Maintenance and Reliability Best Practices. Ramesh Gulati
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need replacement after a specified number of hours of operations, e.g., a pump bearing on a hydraulic system to ensure that the system lasts through its design life. Any time we fail to perform maintenance activities, we may be shortening the operating life of the asset. Over the past few decades, many cost-effective approaches have been developed to ensure an asset reaches or exceeds its design life. Instead of waiting for assets to fail and then fix them, maintenance actions are performed to keep assets in good working condition to provide continuous service.
Why Have a Structured Maintenance Program
The most important reason to have a maintenance program with a structured approach is to ensure that assets don’t fail prematurely, that they keep producing or providing service as intended. Maintenance programs should sustain and improve production capacity and reduce overall facility costs by:
• Reducing production downtime—the result of fewer asset failures.
• Increasing life expectancy of assets, thereby eliminating premature replacement of machinery and assets.
• Reducing overtime costs and providing more economical use of maintenance personnel due to working on a scheduled basis, instead of an unscheduled basis, to repair failures.
• Reducing the cost of repairs by reducing secondary failures. When parts fail in service, they usually damage other parts.
• Reducing product rejects, rework, and scrap due to better overall asset condition.
• Identifying assets with excessive maintenance costs, indicating the need for corrective maintenance, operator training, or replacement of obsolete assets.
• Improving safety and quality conditions.
A structured maintenance program can have different philosophies, approaches, and practices embedded within the program. The basic philosophy is really twofold: Do some form of maintenance to an asset to prevent failure, or allow the asset to run to failure. The basic approaches to maintenance can be grouped into the following categories: condition-based maintenance (also known as predictive maintenance); preventive maintenance; reliability-centered maintenance; risk-based maintenance; proactive maintenance; corrective maintenance to correct discrepancies found during PM, CBM, OBM, etc.; and operator-based maintenance. A brief description of each of these approaches follows, with more details on each covered in later chapters, specifically Chapter 8, “Maintenance Optimization.”
Condition-Based Maintenance
Condition-based maintenance (CBM), also known as predictive maintenance (PdM), attempts to evaluate the condition of an asset by performing periodic or continuous asset monitoring. The ultimate goal of CBM is to identify proactive maintenance actions to be performed at a scheduled time when the maintenance activity is most cost-effective and before the asset fails in service. The “predictive” component stems from the goal of predicting the future trend of the asset’s condition. This approach uses principles of statistical process control, trend analysis, and preselected thresholds to determine at what point, in the future, maintenance activities should be scheduled.
Most CBM inspections are performed while the asset is operating,thereby minimizing disruption of normal system operations. Adoption of CBM/PdM in the maintenance of an asset can result in substantial cost savings and higher system reliability.
There are a number of different CBM/PdM technologies that can be used to evaluate an asset’s condition. A few of the more common technologies (or data) are:
• Vibration analysis
• Infrared (IR) thermography
• Acoustic/ultrasonic—sound-level measurements
• Oil analysis
• Electrical testing—amperage plus other data
• Shock pulse method (SPM)
• Partial discharge and corona detection
• Operational performance data—pressure, temperature, flow rates, etc.
In the CBM approach, the maintenance need is based on the actual condition of the machine rather than on some preset schedule. Activities such as changing oil are based on a predetermined schedule (time), like calendar time or asset runtime. For example, most of us change the oil in our cars every 3,000–5,000 miles (or kM) driven. This is effectively basing the oil change needs on asset runtime. No concern is given to the actual condition and performance capability of the oil. It is changed because it is time to change it. This methodology would be analogous to a preventive maintenance task.
On the other hand, if we ignore the vehicle runtime and have the oil analyzed at some regular period to determine its actual condition and lubrication properties, then we may be able to extend the oil change until the car has been driven 10,000 miles or maybe even more. In new cars today, engine oil change is determined by the remaining oil life calculations based on an algorithm of a parameter such as engine runtime,engine temperature, and engine rpm instead of just miles driven.
This is the advantage of utilizing condition-based maintenance. CBM is used to define needed maintenance tasks based on quantified asset conditions or performance data. The advantages of CBM are many. A well-established CBM program will eliminate or reduce asset failures cost-effectively. It will also help to schedule maintenance activities to minimize overtime cost. In addition, we will be able to minimize inventory and order parts, as required, well ahead of time to support the downstream maintenance needs.
Past studies have shown that a well-implemented CBM program can provide an average savings of 10% (7–15%) over a program utilizing preventive maintenance alone. These savings could easily exceed 30–40% if there is not an effective PM program in place. In fact, independent surveys and technical papers presented at the International Maintenance Conferences 1999–2019 combined with the author’s own experience indicate the following industrial average savings resulting from a well-established condition-based maintenance program:
• Reduction in maintenance costs: 15–30%
• Reduction in downtime: 20–40%
• Increase in production: 15–25%
On the downside, starting a full-blown CBM program utilizing all the mentioned technologies can be quite expensive. Some technology’s test equipment may cost more than $40,000. In addition, training plant personnel to utilize PdM technologies effectively will require considerable funding as well. This is one reason to have an RCM basis for choosing where to apply which CBM technology; it helps to determine the test equipment purchase that can provide the most “bang for your buck.” Program development will require an understanding of predictive maintenance and a firm commitment to make the program work by all facility organizations and management.
How the CBM team should be organized is another issue. We have found that a centralized dedicated team is a good way to start a program. This approach helps in standardizing testing methods and practices.
The CBM approach consists of scheduling maintenance activities only when equipment or operational conditions warrant—by periodically or continuously monitoring the machinery for excessive vibration,temperature, noise, etc. When the condition gets to a level that has been predetermined to be unacceptable, the asset is shut down. The asset is then repaired or has damaged components replaced in order to prevent more costly failures from occurring. This approach works very well if personnel have adequate knowledge, skills, and time to perform the CBM work. In addition, the company must allow asset repairs to be scheduled in an orderly manner. The approach provides some lead time to purchase materials for the necessary repairs, reducing the need for a high parts inventory. Because maintenance work is only performed when it is needed, there is likely to be an increase in production capacity.
Preventive Maintenance
As stated previously, a CBM approach is the preferred approach if your organization can handle the expense of implementing this approach. However, a preventive maintenance (PM) approach is the next best thing and