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Click Below for Additonal SIL Information
Topics
 Articles: SIL
101 | SIL
102 | SIL
103
Calculating
SIL Suitability Levels
Functional
Safety Standards
Choosing
an SIL System
LOPA:
Layer of Protection Analysis
 SIL 102:
Can a Fire and Gas System Be Considered a Safety Instrumented System?
There has been much industry debate over
the categorization of Fire and Gas Systems (FGS) as Safety Instrumented
Systems (SIS), especially as the concept of functional safety matures
in the marketplace. Fire and gas detection field devices and technologies
are fundamentally different from other forms of plant instrumentation.
Incorrect detector placement and poor environmental conditions
can prevent the product from detecting a hazardous gas leak or
flame, even when the unit is functioning properly. When a safety
hazard is undetected, then the appropriate safety action (shut
down, deluge, venting, etc.) cannot be initiated. Because of this,
many end-users and system integrators are wondering if the functional
safety standards are applicable to a FGS.
A legitimate argument can be made both for and against the classification
of a FGS as a SIS. Those that oppose the concept typically believe
that because of the operating characteristics of fire and gas detection
equipment, a FGS should only augment a plant safety system and
not be considered a critical safety function. If a properly functioning
FGS comprises a SIL 2 logic solver, a SIL 2 sensor, and a SIL 2
final element, but fails to see the hazardous gas or flame and
effectively eliminate or mitigate the hazard, then the SIL 2 risk
reduction factor of 100 has not been achieved.
For this reason, many people think that too much emphasis and importance
is placed on the functional safety of the hardware of the system,
which can instill a false sense of security. To eliminate this
false sense of security, many industry experts feel that a FGS
should not need to comply with the IEC 61508 / 61511 standards.
Rather a new performance standard or an addendum to the existing
functional safety standards should be developed specifically for
FGS and adequately address the issue of detector placement, coverage,
and related issues.
The advocates for classifying a FGS as a SIS claim that a system
of gas and flame detectors is an effective mitigation layer of
protection and should fall within the scope of IEC 61508 / 61511.
This philosophy suggests that if a facility chooses to implement
a FGS as a layer of protection while understanding the operating
characteristics of the technology and designing a solution that
optimizes the functionality of the equipment, then the system should
conform to the guidelines put forth in both IEC 61508 / 61511.
A FGS that automatically initiates process actions to prevent or
mitigate a hazardous event and subsequently takes the process to
a safe state can be considered a Safety Instrumented Function (SIF)
or SIS. The FGS would need to be composed of the appropriate logic
solver(s), sensor(s), and final element(s).
From General Monitors’ perspective, correct sensor placement,
proper system utilization, and the installation of a diverse set
of detection technologies, are extremely important issues that
must be considered when determining whether a FGS can technically
be classified as a SIS. If there is incorrect placement of the
gas or flame detectors and hazardous gases and flames are not adequately
detected, then the SIF / SIS will not be effective, regardless
of the system SIL rating.
Correct sensor placement is more important than deciding whether
a FGS should be SIL 2 or SIL 3. The bottom line is you can’t
buy safety out of a box. As with any SIF, the FGS design must be
based on the unique needs of each facility, as well as the operating
requirements and constraints of the fire and gas detection equipment.
It is vital that the strengths and weaknesses of the equipment
are assessed and appropriately applied to the plant application
so the FGS provides optimal protection, coverage, and safety.
No detection technology is perfect, so utilizing a variety of products
and sensing technologies in a FGS improves safety and detection
coverage. Just purchasing a SIL 2 logic solver, a SIL 2 sensor,
and SIL 2 final element, does not guarantee a SIL 2 system. The
equipment and system must be configured, installed, utilized, tested,
and maintained properly.
At General Monitors, our goal is to provide the customer with the
most reliable safety solution possible. We are committed to being
at the forefront of industry trends. We will strive to provide
our customers with diverse technologies and plant safety solutions
that most effectively meet their safety needs, especially in regards
to how fire and gas systems relate to functional safety. We anticipate
that the industry discussions regarding the classification of a
FGS as a SIS will continue to evolve over time.
We will remain responsive to these changes and implement products
and safety system solutions that reduce risk, provide increased
safety, and successfully fit within the published functional safety
guidelines and standards.
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