Flame Detectors Mapping and Coverage Assessment
Introduction
This article delves into the essential methodologies and insights for effective flame detector mapping assessments. When dealing with flammable hydrocarbon releases, the repercussions of their escape and ignition can be catastrophic, leading to turbulent diffusion fires such as pool or jet fires. The detection philosophy serves as a guiding principle, outlining the threshold for detecting fires and the subsequent safety measures to be automatically initiated.
Typical fire detection philosophy outline whether alarms and Fire and Gas System (FGS) actions will occur based on a single detector in alarm state (1ooN) or necessitate multiple detectors in alarm state (e.g., 2ooN). This decision significantly influences the efficacy of response mechanisms and the overall effectiveness of fire detection systems.
Fire-zone and Sub-zone Definitions
A fire zone is a designated area where the presence of flames must be closely monitored and detected. This area could encompass a specific room, a section of a building, or an outdoor space where the risk of fire exists. The delineation of fire zones is typically based on factors such as identified fire hazards and the layout of the facility.
Conversely, subzones represent further subdivisions within a fire zone, aiming to facilitate more detailed monitoring or control. These subzones are typically identified based on increased fire risks or specific operational considerations that require increased surveillance.
It is essential to consider that not all areas or volumes within a facility/zone/subzones contain potential fire sources or leaks. Therefore, as part of the zoning process, it is prudent to exclude such non-operational areas/equipment from the defined zones and subzones. This exclusion helps in reducing the zero visibility region and improves overall coverage by focusing resources on areas where fire hazards are present [3] [4].
Optical Flame Detectors
Optical flame detectors (infrared, visible, and/or ultraviolet sensors or combinations thereof) rely on the perception of electromagnetic radiation emitted by a fire. This includes analysing the fire's signature spectrum, frequency, and distinguishing it from background radiation and heat sources. These detectors have a limited Field of View (FOV) and require a line-of-sight transmission of radiant energy from the source to the detector.
FM3260 Standard and Testing Requirements [1]: The FM3260 standard outlines testing and certification requirements for radiant energy-sensing fire detectors used for automatic fire alarm signalling. The FM approval test assesses the proportional radiant emissions from the test source (flame) to establish the detector's sensitivity during testing. Sensitivity is quantified as the maximum distance from the fire centre at which the flame detector consistently triggers alarm responses within a specified time frame, usually not exceeding thirty seconds. Typically, flame detector sensitivity is evaluated using one or more test fires, such as N-heptane, alcohol, jet fuel, propane, etc. It is essential to recognise that flame detectors are not tested for obstructed flames. Testing for FM3260 certification is conducted under unobstructed conditions. The detector manufacturers do not provide the Radiant Heat Output (RHO) values for the fire under test used during testing. The typical RHO of these test fires ranges from 40-50kW, although different references such as Shell DEP, consultants, researchers and software developers quote different values. The following figure shows the typical field of view of the flame detector within which sensitivity is at least 50% of the on-axis sensitivity/centreline (measured in units of distance) in at least four directions (left, right, up, and down).
Hazard Grade and Fire Performance Targets Determination
FGS performance targets define the capability of an FGS function to detect, raise an alarm, and potentially intervene to mitigate the impact of a fire or gas release under demand conditions. The guiding principle stipulates that installations with higher hazard levels require higher performance levels, whereas those with lower hazards can suffice with lower performance levels, optimising FGS resource allocation. In hydrocarbon processing zones/subzones, hazard grading can be semi-quantitatively assessed using a scoring system. This method categorises fire, combustible gas, and toxic gas risks into high, medium, or low categories, facilitating the determination of FGS performance criteria [2].
Hazard ranking is influenced by various factors including equipment, hazards, consequences, likelihood, occupancy, and special considerations. For each hazard (fire, combustible, and toxic), individual equipment items are assigned adjusted likelihood and consequence scores. The hazard rank is calculated as the sum of these scores, indicating the degree of hazard and the associated risk of fire, combustible gas, or toxic gas hazards. The highest individual value of the baseline hazard rank for all equipment within a given zone defines the zone hazard rank. A higher hazard rank denotes a higher level of risk, necessitating a correspondingly higher performance target for the FGS to mitigate the risk effectively [2].
ISA-TR84.00.07 standard, titled "Guidance on evaluation of fire, combustible gas, and toxic gas system effectiveness," provides comprehensive guidance on hazard ranking procedure [2]. A typical flow chart illustrating the hazard ranking procedure for a fire zone/subzone is presented below:
A correlation exists between the fire hazard rank, fire grade (A, B, C), and the corresponding detection performance target. The recommended fire detection coverage performance targets specified for different fire grades are >=90% for Grade A, 80% for Grade B, and 60% for Grade C [2].
Flame Detector Mapping and Coverage
Typically Engineering consultant proposes the initial detector locations based on generally accepted good engineering practices for the placement of detectors. The detector mapping confirms the selected detector layout is capable of achieving the detector coverage performance targets specified for the fire and gas system if the system is correctly installed, operated and maintained. Flame detector mapping follows geographic coverage assessment. Flame detector coverage is influenced by factors such as the designated FOV and potential visual obstructions. FOVs may vary due to manufacturer specifications, model variations, fire characteristics, detected fuel types, and sensitivity adjustments.
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3D Ray Casting: Typically 3D ray casting technique is employed to simulate the path of rays originating from the detector's location and extending outward into the surrounding environment. These rays represent potential lines of sight from the detector to various points in the monitored space. Rays are cast in multiple directions from the detector's position, covering the entire FOV. The number and direction of rays can vary based on the application's requirements. The rays are traced through the 3D space, checking for intersections with objects or surfaces along their path. Each ray travels until it hits a geometry item or the extent of the FOV.
The coverage calculation methodology outlined in this article is embedded within the Insight Numerics Detect3D software tool [3]. Two distinct flame detector coverage calculation methods are implemented [4].
Rather than relying on the inverse square law, an alternative approach allows for the specification of a static value for the fire size (acceptance criteria) required to trigger the alarm stage/coverage calculation which comes under the Fire Area (fixed) coverage calculation option.
Flame detector coverage results are typically represented through Coverage Tables, Contours, and Iso-volumes, derived from the monitored zone's volume. Coverage contours offer a 2D representation on a specified plane (e.g., XY plane). Iso-volumes provide a 3D visualisation of detector coverage within the defined fire zone.
Layout Optimisation
The calculated coverage results are assessed against performance targets to ascertain the need for additional optimisation of the detector layout. Analysing how disabling individual detectors within a zone impacts its coverage, through an iterative process, helps identify the least effective detectors in the layout (detector ranking). Furthermore, comprehending each detector's contribution to the overall coverage is vital for optimising the detector layout effectively [3] [4].
References:
[1] FM3260: Examining standard for radiant energy sensing fire detectors for automatic fire alarm signaling.
[2] ISA-TR84.00.07 standard, titled "Guidance on evaluation of fire, combustible gas, and toxic gas system effectiveness
[3] Insight Numerics, Detect 3D Software Tool
[5] Shell DEP 32.30.20.11-Gen Fire, Gas and Smoke Detection Systems
#Flame Detector Mapping; #FM3260; #F&G; #Inverse Square Law; #Fire Hazard Grading; #Detect3D; #Detector Performance Targets; # Detector Layout Optimisation
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