One method for optimizing limited resources and reducing the possibility of equipment failures that has been in use in the nuclear industry for some time now is risk-based inspection. Risk-based inspection is a procedure for ranking or prioritizing equipment for inspection purposes, based upon risk. Risk is the combination of probability and consequence. Probability is the likelihood of an event occurring, in this case an equipment failure. Consequence is a measure, both in lives and property, of the damage that would occur if an equipment item failed.

Risk based inspection procedures can be based on either qualitative or quantitative methodologies. Qualitative procedures provide a ranking of equipment, based largely on experience and engineering judgement. Quantitative risk-based methods use several engineering disciplines to set priorities and develop programs for equipment inspection. Some of the engineering disciplines include nondestructive examination, system and component design and analysis, fracture mechanics, probabilistic analysis, failure analysis, and operation of facilities. Quantitative analysis methods are expensive, time consuming, tedious and are outside the scope of this document. Often, insufficient information is available for conducting a quantitative risk analysis. Two organizations that are currently working on quantitative risk-based analysis procedures for use by the chemical industry are the American Society of Mechanical Engineers and the American Petroleum Institute.

RBI Procedure

This document discusses one approach for ranking process equipment, based on internal probability of failure (POF). The procedure is based on an analysis of equipment process and inspection parameters, and ranks equipment on a scale of one to three, with "one" being the highest priority. The procedure requires considerable use of engineering judgement and experience; therefore, the results are dependent on the background and expertise of the analyst. The procedure has been used to rank more than 2000 equipment items and is both practical, effective and efficient.

The POF numerical ranking arrived at using the procedure is not meant to be an indicator of an equipment item's absolute susceptibility to failure. It is intended as a convenient and reproducible means for establishing equipment inspection priorities, based on knowledge. As such, it facilitates the most efficient use of finite inspection monies and personnel where 100% inspection is not practical

The procedure is based on a set of rules heavily dependent on detailed inspection histories, knowledge of corrosion processes, and knowledge of normal and upset conditions. As such, the equipment rankings will not stay constant, but will require updating as additional knowledge is gained, process conditions change and equipment ages. Maximum benefits of the procedure depend on fixed equipment inspection programs that permit the capture, documentation, and retrieval of inspection, maintenance, and corrosion/failure mechanism information.

The POF procedure is one-half of a risk-based inspection procedure. The POF ranking is combined with a consequence ranking to provide a true risk-based ranking. A procedure for ranking equipment based on consequence is also outside the scope of this document; however, a similar procedure can be developed, using such process attributes as flammability, reactivity, corrosivity, density, toxicity, etc. The three rankings developed from the consequence analysis can be combined with the probability rankings in a three-by-three matrix. Forty-five degree parallel constant-risk lines result in five risk categories. These categories can be used to establish inspection intervals, etc.

The equipment covered by the procedure includes all fixed equipment to consider items designed and constructed to ASME Section VIII, "Rules for Construction of Pressure Equipment", divs. 1 and 2. They are not appropriate for rotating equipment. The fixed equipment to consider on the procedure would normally be based on OSHA's hazardous chemicals guideline; however, it can be used to rank all equipment, if desired.

As failures can occur from both internal and external causes, a similar procedure is required to rank equipment based on external failures. This internal POF ranking procedure is used for shutdown inspection planning. The external POF ranking procedure is used to plan onstream inspections.