To determine the probability of a failure, two fundamental issues must be considered:
What are the specific forms of corrosion and their rates?
What is the possible effectiveness of inspection?
The input of corrosion experts is required to identify the relevant forms of corrosion in a given situation and to determine the key variables affecting the propagation rate. It is also important to realize that full consensus and supporting data on the variables involved is highly unlikely in real-life complex systems and that simplification will often be necessary.
One semi-quantitative approach for ranking process equipment is 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 a fair degree of engineering judgment and experience and, as such, is dependent on the background and expertise of the analyst. This procedure can facilitate an efficient use of finite inspection resources when 100% inspection is not practical.
The POF approach is based on a set of rules heavily dependent on detailed inspection histories, knowledge of corrosion processes, and knowledge of normal and upset conditions. The equipment rankings may have to be changed and could 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.