor in selection.
These are only a few ways a chemical
producer can determine the type
of sensors they invest in. What is
clear, however, is that chemical producers
will look very carefully at all
options when selecting sensors for
a comprehensive SIS system to ensure
the best possible system based
on the SIL requirements and the effective
operations of the plant.
The vortex meter option
In the past, vortex meters have gained
the reputation as being unreliable and a
poor choice for chemical applications.
This was mainly due to early technology
and application challenges, which
included low-flow cutoffs that didn't
measure down to zero flowrate, extreme
vibration applications that might
not have been suitable due to the
potential for inaccurate flowrate measurements,
and some designs that
were prone to plugging.
In recent years, many suppliers of
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Chemical Engineering March 2019

Table of Contents for the Digital Edition of Chemical Engineering March 2019