Flame Detector False Trips and Bypass in a Spray Booth
A solvent spray booth 6 m by 4 m by 3 m is protected by four UV/IR flame detectors tied to a deluge suppression system. The booth has a robotic arm that moves along the centerline. One detector is mounted behind the robot's home position, and another is aimed through a window at a welding station 8 m away. The system has tripped twice in three months, both times when welding started in the adjacent bay. The owner has started using the bypass switch during production and asks you to disconnect the 'bad' detectors so the line can run. The suppression system is a pre-action deluge with a 30 s delay. The AHJ requires flame detection because of the solvent load. The booth is normally occupied by one operator during loading.
1. Evaluate the detector placement and explain the false trips.
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Flame detectors must have an unobstructed line of sight to the hazard and must not view extraneous sources that mimic flame. The detector behind the robot's home position is blocked during part of the cycle and cannot be relied on. The detector aimed through a window at a welding station is seeing UV and IR energy from welding, which is a classic false-trip source. The window may also attenuate or reflect the signal. Two trips coincident with welding confirm the cause. Do not simply disconnect the detectors. Relocate them to maintain a clear view of the booth hazard, avoid windows and reflective surfaces, and follow the manufacturer's cone of vision and spacing. Consider cross-zoning or voting so two detectors must confirm before release, and shield the detectors from the welding arc. Verify that the revised layout still covers the entire booth, including the robot's travel path and the operator loading area.
2. What is the proper response to the bypass and the request to disconnect detectors?
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The bypass defeats the AHJ-required protection and must not be used for production convenience. Disconnecting the 'bad' detectors would leave the booth partially unprotected and create an unreported impairment. If the system must be taken out of service for repairs, follow a documented impairment procedure: notify the AHJ and insurer, provide a fire watch, remove or control ignition sources, and restore protection as soon as possible. Investigate the false trips and correct the placement or sensitivity instead of disabling devices. Use cross-zoning, shielding, or a different detector technology only if it maintains the required response to a real fire. Document every bypass, its duration, and the compensating measures. The owner's production pressure does not justify an undocumented disablement, and the technician should refuse to disconnect detectors without an approved design change and AHJ acceptance.
3. Describe the test and acceptance method for the flame detectors and their interface with the deluge system.
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Use the manufacturer's listed test lamp at the specified distance and angle for each detector; do not use an open flame. Verify that each detector alarms at the panel and that the pre-action deluge sequence initiates, including the 30 s delay, alarm annunciation, and valve operation. Confirm supervision and trouble signals for detector disconnection and power loss. Test the cross-zoning or voting logic if provided, and verify that a single detector does not release the system when two-detector confirmation is required. Re-check line of sight after the robot moves through its full cycle and after any shielding is installed. Verify that manual release and abort functions, if provided, operate as listed. Document the test results, detector locations, and any adjustments. Acceptance requires that the detectors detect a real flame-like signal while rejecting the welding source under normal production conditions.