Discussion on acoustic diagnostic methods for pump set failures

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I. Centrifugal pumps – Diagnosing faults in centrifugal pumps using sound is a practical method based on vibration and noise characteristics; different faults produce specific acoustic features. The following are the specific methods and case analyses. 1. Acoustic characteristics of cavitation faults: high-frequency explosive noises (similar to the sound of gravel flowing) accompanied by irregular crackling sounds, which usually intensify as the opening degree of the outlet valve increases. Example: A chemical plant’s water pump was producing frequent ‘clicking’ sounds during operation, and the gauge pointer was fluctuating. Inspection revealed that the clogged inlet filter caused the suction pressure to drop below the saturated steam pressure, resulting in cavitation. The noise disappeared after cleaning the filter. 2. Sound characteristics of impeller imbalance or wear: low-frequency periodic rumbling (1× rotational frequency; for example, a pump running at 1450 rpm produces 24 Hz), accompanied by harmonics of the blade passage frequency (number of blades × rotational frequency). Example: A certain cooling water pump emits a regular \"buzzing\" sound, and vibration analysis reveals an imbalance in the mass of the impeller. Inspection revealed severe scaling on the impeller; noise levels were reduced by 15 dB after dynamic balancing. 3. Sound characteristics of bearing failures: - Outer ring damage: Constant high-frequency metallic screeching (1~3kHz), at intervals corresponding to the bearing outer ring failure frequency (BPFO). - Ball damage: Irregular clicking sound, with a frequency equal to the BSF (ball rotation frequency). Example: An 8kHz screeching sound was heard from the bearing area of a water supply pump, and spectral analysis revealed a prominent BPFO component. The abnormal noise disappeared after replacing the bearing. 4. Characteristics of mechanical looseness sounds: wide-band random impact noises (such as a \"clanging\" sound), possibly accompanied by 2× frequency vibration. Example: The foundation bolts of a fire pump became loose, causing intermittent metallic clanging sounds during operation. After tightening the bolts, the 2× rotation frequency component in the noise spectrum decreased significantly. 5. Characteristics of the sound produced by mouth ring friction: a high-frequency, continuous hissing sound (similar to metal scraping), with frequencies concentrated between 500 and 2000 Hz. Example: After maintenance, a boiler feed pump produced a harsh rubbing sound; upon disassembly, it was found that the clearance of the mouth ring was too small, causing contact and friction between the moving and stationary components. Noise is eliminated after adjusting the gap to the design value. 6. Diagnostic tools and steps 1) Initial assessment: Use a sound detector to locate the source of abnormal noises (bearing housing/pump casing/inlet and outlet). 2) Spectral analysis: A portable acoustic camera or vibration analyzer is used to capture the characteristic frequencies. 3) Comparative verification: Compare the noise spectrum under normal operating conditions (such as by using previously saved baseline data). 4) Auxiliary parameters: Comprehensive judgment is made by considering changes in pressure, flow rate, and current. 7. Precautions 1) Environmental noise interference must be eliminated (such as pipe resonance, motor noise). 2) High-frequency sound waves attenuate rapidly, requiring measurement close to the sound source. 3) Regularly establishing an acoustic fingerprint database for pump units can improve diagnostic efficiency. By systematically analyzing sound characteristics, it is possible to predict over 80% of common faults, thereby significantly reducing unplanned downtime. II. To diagnose faults in the motor of a centrifugal pump using acoustic methods, it is necessary to take into account both the mechanical and electrical characteristics as well as their acoustic manifestations. Abnormal noises from electric motors usually indicate issues such as bearing damage, electrical imbalances, or mechanical misalignment. Below are specific methods along with example analyses: 1. Bearing failures 1.1 Noise characteristics – Outer ring damage: Continuous high-frequency whistling (1–3 kHz), with the frequency component corresponding to the bearing outer ring failure frequency (BPFO). - Inner ring damage: a modulated clicking sound with a frequency corresponding to the inner ring failure frequency (BPFI), which becomes more noticeable as the load increases. - Cage damage: Irregular “clattering” sounds at the cage failure frequency (FTF). 1.2 Example: A centrifugal pump motor in a wastewater treatment plant was emitting an 8kHz screeching sound; the spectrum analysis of the acceleration sensor revealed a prominent BPFO component at 112Hz, and inspection revealed that the outer ring of the bearing had peeled off. The noise disappeared after replacing the SKF 6311 bearing. 2. Rotor bar breakage or eccentricity 2.1 Sound characteristics – Rotor bar breakage: a periodic \"buzzing\" sound combined with electromagnetic noise; slip frequency sidebands can be seen in the spectrum (f_sideband = f_power supply ± 2sfr, where s is the slip rate). - Uneven air gap: sharp electromagnetic noise (50/60Hz and their harmonics), accompanied by vibrations at 2×power frequency. 2.2 When the motor of a certain chemical pump in operation is under load, it emits a regular electromagnetic humming sound; current analysis reveals a component at 54Hz (50Hz + 2×2Hz slip). Upon unpacking and inspection, it was found that 3 bars of the rotor were broken; after replacing the rotor, the current noise spectrum returned to normal. 3. Stator winding faults
3.1 Sound characteristics
- Inter-turn short circuit: High-frequency “hissing” discharge sound, accompanied by local overheating. - Interphase imbalance: Low-frequency (100/120 Hz) electromagnetic humming; negative-sequence components appear in the current spectrum. 3.2 When the motor of a certain water supply pump was in operation, it emitted intermittent hissing sounds; the infrared thermal imager showed that the temperature rise of stator phase B exceeded the limit by 20 K. The insulation test revealed an inter-turn short circuit; noise was eliminated after rewinding the stator. 4. Mechanical misalignment 4.1 Sound characteristics – An axial \"clunking\" sound at a frequency of 2× the rotation frequency, with a higher sound pressure level on the coupling side. - It is often accompanied by excessive pump body vibration (per ISO 10816-3 standard). 4.2 Example: At the motor end of a booster pump in an oil pipeline, two impact sounds were produced per revolution. Detection using a laser aligner revealed an axial deviation of 0.15 mm (three times the allowable limit). After realignment, the noise at 2× frequency (49.6 Hz) decreased by 12 dB. 5. Diagnostic tools and procedures – Stethoscopic localization: Use a mechanical stethoscope to identify the source of noise (bearing end/stator housing/coupling side). Spectral analysis: - Electrical faults: Focus on 50/60Hz harmonics and their sidebands. - Mechanical failures: Analyze the rotation frequency, bearing failure frequency, and their harmonics. Baseline comparison: Compare the acoustic characteristics of the motor in no-load/load conditions. Comprehensive verification: Combining current spectrum analysis (CSA), vibration data, and infrared detection. 6. Analysis of typical case combinations 6.1 The cooling water pump motor in a certain steel plant exhibited the following acoustic characteristics: High frequencies: 7.2 kHz whistling (bearing BPFO at 158 Hz); Low frequencies: 100 Hz electromagnetic humming (negative sequence component of stator current +5%); Impact sounds: 2× rotational frequency (49 Hz); Axial vibration. 6.2 Diagnostic results: Damage to the outer ring of the bearing + slight imbalance in the stator windings + misalignment of the coupling. The high-frequency noise disappeared after replacing the bearings first; then the electrical connections were adjusted and the pump and motor were realigned, thereby resolving the issue comprehensively. 7. Precautions: 1) The noise from the motor cooling fan can mask early bearing failures (the fan needs to be stopped temporarily for inspection). 2) Power supply harmonics may interfere with electromagnetic noise diagnosis (it is recommended to record the voltage waveform simultaneously). 3) Permanent magnet motors require additional inspection for torque ripple noise caused by demagnetization. System acoustic analysis can provide a warning of motor failures 2 to 3 months in advance, preventing damage to the driven equipment (pumps) as well. Recommendation: Conduct quick screenings quarterly using a portable acoustic camera (such as the FLIR Si124).