Case Study Spotlight: Boiler Feed Pump 1B – Misalignment of Booster Pump (Jan 2011)

How vibration trending, 1× spectrum, and phase analysis confirmed misalignment—and how simple mechanical fixes restored stable operation.

 

Why this matters

Boiler feed pumps (BFPs) are among the most critical auxiliaries in a thermal power station because they deliver feedwater to the boiler at high pressure. Loss of a BFP can quickly become a generation risk if standby redundancy is compromised. This case study shows how condition monitoring detected a developing problem early, confirmed the fault mechanism, and validated the repair.

1) Background and initial symptoms

After planned servicing during a unit overhaul in December 2010, routine vibration monitoring identified an abnormal rise in horizontal velocity on the booster pump bearings. Importantly, the motor, hydraulic coupling, and main pump vibration levels remained normal, and operating parameters such as pressure and temperature were also normal. Because the vibration trend continued to increase, the team decided to change over the pump for inspection.

Key observations (pre-maintenance)

·       Horizontal velocity at booster pump DE and NDE increased from typical values to 4.8 and 4.1 mm/s RMS, respectively, and later reached 5.9 mm/s RMS at the DE.

·       The abnormality was localized to the booster pump: other drive-group components showed normal vibration behavior.

2) Diagnostic workflow (what the vibration data was telling us)

The team followed a simple but powerful sequence: (i) confirm the trend change, (ii) look for the dominant spectral component, and (iii) use phase to validate the fault type.

2.1 Trend + overall levels

Overall velocity levels at the booster pump were significantly elevated compared with the normal range (about 2.5–3.0 mm/s RMS), prompting spectrum analysis.

2.2 Frequency domain (FFT spectrum)

Spectrum analysis showed a clear increase at 1× running speed (around 1480 rpm). A dominant 1× component on a coupled machine train is a classic indicator of alignment-related forcing, especially when it grows without corresponding increases at bearing defect frequencies.

2.3 Phase analysis (the clincher)

To confirm misalignment, phase across the coupling was measured. A phase difference of approximately 180° across the coupling strongly supported a misalignment condition between the booster pump and motor.

3) What was found during maintenance

During decoupling and inspection, the bearings showed no scoring and housings were acceptable. However, two mechanical contributors to misalignment/instability were discovered:

·       Six flexible coupling bolts were found sheared/damaged.

·       Foundation bolt shims below the suction strainer were loose.

4) Corrective actions (repair + alignment)

The corrective work focused on restoring mechanical integrity and re-establishing precise alignment:

1.       Decoupled the booster pump from the drive group and recorded initial alignment readings for reference.

2.       Replaced the sheared coupling bolts with new bolts.

3.       Adjusted shimming at the suction strainer (trial 5 mm shims at all four corners, then refined to 4 mm shims) to achieve precise alignment and allow smooth expansion.

4.       Completed alignment of the booster pump with the rest of the drive group (dial gauge method described in the case study) and performed a trial run; vibrations were within limits.

5) Results and verification

Post-maintenance measurements confirmed that vibration returned to normal levels after bolt replacement and alignment work, and post-maintenance spectra did not show misalignment/looseness indicators.

Selected before/after vibration levels (velocity, mm/s RMS)

Component	Point	Direction	30/12/2010 (Initial)	03/01/2011 (After re-alignment)
Booster pump	DE	H	5.9	2.9
Booster pump	DE	V	2.8	1.4
Booster pump	DE	A	2.0	1.3
Booster pump	NDE	H	4.1	2.5
Booster pump	NDE	V	3.2	2.7
Booster pump	NDE	A	1.2	1.1

 

6) What this case teaches (practical takeaways)

·       Localize the fault: when one element in a drive group deviates while others stay normal, focus diagnostics on that element and its interface (coupling, base, soft-foot, shims).

·       Use phase to separate ‘looks like misalignment’ from ‘is misalignment’: a strong 1× peak plus ~180° coupling phase difference is compelling evidence.

·       Inspect what the data is pointing to: coupling hardware and base/shimming issues can turn an alignment problem into a reliability risk quickly.

·       Always close the loop: the repair is not complete until post-maintenance readings and spectra verify stability.

 

 

Business impact (why condition monitoring pays)

By acting on the rising trend, confirming misalignment through spectrum and phase analysis, and correcting the hardware/alignment issues, the maintenance team restored the BFP to service with stable vibration. The case study reports this proactive intervention helped avoid significant losses and estimates the cost impact at approximately Rs. 73 lakhs (including generation loss and associated maintenance costs).

 

Reference

Bari, H. and Thaker, J. (2025) Condition Monitoring in Thermal Power Stations: Case Studies. 1st edn. Boca Raton, FL: CRC Press. DOI: 10.1201/9781003432616.

 

Figures

Note: Baseline value represents the typical operating range (approximately 2.5-3.0 mm/s RMS) noted in the case study; dates are shown for illustration based on the case timeline (late Dec 2010 to early Jan 2011).