Mean time to repair

The mean time between failures MTBF is used as a measure of system reflabiUty, whereas the mean time to repair MTTR is taken as a measure for maintainabihty. Eor example, a system with an MTBF of 1200 h and a MTTR of 25 h would have an availabihty of 0.98. Furthermore, if only an MTBF of 800 h could be achieved, the same availabihty would be realized if the maintainabihty could be improved to the point where the MTTR was 16 h. Such trade-offs are illustrated in Figure 3, where each curve is at a constant availabihty. 

Fig. 3. System availability trade-off curves. MTBF = mean time between failures MTTR = mean time to repair.

Equation 2.5-43 is a definition of availability." Since 1/p = MTTR (mean time to repair) and 1/A, = MTTF (mean time to failure) A more conventional definition is given by 2.5-44. 

With the Industrial Revolution, life became more complex but it was not until World War II that reliability engineering was needed to keep the complex airplanes, tanks, vehicles and ships operating. Of particular concern was the reliability of radar. Prior to this time equipment was known qualitatively to be reliable or unreliable. To quantify reliability requires collecting statistics on part failures in order to calculate the mean time to failure and the mean time to repair. Since then, NASA and the military has included reliability specifications in procurements thereby sustaining the collection and evaluation of data build statistical accuracy although it adds to the cost. 

The GIDEP Reliability-maintainability Data Bank (RMDB) has failure rates, failure modes, replacement rates, mean time between failure (MTBF) and mean time to repair (MTTR) on components, equipment, subsystems and systems. The RMDB includes field experience data, laboratory accelerated life test data, reliability and maintainability demonstration test results. The... 

Accident progression scenarios are developed and modeled as event trees for each of these accident classes. System fault trees are developed to the component level for each branch point, and the plant response to the failure is identified. Generic subtrees are linked to the system fault trees. An example is "loss of clcciric power" which is analyzed in a Markov model that considers the frequencies of lo,sing normal power, the probabilities of failure of emergency power, and the mean times to repair parts of the electric power supply. 

Mulvihill, R. J. and Y. G. Mody. Failure Rate Distribution and Mean Time to Repair Estimates for Heat Exchanger and Condenser. SAN/RA50239, Pickard, Lowe and Garrick, Inc., November 1981. 

Predictive maintenance was shown to reduce the actual time required to repair or rebuild plant equipment. The average improvement in mean-time-to-repair, MTTR, was a reduction of 60 per cent. To determine the average improvement, actual repair times before the predictive maintenance program were compared to the actual time... 

MTTR mean time to repair PET photoinduced electron transfer ... 

Second, guaranteeing availability as high as 99-9999 percent requires a tremendous amount of performance data on every single piece of equipment. Accurate risk-assessment analysis requires reliable data such as mean time between failure (how long a component is likely to run before breaking down) and mean time to repair (how long it will take to fix a component that has broken down). Analysts would prefer to have as much as i million hours of data on each and every system component. That takes years to... 

Many of the equipment vendors have developed cost of ownership (COO) models, some traceable, at lease in part, to SEMATECH. These COO models may be used to account for all aspects of amortized costs and provide a user with a highly accurate anticipated cost schedule. At a minimum a COO model should include the cost of the system, utilities, facilitization, mean-time-between-failures, mean-time-to-repair, preventative maintenance, personnel, all consumable safety costs (including that of required support equipment), reactant, and substrate costs. Each of these parameters" should be well defined and guaranteed, and the user of such models should precisely understand how up-time, mean-time-to-repair, and other terms are defined. A 90% uptime schedule is useless if the system is routinely defined to be out of service, for maintenance, 25 % of the time. 

Mean time between failures. Mean time to repair. Medium voltage. 

MTBF—mean time between failures MTTR—mean time to repair FMEA—failure mode effect analysis Uptime of equipment or downtime avoided... 

Quality of pmduct pertains to product reliability, value, functionality, and dependability. End users also utilize mean-time-to-failure, mean-time-to-repair, and maintenance cycle time and costs to evaluate the quality of products. 

Maintainability measures. Maintainability function based on time to repair distribution, mean time to repair (MTTR), percentile of time to repair, and maintenance ratio. 

If the repair time T follows the exponential distribution with mean time to repair, MTTR, H fx, where fx is the repair rate, then... 

Availability, in general, is defined as the ability of the plant/equipment to perform its required function over a stated period of time. Maintainability is the probability that a failed item can be restored to operation effectiveness within a given period of time when repair action is performed as per the specified procedure (Smith, 2011). Software is available for performing RAM studies. For smaller projects, spreadsheets can be used. Reliability and process safety are interlinked, and so combined RAM and safety (RAMS) studies can be performed with the RAMS software (Sikos and Klemes, 2010). It considers many factors affecting the plant performance such as equipment performance, redundancy, demand requirements and logistics. RAM analysis is based on statistical failure data such as mean time between failures (MTBF), mean time to repair (MTTR), mean time to failure (MTTF) and mean down time (MDT). Wherever possible, failure data available within the company should be used for RAM/RAMS study. If not, typical failure data available in the literature/software can be used. 

In the past, the acronym MTTR stood for "Mean Time To Repair." The term was changed in lEC 61508 because of confusion as to what was included. Some thought that Mean Time To Repair included only actual repair time. Others interpreted the term to include both time to detect a failure (diagnostic time) and actual repair time. The term "Mean Dead Time (MDT)" is commonly used in some parts of the world and means the same as Mean Time To Restore. 

In Eq. (9.98)is the failure rate. Analogously one describes repair by the repair rate [t, which is the reciprocal value of the mean duration of repair (mean time to repair MTTR), i.e. = 1/Tr... 

The expected value of the time from a failure until the system ean be executed again is denoted MTTR (mean time to repair). This term is not dependent on the number of remaining faults in the system. 

In the case mentioned, it is admitted that the failures of all the machines occur independently, with the same failure rate X. It is also considered that the repair rate fi, equivalent to inverse of the mean time to repair 1/MTTR, is identical for all the types of repair services and independent of the technician who executes the service. In case different failure rates are observed for each machine or group of machines, a more complex model could be elaborated, considering multiple class queuing models (Menasce et al., 2004) for the distinct demands of services. In case it is necessary to distinguish repair rates for each technician, a heterogeneous multi-server model could be defined to represent the individual rates. 

The vectors D, i = 1,2,..., C, of the components degradation states. The maintenance actions, in practice, may depend on the values of these vectors, e.g. a repair action is foreseen if the state achieved by the y-th degradation mechanism, Z)j, is greater than a threshold dy The repair rates also depend on the values of the vectors Z> , e.g., the Mean Time To Repair (MTTR) of the degradation process j may be larger than that of the degradation process k. 

The expected values EU and ED are commonly referred to as the Mean Time To Failure (MTTF) and Mean Time To Repair (MTTR), respectively. 

As soon as a failure is detected (either through diagnosis or within the proof test) it is repaired in an assumed down time (Mean Time To Repair = MTTR). 

The values of the failure rates, X, and mean times to repair of the traction and breaking modules are given in Table 1. These values were directly provided by Ansal-doBreda. Moreover, due to these values, the state TTB is considered unreachable. 

Mean times to repair of the traction and brake modules are different. So, to completely describe the state of the fleet, it is necessary to specify the failure mode of the c trams that are kipping the queue and the order according to which they will be served. In this paper, the model was developed associating to each pair d, c the number, m, of possible different configuration of the queue. The number of states considered for each pair [d, c is reported in the Table 2, where only the pairs that satisfy the constrain (3) are taken into account ... 

The simulation experiments are made for the serial mechanical system with six elements. The mean time between the faults and the mean time to repair is of exponential probability distribution. 

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