Often, the implementation of a predictive-maintenance (PdM) program is based on identifying imminent failures and preventing them — or by predicting a best estimate of an asset’s remaining useful life. When this approach is used, the program’s success typically is measured by the identification of asset issues, and by calculating the cost of avoiding failures and lost production. The savings are used to justify the program.
A real-life analogy can help illustrate this approach. Imagine a patient goes to the dentist, and the dentist finds cavities. The patient may ask how long it is possible to delay cavity repairs and still retain the affected teeth. In the end though, he or she will still follow the dentist’s suggestion to have the cavities filled. Tooth removal would be a last resort to avoid a costly or painful experience.
If that same patient receives a clean bill of health from the dentist on multiple visits, how might this change the patient’s behavior? Would he or she decrease the frequency of visits? Perhaps stop seeing the dentist altogether? Could this behavior result in a missed opportunity to intervene before another tooth needs to be removed?
There is value in validating good dental health and being advised of possible issues that could be prevented with proactive care. Similarly, organizations must resist the temptation to reduce or eliminate elements of a predictive-maintenance program simply because they are not seeing obvious savings.
Knowing the plant is running with healthy equipment can be more important than knowing when problems are evident. When properly facilitated and maintained, a predictive-maintenance program that is not regularly identifying faults is still effective and essential. Instead of reducing or eliminating an effective PdM program, it should evolve into an condition-monitoring program that is able to detect potential failures, identify lost performance and validate continued trouble-free operation.
Mistaking PdM Success for Inefficacy
New PdM programs typically identify many issues. As these issues are remedied, the resulting improvements allow the reliability team to claim significant cost avoidance. As the program matures, however, the number of new issues discovered is reduced due to the assets operating in a healthier state. Defects have been eliminated, operational problems have been rectified and equipment “infant-mortality“ issues have been addressed.
Once a PdM program stops finding defects — a direct result of the program being effective — it can be tempting to call for reduced spending. The value of the cost avoidances may not always support the PdM program expenditures; at that point, management may perceive that the program is not paying for itself and call for cuts.
This type of reduction can result in staffing cutbacks, the elimination of program assets and extensions of inspection frequencies. The results of these reductions may not be readily evident to management; they typically will only notice the expense reductions. As time goes on, however, when asset failures result in adverse effects on operations, management may decide the PdM program is ineffective. The thinking may go, “We have a PdM program, but it did not notify us of an issue on a critical asset, and we had the failure anyway” — all while forgetting the reductions in PdM investment.
The reality, however, is simply that reduced attention and spending have allowed the PdM program to fall behind to the point where it is no longer operating proactively.
Expanding the Benefits of PdM
The best approach to stay in charge of plant health is to be proactive. Instead of cutting the number of assets examined, reducing maintenance staff and decreasing data-collection frequency, organizations should treat plateauing PdM programs as a foundation for a condition-monitoring program. Creating a robust condition-monitoring program will help plant personnel look beyond individual assets to overall plant health and performance. This widening of scope can further increase asset and plant health, leading to higher levels of performance and increased cost savings.
Often, predictive-maintenance programs are developed with a singular view of finding issues, reporting them and acting to mitigate their effects. In these scenarios, assets without any concerns are not of primary focus because, if an asset has no issues, nothing needs to be done. With utilization of process data, however, plant personnel can have a full picture of each asset’s health and overall system performance, providing opportunities for improvement.
For example, suppose a key asset such as a 100-gal/min pump is functioning well enough that the PdM program does not flag it. A small flaw, however, has the pump operating at 90 percent efficiency. In this circumstance, the pump may appear healthy, but in reality, the plant is losing 10 percent pumping capacity. It may not be readily evident, but the plant is losing production rate, reducing product yield and introducing quality concerns. These financial losses will never be regained.
A condition-monitoring program focuses on knowing the status and health of all assets and, ultimately, system performance and health. Paying attention to asset health allows reliability teams to ascertain, as early in the detection process as possible, if an item of concern is developing. In the case of the underperforming pump, a condition-monitoring system will alert plant personnel that the pump is operating but with degraded performance.
Early notification provides time for confirmation, evaluation, root-cause analysis, planning, scheduling and work approvals. It also can lower costs due to better inventory utilization: More parts can be purchased as needed rather than being kept on hand to accommodate lead-time issues.
Condition Monitoring for Process Heating
A move from PdM to condition monitoring is a move toward holistic asset health and performance monitoring. The more plant personnel knows about the health of plant assets, the better position they will be in to understand the status of the assets and, ultimately, the health and performance of the plant’s production systems. As a plant makes a move toward condition monitoring from PdM, the metrics for success also change.
With a condition-monitoring system, organizations typically measure overall plant health with an emphasis on how early in the process an issue can be identified — classified by zone (figure 1).
In the proactive zone, issues are discovered before they are evident to the operator, allowing time for root causes to be identified. At this stage, no significant loss of equipment life has occurred and, most importantly, the issue has not affected the process. Identifying as many faults as possible in the proactive zone helps maintain the highest levels of performance.
An issue not fixed in the proactive zone will move to the remediation zone. The remediation zone occurs when an issue has been identified and may be evident to the operator. At this stage, the issue has begun to affect equipment life, and some performance impact has likely begun.
The final zone is the reactive zone. In the reactive zone, the asset has failed to meet requirements regarding availability, production rate or process quality. The asset will have a decreased life.
A uniform system for displaying plant health can be accomplished with intuitive designations. Green status means an asset has no issues or concerns. Yellow-status assets have developing areas of concern. Red status signifies an asset has a serious problem or has failed.
A system such as green-yellow-red identifiers (figure 2) for overall asset health helps personnel to prioritize work and maintain plant-wide health and performance. With such a system, operators and technicians do not have to analyze alerts and events to identify where problems exist. Instead, they simply strive to keep all assets in the green zone. With such a system, operations — and management — can quickly confirm that processes will be available at full production rate with optimal quality.
The Value of Condition Monitoring
When plant personnel are aware of plant asset health, they can make decisions quickly as abnormalities begin to develop. Reliability and maintenance teams can take proactive steps to mitigate or perform repairs. Also, when maintenance personnel have enough warning, they can execute their work more efficiently. Technicians can thoroughly review the bill of materials for an asset and determine the proper materials needed, safely plan required activities, provide additional training as needed and schedule service at a time that best accommodates production activities and maintenance utilization.
Ideally, implementing a high quality condition-monitoring program will help an organization move quickly from reactive maintenance to a condition-monitoring profile. Continued efforts to improve condition monitoring will help the organization move to a proactive-maintenance profile.
The value of a condition-monitoring program can be supported by illustrating increased performance using overall equipment effectiveness (OEE). This holistic measurement encompasses asset and materials availability, production rate or yield, and end quality. OEE provides a view of process results and covers operations, procurement, logistics and maintenance. It serves as a tool for each group to focus on a common goal (figure 3).
In conclusion, most people would prefer to receive a good physical examination result from a doctor rather than discovering a heart condition needing immediate intervention. Likewise, knowing production equipment is healthy can provide more benefits than simply identifying problems with it once it is failing.
Plants will always see higher reliability and increased performance from assets remaining in good health. Condition monitoring allows organizations to continue to improve on gains made with their PdM programs. In addition, condition monitoring allows them to push for higher availability and better quality, with even small improvements having the potential to provide savings.
Christopher Jensen is a senior advisor with the professional services team of Emerson Automation Solutions. H.P. Slater is an independent consultant. Emerson Automation Solutions, Round Rock, Texas, can be reached at 512-834-7039 or visit emerson.com.