Maintenance and Repair strategies for Reliability.pptx

MTBF, MTTR, MTTF, FIT

1. MTBF, MTTR, MTTF AND FIT MTBF, MTTR, MTTF and FIT are reliability terms based on methods and procedures for lifecycle predictions for a product. Customers often must include reliability data when determining what product to buy for their application. MTBF (Mean Time Between Failure), MTTR (Mean Time To Repair), MTTF (Mean Time To Failure) and FIT (Failure In Time) are ways of providing a numeric value based on a compilation of data to quantify a failure rate and the resulting time of expected performance. The numeric value can be expressed using any measure of time, but hours is the most common unit in practice.

2. Mean Time Between Failure (MTBF) is a reliability term used to provide the amount of failures per million hours for a product. This is the most common inquiry about a product’s life span, and is important in the decision-making process of the end user. MTBF is more important for industries and integrators than for consumers. Most consumers are price driven and will not take MTBF into consideration, nor is the data often readily available. On the other hand, when equipment such as media converters or switches must be installed into mission critical applications, MTBF becomes very important. In addition, MTBF may be an expected line item in an RFQ (Request For Quote). Without the proper data, a manufacturer’s piece of equipment would be immediately disqualified. Mean Time To Repair (MTTR) is the time needed to repair a failed hardware module. In an operational system, repair generally means replacing a failed hardware part. Thus, hardware MTTR could be viewed as mean time to replace a failed hardware module. Taking too long to repair a product drives up the cost of the installation in the long run, due to down time until the new part arrives and the possible window of time required to schedule the installation. To avoid MTTR, many companies purchase spare products so that a replacement can be installed quickly. Generally, however, customers will inquire about the turn-around time of repairing a product, and indirectly, that can fall into the MTTR category.

3. Mean Time To Failure (MTTF) is a basic measure of reliability for non-repairable systems. It is the mean time expected until the first failure of a piece of equipment. MTTF is a statistical value and is meant to be the mean over a long period of time and a large number of units. Technically, MTBF should be used only in reference to a repairable item, while MTTF should be used for non-repairable items. However, MTBF is commonly used for both repairable and non-repairable items. Failure In Time (FIT) is another way of reporting MTBF. FIT reports the number of expected failures per one billion hours of operation for a device. This term is used particularly by the semiconductor industry but is also used by component manufacturers. FIT can be quantified in a number of ways: 1000 devices for 1 million hours or 1 million devices for 1000 hours each, and other combinations. FIT and CL (Confidence Limits) are often provided together. In common usage, a claim to 95% confidence in something is normally taken as indicating virtual certainty. In statistics, a claim to 95% confidence simply means that the researcher has seen something occur that only happens one time in twenty or less. For example, component manufacturers will take a small sampling of a component, test x number of hours, and then determine if there were any failures in the test bed. Based on the number of failures that occur, the CL will then be provided as well.

4. MTTR Mean Time To Repair ( MTTR ) is a basic measure of the maintainability of repairable items. It represents the average time required to repair a failed component or device . The most common failure related metric is also mostly used incorrectly. “Mean time between failures” or “MTBF” refers to the amount of time that elapses between one failure and the next. Mathematically, this is the sum of MTTF and  MTTR , the total time required for a device to fail and that failure to be repaired

5. Mean time to repair (MTTR) is the average time required to troubleshoot and repair failed equipment and return it to normal operating conditions. It is a basic technical measure of the maintainability of equipment and repairable parts. Maintenance time is defined as the time between the start of the incident and the moment the system is returned to production (i.e. how long the equipment is out of production). This includes notification time, diagnostic time, fix time, wait time (cool down), reassembly, alignment, calibration, test time, back to production, etc. It generally does not take into account lead-time for parts. Mean time to repair ultimately reflects how well an organization can respond to a problem and repair it.

6. How is mean time to repair calculated? Expressed mathematically, it is the total maintenance time divided by the total number of maintenance actions over a specific period. Over the lifetime of an asset, each failure will vary depending on the severity of the issue. Some issues will require a simple parts swap, while others could take days to diagnose and repair. The frequency v.s . repair time plot follows the log-normal distribution. We will have a large number of repairs that are quick to repair, and a small number that take much longer.

7. Why is mean time to repair important? For mission critical equipment, mean time to repair can have a dramatic effect on the organization’s bottom line. Taking too long to repair equipment can mean  product scrap , missed orders and soured business relationships. To limit the impact of mean time to repair, organizations have their own maintenance teams, hold spare parts onsite or run parallel production lines.

8. What can mean time to repair tell you? Prediction of the number of hours that a system or component will be unavailable whilst undergoing maintenance is of vital importance in reliability and availability studies. Mean time to repair yields a lot of information that can help reliability engineers make informed decisions such as repair or replace, hire, optimize maintenance schedules, store parts onsite or switch  parts strategy . For example, as the system ages, it may take longer to repair systems. MTTR will trend upwards prompting the repair versus replace decision.

9. You can also use mean time to repair to predict performance or the life cycle cost of new systems. Equipment manufacturers are now using a modular design philosophy so parts or sub assemblies can be swapped out quickly and easily. Consider being faced with a purchasing decision that involves 2 similar systems – one has a higher MTTR because repairable items are difficult to remove due to their location. The additional time and costs to maintain should be factored into the life of the system to simplify the purchasing decision. Manufacturers also use MTTR to justify redesigning or improving systems. For an accurate calculation of MTTR, we must make the following assumptions: One technician performs all tasks sequentially. Appropriately trained personnel perform the maintenance .

10. MTBF , the most well-known term, is usually used for repairable systems and is also widely used for the case where the failure distribution is exponential. Mean time between replacements and  MTBF  with scheduled replacements are applied to repairable systems with scheduled preventive  maintenance . Mean time between failures  ( MTBF ) is the predicted elapsed time between inherent failures of a system during operation.  MTBF  can be  calculated  as the arithmetic mean (average) time between failures of a system. The term is used in both plant and equipment maintenance contexts . In reliability analysis,  MTTF  is the average time that an item will function before it fails. It is the mean lifetime of the item. With censored data, the arithmetic average of the data does not provide a good measure of the center because at least some of the failure times are unknown.

11. he data you need for the  calculation  is the total hours of operation of the equipment divided by the number of failures – pretty simple. So if you have 100 pumps, and all but one runs for 100 hours. The one fails at say 50 hours. Then the calculation  is (( 99 x 100 ) + (1 x 50)) / 1 failure = for  mttf  of 9950 hours . Mean time between repair  ( MTBR ) is one of several related metrics that helps to provide information on operating reliability for IT products and systems.  MTBR  is often defined as the average operating calendar time between required repairs for a given product or set of products . MTBF  ( mean time between failures ) is a measure of how reliable a hardware product or component is. For most components, the measure is typically in thousands or even tens of thousands of hours  between failures . For example, a hard disk drive may have a  mean time between failures  of 300,000 hours.

12. Failure rate  is the frequency with which an engineered system or component fails, expressed in  failures  per unit of time. It is often denoted by the Greek letter λ (lambda) and is highly used in reliability engineering . Availability  is the probability that a  system  will work as required when required during the period of a mission. The mission could be the 18-hour span of an aircraft flight. ...  Availability  includes non-operational periods associated with reliability, maintenance, and logistics .

13. Measuring Availability Mean time to failure (MTTF) Mean time to repair (MTTR) MTBF = MTTF + MTTR Availability = MTTF / (MTTF + MTTR) Suppose OS crashes once per month, takes 10min to reboot. MTTF = 720 hours = 43,200 minutes MTTR = 10 minutes Availability = 43200 / 43210 = 0.997 (~ “ 3 nines ” ) 13

14. Maintenance and Repair Time for Components in Technological Facilities Information on repair time data for various engineering components is available in literature. Active repair time needed for Craftsmen to restore engineering component to working order is vital for sustainability of any engineering company.

15. Methods to Obtain Repair Times Data There are several recognised methods to obtain repair times data : We can elicit repair time estimates from plant personnel since they have experience with the tasks and the work in general. Published data from plant operating experiences Repair time estimation using maintenance time methods, summing the time of individual actions the worker must take to complete a repair. Perhaps the best method is to evaluate historical data from facility operations experience. This is highly pertinent if the facility records available to the analyst are from the correct industry and if the facility has not changed its equipment. However, such evaluations can be exhaustive, so the literature search approach of determining if any finished date are available is a low-cost option then can serve the analyst well until more detailed data are needed.

16. Factors Affecting Repair Time Subcontracted workers versus permanent staff at a plant Skill level of the workers involved in a task Number of workers assigned to the task Workers familiarity with the equipment Equipment accessibility Radiological conditions Toxicological conditions Environment (room temperature, humidity, lighting etc ) Severity spectrum of equipment failures that the staff are repairing, maintenance errors, quality of spare parts (spare parts breakage upon a return-to- service test greatly extends the outage time)

17. Some Definitions There are a number of definitions regarding maintenance times: The Mean Down Time (MDT) is the time from a component or system failing to perform its function to the component or system being returned to service. The MDT usually includes more than spare parts procurement as part of the logistic delays. The logistics can include completing the maintenance work order, ensuring this maintenance activities will not endanger the plant or other personnel, a radiological safety review, an industrial safety review, and an environmental safety review. These can all take time, perhaps days.

18. Some Definitions The Mean Time to Repair (MTTR), is usually referred to as the active repair time. This is the time when Craftsmen are at the component, performing the actions to return the component to service. The MTTR is sometimes referred to as the “hands-on-time”. Some authors use MTTR to signify the “mean time to restore” the component to service. In that case, the meaning is the time from component failure to the time it is retuned to service, so the mean time to restore includes (next slide)

19. Elements of MTTR The time for personnel access Diagnosis Procurement of spare parts or equipment The active repair time The testing of the component The component or system return to service time Some authors use mean time to restore as an equivalent to the (MDT)

20. Elements of MTTR The basic MTTR is found by decomposition into its basic elements and estimating time needed for each element. The equation below illustrates this:  

21. Mean Time to Repair (MTTR)   We can with data for these figure calculate the MTTR, or mean active corrective maintenance time MTTR as mentioned earlier is the actual time it takes to perform corrective maintenance. It is thus a maintenance activity that takes place after failure has been discovered. The time unit is usually, if not prescribed other, in hours.  

22. Mean Maintenance Time To be able to calculate the mean maintenance time , that is to say the total time of maintenance considering both corrective and preventive maintenance tasks we first need to calculate the mean active preventive maintenance time. All in all we can calculate the mean maintenance time considering both corrective and preventive maintenance  

23. Diagnosis Technologies The most commonly applied condition-based maintenance techniques are : Vibration analysis Oil analysis Thermography Ultrasonic Electrical effects and penetrates

24. Vibration Analysis Vibration can be defined as the movement of a mass from its point of rest through all positions back to the point of rest, where it is ready to repeat the cycle. The time it takes to do this is its period, and the number of repetitions of this cycle in a given time is its frequency. The severity of vibration is determined by the amplitude – or maximum movement – its peak velocity and peak acceleration. Vibration analysis in condition monitoring is accomplished by company vibration characteristics of current operation to a baseline, measured when the machinery was known to be operating normally. The selection of the specific parameters to be measured depends primarily on the frequency of the vibration

25. Diagnostic Technologies Vibration analysis techniques can be used to monitor the performance of mechanical equipment that rotates, reciprocates or has other dynamic actions. Example includes: Gearboxes Roller bearings Motors Pumps fans Turbines Belt or chain drives Compressors Conveyors Reciprocating engines Source: College of Eng – Industrial Eng Indexing machines

26. Oil Analysis Ferrography and magnetic chip detection examine the iron-based wear particles in lubrication oil to determine the type and extend of wear , and can help determine the specific component that is wearing . Spectrometric oil analysis measures the presence and amounts of contaminants in the oil through atomic emissions or absorption spectrometry . It is useful for determining not only iron, but also other metallic and non metallic elements, which can be related to the composition of the various machine components, such as bearings, bushing, piston rings etc. It is useful when wear particles are initially being generated in the early stages of the failure, as they are small. Chromatograph measures the changes in lubrication properties , including viscosity, flash point , pH , and water content and insoluble , through selective absorption and analysis.

27. Thermograph The most common uses for thermography, which measures the surface temperature through the measurement of infra-red radiation , are for determining poor electrical connections and hot sports , furnace and kilm refractory wear and critical boiler and turbine components overheating . An infra-red camera shows surface temperature variations , calibrated to provide the absolute temperature or temperature gradient through black and while or colour variation.

28. Ultrasonic There are several techniques for ultrasonic testing, but they are used to determine faults or anomalies in wilds , pipes, tubes, structures, shafts etc. Cracks , gaps, erosion, corrosion and inclusions are discovered by transmitting ultrasonic pulses or waves through the materials and assessing the resultant signature to determine the location and severity of the discontinuity. This techniques is also used to measure flow rate.

29. Penetrates Electrostatic and liquid dye penetrants are used to detect cracks, and discontinuities on surfaces, caused in manufacturing, by wear, fatigue, maintenance and overhaul procedures, corrosion or general weathering. The penetrants is applied and allowed to penetrate into the anomalies. The surface is cleaned and the penetrant revealed through direct visual , fluorescent or electrostatic techniques.

30. Problem A factory has 200 machines and the Maintenance Engineer Supervises the repair crews who repair malfunctioning machines. The manufacture policy is to repair the broken down machines and bring back in production within 2 hrs on the average. If average breakdown rate is 3.5 machine per hour on the average. How many repair crews are required? The formula for average repair rate is  

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