Failure occurrence

We can demonstrate the notions of risk and risk assessment using Figure 1.18. For a given probability of failure occurrence and severity of consequence, it is possible to map the general relationship of risk and what this means in terms of the action required to eliminate the risk. 

Some companies make piping service evaluations. These evaluations are used to categorize piping systems and determine the initial piping inspection intervals and degree of inspections for critical piping systems. A team determines which inspection tasks are needed to lower the likelihood of failure occurrence. A matrix is established to determine the type of inspection and the interval between inspections. This service evaluation is beyond the scope of this chapter. 

In our example, severity scores range from a moderate 5 to the highest severity of 10 because the customer could get injured by some failures. Occurrence ratings range from 3 to 7, predicting that some causes would occur very infrequently, while others could become a chronic issue. 

Clinically, there are three different courses of disease following the onset of jaundice .) fulminant or hyperacute liver failure (= occurrence of hepatic encephalopathy in the 1 week), (2.) acute liver failure (= occurrence of hepatic encephalopathy between the and 4 week), and (5.) subacute liver failure (= occurrence of hepatic encephalopathy between the 5 and 8 week). Surprisingly, however, it could be shown that 30-40% of the hyperacute forms survived in spite of the development of hepatic coma and cerebral oedema. As opposed to this, the subacute forms displayed a survival rate of only 10-20%, despite a lower frequency of cerebral oedema and better liver function, (s. tab. 20.1)... 

Sometimes there is a need to define the availability with respect to operating time and corrective maintenance when the system is operating in an ideal support environment. This form of availability, called inherent availability (A,), is useful for determining certain figures of merit for the system per se, such as frequency and type of failure occurrence, repairability (active repair time), and analysis of maintenance actions. Thus, A, is given by... 

Collection consistency. Data shall be collected and reported in the same way all the time, for example, the time for failure occurrence has to be reported with enough accuracy. 

As the reliability is related to the usage, it cannot be measured directly. Instead, it must be calculated from other measurements on the software. A measure often used is the failure occurrence, or more precisely the time between them, of the software which is related to the usage of the software. 

The shape factor a, which is the density, distribution and failure rate function, and represents specific regularity failure occurrence, in function with the Peta parameter and acquires three different shapes P <1, P = 1, P> 1, which defines the "bathtub curve" as shown in figure 4. 

Step lb is therefore based on the application of a lessons learned checklist, an example of which is contained in Table 6.1. Its purpose is to identify potential sources of systemic errors (which could lead to systematic failures). It considers the possibility of requirement, design and implementation errors (as weU as reasonably anticipated crew errors after the failure occurrence of a failure condition). The checklist may be applied to consider system architecture vulnerabilities as well as physical installation vnlnerabilities in the ZSA (see Chapter 8). 

Random fadures occur as a result of physical causes typically traceable to the usage of the system. The metrics used to estimate the random failure occurrence and to decide their acceptability is usually based on service experience (e.g., failure rates), predictive probabilities calculations (e.g., piece part FMEA, MTBF evaluation, etc.) and accelerated life testing run by manufacturers ahead of service experience. 

The design of equipment (including the monitoring/feedback loop to its operators) procedures (e.g. how to deal with failure occurrence) and training (e.g. ensure timely intervention) has a major influence on the likelihood of an accident due to operator inability to cope with developing situation (such as too fast or intermittent flow of information, or out of reach/view location of controls). 

An example is represented by the node System prognosis . Potentially, the node can represent the remaining life second by second but in this application the node has only four states. One of them is the state OK that means that the chiller operates in a correct way. The other three are some time intervals that represent a discretization of the remaining useful life before the failure occurrence. The interval choice has been made from the maintenance point of view considering the restriction of the test bed unit. 

One of the major obstacles in applying simulation methods for the reliabihty analysis of engineered systems and structures is the challenge posed by the estimation of small failure probabdities the simulation of the rare events of failure occurrence implies a significant computational burden (Schueller 2007). 

The non-compliance with the manufacturing process itself, thereby shortening the programming time, might not be a serious problem. More related circumstances might be the real problem. The first one is the fact that the items have been mounted in systems and they have been in operation. Another quite serious problem is the fact that an item function failure can result in failure occurrence on the device which is supposed to perform a system s step function. The failure of this kind might result in an accident. Moreover, it breaks the confidence in the step function which leads to the lack of confidence in a system as a whole (Finn 1998). 

NOTE A test plan type for an item accelerated test is called the [k R tq] type when failed items after failure occurrence are replaced (see Fig. 1). Replacing failed items after failure occurrence is the main principle of the test. The principle of the test acceleration is represented by increased condition effect - heat in our case. This is due to simulation of the presumably real condition of the item operation and its behaviour... 

NOTE Following the standard it is assumed that for electronic items the calculation based on an exponential probability model of failure occurrence might be used. 

The results of module functionality checks will be recorded during the test using computer technology. If the module auto check shows deviation from the required state, the module will be considered to be in fault and a failure detection time is viewed as a performance time during which the module functionality was checked and deviation from the required state was recorded. An identified failure category is also recorded. After failure occurrence a module is removed from the test and replaced by a new one. 

We apphed this method on three lots of semiconductor device having adequate dimensions, and the time of failure occurrences is observed. 

Ammunition fulfils its function only at a shot. The lasting of a shot is very short in order of several milhseconds. That is why we use a failure occurrence probability as an indicator for safety risks. The ammimition belongs to the gronp of one-shot weapon systems (Vintr Vahs 2007). 

The quantification of measures of failure occurrence probability Qi(t) - Qiv(t) includes only those failures that belong to the given failure category. 

Failures of category II must also be extremely improbable, but the probability of failure occurrence is in one order lower than in the category III. That is why Qn must be lower than 10 during the whole period of ammunition use or in the period of its retirement (disposal). 

Failures of category I having the most serious consequences for persons, assets and environment must be extremely improbable. The probability of failure occurrence Qi must be lower than 1.10 in all climatic (weather), mechanical and electrical environments, in all defined regimes of use (storing, transport, manipulation and shooting) as well as in the defined way of disposal (liquidation). This definition of the requirement for safety risk tolerates ammunition failure resulting in user s death not more than once in 100 milliard of shots. 

Failures of category IV are without an impact on the safety, and so they can be probable on the level that is acceptable for the user. From the point of view of safety, ie probability of failure occurrence Qjv can be higher than 10 during the whole period of ammunition use retirement (disposal). See Fig. 4. 

The mission model is created in three steps. The vehicle operation model is designed in the first step. The input variables are the probability of failure occurrence and the probability of failure repair with utilization of maintenance resources. The maintenance model is... 

With the assumption that failure occurrence probability and time to repair probability have exponential distribution and maintenance costs probabihty has normal distribution (Sirok Neugebauer, 2005), then equation (17) transforms to form ... 

To start the cascade, an initial disturbance imposes on each component an additional load D. If the sum of the initial load Lj of component j and the disturbance D is larger than a component load thresholdi/ ", component j fails. This failure occurrence leads to the redistribution of an additional load P on the neighboring component which may, in turn, get overloaded and fail within a cascade which follows the connection pattern 0 f the network system. As the components become progressively more loaded, the cascade continues. 

Failure occurrences for different demands are statistically independent, i.e. the failure probability of one demand does not affect the failure probabilities of other demands. 

The failure propagation in Eq. (1) is computed based on the knowledge of the probabilities of input error causing a failure / (El.) in component Cj. System reliability in Eq. (2) is also computed based on the knowledge of the failure occurrence probabilities of system components. Our model assumes the availability of these parameters. However, several data change probability models [1,14,29] and component reliability models [8,6,13] can be adopted to estimate these parameters for the individual components. 

The probability of the system failure occurrence by time t will then be... 

Equipment failure—occurrence when equipment has broken, ruptured, or is no longer responding to its design specifications. 

As already discussed in Chapter I in connection with risk matrix, in qualitative analysis, likelihood is estimated and categorized based on experience and judgment applicable for the project. Also these risk categorizations may he done on a quantitative basis as already discussed (say once in a year, etc.). In quantitative analysis the same is done based on previous records or a failure database for which quantitative PHA may be helpful. Failure occurrence data from other plants within or outside the company could be a good source of data. 

The probability of component-failure occurrence is largely determined by means of deterministic stress analyses which supply detailed information of the damage mechanisms of a specific component and their significance for the component s overall failure behavior. Thus the findings obtained in stress analysis and the results of fault-tree analysis allow knowledge-based determination of the optimum method to safeguard the function of a component. 

Faults for the simulation of a faulty circuit (faulty machine) can be introduced as automatic fault generation or by presetting each single or multiple fault individually. The failure occurrence times can be chosen at will, that is general all-including and special simulation runs are possible. 

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