Revealed failures

Besides occasionally sharing their success stories with mentees, mentors might also want to reveal failures and mistakes they have made. This provides mentees with a more realistic view of certain business situations. They are then in a better position to appreciate the problems they might encounter whilst pursuing a goal. 

Example 11-2 assumes that all failures in either the alarm or the shutdown system are immediately obvious to the operator and are fixed in a negligible amount of time. Emergency alarms and shutdown systems are used only when a dangerous situation occurs. It is possible for the equipment to fail without the operator being aware of the situation. This is called an unrevealed failure. Without regular and reliable equipment testing, alarm and emergency systems can fail without notice. Failures that are immediately obvious are called revealed failures. 

Figure 11-6 shows the nomenclature for revealed failures. The time that the component is operational is called the period of operation and is denoted by r0. After a failure occurs, a period of time, called the period of inactivity or downtime (xr), is required to repair the component. The MTBF is the sum of the period of operation and the downtime, as shown. 

Figure 11-6 Component cycles for revealed failures. A failure requires a period of time for repair.

For revealed failures the period of inactivity or downtime for a particular component is computed by averaging the inactive period for a number of failures ... 

By combining Equations 11-16 and 11-17 with the result of Equation 11-14, we can write the equations for the availability and unavailability for revealed failures ... 

Potash Mine Explosion Revealed (Failure to carry away combustible gas-air mixtures by ventilation before they accumulate in explosive quantities), C EN, Oct 14, 1963 16) Anon, "Solvent-Oxidizer Mixtures are Explosives (Violent and unexplained reaction between nitrogen tetroxide and many halogenated hydrocarbons), C EN,... 

If any batch or batches of supplied product(s) are documented by the procurement agency not to be in compliance with the agreed specifications of the product or to reveal failure(s) regarding safety, performance or quality of the device. 

Typical stress-strain curves obtained for the minicomposites are shown on figures 3 and 4. The minicomposites reinforced with Hi-Nicalon S fibers exhibit conventional composite behavior that has been previously observed on SiC/SiC CVI minicomposites reinforced with Nicalon or Hi-Nicalon fibers [6, 7], The forcc-deformation curves arc markedly non linear, and they display the following typical features that reveal failures ... 

Biswas et al. 2003) analyze the availability of a periodically tested system that undergoes a fixed number of imperfect repairs before a perfect repair is carried out. (Badia and Berrade 2006a) and (Badia and Berrade 2006b) study the cost function and analyze the optimum maintenance of a system subject to both revealed and unrevealed failures. In the former case a perfect restoration follows the A unrevealed failure whereas the maintenance policy includes a perfect repair after the A revealed failure in the latter. 

Figure 1. Finite state diagram and petri net representations of a simple repairable (non-) revealed failure mode.

The new approach is applicable for low demand SIFs, which are the most common SIFs in the od and gas industry. A low demand SIF is passive during normal operation and is intended to respond if a specific hazardous event occurs. In passive mode, limited information is available about the system state. Regular functional testing is therefore important to reveal failures that otherwise would be hidden until the next demand. The approach presented in this paper suggests criteria for how to adjust the functional test intervals, based on operational experience. 

Dynamic tests to verify the dynamic behavior under realistic conditions and reveal failures to meet the SRECS functional specification. 

Functional testing to reveal failures during the specification, design and integration phases, and to avoid failures during validation of SRECS software and hardware. This shall include verification (e.g., by inspection or test) to assess whether the SRECS is protected against adverse environmental influences and shall be based upon the SRS. 

When a fault tolerant architecture is allowed to degrade to a less safe architecture rather than automatically tripping the process, repair in a timely fashion is essential to maintain the SIL of the SIF or SIS. In such an application, diagnostics are used to reveal failures that would otherwise be undetected. The mean time to repair (MTTR) used in the calculations should include all of the time necessary to restore the equipment to full operating health. This would include the diagnostic detection time, any troubleshooting time, and the repair time necessary to correct the failure and return the equipment to service. 

The details of any demands on the system, and system performance on demand, should be recorded including data on any spurious trips, any revealed failures of the system or its components and, in particular, any failures identified during proof testing. 

The principal activity of maintenance is proof testing to identify any dangerous un-revealed failures. See Proof testing in this appendix. 

For revealed failures the MDT consists of the active mean time to repair (MTTR) PLUS any logistic delays (e.g., travel, site access, spares procurement, administration). For unrevealed failures the MDT is related to the proof-test interval (T), PLUS the active MTTR, PLUS any logistic delays. The way in which failure is defined determines, to some extent, what is included in the down time. If the unavailability of a process is confined to failures while production is in progress then outage due to scheduled preventive maintenance is not included in the definition of failure. However, the definition of dormant failures of redundant units affects the overall unavailability (as calculated by the equations in the next Section). 

Tables 5.1 and 5.2 provide the failure rate and unavailability equations for simplex and parallel (redundant) identical subsystems for revealed failures having a mean down time of MDT. However, it is worth mentioning that, as with all redundant systems, the total system failure rate (or PFD) will be dominated by the effect of common cause failure dealt with later in this chapter.

Even if failures in operating are most often revealed, and failure on stand-by failures unrevealed, they are different concepts that should not be confused. A (too) fast reading of standards often leads to an assimilation of revealed failure s safe failure, which of course is true only after an action has been successfully undertaken... 

The revealed failures are shown in Figure 8.4 where the PFD(t) quickly converges to an asymptotic value equal to A,/p. 

The above results are typical of SIS including components tested periodically but the behavior is very different when the system contains only safe failures, revealed and repaired quickly. With the fault tree above but with revealed failures (A.SD = lO /hour, p= 0.1/hour) and a factor p of 5% we can find the results shown in Figure 8.18. 

Identical redundancy involves the use of elements identical in design, construction and in function with the objective to make the system more robust for self-revealing failures. Diverse redundancy uses non-identical elements and provides a greater degree of protection against the potential for common cause faults. It can apply to hardware as well as to software. 

Testimony before the Commission revealed failures in communication that resulted in a decision to launch 51-L based on incomplete and sometimes misleading information, a conflict between engineering data and management judgments, and a NASA management structure that permitted internal flight safety problems to bypass key shuttle managers. 

What elements of the social order should be restructured. The litany of social and work factors contributing to stress in the office were endless and diffuse. Every facet of workplace conditions was involved from environmental quality issues (temperature and overcrowding) to labor relations (lack of respect and no promotions) to job design (constant sitting and repetitive work). Stress was the expression of the dispersed quality of office oppression, and thus its presence revealed failures of the social order, not the worker. 

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