Pump Bearing Alarm After Sensor Change
A 75 kW horizontal centrifugal pump, driven at 2970 rpm through a flexible coupling, has a new vibration alarm. The pump drive-end bearing horizontal reading is 4.8 mm/s RMS over 10–1000 Hz, up from a baseline of 2.1 mm/s. The spectrum shows 1x at 49.5 Hz of 1.2 mm/s, 2x at 99 Hz of 0.8 mm/s, and a cluster of peaks at 312 Hz with harmonics plus a raised noise floor above 1 kHz. The accelerometer was attached with a magnetic base; the baseline survey used a stud-mounted accelerometer. Bearing temperature is 68°C, up 6°C from normal. The machine is a 75 kW unit in a process plant, and production cannot stop without justification. ISO 20816-1 Group 2 zone boundaries are A/B 1.4, B/C 2.8, and C/D 4.5 mm/s RMS. The maintenance supervisor wants to run until the next planned outage in three weeks.
1. Classify the 4.8 mm/s reading under ISO 20816-1 and state whether it alone justifies immediate shutdown.
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Under ISO 20816-1 the pump is Group 2 (15–300 kW). The C/D boundary is 4.5 mm/s RMS, so 4.8 mm/s is in Zone D. Zone D is normally described as unsuitable for long-term operation and calls for immediate investigation, but the standard is not a shutdown mandate by itself. The reading is only 0.3 mm/s above the boundary, and it was taken with a different mounting than the baseline. The first action is to verify the measurement, not to condemn the bearing. If the stud-mounted repeat confirms 4.8 mm/s, the machine should be scheduled for prompt repair and should not run unattended for three weeks.
2. How would you distinguish a real bearing defect from a magnetic-mount artifact? Include the specific checks you would make.
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Check the mounting first. A magnetic base can add a low-frequency mounting resonance and give poor repeatability above 1 kHz; it can also attenuate or amplify high-frequency peaks. Remove the magnetic base, prepare the surface, and use a stud, adhesive pad, or flat magnet with a hard coupling, then repeat the measurement at the same point and direction. Perform a bump test or check the mounting resonance. Compare the spectrum with the baseline: a real bearing defect shows discrete BPFO or BPFI peaks with harmonics and sidebands, often with a raised high-frequency floor, and it should repeat with the stud mount. A mounting artifact tends to change or disappear with the mounting method and may show broadband energy not tied to bearing frequencies. Also check coherence, accelerometer cable noise, and bearing temperature trend. Enveloping or high-frequency acceleration is useful confirmation.
3. What recommendation would you give for the next three weeks, and what evidence would change it?
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Recommend a confirmation measurement within one shift using a stud-mounted accelerometer and acceleration enveloping, plus lubrication check and temperature trend. If the bearing frequencies are confirmed and temperature continues to rise, plan a controlled shutdown before three weeks; continued operation may be acceptable only with daily monitoring, reduced load, and a defined trip limit. If the stud-mounted spectrum is normal and the high-frequency energy was a magnetic-mount artifact, the machine can run to the planned outage with normal monitoring. The decision should be based on confirmed data, trend, and consequence of failure, not on the single 4.8 mm/s number.