Last year in this department, we ran a three-part series on “Competitive Maintenance Strategies” (March, April, and May 2010, available in our archives at http://www.powermag.com). We began that series with a description of the three typical approaches to equipment maintenance (corrective, preventive, and predictive) and then progressed into the essential requirements for a robust reliability-centered maintenance program. The series concluded with a series of equipment-specific suggestions used by many plants to cut their operation and maintenance (O&M) costs while achieving high plant reliabilities.
This year’s series will focus on predictive maintenance (PdM), also known as condition-based maintenance. PdM requires maintenance on equipment only when the condition of the equipment warrants it, rather than at a predetermined maintenance interval—the type that a car manufacturer uses to specify the timing of oil changes. PdM requires an investment in equipment and training to routinely monitor operating equipment, but a well-defined and -executed PdM program saves time and money by reducing unneeded time-based maintenance tasks and by identifying and fixing problems before they occur.
Perhaps the most significant example of the benefits of a robust PdM program that I am aware of was described in “Entergy’s ‘Big Catch’” (October 2008). In that instance, a vibration anomaly in the Waterford Unit 2 steam turbine bearing, below alarm limits, was intercepted. The turbine was quickly shut down, and an examination of the generator rotor shaft found a crack that extended 180 degrees around the shaft and at least 1.5 inches deep (Figure 1). By catching the problem early, a potential catastrophic failure was averted that would have caused an extended outage, cost between $20 million and $40 million to repair, and perhaps injured or killed plant staff.
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| 1. The “Big Catch.” A dollar bill is inserted into a 180-degree, 1.5-inch-deep crack in Entergy’s Waterford 2 generator shaft. The utility’s Performance Monitoring and Diagnostic Center first identified an early vibration transient, which was confirmed by the plant’s PdM staff. That discovery allowed the unit to be safely shut down before a catastrophic failure occurred. The photo speaks volumes about the value of a healthy PdM program. Courtesy: Entergy |
Developing Goals and Objectives
Many plants have, or are implementing or expanding, a PdM system. Others are considering upgrading, in whole or piecemeal, an old preventive maintenance system to a conditioned-based maintenance program. Regardless, we want to begin this PdM system discussion at the beginning, the implementation phase, rather than jumping directly into PdM techniques and practices. We want the discussion to be inclusive, especially for those considering adopting a PdM approach to equipment maintenance.
The first step in developing an effective PdM program is to define your goals or objectives in realistic terms. That requires you to have a good idea of what can be accomplished within your organization, and how fast. If managers expect too much, the program will be a disappointment; expecting too little may mean that the program does not receive serious consideration.
Whatever the goal, it is critical that it be measurable. All data must be easily and accurately collected, and any assumptions made in data analysis must be agreed upon. For example, it is often best to select a value that is already being measured and is important to plant management, such as equipment availability, hours of machine downtime, steam-turbine maintenance cost, and the like. It is also important to define all of the critical terms and assumptions used in the analysis. A goal could be something like “Reduce maintenance expenses over the next 12 months by 15%.”
Next, you should determine specific actions that must be taken to achieve the defined goals and objectives. To do so, you must know where you are today. Collect historical data on maintenance costs, labor, overtime, spare parts inventory levels, frequency of machine repairs, or whatever other information is pertinent to the specific goals and objectives. This data can be obtained from purchasing, engineering, maintenance management, personnel, accounting, and operations.
A key requirement is to identify machines that have either an excessive failure rate or a high historical cost of repair. These machines should be the focus of attention at the start because they are the most likely to contribute early and dramatic successes as well as highly visible cost savings. Machines should also be included that have patterns of common problems that can be easily fixed, such as misalignment and unbalance.
After determining which problems will be the easiest to solve, you should next identify the technologies that offer the greatest promise of solving those problems. There are more than a dozen PdM technologies, and eventually you should become familiar with all of them, but the core of most power plant PdM programs is vibration analysis, thermographic analysis, ultrasonic analysis, oil analysis and lubrication, and root cause analysis, each of which will be discussed in detail in future articles (Figure 2).
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| 2. Five PdM tools. These five tools must be the focus of every predictive maintenance program. Source: POWER |
At this point, it’s time to develop monitoring hardware and software specifications; determine training requirements; and select the monitoring points, data-collection routes, sample frequencies, and alarm limits. This is an excellent time to get the rest of the maintenance staff and operations involved, as well as those on the condition-monitoring team. Schedule individual and group sessions to explain the goals and objectives of the program, and listen to the comments and concerns of each group. Enlisting their support at this early stage will often ensure their support later on, and it will allow you to take advantage of their practical experience with the machines that are going to be monitored.
For example, ask what the most common problems are and how they would solve them. Ask if there are any concerns about job responsibilities or the threat of discipline if a lot of problems are found. Ask if there is any machine condition information that would make their jobs simpler or more effective.
This is also a critical time for the continued development of plant management support. After management’s agreement on the goals and objectives, you have to convince them that your implementation plans are economically sound and realistic. An effective machine condition-monitoring program is a long-term commitment, and often it may take six to 12 months for start-up and initial data collection/trending plus 12 to 18 months to achieve a significant return on investment (ROI).
In addition to providing hardware and software, management must also provide the people and training resources to make the capital investment worthwhile. A common mistake is to assign people to machine condition monitoring on a part-time basis during program start-up, feeling that as the program grows, they will spend more time on it. Often, the result is that these people aren’t available when they are needed, making it difficult, if not impossible, to take consistent, timely data readings. There should also be an understanding that initial data collection and entry can be a very time-consuming task, and temporary clerical support or overtime may be required to get the job done properly. Perhaps the greatest mistake is not providing enough training in data-collection and data-analysis techniques.
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