Before taking the drastic steps of troubleshooting mounting or cable issues, or even upgrading the transducers, try this easy option: increase the supplied excitation current to the accelerometer probe.
Clients often have vibration waveform data that is erratic, unreasonably or impossibly high, or a velocity spectrum with the “ski slope” effect, as seen in figure one. Most of these inaccuracies can be traced back to mathematical errors – errors which are compounded when used to process flawed acceleration data. Although software keeps improving, no software will be helpful when processing inaccurate data. Therefore, it is necessary to provide the best data possible for any analysis. The mathematical transformation of acceleration data to vibration spectrum data is very sensitive and highly effected by noise and instability in the system. Generally, mathematical inaccuracies increase the more the data is processed, causing false waveform RMS values and incorrect spectrums because the errors from the integrated noise will have larger magnitudes than the actual equipment vibrations. Increasing the power to the vibration probe loop can stabilize signals and the data that will be received for processing.
Fig. 1: Example of Vibration Frequency Spectrum from Acceleration Data that shows “Ski Slope” effect.
Looking at the specification sheet for PCB’s 351B41 cryogenic sensor as an example, it is known that the constant current excitation range is from 2-20 mA. Most rack systems are configured to output 2-3.5 mA, but this can be increased through modifying the configuration of the rack or by adding a constant current source to the system. Increasing the supply current has been shown to improve signal quality – especially on in-tank retractable type pumps where the rack is hundreds of meters from the actual accelerometer probe.
The effects of noise and drift in the system can be eliminated by increasing the excitation current to varying amounts, depending on the system. Of course, it is important to ensure that the output does not exceed the rated excitation current of the transducer or the system. Increasing the output current to the transducer has stabilized both vibration velocity time waveforms and also eliminated or greatly reduced “ski slope” issues or lower ghost frequencies in velocity spectrums at multiple LNG regasification plants.
Looks can be deceiving. Whether the pumps have recently been commissioned or they have been running fine for years, make sure to check the spectrums for 1x prevalence to ensure you do not have faulty data. There have been documented cases in which the waveform RMS values for these pumps were within acceptable ranges, but the data was nonetheless faulty. After further analysis it was clear that the data was faulty. Despite the sum of the energy of the spectrum giving an overall acceptable value, the spectrum had little to no energy content at the operating speed of the pump.
Of course, increasing the excitation current is not a cure-all. Problems with cables, mounting, or transducer overload can cause these same issues. But the increase of the current supply can be done without ever needing to take your valuable equipment offline.
If you are experiencing issues with your cryogenic submerged motor pump’s vibration monitoring system or would benefit from a specialty review of the data, feel free to contact NICHE Cryogenic Solutions.
Chuck Blackett
Chief Engineer - NICHE Cryogenic Solutions
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