Keeping fire and gas detection reliable: a maintenance checklist

Gas detectors drift over time, and some sensors can be poisoned or inhibited by substances in the air, such as silicones in the case of catalytic sensors. Routine checks keep them honest:

• Bump test: expose the detector to a known concentration of test gas and confirm that it responds and alarms. It is quick and can be done often.
• Calibration: adjust zero and span against certified test gas, at the interval set by the manufacturer and the site procedures.
• Sensor life: track the age of each sensor and replace it before the end of its expected life.
• Open-path detectors: check alignment and signal strength, and clean the optics.
• Housekeeping: after every test, check that sinters and weather guards are clean and that calibration caps have been removed.

Optical flame detectors depend on a clear view: dirt, oil mist or salt on the window reduces their sensitivity. Many have a built-in optical self-test, but they still need periodic functional tests with a suitable test lamp.

• Clean detector windows and check that nothing new blocks the field of view.
• Functionally test smoke and heat detectors in buildings and equipment rooms.
• Check the pipework, airflow and filters of aspirating smoke detection systems.

A working detector is only half the system. Periodically prove the full chain: the detector input, the logic in the fire and gas panel, the alarms in the control room and the field, and the executive actions such as shutdowns, damper closure or the release signal to a suppression system. This is usually done against the plant’s cause and effect matrix. Any output that must not operate during the test, such as a suppression release, is isolated under a controlled permit, and every inhibit is removed and checked before the system returns to normal service.

Good records turn maintenance into evidence. Log every test and calibration with the date, the result, any adjustment and the test gas certificate used. Over time they show which detectors drift or fail, so intervals can be adjusted and problems fixed at the source.

IEC 60079-29-2 gives guidance on the selection, installation, use and maintenance of flammable gas and oxygen detectors, and IEC 62990-2 does the same for toxic gas detectors, which matters where hydrogen sulphide is present. Where fire and gas functions are safety instrumented functions, IEC 61511 also applies. Fire alarm systems follow the applicable code, such as NFPA 72 or BS 5839, and the requirements of the local civil defence authority.