Chemical Engineering May 2016 - 73

FIGURE 5. Three-dimensional mapping of a facility
is useful in determining the most appropriate
installation locations for flame detectors
FIGURE 4. Field of view is an important factor to consider in the installation of flame-detection equipment.
This diagram shows the distance a flame can be detected at various angles. For example, at 0 deg,
a flame can be detected at 230 ft, and at a 50-deg angle, it can be detected at 50 ft (in this figure, the
degree symbol ° is used for angle degrees, and the prime symbol ' is used for feet)
est reliability standards, continuous
optical-path monitoring (COPM) diagnostics
are often built into optical
flame detectors. The self-check
procedure is designed to ensure
that the optical path is clear, the
detectors are functioning, and additionally,
that the electronic circuitry
is operational.
Self-check routines are programmed
into the flame detector's
control circuitry to activate about
once every minute. If the same fault
occurs twice in a row, then a fault is
indicated via a 0-20-mA output or
a digital communications protocol,
such as HART or Modbus.
SIL/SIS standards. When plant
safety engineers choose detectors
certified to safety integrity levels
(SIL) and properly integrate them
into safety-instrumented systems
(SIS), they have again added another
layer of safety. Certification
to these standards plays a valuable
role in effective industrial gas and
flame detection.
Normative standards establish
minimum requirements for the design,
fabrication and performance of
flame detectors and other safety devices
as necessary to maintain protection
of personnel and property.
The ANSI/ISA S84.00.01 standard
was enacted to drive the classification
of SIS for the process industries
within the U.S., as well as the norms
introduced by the IEC (IEC 61508
and IEC 61511).
Together, these standards have
introduced several specifications
72
that address safety and reliability
based on optimizing processes for
risk. The IEC 61508 standard is a
risk-based approach for determining
the SIL of safety-instrumented
functions. Unlike other international
standards, IEC 61508 takes a holistic
approach when quantifying
the safety performance of electrical
control systems - the design
concept, the management of the
design process and the operations
and maintenance of the system
throughout its lifecycle are within
the scope.
Location and installation
A variety of processes and sources
within the plant environment can
lead to flame and fire incidents,
including leaking tanks, pipes,
valves, pumps and so on. Accurate
detection while avoiding false
alarms is also important because
false alarms result in unnecessary
process or plant shutdowns, slowing
production and requiring timeconsuming
reviews, paperwork
and reporting.
False alarms can, over time, provide
a false sense of security, because
employees can become complacent
if alarms go off frequently for
no apparent reason and are continually
ignored. The problem is that personnel
alone cannot really determine
the difference between a false alarm
and a serious accident that is about
to happen.
Fixed flame- and gas-detector
systems are designed and installed
to protect large and complex areas
filled with process equipment from
the risks of flames, explosions and
toxic gases. For these systems to be
effective, their location and installation
are important so that they offer
a high likelihood of detecting flame
and gas hazards within monitored
process areas.
Three-dimensional mapping. Determining
the optimal quantity and
location of flame and gas detectors
is therefore critical to ensure
the detection system's effectiveness.
Flame and gas three-dimensional
mapping is a solution that
assists in the evaluation of flame
and gas risks within a process facility
and also reduces these risks
toward an acceptable risk profile.
Flame and gas mapping includes
the placement of detectors in appropriate
locations to achieve the
best possible detection coverage
(Figure 5).
The use of three-dimensional
flame and gas mapping helps
plant, process and safety engineers
in a number of ways. First,
mapping helps to increase plant
safety by improving the likelihood
of detecting flame and gas hazards.
Also, it allows facilities to
quantify their risk of a fire or a gas
leak, and then assess the overall
effectiveness of their flame- and
gas-detection coverage. For new
installations, mapping can help improve
the design of new fire and
gas systems to mitigate risks from
accidental gas releases or fires.
For existing installations, mapping
provides a method for assessing
the risk-reduction performance
of existing fire- and gas-detector
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