HSE Reminds Fixed Gas Detector Users to Revisit Alarm Settings

A new document from Britain's OSHA agency says setting alarm points "should not be 'fit and forget'" because initial alarm set-points may be adjusted up or down due to process-related events and instrument characteristics.

A new document published by Britain's Health and Safety Executive reminds users of toxic gas detectors about good practices when setting alarm points. It points out guidance is available on the HSE website for setting alarm levels for predominantly flammable gas detectors, but not for toxic gas detectors (although alarm levels are specified for certain gases in specific industries, such as chlorine plants and hydrogen sulfide offshore). "There is a need, therefore, to produce a framework by which the actual intended function of the toxic gas detection system can be established and, following on from that, which factors need to be considered to set appropriate alarm levels in order to carry out detection and warning," it states.

A key point in the document is the reminder for employers using fixed toxic gas detectors that setting alarm points "should not be 'fit and forget'" because initial alarm set-points may be adjusted up or down due to process-related events and instrument characteristics. "This requires analysis of the data over a suitable period," it adds.

The document also says guidance found in standards, such as BS EN 45544-4 relating to toxic gas detectors, "is not comprehensive enough. While each application of toxic gas detector is usually different from the others, general background information and guiding principles would assist HSE inspectors and safety practitioners."

The document lists 13 factors that relate to setting an alarm point:

1. Whether detectors should be fixed and/or whether personnel should be issued with portable, which includes personal, detectors.
2. The location of the fixed detector and whether the work area is occupied or unoccupied.
3. Whether the fixed detector is a point or open-path (also known as beam or line of sight detectors) detector.
4. Whether egress is difficult and/or time-consuming or there is an emergency.
5. Whether Workplace Exposure Limit (WELs), other Exposure Limit values, or other health-based levels (e.g., Immediate Danger to Life and Health -- IDLH) exist.
6. Instantaneous or time-weighted average (TWA) alarm.
7. Background variations and events from the process, which may trigger "spurious" alarms.
8. Effects of interferents.
9. False alarms caused by instrumental effects.
10. The potential rate of gas buildup (considering short-term peaks in point 7).
11. Time to alarm of the detection system.
12. Number of alarm levels (e.g., high and low levels).
13. Mixture of gases/vapors.

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