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Gas, Smoke, and Fire Detectors

There are many of these items installed in an offshore platform. Most of them will be in naturally ventilated process modules. A few would be located in closed acoustic enclosures of equipment such as gas turbines. Some would be in artificially ventilated areas such as the living quarters. Such differences can produce significant variations in MTBFs. Hence, the data sets are sorted using external environmental considerations, as well as by type of detector.

We count the number of installed items in each such set (P), Next, we note the number of recorded failures over a given period, say one year (for the failure mode—failed to detect, Q). This number is obtained from the periodic tests done every 2 or 3 months. The reliability parameters are computed thus:

MTBF (fail to detect) = Px1 year/Q years between failures

Failure Rate (fail to detect) = Q/(Px1 year) failures/year

These items may also fail by detecting unsafe conditions when in fact there are none. Such failures are termed spurious or nuisance events. These are also unacceptable, as they can cause production losses or divert operators' at?tention unnecessarily. Spurious failure rates can be computed using the same method as above. For this purpose we need the number of false detections during the year. This number has to be obtained from the operations log.

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