Pump Vibration Near a Lateral Critical Speed
A newly commissioned API 610 OH2 pump runs at 2,980 rpm and delivers 180 m3/h at 65 m head. A route vibration survey shows overall 1x vibration of 8.4 mm/s RMS on the drive-end bearing, with stable phase and little 2x or harmonic content. The amplitude was 3.0 mm/s at start-up and has risen gradually over three weeks; it increases when the discharge valve opens from 60% to 100% and falls when the pump is run at 2,700 rpm. A bump test with the pump stopped gives a first lateral natural frequency of 47.8 Hz. The baseplate is grouted, but the suction line has an unbraced 3 m horizontal run and the discharge expansion joint is compressed. Alignment is within 0.05 mm, and the rotor was balanced to ISO 1940 grade G2.5 at the shop.
1. Calculate the separation margin and magnification factor. Decide whether this is resonance, unbalance, looseness or cavitation, and justify the diagnosis.
Read the worked answer
Running speed is 2,980/60 = 49.67 Hz. The natural frequency is 47.8 Hz, so the separation margin is (49.67 - 47.8)/49.67 = 3.8%. The frequency ratio is r = 49.67/47.8 = 1.039, and the magnification factor is 1/|1 - r^2| = 1/|1 - 1.0797| = 12.5. A 3.8% separation is far below the margin normally required by pump specifications, often at least 15%. The vibration is dominated by 1x with stable phase, grows with flow and disappears at reduced speed; that is consistent with residual unbalance exciting a lightly damped resonance. Looseness would normally show multiple harmonics and phase changes; cavitation would show broadband high-frequency energy and flow-dependent noise. The shop balance grade does not protect against a structural resonance.
2. Recommend a corrective-action sequence. Explain why trim balancing alone is not a permanent repair.
Read the worked answer
First stiffen the suction line with a brace near the pump flange, inspect the baseplate grout and correct the compressed discharge expansion joint. The objective is to move the first lateral natural frequency at least 15% above 49.67 Hz, or well below it, and then repeat the bump test with the pump installed. Trim balancing can reduce the excitation force temporarily and lower vibration enough to run, but it does not change the 3.8% separation or the 12.5 magnification. A small residual force will still be amplified, and the natural frequency can drift with piping, temperature or foundation changes. After the structural fix, recheck alignment and balance, then run a speed sweep to confirm no critical speed lies in the operating range.
3. If production cannot stop immediately, propose a temporary operating window and monitoring limits.
Read the worked answer
Run at 2,700 rpm, which is 45 Hz, and avoid the 2,850-3,050 rpm band where the 1x component approaches 47.8 Hz. Limit discharge flow to 60-70% and monitor 1x amplitude, phase and overall vibration daily. Set an alarm at 7.1 mm/s RMS and a trip or controlled shutdown at 11 mm/s RMS, consistent with the applicable machine class. Temporary operation is acceptable only while the suction brace and foundation repair are scheduled; it does not correct the root cause. If phase becomes unstable or 2x content appears, stop the pump and inspect for looseness or cracking.