Experience of failure

An engineer surveyor employed by an insurance company will often be used to investigate claims, and each claim investigation adds something to his experience of failures. At the same time, he must observe strict confidentiality. To quote from a quality assurance manual ... 

The design and construction methods of the caisson breakwater have developed rapidly in the latter half of the 20th century through many bitter experiences of failures in Japan. Especially, the design technique for the caisson breakwaters is nearly established including the calculation method for determining the wave forces acting on the breakwater. 

The work of the supervisor or inspector is sometimes boring. The monotony of the checking process leads to lack of thoroughness. He is deprived of the best teacher, i.e. experience of failure. 

Field experience has revealed that one of the major causes of failure of an HT motor is weak insulation, caused by environmental pollution and ageing. 

The causes of low pressure, for example, could be cither hydraulic or mechanical. In many cases of failure analysis, asking Wliy. and Wliat and answering those questions, until you can no longer ask why , will almost always get you to the answer. If all evidence leads to a mechanical reason for the failure, the problem is probably maintenance induced. If the evidence leads to a hydraulic reason for the failure, the problem is eitJier operations or design induced. In cases where the reason for failure was not determined, a more extensive analysis is necessary. The additional analysis is recommended to take advantage of the pump supplier experience in identifying the root cause. 

We have already seen elements of the CA approach when considering the costs due to safety critical failures. A further insight into the way that failure costs can be estimated for non-safety critical failures is also used to support the CA methodology. Estimates for the costs of failure in this category are based on the experiences of a sample of industrial businesses and published material as follows. 

The experiences of industrial collaborators and surveys of UK businesses suggest that failure costs are the main obstacle to reducing the costs of quality. There is, therefore, a need for design methods and guidelines to give businesses the foresight... 

The frequency of an initiating event is usually based on industrial experience. If the process is new or rare, it may be estimated by a system model of the process steps (e.g., a fault tree) and using data from similar experience to give the probability of failure of the steps. Either of these estimates should consider the possibility of mitigating actions to prevent the hazard from having detrimental effects. 

Potential effects of failure in terms of what the customer might notice or experience... 

Emergency Diesel Generator Operating Experience Nuclear 600 occurrences of failure from event reports and questionnaires Diesel generators 95. 

Other reports used within facilities record failures of particular interest because of failure mode and system or equipment affected. Some facilities may issue special reports when the plant experiences a shutdown (outage report) or when the occurrence is sufficiently unique or troublesome to warrant further investigation (unusual event report). In general, these reports can be characterized by their relatively restrictive focus (when compared to the maintenance records) and their smaller number. 

The most desirable source of equipment reliability data for a CPQRA is the operating experience of the process and plant being studied. Therefore, a chapter of this book provides information that will help an engineer locate raw plant reliability data and convert them to failure rates. However, the quality and confidenee level of the plant-specific data may be questionable because of operating and maintenance procedures, short relevant operating experience, and limited pieces of equipment available for study. The best data to use in a CPQRA are often a combination of generic and plant-specific data. 

Performance Properties. Results of years of field experience have revealed that to reduce the risk of failure, the minimum yield strength should be used instead of average yield strength to determine the performance properties of casing. 

Experience shows that some machines have more frequent failures than do others. Obviously, different failure modes have different frequencies of occurrence. This is usually described as mean-time-between-failure (MTBF) and expresses the probability of machinery failure and breakdown events as a function of time. This is of particular interest to the maintenance failure analyst and troubleshooter who have to grapple with the realization that some machinery failure modes appear slowly and predictably whilst others occur randomly and unpredictably. In most cases, both types of failures have been encountered. 

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