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Proceedings of the Eighth International Conference on Probabilistic Safety Assessment & Management (PSAM)

Michael G. Stamatelatos
Michael G. Stamatelatos
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Harold S. Blackman
Harold S. Blackman
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ASME Press
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While inspection and maintenance strategies have been widely studied for monitored units, less attention has been given to periodically tested components (PTC). The purpose of this paper is to present a model to identify the optimal time between surveillance tests of components whose failures are detected upon inspection and are periodically tested to ensure high availability. This type of equipment includes emergency and spare units, as well as hardware components with hidden failures in normal operation.

The analytical model is based on the component availability during the renewal cycle. It takes into account costs associated with surveillance tests and repairs, as well as the potential losses related to unavailability. The assumptions considered for this analysis are:

1. The component has failures which are only detected upon inspection. Inspection tasks are perfect.

2. Inspections are carried out every T units of time (constant interval). Repairs are conducted in case of failure detection.

3. Inspection and repair durations are not negligible but constant (deterministic).

4. Component is under aging process and remains “as old” after surveillance tests and repairs. Inspection and repair tasks do not deteriorate the component.

5. Periodic preventive overhaul is performed after every N inspection cycles regardless of the unit condition. Component returns to “as new” after overhaul.

6. Component unavailability may cause losses with certain conditional probability. These losses are independent of component age.

7. Direct inspection and repair costs may increase after every test cycle. Inflation effects are negligible.

The model to identify the optimal inspection interval is based on minimizing the total cost per unit time (cost rate function) for the renewal cycle.

The expected downtime and the expected cycle length are calculated considering random times between failures and “as bad as old” process after test cycles. A conditional Weibull density function is assumed for the interarrival times between failures.

Allowing for uncertainty in the Weibull parameters α and β, and using arbitrary cost values, the model was applied to evaluate the effect of failure rate, costs, and overhaul frequency (N) on the optimal inspection policy of PTC.

The effect of overhaul frequency on the optimal test interval, Topt, was studied considering different values of N. Results obtained from diverse sets of costs and Weibull parameters suggest that Topt decreases and tends to an asymptotic value when N is increased.

The effect of N on the total cost per unit time was studied by evaluating the cost rate function, crf, in Topt for different values of N. The analysis reveals that an optimal N can be identified. Thus, the use of the model provides not only an optimal time between surveillance tests, but also an optimal overhaul frequency.

A practical numerical example carried out for a typical PTC (a safety relief valve) with mean time between failures equal to 105 days shows that the optimal policy is to inspect every 4 years, with overhaul every two test cycles.

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