Operations & Maintenance

Jim Sharkey, Senior Project Manager of Electric Power Research Institute (EPRI), writes on the correct set of responses that should be taken in accordance with machine vibration data at a nuclear energy facility.

By Jim Sharkey, Senior Project Manager, EPRI

Once elements of a reliability-centered maintenance program have been implemented at a nuclear station, many condition directed maintenance tasks will be triggered by machine vibration data. It becomes important, then, to understand the actions that should be taken in response to these data.

Data overload

Confronted with a large and expanding vibration database, a vibration analyst may be overcome with questions of where he should concentrate his attention and what actions he should take to ensure important data is not ignored. Distilling techniques can be used to reduce information from an existing vibration da¬tabase in a manner that yields necessary maintenance decisions.

It is not possible for the vibration analyst to individually inspect and resolve every subtle change in amplitude in his station’s vibration database. Action thresholds can be used to determine whether a given amplitude measurement warrants additional investigation. Three action thresholds are common in the nuclear industry.

Alert

 “Alert” refers to that level of overall vibration that simply warrants additional evaluation by the analyst. It is commonly presented in units of overall velocity, and within a frequency range of approxi¬mately 10Hz to 1000Hz. Machine vibration exceeding the alert value is treated as a symptom of a potential machine fault.

Resultant evaluation is left to the skill of the analyst. The analyst will probably want to investigate machine and vibration characteristics other than amplitude prior to making a maintenance recommendation. Alert vibration levels do not necessarily reflect degradation or warrant corrective maintenance.

Danger

 “Danger” refers to a higher amplitude of overall vibration that is not just a symptom of degradation, but of such severity that it is the actual problem itself. In this case, vibration may cause damage to the machine. This is the last warning the vibration analyst is able to provide his station of pending machine failure. The threshold is commonly presented in units of overall velocity, and within a frequency range of approximately 10Hz to 1000Hz.
Alert and danger thresholds

Alert and danger thresholds are insufficient to identify early stages of rolling-element bearing degradation. Damage to contact surfaces in these types of bearings may occur under the influence of load, or in the presence of inadequate lubrication, contamination, circulating currents, or background vibration.

Early stages of rolling-element bearing degradation will be initially noticed in higher frequencies of vibration (i.e. > 1000 Hz), and be most clearly presented in units of acceleration.

Once an action threshold is exceeded, the vibration analyst uses his training and experience to determine the reason a machine’s vibration condition has exceeded the threshold. The result of his in-vestigation may lead to a maintenance recommendation delivered to station management.

Effective recommendations

The importance of creating an effective maintenance recommendation should not be understated, as it may be the single barrier against equipment failure and the resul¬tant loss of those system functions it supports.

A five-step process can be used to promote a systematic approach to vibration analysis. Note that these steps are not prescriptive and are not intended to replace the analyst’s independent analysis.

1. Confirm the vibration condition
2. Review vibration and other data
3. Evaluate the vibration condition against likely causes
4. Report evaluation results to the station organization
5. Inspect forensic results

More information on this approach can be obtained in an EPRI white paper titled, “Developing Maintenance Decisions Through Systematic Use of Machine Vibration Data,” EPRI Report No. 10222884, available at www.epri.com.

The white paper includes an example machine vibration review sheet for documenting and analyzing vibration data, and a vibration fault matrix that itemizes 21 potential machine faults and common symptoms.