Chapter 3

1. Chapter 3 • Preventive Maintenance : Concepts, Modeling and Analysis

2. Chapter Objectives 1. Enable students to understand the Basic Tools for developing Pm Programs. 2. Enable students to diagnose if a plant needs improvement in its PM program

3. Chapter3 Objectives 3. Enable students to develop planned maintenance programs. 4. Enable students to formulate models to determine PM intervals 5. Enable students to formulate and solve inspection models.

4. Preventive Maintenance Preventive maintenance is a series of preplanned tasks performed to counteract known causes of potential failures of those functions.

5. Preventive Maintenance Preventive maintenance is the preferred approach to asset management: ·     It can prevent premature failure and reduce its frequency ·    It can reduce the severity of failure and mitigate its consequences ·     It can provide warning of an impending or incipient failure to allow planned repair ·    It can reduce the overall cost of asset management

6. Preventive Preventive maintenance maintenance Conditio Conditio Statistically and Statistically and reliability based reliability based n n based based Off line Off line On line On line Time Time Use Use based based based based

7. Why PM is Preferred 1. The frequency of premature failures can be reduced through proper lubrication, adjustments and cleaning. 2. If failure can not be prevented periodic inspections can help reduce its severity.

8. Why PM is Preferred 3. Warning of impeding failure can be detected by monitoring gradual degradation of function or parameter. 4. The cost of planned maintenance is always cheaper than emergency maintenance

9. Critical Issues Regarding PM 1. What PM tasks should be performed to prevent failure? . Time based tasks . Condition based tasks Discuss situation where each one is applicable.

10. Diagnostic Technologies The most commonly applied condition-based maintenance techniques are vibration analysis, oil analysis, thermography, ultrasonics, electrical effects and penetrants.

11. 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

12. Vibration analysis 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 comparing 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.

13. Vibration analysis Vibration analysis techniques can be used to monitor the performance of mechanical equipment that rotates, reciprocates or has other dynamic actions. Examples include gearboxes, roller bearings, motors, pumps, fans, turbines, belt or chain drives, compressors, generators, conveyors, reciprocating engines and indexing machines

14. Oil analysis Ferrography and magnetic chip detection examine the iron-based wear particles in lubrication oils to determine the type and extent of wear, and can help determine the specific component that is wearing.

15. Oil Analysis Spectrometric oil analysis measures the presence and amounts of contaminants in the oil through atomic emission or absorption spectrometry.

16. Oil analysis It is useful for determining not only iron, but also other metallic and not metallic elements, which can be related to the composition of the various machine components, like bearings, bushings, piston rings, etc. It is useful when wear particles are initially being generated in the early stages of failure, as they are small.

17. Oil analysis Chromatography measures the changes in lubricant properties, including viscosity, flash point, pH, water content and insoluble, through selective absorption and analysis.

18. Thermography 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 spots, furnace and kiln refractory wear and critical boiler and turbine component overheating. An infra-red camera shows surface temperature variations, calibrated to provide the absolute temperature or temperature gradients through black and white or color variations.

19. Ultrasonic There are several techniques for ultrasonic testing, but they all are used to determine faults or anomalies in welds, coatings, piping, tubes, structures, shafts, etc. Cracks, gaps, buildups, erosion, corrosion and inclusions are discovered by transmitting ultrasonic pulses or waves through the material and assessing the resultant signature to determine the location and severity of the discontinuity. This technique is also used to measure flow rates.

20. Electrical Effects Monitoring There are several tests for corrosion using a simple electric circuit monitored by varying degrees of sophisticated instrumentation. The Corrator uses the electro-chemical polarization method in a vessel with corrosive liquid. The Corrometer uses the electrical resistance across a probe inserted in the active environment eg. refinery process equipment.

21. Electrical Effects Monitoring The most common for monitoring or testing motors or generators are voltage generators, including mergers. These measure the resistance of insulation, and apply a test voltage from 250 volts to 10,000 volts.

22. Penetrants 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 penetrant is applied and allowed to penetrate into the anomalies. The surface is cleaned and the penetrant revealed through direct visual, fluorescent or electrostatic techniques.

23. PLANNED MAINTENANCE Maintenance work carried out with forethought control and record. It can be applied to any of maintenance provided that (a) The maintenance policy has been considered carefully. (b) The maintenance policy is planned in advance.

24. PLANNED MAINTENANCE (c) The work is controlled and directed to conform to the prearranged plan. (d) Historical and statistical record are complied and maintained to assess the results and provide guide for future policy.

25. BENEFITS OF PLANNED MAINTENANCE 1. Greater Plant Availability 2. Less Costly 3. High Level of Output 4. Greater Utilization 5. Servicing and Adjustment is not overlooked

26. BENEFITS OF PLANNED MAINTENANCE 6. Improved Budget Control 7. Improved Stocks and Spares Control 8. Provision of Information for Realistic Forecasts 9. Focusing Attn. on Frequently Recurring Jobs.

27. ELEMENTS OF PLANNED MAINTENANCE (PM) 1. Leader 2. List of all Facilities and their Importance (What) 3. Identification (Coding) 4. Facility Register

28. ELEMENTS OF PLANNED MAINTENANCE (PM) 5. Maintenance Schedule 6. Job Specification 7. Control 8. History Record

29. Facility Inventory The facility inventory is a list of all facilities including all parts of a site and content. It is made for purpose of identification. An inventory sheet of all equipment should be developed showing equipment identification, description of facility, location, type and priority (importance).

30. Identification (Coding) It is essential to develop a system by which each equipment is identified uniquely. A coding system that help in this identification process should be established. The code should indicate location, machine type and machine number. This coding system differ from plant to plant and its design should reflect the nature of the facility.

31. Facility Registor The facility register is a file (electronic or hard copy) including technical detail about items that are included in the maintenance plan. These items are the first to be fed to the maintenance information system.

32. Equipment Record The equipment (item) record should include, identification number, location, type of equipment, manufacturer, date of manufacturing, serial number, specification, size, capacity, speed, weight, power service requirement, connection details, foundation detail, overall dimension, clearance, reference drawing number, reference number for service manuals, interchangeability with other units, etc.

33. Maintenance Schedule A maintenance schedule must be developed for each equipment in the program. The schedule is a comprehensive list of maintenance tasks to be carried on the equipment.

34. Maintenance Schedule The schedule include the name and identification number of the equipment, location, reference number of the schedule, detailed list of tasks to be carried out (inspections, preventive maintenance, replacements), the frequency of each task, the crafts needed to carry out the task, time for each task, special tools needed, material needed and details of any contract maintenance.

35. Job Specification The job specification is a document describing the procedure for each task.. The job specification should indicate: the identification number of the item (equipment), the location of the item, the maintenance schedule reference, the job specification reference number, the frequency of the job, crafts required for the job, the details of the task, components to be replaced, special tools and equipment needed, reference drawings, manuals and safety procedures to be followed.

36. Maintenance Program The maintenance program is a list allocating maintenance tasks to specific time period. When developing the maintenance program a great deal of coordination must be done in order to balance the work load and meet production requirements. This is the stage when the planned maintenance is scheduled for execution.

37. Program Control The maintenance program developed must be executed as planned. Close monitoring is needed in order to observe any deviation from the schedule. If deviations are observed a control action is needed

38. A SYSTEMATIC SIX STEP METHOD(TOP-TO-BOTTOM) 1. Determine Critical Plant Units and Operation Windows. 2. Classify the Plant into Constituent Items. 3. Determine and Rank Effective Procedure.

39. A SYSTEMATIC SIX STEP METHOD(TOP-TO-BOTTOM) 4. Establish Plan for Identified Work. 5. Establish a schedule for On-Line Maintenance 6. Establish Corrective Maintenance Guidance

40. Mathematical Models for Optimum PM Polices Two well known polices which are: 1. Age based policy (Type 1 policy) 2. Constant interval replacement polices (type II policy)

41. Age Based Policy • Policy I (age preventive replacement) is defined as follows: perform preventive replacement after tp hours of continuing operation without failure; tp could be finite or infinite. In case of an infinite tp no preventive maintenance (replacement) is scheduled. If the system fails prior to tp hours having elapsed, perform maintenance (replacement) at the time of failure and reschedule the preventive maintenance after tp operation hours.

42. Notation • Cp = cost of preventive maintenance • Cf= cost of breakdown (failure) maintenance • f(t) = time to failure probability density function (p.d.f.). • F(t) = equipment or system failure distribution, it is the integral of f(t) • r(t) = failure rate function

43. Notations • N(tp) = number of failures in the interval (0,tp); N(tp) is a random variable. • H(tp) = expected number of failures in the interval (0,tp). • R(t) = reliability or survival function. • M(tp)= expected value of the truncated distribution with p.d.f. f(t) truncated at tp. • M(tp) = • C(tp) = expected cost per cycle • UC(tp) = expected cost per unit time.

44. Failure replacement Failure replacement Preventive replacement Preventive replacement Time Age Based Policy Failure replacement Failure replacement tp tp 0

45. Model Development Expected Cycle Length = tp R(tp)+M(tp)(1-R(tp)) Expected Cycle Length = tp R(tp)+M(tp)(1-R(tp))

46. Golden Section Method • 1. Choose an allowable final tolerance level, , and assume the initial interval where the minimum lies is [a1,b1,] = [a,b] and let 1 = a1+(1-)(b1-a1), 1 = a1+(b1-a1),  = 0.618. Evaluate g(1) and g(1), let k=1 and go to step 2. • 2. If bk-ak <, stop as the optimal solution is . Otherwise, if g( k)>g(k) go to step 3, and if go to step 4

47. Golden Section Continued • 3. Let ak+1=kand bk+1=bk, furthermore, letk+1=k and let k+1=ak+1+(bk+1-ak+1). Evaluate g(k+1,), and go to step 5. • 4. Let ak+1=akand bk+1= k, furthermore let k+1=k and let k+1= ak+1+(1-)(bk+1- ak+1), evaluate g( k+1), and go to step 5. • 5. Replace k by k+1 and go to step 1. • For more on the properties and the convergence of the above algorithm see Bazarra et al. [4].

48. Example on Policy 1 • An equipment has a time to failure density function f(t) that follows a uniform distribution between [0,10] weeks. The cost of preventive replacement is $5 and the cost of failure replacement is $50. Determine tp, the optimal time of preventive replacement.

49. Cost Function

50. Applying Golden Section the next interval is