Systems failure

The resulting fault tree is shown in Figure 6, in which the top event is defined in terms of two intermediate events failure of the tank system or failure of the pumping system. Failure in either system would contribute to the overall system failure. The intermediate events are then further defined in terms of basic events. All of the basic events are related by AND gates because the overall system failure requires the failure of all of the individual components. Failures of the tanks and pumps are basic events because, without additional information, these events cannot be resolved any further. 

Fig. 10. Reliability and failure probability computations for components ia (a) series linkages where the failure of either component adds to the total system failure, and (b) parallel linkages where failure of the system requires the failure of both components. There is no convenient way to combine the failure rate...

C. B. Boyer, HotIsostatie Pressure Systems Failures andMeeidentHistory, BatteUe Memorial Institute, Columbus, Ohio, 1987. 

On complex systems, which are repaired as they fail and placed back in service, the time between system failures can be reasonably well modeled by the exponential distribution (14,15). 

Example 5. There are six dynamometers available for engine testing. The test duration is set at 200 h which is assumed to be equivalent to 20,000 km of customer use. Failed engines are removed from testing for analysis and replaced. The objective of the test is to analy2e the emission-control system. Failure is defined as the time at which certain emission levels are exceeded. 

In solid drumming Check liner position and integrity before filling systems, failure of liner allowing powder to blow out of the container. CCPS G-3 CCPS G-22 CCPS G-29... 

For open filters, or when opening closed filters, solvent is flammable and may be above flash point with air present. For open filters, vent system failure may increase solvent vapor concentration, resulting in a fire or explosion. 

Same sensor used for basic process control system and safety instrumented system. Failure of sensor leads to loss of control system and safety system functionality. 

There are a variety of ways to express absolute QRA results. Absolute frequency results are estimates of the statistical likelihood of an accident occurring. Table 3 contains examples of typical statements of absolute frequency estimates. These estimates for complex system failures are usually synthesized using basic equipment failure and operator error data. Depending upon the availability, specificity, and quality of failure data, the estimates may have considerable statistical uncertainty (e.g., factors of 10 or more because of uncertainties in the input data alone). When reporting single-point estimates or best estimates of the expected frequency of rare events (i.e., events not expected to occur within the operating life of a plant), analysts sometimes provide a measure of the sensitivity of the results arising from data uncertainties. 

The probability a specific component (or collection of components) is contributing to a system failure, given that the system is failed... 

Bignell, V. and Fortune, J. 1992 Understanding Systems Failures. Manchester Manchester University Press. 

Any graceful degradation features and repair philosophy after system failure. 

Boolean equations show how component failures can fail a system. A minimal cut. he smallest combination of component failures that can fail a system. It is the set of non-sup us components, such as in the previous example, with the superfluous combination Y Z(X Y,. Z) e uded. If they all occurred they would cause the top event to occur. One-component minimtil cut s( if there are any, are single failures that cause system failure. Two-component minimal cutsets ai tairs of components, if they occur together cause system failure. Triple-components minimal Cl sts are sets of three components that, if they fail together cause system failure, and so on to hi er cutsets... 

The disadvantage of Laplacian probability is its use is limited to calculating the probability of processes for which all outcomes are known and equally probable. This eliminates the use of Laplacian probability for determining the probability of process system failure. 

The Boolean equation for the probability of a chemical process system failure is R = A (B-i-C (D-rE (B-l-F G+C). Using Table 2.1-1, factor (he equation into a sum of products to get the mincut representation with each of the products representing an accident sequence. 

The objective is to estimate, numerically, the probability that a system composed of many components will fail. The obvious question is, "Why don t you just estimate the failure rate of the system from operating experience " There are three reasons IJ the system may not exist, so new data are not available, 2) the injuries and fatalities from the developmental learning experience are unacceptable - the risk must be known ahead of time, and 3) by designing redundancy, the probability of the system failing can be made acceptably remote in which case system failure data caimot be collected directly. The only practical way uses part failure statistics in a system model to estimate the system s reliability. 

Initially, a system s hierarchy is identified for subsystems, sub-subsystems and so on to the components for which data must be found. The top event specifies system failure subsystems required for operation of the system in the mode specified are input to the top event s OR gate. Redundancy is represented by the redundant systems inputting an AND gate. This process of grouping subsystems under OR gates, if they can individually fail a function, or under AND gates if concurrent failures are necessary, is continued to the component or support system level until the tree is completed. This process grades the hierarchy from top to bottom, down the fault tree. 

Function event trees include primarily the engineered safety features of the plant, but other systems provide necessary support functions. For example, electric power system failure amid reduce the effectiveness of the RCS heat-removal function after a transient or small UJ( A. Therefore, EP should be included among the systems that perform this safety function. Siipfiort systems such as component-cooling water and electric power do not perform safety functions directly. However, they significantly contribute to the unavailability of a system or group of systems that perform safety functions. It is necessary, therefore, to identify support systems for each frontline ssstcm and include them in the system analysis. 

USC may be modeled as a power-series expansion of non-CCF component failure nates. No a priori physical information is introduced, so the methods are ultimately dependent on the accuracy of data to support such an expansion. A fundamental problem with this method is that if the system failure rate were known such as is required for the fitting process then it would not be neces.sary to construct a model. In practice information on common cause coupling in systems cannot be determined directly. NUREG/CR-2300 calls this "Type 3" CCF. 

The overall system failure rate including common modes was estimated to be the geometric mean of these extremes (equation... 

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