Motor Testing IDs Pump Woes

Electrical-signature analysis detects what other pump tests couldn’t at nuclear power facility.

A nuclear-power-generating facility in the Northeastern United States relies on four identical vertical pumps to feed water to a heat exchanger that cools the sealed ultrapure water/steam driving the site’s turbine generators. Featuring 30-ft.-long shafts, these service-water pumps were installed when the plant was built in the early 1970s. Over time, two of the four began exhibiting performance issues. The situation motivated the site’s maintenance team to conduct a performance study.

The approach

The plan was to analyze a number of measurements, including vibration, pressure, and flow. Since the team also wanted to include electrical-signature analysis as part of the performance study, they contacted Bob Dunn at I&E Central (ie-central.com, Rochester, NY). The company provides on-site predictive-maintenance (PdM) services and supplies industrial test equipment for use by PdM, electrical, and reliability professionals. Dunn visited the power plant and analyzed two of its 350-hp, 480-V motors.

The process

Electrical-signature analysis is performed by connecting current sensors and voltage leads to the motor, and then capturing high-resolution current and voltage waveforms that can be analyzed as FFTs (Fast Fourier Transforms) and raw-waveform data. The resulting data can provide insight into an entire motor system, both mechanically and electrically, from the incoming power through the driven load. This type of technology actually uses the motor as a transducer for the mechanical analysis. Any mechanical phenomena will be modulated onto the current waveform where they can be detected and analyzed.

Dunn turned to the ALL-TEST Pro On-Line II (ATPOL II) energized motor-testing instrument from ALL-TEST Pro LLC (alltestpro.com, Old Saybrook, CT) to perform the electrical-signature analysis. He connected the device to the system, which was set to a flow rate of 5,600 gpm for the test. The motor’s terminal box remained open, and the data collector was placed nearby. Because the ATPOL II collects and transmits data with Bluetooth, Dunn and the maintenance team were able to position themselves at a safe distance.

The data was collected in two segments. The first was a high-resolution, low fmax (100 Hz) current acquisition of 50 sec., which primarily shows issues at running speed (misalignment, unbalance), and below the synchronous speed (rotor bar or load related issues).

The second segment was a high-frequency capture and FFT of voltage and current that shows:

• electrical issues including power harmonics, power-factor issues, voltage, and current, versus nameplate and balance
• high-frequency mechanical faults,.

The results

Results of the electrical-signature analysis revealed:

• The motor was electrically perfect with balanced current and voltage, and 90+ power factor.
• The motor and pump were mechanically excellent with no indications of misalignment or unbalance, bearing issues, rotor, or stator issues.

Tests showed a significant fluctuation in the current draw, pulsing at about 10 Hz. Dunn and the team collectively determined that this was caused by non-laminar flow, or turbulence in the system.

The indication of turbulence on the power plant’s two under-performing vertical service water pumps was a revelation. More surprising to the maintenance team was the fact that the turbulence had not been detected with any of the other tests.

One of the key benefits of electrical-signature analysis is that it can show mechanical issues in the driven load—even in the case of a vertical pump with an impeller some 30 ft. below the surface. This is in addition to detailed information on other parts (mechanical and electrical) of a system. Once the maintenance team understood the root cause of its pump problems, they were able to take the necessary steps to remedy the situation and get the units back to operating in a way that meets the power plant’s performance requirements. EP

Lessons Learned

Monitoring your industrial equipment and evaluating performance issues before catastrophic failure occurs can save costly downtime and unwanted expenses.

Performing electrical-signature analysis along with vibration analysis can provide more of an in-depth look at what is really going on with equipment. Making sure you have the proper tools to perform these kinds of analyses is critical to any condition-based maintenance/predictive-maintenance program.

For more testing-solution information, visit alltestpro.com.