Redundant system

In usiag these failure rates an exponential distribution for time to failure was assumed. Such an assumption should be made with caution. Parallel Systems. A parallel (or redundant) system is not considered to be ia a failed state unless all subsystems have failed. The system rehabihty is calculated as... 

System rehabihty is improved by providing alternative means for performing the same task. For example, automobiles were equipped with hand cranks even though they had electric starters. This back-up equipment was provided because at that time starters were unrehable. In contemporary system design, factors such as added cost, weight, and space may prohibit the use of redundant systems. 

Special high rehabiUty systems have been developed for safety shutdowns, eg, triple modular redundant systems (TMRS), which are designed to be... 

Fig. 5. Stmcture of a triple modular redundant system (TMRS). See text.

Another difficulty is assessing the potential for human errors. If redundancy is accompanied with increased complexity, the resulting increased potential for human errors must be taken into consideration. Redundant systems require maintenance procedures that can correct problems in one part of the system while the remainder of the system is in full operation. When conducting maintenance in such situations, the consequences of human errors can be rather unpleasant. 

Odor control by the addition of dilution air involves a problem associated with the breakdown of the dilution system. If a dilution fan, motor, or control system fails, the odorous material will be released to the atmosphere. If the odor is objectionable, complaints will be noted immediately. Good operation and maintenance of the dilution system becomes an absolute requirement, and redundant systems should be considered. 

Gas compressors operating on highly toxic or flammable gases may require redundant systems to assure no leakages. In many applications, such as refrigeration gas, buffer seals are required with the liquid-buffered face seal. A popular technique is to use a buffered labyrinth seal with a liquid seal. 

The proteetive system is independent of the eontrol system and provides proteetion from over-speed, over-temperature, vibration, loss of flame, and loss of lubrieation. The over-speed proteetion system generally has a trans-dueer mounted on the aeeessory gear or shaft, and trips the gas turbine at approximately 10% of maximum design speed. The over-temperature system has thermoeouples similar to the normal temperature eontrols with a similar redundant system. The flame deteetion system eonsists of at least two ultraviolet flame deteetors to sense a flame in the eombustion eans. 

Diversity. Diversity is applied to redundant systems or components that... 

The other caution relates to redundant systems. Figure 3.4.1-1 shows very simple auiling swicm.s that pump... 

I f System B with the redundant systems, the probabilities are multi >. 1E-... 

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. 

KDC has a cause and effect relationship between as the primary cause leading to secondary failures. Besides its drastic operational effects on redundant systems, the numerical etlects that reduce sy.stem reliability are pronounced Equation 2.4-5 shows that the probability ut failing a redundant. system composed of n components is the component probability raised to the n-th power. If a common clement couples the subsystems. Equation 2.4-5 is not correct and the failure rate is the failure rate of the common element. KDC is very serious because the time from primary failure to secondary failures may be too short to mitigate. The PSA Procedures Guide (NUREG,/CR-2.3(X)) cl.issities this type as "Type 2."... 

The probability of the redundant system failing is the product of the single system time-dependent unavailability which includes approximately linearity between test and repair. There are two ways of performing the surveillance tests on these two systems sequential and staggered testing. 

The binomial distribution can be used to calculate tlie reliability of a redundant syslein. A redundant system consisting of n identical components is a system tliat fails only if more tlian r components fail. Familiar examples include single-usage equipment such as missile engines, sliort-life batteries, and flash bulbs, which are required to operate for one time period and are not reused. Associate "success with tlie failure of a component. Assume tliat tlie n components are independent witli respect to failure, and tliat tlie reliability of each is 1 - p. Then X, tlie number of failures, has tlie binomial pdf of Eq. (20.5.2) and the reliability of the redundtuit system is... 

For example, consider tlie case of a standby redundancy system consisting of one operating pump and two on standby so that tlie system can survive two failures. Assume tlutt tlie pumps are independent with respect to failure and tliat each lias a probability of failure of 0.1. Thus, n = 3, r = 2, 1 - p = 0.9, and tlie reliability of the system is found from Eq. (20.5.3)... 

In addition to tlie applications cited above, tlie Poisson distribution can be used to obtain tlie reliability of a standby redundancy system, in which one unit is in tlie operating mode and n identical units are in standby mode. Unlike a... 

Tliis is tlie probability if n or fewer failures in the time period specified by t. For example, consider tlie case of a standby redundancy system with one operating unit and one on standby (i.e., a system tliat can survive one failure). If the failure rate is 2 units per year, then tlie 6-nionth reliability of the system is... 

Class 111 or serious machinery include major plant equipment that do not have a dramatic impact on production but that contribute to maintenance costs. An example of the serious classification would be a redundant system. Since the inline spare could maintain production, loss of one component would not affect production. However, the failure would have a direct impact on maintenance cost. 

Reliability is a function the system failure rate or its reciprocal, mean time between failures (MTBF). The system failure rate in non-redundant systems is numerically equal to the sum of component failure rates. 

A traditional approach to fault diagnosis in the wider application context is based on hardware i.e. physical) redundancy methods which use multiple lines of sensors, actuators, computers and software to measure and/or control a particular variable. Typically, a voting scheme is applied to the hardware redundant system to decide if and when a fault has occurred and its likely location amongst redundant system components. The use of multiple redundancy in this way is common, for example with digital fly-by-wire flight control... 

Early approaches to fault diagnosis were often based on the so-called physical redundancy [11], i.e., the duplication of sensors, actuators, computers, and softwares to measure and/or control a variable. Typically, a voting scheme is applied to the redundant system to detect and isolate a fault. The physical redundant methods are very reliable, but they need extra equipment and extra maintenance costs. Thus, in the last years, researchers focused their attention on techniques not requiring extra equipment. These techniques can be classified into two general categories, model-free data-driven approaches and model-based approaches. 

In the event of a disaster, dedicated redundant systems at a separate locality which must be far enough distant not to have been affected by the disaster are brought on-line. Users are either relocated to the backup facility or are provided remote access to the backup system via some sort... 

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