Below for Additonal Topics of Interest
101 | SIL 102 | SIL 103 | SIL 104
SIL Suitability Levels
a SIL System
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.
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 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.
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.
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.
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.
Copyright ©2008 General Monitors, Inc. All logos, brand and product names
are registered trademarks of their perspective owners. All rights reserved.
Questions or comments to email@example.com