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Predictive Maintenance Or Condition based Maintenance • The success of any such organization depends on continuous, safe and cost effective operation of its plant, • where breakdowns without any preliminary warnings have to be avoided by all means. • This is important to know the exact condition of the critical parts/components in a continuous running plant, which can fail, and thus cause a breakdown without warning.

4.1 Predictive Maintenance or Condition Based Maintenance

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Predictive Maintenance or Condition Based Maintenance

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  • Predictive Maintenance Or Condition based MaintenanceThe success of any such organization depends on continuous, safe and cost effective operation of its plant, where breakdowns without any preliminary warnings have to be avoided by all means. This is important to know the exact condition of the critical parts/components in a continuous running plant, which can fail, and thus cause a breakdown without warning.

  • Contd This means diagnosing the exact conditions of the plant/equipment by means of on-stream, non-destructive testing to avoid unnecessary shutdowns.

    Condition monitoring of a machine or equipment can be easily compared to the health monitoring of an individual by diagnostic tests/checks/instrument.

  • What should be the optimal maintenance interval and how can it be decided upon and arrived at?

    Over-maintenance or too frequent maintenance increases expensive down time resulting in added cost due to expenditure incurred and waste of precious time. Under-maintenance or too long an interval between two successive checks may also result in high incidence of failures, which is not acceptable.

  • In a preventive maintenance system, a machine is taken on preventive maintenance checks, depending on a predetermined cycle, irrespective of its actual condition, whereas in the case of predictive maintenance, a machine is taken for maintenance only when its status or condition so demands (this is also known as condition-based maintenance.)

  • The objectives are to provide increased on-stream plant availability and reliability, increased plant and personnel safety and optimal maintenance costs. A given machine can continue to be kept in operation as long as the monitored parameters continue to remain within the laid down limits.

  • ContdCondition monitoring tells us in advance: Maintenance is carried out when the condition so warrants it. Thus, one has enough lead-time to plan accurately and in detail for this choice, thus reducing the down time and ensuring the optimal utilization of maintenance manpower

  • Contdb) Spare part costs can be reduced, and the stocking of spare parts can also be better controlled and managed, depending of course, upon the lead time available obtain spares. C) The requirements of spare parts, the consumables and of requisite tools can therefore be planned and obtained well in time, thus reducing the total down time.

  • Methods of Condition Monitoring Condition monitoring can be carried out when the equipment is in operation, which is known as on-line. Some of the parameters that are possible to be monitored are speed, temperature, vibration, power, and pressure. Off-line monitoring is carried out when the machine is down for whatever reason.

  • Contd The question is as to which of the machinery and plant have to be chosen for condition monitoring? The entire plant, equipment and machinery has to be divided into three broad categories:

    Critical (Category A)Semi-critical (Category B)Non-critical (category C)

  • Some of the major criteria for this decision-making process will have to be thought out with care and clarityCriticality of the machine/equipment with regard to the process or production. If machine is unserviceable and cannot be short-circuited or its task cannot be switched over to another machine then it is critical.

  • Contdii. Availability of standby. If the original equipment is critical, then the standby will also be critical equipment.

    iii. Hazards involved in the case of failure. Find out how dangerous the particular equipment can be in case of failure. iv. Downstream effects. What will be the effect of a breakdown, down the stream on production, on process materials? How expensive will it be?

  • Contdv. Cost of down time. How high is it? If it is disproportionately high, then condition monitoring is needed.

    iv. Cost of condition monitoring equipment. This is to be weighed against the costs that would be incurred in case condition monitoring is not required.

  • Non-destructive testing methodologies have been designed for the accurate measurement of characteristics of parts of a machine, without in any way affecting that part physically or functionally. Parameters in order to monitor the health of a machine Process variables include pressure, temperature, flow of process fluids, lubricating oil, and coolant circulation. Non-destructive testing

  • ContdMechanical running condition variables are: Temperature of vulnerable machine component Mechanical vibrations Machine RPMThe relative motion of moving elements Crack detection.

  • Contdii. Rotating machinery malfunctions include imbalance, misalignments, foundation problems, fracture and wear of casing due to a variety of loads such as thermal loads, and bearing failures.

  • Non-Destructive Testing (NDT) Methodologies In order to monitor the parameters, a variety of non-destructive testing methodologies have been developed. Boroscope can be inserted into inaccessible places through small openings, and with its aid one can see a highly magnified image of the part under examination.Flexi scope is an instrument that is used to see contoured surfaces and U-bends.

  • ContdLiquid Dye penetrate can seep into minute surface openings by capillary action to detect surface cracks, porosity and lamination.Magnetic particle Detection is used for locating surface and sub-surface discontinuities in ferromagnetic materials.Eddy Current Testing is employed to measure electrical conductivity, magnetic permeability, grain size, heat treatment condition and hardness

  • ContdUltrasonic Devise uses beams of high frequency waves to detect surface/sub-surface flaws. The waves travel through the material with attendant loss of energy and are reflected at the inner faces.

    Radiography The radiation can be record on film, or on to photosensitive paper and viewed through a radiographic viewer to locate defects.

  • ContdCreep Tester permanent deformation and damage may occur when an item has been operating at high temperatures and stresses. The creep tester measures the changes that have come about in the dimensions.Spark Testing. A visual examination of the spark pattern is used as an indicator for classification of ferrous alloys according to their chemical composition.

  • ContdLeak Testing The assembly is filled up with air up to a specific pressure, and then a testing is done with a soap solution, halogen helium or freon, to locate the leak.Hardness Testers such as the Rockwell hardness tester, the rebound hardness tester and other allied tools, are available.

  • ContdThermal Testing . Making use of contact thermometers, heat sensitive paint, heat sensitive stickers, infrared thermometers or optical pyrometers can do the measurements.Acoustic Emission Testing is defined as a high frequency stress wave generated by the rapid release of strain energy that occurs within a material during crack development or plastic deformation. This method is capable of detecting the minutest of increasing flaws.

  • ContdIn Situ Metallography The particular part to be examined is polished and etched in its location and the surface structure is transferred to a thin film using suitable chemicals. Then it is examined under a microscope to review the metallurgical changes.

    Strain Monitoring is used to monitor the condition of parts subject to strains due to variations in their operating conditions. This is used extensively in aircraft testing during design stages and after overhauls during test flights.

  • Vibration Monitoring Vibration in rotating machines is caused due to factors such as imbalance, misalignment, looseness, cavitations, turbulence, wear, bearing damage, coupling damage, and rubbing.

    The major characteristics of vibration analyses are displacement, velocity and acceleration (which sow the presence of vibration); frequency (which shows what causes vibration);

  • Spectrometric Oil Analysis Procedure (SOAP) This is a technique used to monitor the condition of machines by analyzing the concentration of metal elements present within the used oil samples taken and analyzed at regular intervals with the help of a spectrometer.

  • Long-range maintenance planning entails extensive consideration and precise assessment of maintenance requirements for the proposed production plants. Short range planning makes arrangements for taking care of overhaul of newer equipment, the installation of testing facilities, the creation of facilities matching the needs of the newer NDT and surveillance techniques, and the training of manpower to meet the demands placed on them to man these services.

  • Elements that go into a planned maintenance system

    Maintenance plan is essential to prepare a Master plan for the year, and then broken up into monthly, weekly and daily plans.Maintenance schedules have to be made for corrective and preventive maintenance, for inspection, lubrication and overhaul.

  • Contdiii. Maintenance control system is necessary to indicate and initiate activities at scheduled periods and to record and assess its progress and control it as per the periodic plans.iv. Maintenance work specification involves preparation of documents defining the exact work to be done for each type of requirement. v. Resource schedule deals with the requirements and their distribution for optimized manpower allocation and other critical resources.

  • Contdvi. Liaison with production An accepted and agreed upon principle and methodology has to be laid down. production will accept and permit the time frame within which maintenance will be carried out. vii. Maintenance records A correct documentation of each and every maintenance activity has to be done and its analysis starting with the Facility Register, which lists all items that have to be maintained..

  • Contdviii. Maintenance spares provisioning cell has to constantly interact with maintenance and revise, update and indent all materials required to meet maintenance tasks.

    ix. Maintenance support organization will provide support in respect of technical information, drawings and complete rage of technical literature, and make arrangements for tools, tackles and test equipment and calibration facilities.

  • Contdx. Cost control involves a system to correctly estimate maintenance costs, and to control cost. xi. Management information system entaits the creation of reporting system to management. xiii. Training is intended to make the personnel attain the required standards for operating the management

  • A practical and acceptable schedule can be developed by following a few simple points. Schedules must be made in two parts: the long term schedule to be made 8 to 12 weeks in advance, and the short term schedule, just a week or two in advance. The schedule should be based on preplanned methods, activities, and, if possible, on scientifically determined time standards.

  • ContdC) The total system ought to be clear and understandable and at the same time simple to operate and to implement. d) Revisions in schedules are expected, and hence, they must be kept flexible enough to accommodate any change.

  • Contde) Visual aids like wall charts or scheduling boards, should be made use of, which show job status, the date of issue of work orders, the work progress long pending jobs, etcf) The maintenance requirements have to be carefully balanced against the available capacity to accomplish the tasks.

  • Contd

    g) Important information like backlog report, manpower availability report, and materials and spares availability report are of great use. Backlog report is a list of unattended, or if attended, then incomplete jobs with indication of estimated man-hours required for each. h) Production plan is issued by Operations Department and includes machine loading and utilization plans. It helps maintenance staff to schedule machine repairs to a convenient time and opportune moment.

  • Control System for Planned Maintenance Basic model of a control system for planned maintenance is given in the nerve center is the maintenance Control Center which has all the important and valuable master-records such as Maintenance schedules Facilities records History records Work orders/Job cards Work specification Records for defect analysis Monthly performance reports etc.

  • Maintenance turnaround is defined as the planned outage to completely overhaul the plant or machine, so that it will become almost new and will have al full operating life. Special attentions needed while preparing for a turnaround are: Planning the number of days is an important criterion, as it will ultimately decide the availability of on-stream days. Material equipment planning. All materials need is predicted on the basis of give-off-lists.

  • Contdiii. Indigenous manufacture relates to those components that would be needed, but are not available through spare parts vendors. iv. Pre-shutdown phase. A detailed briefing is to be carried out among all concerned personnel.v. Shutdown. Actual shouting down of the plant has to be done following correct laid down procedure.

  • Contdvi. Turnaround. Actual work is performed as per plans during this phase. vii. Start-up. The times when maintenance completes its task and operations personnel take over is know as the star-up time. viii. Post-turnaround phase. All aspects of the turnaround performance are critically examined and analyzed, and improvements to be made in future noted down for the next turnaround.

  • Inspection is the examination of certain critical machines or parts thereof, equipment or machines to determine their state or condition. It is useful for planning and programming of actions, which help to maintain accuracy of the machine, and the prevention of unwarranted breakdowns. The inspection routine would involve carrying out checks forAny abnormal vibrations, or any abnormal noise;

  • ContdThe temperatures of all the bearings to ascertain that they are at acceptable levels;Leakages form if they are excessive; Oil levels in cup; and Grease nipples to ensure that they are not dry.

  • The frequency of inspection is determined by an engineering analysis, which considers the following parameters: Age of the machine, its condition and value.Severity and intensity of service.Hours of utilization-one, two or three shifts?

  • ContdSusceptibility to wear and tear-is the machine subjected to dirt, friction, fatigue, stress, corrosion, smoke, and ash?

    e) Susceptibility to damage-is the machine subjected to severe vibration, overloading abuse, heat and freezing cold?

  • Contdf) Susceptibility to losing adjustment during use-will the maladjustment or non-alignment affect the accuracy or functioning?

    g) Safety requirements and considerations

    h) Criticality of item-if very critical, then the item may need daily inspection

  • Lubrication plays a very important and effective role in panned or preventive maintenance. Proper lubrication helps to prevent rust formation; reduce frication, thus reducing wear scouring and seizures, and economizes on power consumption; reduce heat; wash away worn material and particles; and increase equipment life.

  • In order to calibrate, it is necessary to decide on and lay down a standard. Once the parameters have been fixed, the maintenance personnel should endeavor to calibrate in accordance with these standards. There are two types of standards for calibration, namely, the primary standards and the secondary standards.

  • Cont

    Primary standards are laid down by the relevant body of standardization in all technologically advanced countries. Secondary standards are designed to the closest possible tolerance to the primary standards and put into actual use.

  • A manual or document describing its calibration system, must contain the following information/details

    A list of all equipment that needs calibration The calibration procedure to be adopted Calibration procedure for each of the standards used Calibration intervals for each of the above equipment

  • Contde) The necessary environmental conditions for calibration f) Method of implement the calibration system and its coordination with user departments g) Method for providing timely connective actions

  • ContdSystem of ensuring that calibration tags and records are affixed for proper implantation and control

    i) Designated facilities for calibration within and outside the organization

  • Quality of maintenance. As the quality of a product must be designed and built into the system, process, or method of maintenance for quality cannot be inspected. There are three principles of quality audit: Prevention is not only better but easier then cure Defects should be detected at the start and not at the time of inspection.Quality should be built into the process or system; this is better since inspecting it is difficult.

  • The total quality of maintenance depends upon will-designed plans, systems, and procedures, and the use of proper tools and test equipment, the adoption of correct technical practices, and the creation of a conducive environment for good maintenance, catering to the needs of trained and motivated manpower, with the top management giving support and realizing the importance of maintenance.

  • The Chief Maintenance manager is responsible to plan and train his personnel. Training is a continuing process and it can be successfully planned and impended only if the organization has a proper training policy and has the support is its top management. There are three levels at which training has to be imparted: Worker-level training. In the first instance, along with general training specific trade knowledge and skills have to be imparted,

  • Contd

    and in the second case, specialized knowledge of a machine or process will be the focus of training.

    ii. Supervisory training., It is essential that the supervisor he has a good knowledge of the job and in fact be the best man on the floor in terms of problem identification and problem solving

  • Contdiii. Executive training. Executive training has to be divided into separate categories, depending upon the hierarchical segment at which it is being directed (top level management, middle level maintenance executives or junior maintenance executives)

  • It is a common complaint that the maintenance workers often work below expected levels of performance. An incentive is a plan to compensate workers apportion to their outputs on a reasonably direct and continuing basis. Incentives can be financial or non financial. The non-financial incentives are designed to provide psychological motivation and have an emotional appeal.

  • There are two types of incentive plans.Direct incentive plans. These are usually based on the measurement of maintenance work, by any of the measurement techniques in use, like work estimating, activity sampling (or work sampling), analysis of past performance, and standard data, etc. these are now briefly dealt with.

  • Contdb) Indirect pans. Indirect incentives are based on broad indications of maintenance costs. They depend upon cost increases of total maintenance cost including labor, material, and overhead costs. Such indices reflect ratio changes in maintenance costs over period of time stretching to a few years