Redundancy fail-safe

MATERIALS HANDLING Controls Ranges, Redundancy, Fail-Safe... 

The transfer of material to or from one of these units is only performed when it can safely be done. A redundant fail safe instrumentation detects any increase of the fission rate should this kind of accident occur. Special alarm systems and escape ways are carefully designed. 

Tolerate the Hazard. The design needs to be fault tolerant. That means, in the presence of a hardware/software fault, the software still provides continuous correct execution. Consider hazard conditions to software logic created by equipment wear and tear, or unexpected failures. Consider alternate approaches to minimize risk from hazards that cannot be eliminated. Such approaches include interlocks, redundancy, fail-safe design, system protection, and procedures. 

Reactivity control a) Reactor regulatmg system b) Shutoff rods (28) c) Poison mjecnon d) Loss of DjO moderator Active Active/Passive Passive Passive power changes-set back and trip I Fail safe principle, redundancy Fail-safe prmciple, redundancy Loss of D20 moderator leads to subcrihcality ... 

Most HWRs have two, independent, diverse, reliable, testable, redundant, fail-safe SDSs (as well as the control system). The two systems do not share instrumentation, logic actuation devices, or in-core components. One system uses rods, the other liquid poison injection. Each of the SDSs is effective, by itself, for all design basis accidents. With each one demonstrated by on-power testing to a reliability of 999 times out of 1000 attempts, the risk of a transient or accident occurring without shutdown is negligible. 

Entry 3.6 again demonstrates how human error can lead to a catastrophic accident Of course, the best solution is to design the hazard out. In this case, you cannot take away the potential but have added numerous levels of redundant fail-safe mechanisms to prevent the mishap from occurring. Note that the ready tank already has a liquid-level indicator, but if it is only tied back to an alarm, it does not guarantee that the hazard will be prevented. It only means that the operator will be warned. The overfill hazard is too significant to be left to an alarm indication alone. 

Alternative approaches to minimise risk from hazards that cannot be eliminated must be considered. Such approaches include interlocks, redundancy, fail-safe design, system protection, fire suppression and protechve clothing, equipment,... 

The basic approach is to direct the system to the safest operating level relative to people or the environment when any emergency condition is detected, including power loss. An important concept of process control safety is to have adequate redundancy to reduce unwanted shutdowns and maintain an adequate level of certainty that a safe state will result if a real emergency does occur. As far as possible, instruments should be of the fail-safe type. 

Preferentially, instrumentation should exist to indicate, control, and alarm COPs at the N-E-L. Controls should be established to assure that these requirements can be met if the instrumentation fails, Consideration should be given to fail safe and/or redundant instrumentation. On-stream analyzers should be considered for COPs measured by laboratory analyses. 

Ranges, redundancy and fail-safe Frequency, adequacy... 

Installed spare capacity, including fail-safe redundancy features Future expansion with any imphcations for reduced performance Plant external I/O signals (type, format, range, accuracy, timing)... 

Multiple use of sensors does not necessarily mean that fewer sensor elements are required, because overall sensor performance has to be improved at the same time. The strong demand for fail-safe sensors in safety-related applications will lead to systems using additional sensor signals for increased reliability and based on partial redundancy or on systems to crosscheck the plausibility of the individual signals. Another way of increasing a sensor s accuracy is by using information from multiple sensors. 

Strand, F.L., Segarra, A.C., Zuccarelli, L.A., Kume, J. and Rose, K.J. (1990b) Neuropeptides fail-safe, redundant or special . In D. Gupta, H.a> Wollman and M.B. Ranke (Eds.), Neuroendocrinology New Frontiers, Brain Research Promotion, London, Tubingen, pp. 19-28. 

Although redundant measurements and equipment such as fail-safe devices and the like are often mandatory, accidents still happen. The 2010 Macondo... 

Single or multiple barriers Redundancy Diversity Fail-safe design Fire-safe design... 

Accident safe design with special equipment, double containment, fire protection, alarms, shutdown Special provisions, fail safe, fire safe redundancy, diversity Marking, labeling Experience, training... 

Specific health problems associated with the workplace have contributed to (he development of the modem safety and health movement. These problems include lung diseases in miners, can-c s caused by contact with various industrial chemicals, and lung cancer tied to asbestos. Widely used accident prevention techniques include failure minimization, fail-safe designs, isolation, lockouts, screening, personal protective equipment, redundancy, and timed replacements. 

Actual failures of instruments can be classified as "fail-safe," "fail-danger," or another failure mode. Such failure modes will be defined in this chapter in the context of an individual instrument. Note that sometimes the application must be understood before these classifications can be made. It must be remembered that the safety instrumented function may or may not fail when one instrument has failed. A redundant architecture may compensate for instrument failures. 

Most practitioners define "Fail-Safe" for an instrument as a failure that causes a "false or spurious" trip of a safety instrumented function unless that trip is prevented by the architecture of the safety instrumented function. Many formal definitions have been attempted that include "a failure which causes the system to go to a safe state or increases the probability of going to a safe state." This definition is useful at the system level and includes many cases where redundant architectures are used. 

If protective measures are implemented through devices with fail-safe -behaviour (a failure in the safe direction, e.g. opening of a safety discharge valve on failure of compressed air) or a self-announcing device (including the measures to be taken, when the failure is announced), frequently the redundancy is not implemented. 

In summary, systems which are susceptible to failure, especially those left unattended while in operation should include fail-safe precautions with redundant safeguards should a primary safety feature fail. 

The acceptance of a qualitative or quantitative failure probability declaration is often based on the assumption that failures are independent (AMC25.1309). Independency is often accomplished in duplication of systems/components. Redundancy, and the independence thereof, is a key feature in fail safe designs of system requiting high level of functional integrity. 

No discussion of crew responses to system failure is complete without considering the Fail Safe design concept. A system is fail safe if, in the event of a failure, the system or component automatically reverts to one of a small set of states known to be safe and thereafter operates in a highly restricted mode. This may involve complete loss of functionality, or reverting to back-up/redundant features. See AMC25.1309 (Amend 17, para 6b) and Kritzinger (2006, Chapter 7) for more information on this topic. 

Considering the relative ease with which prototypes can be developed on the microscale, custom EC electrode cOTifigmations mated perfectly to the CE separatitm platform are possible. For example, devices with multiple separation channels may be outfitted with multiple EC cells, each identical to the other to establish redundancy for fail-safe detection. Each channel could run simultaneously, and statistical calculations (running averages, standard deviations, etc.) could be used to enhance both the reliability of detection and the robustness of the detection system. The development of such advanced LOC instruments is expected to be central to their practical utilization in applications involving remote and unattended sensing. 

Sinale Failure Criterion Graded approach is dependent on SSC classification and operational mode (passive vs. active). Any of the following Fail safe. Redundancy or diversity. Separation or isolation (including interfaces and boundaries). Evaluate for common failure modes, independence, and reliability. Backfit only if risk/cost effective. 

However the ratio is not known and we wfll consider the worst case to design of the Markov model. It means the case of standby redundancy with fully loaded reserve (with fail-safe switch). We can calculate from the formula above that mean time to failure for main filament is 400 hours and for reserve filament 400 hours as well MTTFp = 400 hours). 

The latches are also of fail-safe design, because loss of eiectri cal power or burnout of the latch solenoids will permit the rods to drop. The IXX safety circuit utilizes two sets of closed contacts in series from the primary sensing elements such as power relays, Beckmans, Paneilit, etc. The opening of any one contact will de-energize the associated IXX or IXXA relay and scram the VSR s, This is a form of parallel redundancy. In this case, fail safe operation, i,e rod insertion, will occur for almost any circuit contact or relay malfunction. 

The balls are held in the hoppers by gates actuated by two independently powered solenoid latches. Each of these solenoids is on a different d-c power supply, De-energization of both solenoids is required to drop the balls. This can be classed as a parallel fail-safe redundant system two components operate in parallel, and the failure of a single power supply or solenoid will not permit the balls to drop. Failsafe in this instance has a somewhat different meaning than for the VSR system where a similar failure would result in rod insertion. An inadvertent ball drop, howevei presents... 

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