Understanding preventive and predictive maintenance
Preventive (PM) and predictive maintenance (PdM) can improve productivity, delivery, customer satisfaction, and profits.
Preventive (PM) and predictive maintenance (PdM) is a series of tasks and company policies that, if followed, can improve productivity, delivery, customer satisfaction, and profits. Three critical guidelines help maintenance managers effectively use these principles:
- Maintain the production and plant utility systems equipment as close to brand new condition as possible. Have all equipment ready to start up and run with no unplanned shutdowns.
- Maintain the production equipment and plant utility systems equipment in the best possible operating condition for their intended use while the machines are in service.
- Complete all PM and PdM work on a regularly scheduled basis without exceeding a point of diminishing returns on investment of labor, tools, and materials required.
What’s the difference between preventive and predictive maintenance?
Preventive maintenance tasks are completed when machines or equipment are shut down and predictive maintenance activities are carried out as they are running in normal modes.
With PM and PdM systems – as in all systems and processes of work – there are Who, What, When, Where and Why questions to answer before any actual work begins. With the above three guidelines, we have already defined the “Why” question. The “Who” question relates to several different types of members of the PM and PDM team.
- Someone who has an abundance of maintenance and plant engineering experience should write individual tasks. To receive the expected results from the investments made in the PM and PDM protocol, the person writing the details of what needs to be done must have a deep understanding of the many aspects of machines. Aging, wear, component material fatigue patterns, effects of dirt and other contaminants, heat/cold, humidity, effects of chemical contact, vibration, lubrication practices, measurement processes, maximum safety methods, work efficiency standards, work schedules, people skills, and plant processes are all factors that must be carefully considered.
- The mechanics and electricians performing the PM and PdM work must be of high caliber and possess the skill levels of a maintenance department. On average, 85% to 90% of PM and PdM work orders call for machine inspection work and only 10% to 15% lubrication work. The people performing these tasks must fully understand machine and machine component operations before they can effectively inspect for specific problems and negative operating trends.
- The management person directly responsible for the people performing the PM and PdM work must understand the work to be accomplished and set the performance standards, goals and expectations. They must be able to monitor the quality and quantity of work completed as well as measure the results. Also, they should be able to make ongoing changes and improvements to the individual PM tasks as part of an overall continuous improvement effort. These changes are dictated by the results of measurements and changes in the plant processes and equipment. Finally, the management person should also be able to complete component failure analyses. Determining why and how a component failed, is the first step in determining how to prevent subsequent failure.
- The plant upper management must view the PM and PdM work system as required constant work practices are as important to the production process as any other function. This will require a minimal planned downtime of equipment to accomplish all PM and PDM work.
- Production employees also are a significant part of PM systems. They can be, and many times are, the first to see changes in the equipment they operate. Total productive maintenance (TPM) represents the active participation of all production employees in various machine set-ups, inspections and, in some cases, the lubrication of machinery. The amount of participation in TPM varies with the complexity of the equipment, the types of processes involved and the overall skill levels of the workforce. Training the workforce and setting its expectations vary with the philosophies of the plant management of each facility.
- Contractors should be included in some PM work, especially for PdM work. Many contractors can supply cost-effective services in some of the more specialized inspections and tests required. For example, many plants do not have trained refrigeration mechanics and they hire contractors to do the scheduled PM on HVAC units for the plant systems. Other contractor applications may be for an air compressor and air dryer PM. In addition, vibration and ultrasonic analyzing is a highly specialized aspect of PdM and requires extensive training and high-test equipment purchase and upkeep costs.
The “what” and “where” of PM consists of what equipment is to be inspected and lubricated and to what extent and detail the work is to be performed. Typically detailed PM and PdM procedures are written for all production equipment to ensure that all machine components are inspected and lubed for maximum sustained operation. This process should also be applied to other plant equipment systems, such as machines that supply the plant utilities including air compressors, air dryers, boilers, electrical substations, motor control centers, and wastewater treatment. Plant safety systems also should be included, including natural and propane gas systems, tanks, fire alarms, suppression systems, emergency lighting, overhead cranes and hoists, and ceiling mounted items such as lights, fans, and piping. Many plants also choose to include PM work on HVAC systems, overhead door and dock plates, forklifts, company vehicles, truck fleets, roof leak detection, air emissions, and other systems usually not considered until there is a problem.
Write it down, make it protocol
The detail and extent of the PM and PDM work vary with the type of equipment involved. The written PM procedure is the document telling workers what needs to be done. This document needs to contain all of the tasks that will provide the most thorough inspections and lubrication of machines in planned downtime, without exceeding the point of diminishing ROI. Generally, the PM inspections of most machines are one of two types. The first is for anything that moves or causes some other machine part to move. This needs to be inspected for damage, wear, loose and missing fasteners, etc., and proper lubrication performed. The second is the inspection of static, non-moving machine components such as wiring, plumbing lines, and hoses, structural support members, etc., for damage, cracked welds, loose and missing fasteners, etc. These procedures may be lengthy or simple, depending on the type of machines involved. The main concern is the person writing the procedures is very experienced in plant maintenance and plant engineering. This person must understand and be experienced in all phases of static and dynamic machine principals and actual machine degradation analysis.
“When” PM should be performed depends on several factors. Some machines are simple in design and function and some are not. Typically, there are items to be inspected and lubricated on a daily basis. Other inspections and lubes are progressively more detailed and regularly performed on a bi-weekly, monthly, quarterly, semi-annual and/or annual basis, depending on what is required.
In addition to the machine design and basic function, several other factors help in determining the best time interval between PM tasks. One is the amount of time the machine runs between regularly scheduled shutdowns and/or how much time is available for PM. Does the machine run 24 hours per day, seven days per week, or eight hours per day five days per week? Another factor is the environment in which the machine runs. Is it humid and damp, or extremely hot and dry? Does the machine receive shock loads or run with moderate to high vibration levels? Is the machine subject to chemical spillage or leakage, ultraviolet light, etc? Good operating and cleaning practices, or the lack of them, have a significant impact on PM scheduling. In addition, planned shutdowns for plant expansions, machine rebuilds, inventories, vacations, etc., also dictate when some of the more involved PM work can be accomplished.
Predictive maintenance for a new decade
Most of the above information relates to preventive maintenance procedures completed on machines while they are shut down. There are other tasks that are considered predictive maintenance (PdM) practices. One of these, usually done while the equipment is shut down, is oil sampling and analysis. Oil samples are taken and sent out to laboratories that specialize in analyzing industrial oils. The cost is relatively inexpensive and provides much valuable information. This process identifies the lubricating ability of the oil; its stability; contents of water, wear metal particles, and dirt; among other aspects.
Another important PdM procedure is vibration analysis. Using a portable vibration analyzer, readings are taken from many points on machines. These readings are direct and extremely accurate measurements of the vibration amounts and frequencies produced by the moving parts of the machine. These vibration amounts and frequencies are used to tell if any parts need replacing or adjustment, and exactly what internal parts are causing problems. Bad bearings, excessively worn gears, and poor coupling alignment can cause excessive vibration, weakened mounting fasteners and many other mechanical problems. Vibration analysis also can tell you if pumps have loose impellers, air cavitation, faulty valves, or mounting problems. In addition, it reveals trending information. When vibration readings are taken, they provide a base number to use as a gauge to determine if and how much internal machine changes are taking place between inspections. This is the very best tool for detecting the correct health of machines and for trending the internal activities inside motors, gearboxes, pumps, large fans, compressors, and many other machine components. There is no better way to detect machine problems before they cause an unplanned shutdown due to a component failure. The fact that vibration analysis is done while the machines are running in their production modes allows the testing to be done at any time and is especially important for any plant that runs 24/7.
Seven big picture considerations
There are many aspects of maintenance and other plant functions that have an effect on the number of machine breakdowns and the length of downtime. Some of these considerations are not normally associated with the term preventive maintenance, but nonetheless contribute to equipment failure. Therefore, they should be considered as a part of a comprehensive approach to preventive and predictive maintenance.
- Proper start-up protocols. Start-up operations; changeover and set-up; and shutdown procedures should be carefully planned and consistently implemented on all machines.
- Procedure lists for unplanned shutdowns. Outline what is to be done when an unplanned power outage occurs and what to do before the power comes back on.
- Emergency management plans for floods, fire, etc.
- Machine component rebuild programs to ensure quality and consistency.
- Machine cleaning practices and procedures.
- Available informational equipment manuals for maintenance personnel.
- Solid troubleshooting skills by all maintenance personnel.
The above information provides an outline of a comprehensive PM and PdM system. This approach leaves little, if nothing, to chance. While there are many variables to take into consideration, if properly designed, instituted and operated, the PM and PdM system will help to ensure dependable and predictable performance from all serviced equipment, machines, and related processes.