ASME PCC-3:2017 pdf free download – Inspection Planning Using Risk-Based Methods
Understanding the two-dimensional aspect of risk allows new insight into the use of risk analysis for inspection prioritization and planning. Figure 2.1 dis- plays the risk associated with the operation of a number of equipment items. Both the probability and conse- quence of failure have been determined for ten equip- mentitems, and the results have beenplotted. The points represent the risk associated with each equipment item. An “iso-risk” line, representing a constant risk level, is also shown on Fig. 2.1. A user-defined acceptable risk level could be plotted as an iso-risk line. In this way the acceptable risk line would separate the unacceptable from the acceptable risk items (i.e., if the iso-risk line on the plot represents the acceptable risk, then equipment items 1, 2, and 3 would pose an unacceptable risk that requires further attention). Often a risk plot is drawn using log-log scales for a better understanding of the relative risks of the items assessed. Risk levels or values may be assigned to each equip- ment item. This may be done graphically by drawing a series of iso-risk lines and identifying the equipment items that fall into each band or it may be done numeri- cally. Either way, a list that is ordered by risk is a risk-based ranking of the equipment items. Using such 2 a list, or plot, an inspection plan may be developed that focuses attention on the items of highest risk.
When the risk associated with individual equipment items is determined and the relative effectiveness of different inspection techniques in reducing risk is esti- mated or quantified, adequate information is available for developing an optimization tool for planning and implementing an RBI program. Inspection affects per- ceived risk; physical actions such as mitigation activities performed as a result of an inspection affect actual risk. Inspections may affect the calculated risk by reducing uncertainty. When there is uncertainty about the risk associated with operating equipment items, the default action should be to make reasonably adverse (conserva- tive) or even “worst-case” assumptions resulting in rela- tively high calculated risk. For example, during an initial analysis one assumption may be that the only credible damage mechanism for a component is general corro- sion (i.e., general metal loss). If examination reveals that no measurable metal loss has actually occurred, then the probability of failure may be reassessed to a lower level with a corresponding reduction in the calculated risk. Figure 2.3 presents stylized curves showing the reduc- tion in risk that should be expected when the degree and frequency of inspection are increased. The upper curve in Fig. 2.3 represents a typical inspection program. Where there is no inspection, there may be a higher level of risk, as indicated on the y-axis. With an initial investment in inspection activities, risk generally is sig- nificantly reduced. A point is reached where additional inspection activity begins to show a diminishing return and, eventually, may produce very little additional per- ceived risk reduction. Any inspection activity beyond this point may actually increase the level of risk. This is because invasive inspections in certain cases may cause additional damage (e.g., introduction of oxygen into boiler feedwater, water contamination in equipment with polythionic acid.