The Application of Risk Based Inspection (RBI) and Evolution to Risk-Centered Maintenance

Part 1 provided a review of RBI for pressure vessels and piping and an introduction to application of RBI to rotating equipment and the differences between approaching the two different types of equipment. As promised, Part 2 will delve more deeply into the details of rotating equipment RBI analysis, highlighting additional differences between this and fixed equipment analysis and cover:

• RBI for Rotating Mechanical Equipment: Likelihood
• Qualitative Risk Assignment
• Forming the Risk Reduction Plan
• Moving from RBI to Risk-CM

RBI for Rotating Mechanical Equipment

Likelihood

Likelihood analysis differs for rotating mechanical equipment and instrumentation. The goal with pressure vessel likelihood assessment is to model damage that can be predicted by inspection and subsequently, prevent release. Rotating mechanical equipment models include inspections (predictive maintenance), timely repair or replacement (preventive maintenance) and redesign (non-maintenance). All three types of likelihood mitigation have the propensity to improve reliability and overall asset utilization.

Likelihood analysis utilizes actual mean time between failure, MTBF, for the item being analyzed. Unlike pressure vessels, obtaining actual mean time between failure is possible and can be calculated or closely estimated. It is highly probable that several failures have already occurred at the particular installation. Data may be extracted from this history directly from the maintenance management software, log books, and operator interview. When the failure history has been obtained, MTBF or probability of failure (PoF), may be obtained by simple calculation. Regardless of whether a screening is utilized or analyzed in a detailed, quantitative level, MTBF is sufficient.

Experience has shown that the maintenance management software may not be sufficiently accurate or complete to use for MTBF calculations. Therefore, the RBI analyst must interview plant experts. Interviews, many times, lead to greater accuracy than the maintenance software. MTBF may be calculated as a simple average of operating intervals between failures, number of failures over fixed time, or extracted from Weibull statistics(2). Also, do not shy away from estimates from the plant experts. Again, they may provide the greatest knowledge versus analytical tools applied. The RBI analyst must judge which value provides the great- est accuracy for that specific installation.