Effective maintenance programs start with a critical equipment list
Assess the relationships between various types of maintenance to determine the best strategy.
By Jeff Townsend, Progress Energy
- An effective maintenance program is founded on a critical equipment list developed through a rigorous analysis of the probability of failure and the consequences of failure.
- Multiple sources of data are available for quantifying the probability and consequences.
- The details of the resulting maintenance program needs to be reviewed no less than annually to accommodate changes in equipment condition and technology.
Preventive maintenance, run-to-failure, zero tolerance for equipment failure, predictive maintenance, operator rounds, thermography and routine maintenance are the tools and programs that facilities use to ensure plant equipment performs effectively and efficiently.
Of course, we all know there’s much more to this picture than a simple question and subsequent answer. What would a perfect power-generating facility look like from an operations and maintenance perspective? What would be the perfect activity balance to maximize resources while operating safely and operating within budget? This can be broken into several categories.
The first category would be management perspective or management protocol with regard to maintenance. Different managers can have various mindsets when it comes to preventive maintenance versus run-to failure models. Some managers established hard lines for the preventive maintenance and corrective maintenance work activity percentages.
Corrective maintenance typically falls into one of two categories. The first category is when equipment that has been well maintained simply breaks or fails to some degree that it requires maintenance. Corrective maintenance to make any needed repairs is scheduled and performed.
The second corrective maintenance category is when equipment is run to failure deliberately before any corrective maintenance is scheduled and performed. This is what we’ll call the run-to-failure model. As indicated earlier, some managers have hard percentage numbers for the relationship between corrective maintenance and preventive maintenance. A typical scenario would be 60% of maintenance work hours are scheduled for preventive maintenance and the remaining 40% of scheduled hours would be for corrective maintenance.
Define the numbers
There are two basic approaches to determining these percentages. One approach holds that the more preventive maintenance performed, the less corrective maintenance will be needed. An example is changing the oil in an automobile. It’s generally accepted that if motor oil is changed regularly, the less engine maintenance will be required. Using the same vehicle example, a person wouldn’t consider changing certain pieces, such as a muffler, until it actually fails. That’s the run-to failure approach given that mufflers rarely fail and the cost to replace one is relatively low. To review the first item in our discussion, management philosophy normally determines the percentage for corrective maintenance work hours and preventive maintenance work hours.
“Determining the consequence level and the failure probability allows us to determine the maintenance frequency.”
The preventive maintenance percentages are the leading indicators for the overall maintenance program. Managers who follow this percentage guideline would expect the corrective maintenance work hours to be 40% of the total work week hours, if 60% of the hours were spent on preventive maintenance. If either percentage fell outside a predetermined tolerance window, the maintenance program would undergo a review.
Now that we’ve defined the two subcategories of preventive maintenance and run-to-failure in a maintenance program, we need to determine the category in which each piece of equipment falls. Most maintenance programs begin by establishing a critical equipment list.
Make a list
The list can be delivered several ways, but most programs use some form of the probability and consequence table to determine the category. This involves a determination that quantifies the probability of equipment failing and the consequences associated with the failure.
Consider a 1-to-10 scale with 1 being a low probability and 10 being the highest failure probability. Management needs to assign the actual scale numbers to use for this determination. Most equipment in a power-generating facility has a low failure probability, or else it wouldn’t have been purchased and installed in the first place. Even so, there will be some equipment with a higher failure probability than others, and these receive a higher probability rating. Because probability failures are normally low, the consequence of a failure number assigned to the piece of equipment carries more weight than the probability.
But we need to use both numbers in our run-to failure versus maintenance performance equation. We’ll use the same 1-to-10 scale for the consequence, with 1 being low-to-no consequence and 10 being high negative consequence.
How do we quantify the equipment failure consequence? Most power-generating facilities use the safety, environmental, equipment and megawatts (SEEM) method. Safety should be the No. 1 criterion when determining negative consequences. Going through the acronym, the second most importance consideration is environmental impact, followed by equipment damage and finally megawatt loss.
Each item is important and should be considered when determining the equipment failure consequences. Determining the consequence level and the failure probability allows us to determine the maintenance frequency. The critical equipment list is the final list of identified equipment with the highest level of probability and consequences.
There are many tools and programs that can help determine equipment failure probability and consequences. The plant’s historical records are a good starting point. Reviewing the equipment past performance, including corrective maintenance work activities performed along with documentation from any preventive maintenance records, helps establish a number for the equipment failure probability and consequence.