ASME PCC-3:2017 pdf free download – Inspection Planning Using Risk-Based Methods
2.3 Inspection Optimization
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, damage to protective coatings or glass-lined vessels, or improper reclosing of inspection openings that may result in leakage of harmful fluids). This situation is represented by the dotted line at the end of the upper curve.
RBI provides a consistent methodology for assessing the optimum combination of methods and frequencies. Each available inspection method may be analyzed and its relative effectiveness in reducing failure probability estimated. Given this information and the cost of each procedure, an optimization program may be developed. The key to developing such a program is the ability to assess the risk associated with each equipment item and then to determine the most appropriate inspection techniques for that equipment item. A conceptual result of this methodology is illustrated by the lower curve in Fig. 2.3. The lower curve indicates that, with the applica- tion of an effective RBI program, lower risks can be achieved with the same level of inspection activity. This is because, through RBI, inspection activities are focused on higher risk items and away from lower risk items. Notallrisks are affected byinspection. Table 2.3 shows seven categories of factors that have contributed to loss of containment events resulting in major insurance losses in petrochemical process plants.