In the previous issue of Asia Pacific Fire Magazine, we discussed the challenges posed by aircraft hangars in relation to flame-detector selection. In this issue William Pittman discusses how 3D flame-detector modelling techniques can be used, in combination with good engineering practice, and detector selection, to provide a cost-effective design and aid compliance with standards.
Case Study Using a Group I Aircraft Hangar
In this case study, a design for a hypothetical ‘Group I’ aircraft hangar using commercially available 3D modelling software and a range of industrial flame detectors is used. The software will provide coverage assessments based on individual flame-detector characteristics.
Please note the case study will largely ignore the provisions of the United Facilities Criteria (UFC) that in some cases allows for activation of the foam system if a single optical flame detector goes into alarm and the system also detects that water is flowing through overhead fire water / sprinkler systems. Furthermore, we will not consider the use of ceiling-mounted smoke and heat detection systems as thermal stratification and other issues sometimes encountered within structures with very high ceilings can delay activation.
Detector modelling and desensitization
The case study compares the coverage achieved by an ‘industry standard multi spectrum infrared’ (MSIR) flame detector to that of two intelligent visual flame detectors (iVFD). As noted previously, the MSIR selected is currently specified for use in the US Military’s UFC.
The Field of View (FOV) of the detector is one of the most important features of a detector in determining coverage, second, perhaps, only to the effective detection range to a 1 ft2 pan fire. Table 1 below gives a summary of the maximum FOV achieved by each detector.
As discussed in previous articles, the effective range of a detector can be reduced by a wide range of environmental factors and its necessary to account for this sensitivity loss when assessing the coverage, a detection system can be expected to provide.
One method, with wide industrial acceptance, for determining the effective detection range of a detector uses three desensitization factors.1,2 These factors, often called F1, F2, and F3, adjust the detection range to account for blinding / interference sources, the impact of dirty optics and loss of off-axis sensitivity respectively.
Table 2 provides a breakdown of the desensitization factors for each detector and the effective ranges used in the assessments. The value ‘X’ is approved by FM Global (FM3260 data) and can be taken from product manuals. In the case of the MSIR, the desensitization factors depend on the sensitivity setting used so Table 2 lists these by sensitivity. Visual Flame Detectors are spatially sensitive and have one only sensitivity setting.
As noted previously, iVFDs are far more resistant to blinding and desensitization than MSIR units. As a result, the iVFDs only suffer a 9.75% loss of effective range where the MSIR loses between 37 and 52% depending on the sensitivity setting.
As the FOV cone for the MSIR and iVFD’s are accurately modelled in the modelling tool used, no adjustment is necessary, and a value of 1.00 is used for F3.
The aircraft hangar is approximately 90 m wide, 80 m deep in the centre, and 50 m deep at the sides with a 23 m ceiling. With a fire zone of approximately 5,500 m2 and room for aircraft having a tail height greater than 28ft, this hangar would be classed as a Group 1 hangar. Figure 2 shows how the hangar might accommodate different aircraft. Commercial aircraft are used in this example, as the overall form factor of large commercial aircraft and larger military cargo planes, like the C-5 galaxy, are sufficiently similar to use for illustrative purposes.
The modelling tool allows the user to assign a risk grade to the aircraft as well as indicating that fires are thought possible in the surrounding area too. How far the graded area extends can vary but 1 to 5m is typical. Ungraded areas represent spaces where fire is either not expected, or not expected to result in significant consequences. Figure 3 shows the graded volumes selected for assessment.
Note that the entire area of the hangar is part of the graded fire zone up to 8m above the local deck (ALD). The volume within 5m of the planes that is assigned to the same risk grade, even if that space is more than 8m ALD. This grading is reasonable given that the principle fire hazard is the release and ignition of liquid hydrocarbons, which can burn on the deck, any aircraft surface or on equipment brought into the hangar for fuelling or maintenance activities.
Having assigned grades, detectors can be positioned to provide optimal coverage. Hazard-mapping software shines at this stage as it allows users to change detector position, run new assessments and immediately see the impact in coverage.
Coverage achieved using MSIR
A design was produced using the MSIR using Very High Sensitivity. This sensitivity setting is generally used for military hangars where the aircraft are not permitted to move under their own power. The UFC requires detectors to be kept in Medium or Low sensitivity settings when aircraft can enter and leave a hangar under their own power. The design is shown in Figure 4.
The FOV cones shown in Figure 4 (and subsequent figures) represent the volume in which, baring visual obstruction, the detector should detect a 40 kW RHO fire in 10 seconds or less. This fire is roughly equivalent to the 1ft2 n-heptane pan fire used in FM 3260 testing.
The hangar height and dimensions make it difficult to achieve good coverage in the middle of the hangar without placing detectors along the back walls. This is not ideal as six of the eight detectors are positioned to ‘look’ beyond the hangar doors. This can be undesirable as when the doors are open the detectors might false alarm to aircraft moving under their own power outside the hangar.
The design was assessed based on the detectors’ ability to detect a 90kW RHO fire, which roughly corresponds to a 2ft square JP-4 or JP-5 fire. The results are shown in Figure 5.
The results using eight-detectors are good with 2ooN coverage achieved for 83% of the graded volume and nearly complete 2ooN coverage for the hangar floor. Most of the areas of low coverage are along the outer edges of the hangar and in the middle, near the hangar door. Most of the coverage loss results from the visual obstruction from the aircraft and the effective range used.
Covering an aircraft hangar becomes more challenging if the aircraft can enter and leave under their own power. Per the requirements of the UFC the detectors must use the ‘medium’ or ‘low’ sensitivity setting. Figure 6 shows how the FOV cone for the MSIR shrinks relative to the size of the hangar when the lower sensitivity settings are used.
Table 3 provides a summary of the coverage provided using each sensitivity setting using the eight-detector MSIR layout.
Coverage Achieved Using iVFDs
By comparison, Figure 7 shows a six-detector layout using iVFD–type 2 detectors. This design eliminates two detectors from the previous example by employing visual flame detectors.
The six-detector layout achieves 87% 2ooN coverage and provides at least 1ooN coverage for all but 1% of the graded volume. Even with two fewer detectors, the layout achieves better coverage than the eight-detector MSIR layout, using the ‘very high’ sensitivity setting.
The coverage provided using iVFD–type 2s can be improved by swapping for the VFD – type 1 devices. Using six iVFD–type 1 detectors, and the same device locations the 2ooN coverage rises to 93%.
For context, Figure 9 shows the coverage achieved using six MSIR’s in the same layout as for the iVFDs. Table 4 summarizes and compares the coverage of all the devices examined.
The 2ooN coverage of the MSIR drops to 70%, falling well below the coverage achieved by the iVFDs with the same detector count. This layout cannot provide timely activation in response to a 90kW RHO flame in the centre of the hangar. The iVFD-Type 1, on the other hand, can detect a 90kW RHO fire at roughly 81m, even after desensitization. So, if two iVFD-Type 1s have a line of sight to the fire, the system will alarm in a timely fashion.
Optical flame detectors have recognized utility in improving the performance of fire-suppression systems in aircraft hangars and protecting the extremely expensive assets inside them. MSIR detectors currently dominate aircraft hangar installations. However, MSIR performance can be bettered using alternative optical flame detection technologies.
The iVFDs can achieve detection at longer ranges with fewer false alarms than MSIR units, even when the MSIR units are set to higher sensitivity settings. This ultimately means that iVFDs can provide adequate coverage of larger hangars with fewer detectors, which lowers upfront and maintenance costs. When MSIRs are used in lower sensitivity settings, as is sometimes required by the UFC, the performance swing is enhanced.
This article has discussed how 3D flame detector modelling techniques can be used, in combination with good engineering practice, and detector selection, to provide a cost-effective design and aid compliance with standards.
For more information, go to www.micropackfireandgas.com
Refer to Part 1 of Article, Asia Pacific Fire, Issue 73.