Fire and Gas Systems - Intro to Risk Based Detector Placement
Shaun Williamson P.L. Eng., CFSE, PMP
Principal | Director of Engineering - Supporting our clients with HAZOP/LOPA, SIL/SIS consulting, Fire & Gas Engineering, Alarm Management, Bowtie, PSI As-Builts.
Intro to Risk Based Detector Placement (FGS-BLG-101)
This article is a continuation on FGS-BLG-100 which discussed Fire and Gas Systems as a “Last Line of Defense”.?For safety critical protection layers, time is of the essence in the event of a fire or gas release.?Without mitigative action, a combustible product release can escalate to a fire or explosion.?Small fires can grow to become large ones, resulting in cascading explosions or BLEVE’s that can wipe out an entire facility and cause substantial loss of life.?Failure to detect these events quickly, has been identified as a common safety issue during the investigation of many past major accidents.?
?One of the most important factors to ensuring effectiveness of a F&G system is to ensure that the sensor is properly placed to see the fire or gas event.?While process sensors require very little thought when placed, F&G sensor placement on the other hand is a highly specialized activity and is easy to get wrong.?Poor detector placement is the greatest factor influencing why fire and gas systems fail.?
?Often, detectors are treated as an afterthought during the engineering phase.?Very few details other than approximate location are typically provided to installers by engineers.?It is typically the electrician that decides the final location of devices and aims the fire detectors.?This is done with no specialty training, understanding of what area the detectors are meant to cover, or what site conditions may impair sensors.
?This article will introduce the concept Fire and Gas detector coverage mapping to help decision makers understand that there is a scientific method to support these critical design decisions, and to maximize the return on investment of F&G systems.?It will also highlight why these safety critical systems require specialized training and experience to design and support installers.
?Fit for Purpose Design
Before any effort is spent to assess what detector coverage is achieved by a proposed design, one of the first things to consider is “what is the design target”.?Detection systems can be expensive to purchase and maintain, therefore it is important to understand which areas actually need coverage.?Low risk areas may not benefit from a high detector coverage, while high risk areas would benefit from the greatest detector density practical to maximize coverage.?Taking a risk-based approach to detector coverage ensures a “Fit for Purpose” design.?
?The ISA TR84.00.07 standard describes a risk assessment process called Hazard Grading that can be used to set detector coverage targets.?Factoring in many variables such as leak probabilities, area occupancy, operating pressures, fluid properties, level of congestion, ignition probabilities, toxicity and more, the user can use a calibrated risk table to determine a Hazard Grade with a corresponding coverage target for each risk zone.?Hazard grades can also be used to determine the design basis coverage scenarios using Relative Heat Output (RHO) as a design input (how big of a fire do I wish to detect).?This in turn allows the designer to consider the size of fire a specific model can “see” using the design basis fire sizing associated with the selected Hazard Grade.?The hazard grading and coverage assessment process will need to be established in a F&G Philosophy before any risk assessment or coverage assessments begins.?The standards do not dictate what the process must be, but rather provides guidance on what it could look like.?The process is up to the end user to determine based on their corporate risk policy.
?3D Detector Coverage Mapping (In Depth Analysis)
Once performance targets have been established, software tools are available to import or setup a 3D model of the hazard area.?Hazard grading information can be calibrated into the software.?Specific models of sensors can be selected allowing the software to take into consideration sensor capabilities and limitations.?Detectors can be placed within the model and configuration and orientation data entered for analysis.?Some major advantages of these software tools that make them worth the time, money and effort to perform 3D mapping include:
This overview is just scratching the surface on coverage mapping and the associated tools.?There are multiple tools available, each with their own strengths and weaknesses.?The standard offers good guidance on processes that can be followed to work with which ever software tool seems to be the best fit for the user’s application.?It should also be considered that 3D mapping may be overkill for some applications.?An experience analyst should be able to provide guidance on the best approach.
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?Qualitative Detector Coverage Assessment (Simplified Analysis)
In some cases, a qualitative approach to detector coverage assessment may be the preferred method to determine if a proposed design achieves a tolerable level of coverage.?After reading about the benefits of using software tools, hopefully it is apparent how complicated coverage assessments can be and how many variables need to be considered when doing this work.?Qualitative coverage assessment therefore places a significant demand on the analyst to take these factors into consideration in their assessment without the support of software tools to simplify this process for them.?Often this will result in a recommendation for more detection since the analyst may need to take a more conservative approach in their estimations.?Qualitative assessments for this reason are most effectively performed by specialists with experience using the semi-quantitative approach and working with software tools since this experience helps the analyst apply their learnings from previous mapping work.?
?A visual representation of detector coverage should be provided to show what coverage has been allocated to detectors and which obstructions have been considered.?Coverage targets should still be assigned using a Hazard Grade, however achieved coverage is an estimation and not quantified by software tools with this method.?Designers should provide similar details to installers that are available from software tools including: location, elevation, orientation, declination and sensitivity settings depending on the sensor type.
?Experience and Independence Should not be Overlooked
While the process described may seem straight forward, it is important that designers have specialized fire and gas systems training to do this properly.?Decisions around where to place detectors can be affected by a wide range of factors that must be considered for the system to work when needed.?
?An independent eye focused on coverage and correct application of technology can be very beneficial.?Avoid some common pitfalls such as: a) avoid using generalist engineering for this specialist activity; b) avoid limiting designers to technologies available from a single manufacturer and their sales rep; b) seek unbiased engineering advice on detector quantities and placement (i.e. companies that do not sell the hardware being recommended).
?Summary
Hopefully this article will help readers recognize the importance of proper F&G detector placement, understand some of the issues contributing to detector effectiveness and what it takes to ensure fit for purpose coverage has been achieved.?These activities can now be factored into new project plans.?For existing installations, it is not too late for a detector coverage study.?Quite often significant improvements can be made simply by re-orienting existing detectors.?Many facilities have aging equipment due for replacement.?This is a good time to ensure that coverage is optimized, detectors are accessible for maintenance and the quantities are appropriate.
?Assuming the topic of detector coverage is now better understood and readers are on the path to ensuring good coverage is achieved, consider other factors that influence the overall F&G system effectiveness.?In future articles, we will discuss other factors such as effectiveness of annunciation systems, and automatic actions that should be considered upon a fire or gas alarm.?
?Feel free to reach out to the author at [email protected] to discuss the information in this article or for support with F&G detector coverage assessments.
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