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.

Online SIL Resource Center
Visit our new SIL Resource Center. You’ll find information regarding our gas and flame products that are SIL suitable, FAQs, common myths and useful links. Be sure to read our article entitled, “SIL 101: How Safe Do I Need To Be?”



Copyright 2007 General Monitors, Inc. All logos, brand and product names are registered trademarks of their perspective owners. All rights reserved. Questions or comments to etech@generalmonitors.com.




 


              

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