How a pump health audit helped a power plant move from a preventative to a predictive maintenance model.
Deciding the right time to service equipment is critical. Refurbishing equipment too soon results in unnecessary costs in service, materials and labor; it can also put equipment at more risk, as many problems have the potential to occur during equipment start-up. Waiting too long increases the likelihood of an online failure, which can be catastrophic and compromise the system’s ability to deliver its required output.
Developing an optimal maintenance frequency requires pump users to accurately assess the current health of the equipment, identify signs of degradation, and understand how this degradation will affect future equipment health. Many times, end users don’t have the necessary instrumentation or experience to perform this assessment. In these situations, completing a comprehensive field performance test can provide the information necessary to make decisions that will best benefit the equipment, system and profitability of the plant.
Maintenance planning
A combined cycle power plant in the Northeastern US adopted this approach to improve maintenance planning for its seasonal outages. The goal was to understand asset health in order to prioritize what equipment needed maintenance in the upcoming outage cycle and to understand the risk of delaying maintenance on any of its assets. The pumps were not instrumented in a way that captured sufficient data to make this assessment, nor did the plant have in-house vibration or hydraulic experts to interpret the required data.
Pump health audit
Understanding that it needed both hydraulic and mechanical performance data to complete its assessment, the plant reached out to Hydro Inc to provide a pump health audit on its two condensate extraction pumps. Hydro’s dedicated field engineering arm, Hydro Reliability Services (HRS), regularly performs extensive field testing with the goal of evaluating system health and identifying potential problems.
Before testing was completed, a preliminary review of the pump and system was performed. This review included an evaluation of the pump and system design and an examination of maintenance and operations historical records. This assessment was necessary to provide a background of information to support evaluation of the test data, identify areas of weakness, and aid development of recommended actions.
Field testing
Field testing was then undertaken on both the pump and the motor under typical operating conditions. The field testing included collecting a wide array of data, including suction flow, suction and discharge pressure, motor volts and amperage, pump speed, vibration signatures (velocity, displacement and acceleration), and thermographic imagery.
While on-site, an inspection of the system was completed to determine installation integrity. This included the sealing system, auxiliary piping, lubrication system, suction and discharge piping, installed instrumentation, baseplate and foundation. The inspection looked for corrosion, leakage and weld degradation; it also examined supporting structures and the foundation grouting.
Results of the field testing clearly indicated that the A condensate pump needed to be refurbished. Fortunately, the B condensate pump was in good condition and could continue to safely operate and deliver the expected flow to the system.
The A pump was classified as being in the “caution” level for hydraulic performance and was operating in a substantially degraded condition, with total developed head (TDH) 12.35% lower and efficiency 15.2% lower than the original tested performance curve. Performance degradation was likely due to increased clearances that resulted in greater internal recirculation; refurbishment of the equipment to design clearances would return the equipment to expected operation
A pump tested performance
Vibration event
Mechanical testing uncovered a vibration event that was suspected to be resonance. This vibration occurred during a period where the flow output was slightly increased. While the vibration was not high enough to be in alert, the pattern of the vibration was concerning. During the vibration event, an increase in subsynchronous energy was observed. This energy, which was located at approximately 54% of run speed or ~0.5x, increased from ~0.01in/s 0-peak to ~0.08in/s 0-peak.
During analysis of the vibration data, HRS’s field engineer concluded that this vibration event was most likely the excitation of a structural resonance that is typically dormant in this asset. While not in a degraded condition, it was assumed that there is no forcing function at 0.5x strong enough to excite the resonance. It was believed that the looseness and excessive running clearances of the pump in the degraded condition created enough sub-synchronous energy to excite this resonance, and the vibration would be resolved by refurbishment.
Elevated vibration amplitude during vibration event
By performing an extensive health assessment on its equipment, the plant was able to move from a preventative maintenance model, based on historical run times, to a predictive maintenance model, based on actual asset condition. This improved maintenance strategy facilitated better outage planning, allowing the plant to spend money where it was most needed and reduce its overall risk.
Additionally, the assessment provided the plant with information that helped it improve operation in the period leading up to the impending refurbishment of the A condensate pump. Because the pumps were operating in parallel with each rated for 100% of the system flow, the plant was able to tailor operation to run the more efficient and reliable asset. This strategy minimized risk of an online failure and reduced energy usage. In the end, better information enabled better decision making, supporting the plant’s goals of increased operational efficiency.
Increased subsynchronous vibration during vibration event.
