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Maintenance, Reliability and Troubleshooting in Rotating Machinery


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turbines used as mechanical drives in compressor trains.

      Once all failure modes and their associated consequences are identified, then a comprehensive maintenance strategy can be developed. If the deterioration of a critical component can be measured and trended while a machine is in operation, then a condition monitoring strategy makes sense. However, if the deterioration of a critical competent cannot be assessed while a machine is in operation, then scheduled outages and inspection should be incorporated into the overall maintenance strategy.

      A possible reliability strategy for a critical compressor might look like this:

      1 Condition monitoringContinuous vibration monitoring with alarms and tripsContinuous temperature monitoring with alarms and tripsContinuous or periodic performance trendingContinuous performance monitoring of compressors if the required instrumentation and analysis software are available

      2 Periodic inspectionsDaily operator walkthroughsWeekly inspections by mechanicsQuarterly performance evaluations by engineering if online performance capabilities are not in place

      3 Scheduled EventsPeriodic borescope inspections of gas turbinesPeriodic water washing of gas turbinesCleaning (air-cooled electric motors, heat exchangers, fin fan coolers, etc.)Scheduled overhauls based on time in service or fired hours

      There is no generic maintenance strategy for all the different types of rotating machinery. Each situation must be evaluated based on the historical failure mode, their consequences, and your maintenance budget. Some trial and error may be needed to identify the right mix of maintenance strategies.

      A safeguard is any hardware and procedure designed to directly prevent an equipment failure or mitigate its impact. Typical safeguards include operating procedures, vibration monitoring systems, relief valves, overspeed trip systems, and surge control systems. One of the deliverables of a machinery reliability assessment is to determine if the current set of safeguards are adequate. If they are not considered adequate, then you must explain why they are not and provide detailed recommendations regarding any additions or modifications to the existing safeguards required to reduce risk to acceptable levels.

      An example of a safeguard is a procedure, alarms, or control system designed to prevent a harmful compressor operating condition. For example, variable speed centrifugal compressors have a maximum rotational speed limit. Above the maximum speed limit there is the potential to permanently damage the compressor rotor due to overstress. Every operating limit typically has a related adverse consequence, i.e., if a certain condition occurs, then something bad could happen. Examples of compressor operating conditions to avoid include:

      1 Critical speeds

      2 Minimum speed

      3 Maximum speed

      4 Surge flow

      5 Stonewall flow (sonic flow conditions)

      6 Maximum discharge temperature limit

      7 Maximum vibration (% of bearing clearance) limit

      8 Radial and thrust bearing temperature limits

      9 Maximum horsepower limit

Graph depicts the centrifugal compressor operating limits. Schematic illustration of the typical centrifugal compressor surge control system.

      Note: Axial compressors have been known to fail catastrophically at high flows due to blade vibration and high temperatures generated when operated beyond the stonewall line.