Mechanical failure probability

The event or scries of events that will initiate an accident has to be identified. An event could be a failure to follow correct safety procedures, improperly repaired equipment, or a safety mechanism The probability tliat tlie accident will occur lias to be determined. For e.xample, if a chemical plant has a 10-year life, wliat is the probability that... 

As von Nimitz points out, only cyclic stresses are directly related to failure probability. These stresses are often produced by pulsations in the fluid system, by mechanical vibrations produced by the mechanical movement of certain equipment components, and as a result of the fluid pulsations. Figure 13-8 lists the sequence of events that leads to most failures of equipment and piping. 

Failure and incident databases are available that provide generic or industry-wide in-service failure rates and on-demand failure probabilities for various components. These can be used for assessing risks of new facilities with no operating experience, as well as for estimating the likelihood of rare events such as vessel mechanical failures at operating conditions and for fully quantitative risk studies. 

During the process operation, water was introduced into the blender, probably as a result of a mechanical failure. Operators noticed the production of heat and the release of foul-smelling gas. During an emergency operation to offload the blender of its reacting contents, the material ignited and a deflagration occurred. The most likely cause of this incident was the inadvertent introduction of water into water-reactive materials (USEPA-OSHA, 1997). 

The significance of ammonia in clinical disease was broadened markedly when it was discovered that ammonia poisoning might be the mechanism of the cerebral symptoms associated with chronic heart failure. It has been known for a long time that the mental symptoms associated with heart failure could not be correlated with the oxygen supply to the brain, which remains, in most cases, adequate. The work of A. N. Bessman (B4) demonstrated that the ammonia content of the blood was elevated in heart failure, probably due to the chronic passive congestion of the liver which prevented the liver from removing the normally formed ammonia from the portal system. When the blood ammonia fell, the mental symptoms of heart failure were relieved. This has been confirmed by Calkins and Delph (Cl), who studied twenty-six cases of heart failure and found the blood ammonia to be elevated only in the two patients who had mental symptoms. 

The previous tables summarise criteria and parameters for the failure probabilities calculation, beginning with a list of failure probability model definitions in Table 1, followed by specific data on mechanical equipment in Table 2, and ending with some unavailability reported values in Table 3. 

This crude distinction between adhesive and cohesive wear mechanisms is probably oversimplified in the sense that it neglects many aspects of the interactions between bulk deformation modes and interface rheology. It has, however, the merit of making a clear distinction between wear processes which can, to some extent, be related to known bulk failure properties and... 

Besides its usual indications, topical glyceryl trinitrate has been used to prevent intravenous infusion failure because of phlebitis and extravasation, which occur in 30-60% of patients and can cause discomfort and harm, such as pulmonary embolism, septicemia, and increased mortality. Local application of glyceryl trinitrate patches was efficacious in several studies (11) the mechanism is probably vasodilatation and increased capillary flow. Glyceryl trinitrate is safe, since it causes only skin rashes and transient headache. 

The mechanical performance of femoral heads and acetabular cups made from alumina have been the subject of intense research and development effort as it is crucial for the longevity of the endoprosthetic hip implant. Hence, the failure probability of these ceramic construction parts has been investigated and expressed by the Weibull probability density function. As safe design of... 

The Weibull distribution is the state of the art statistics in the mechanical design process of ceramic components [1 - 3]. Strength testing and data evaluation are standardised. A sample of at least 30 specimens has to be tested. The range of measured failure probabilities increases with the sample size [3, 13] and is - for a sample of 30 specimens - very limited (it is between 1/60 and 59/60). To determine the design stress, the measured data have to be extrapolated with respect to the volume and to the tolerated failure probability. This often results in a very large extrapolation span [3]. 

Au, S. K. and Beck, J. L. Estimation of small failure probabilities in high dimensions by subset simulation. Probabilistic Engineering Mechanics 16(4) (2001), 263—277. 

It is also possible to carry out a probabilistic analysis for subsea pipelines in relation to the requirements of fitness for service or life extension. This approach is supported by Hopkins et al. (2001) where the uncertainty values in relation to the inspection data, material strength, and so forth are accounted for The statistics of the input parameters and the engineering models then determine, by probabilistic analysis, the failure probability for each failure mode or mechanism and the variation of the failure probability over time. Once the uncertainty of each input value has been described statistically, a Monte Carlo simulation can be used to predict the growth rate of known defects over time. 

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