Key Causes of Vibration in Split Case Pumps and How to Detect Them

Vibration in a split case pump is not only undesirable but can also cause severe damage to pump components, increase energy consumption, and lead to unplanned downtime. Unacceptable vibration can result in noise pollution, reduced operational efficiency, and even catastrophic equipment failure. This guide explores the most common causes of vibration in split case pumps and how to recognize the signs before damage occurs.

Main Causes of Vibration in Split Case Pumps

• Cavitation

Cavitation generates random high-frequency broadband energy, often combined with blade pass frequency harmonics. It is typically caused by insufficient net positive suction head (NPSH). When local areas inside the pump experience a drop in pressure below the liquid’s vapor pressure at pumping temperature, vapor bubbles form. These bubbles collapse when they move into higher-pressure areas, creating shock waves that can severely damage pump components. In split case pumps, cavitation often sounds like marbles or gravel passing through the pump. Maintaining NPSHA above NPSHR is essential to prevent cavitation.

• Pump Flow Pulsation

Flow pulsation occurs when a pump operates near its shut-off head. The resulting vibration waveform is sinusoidal, with spectra dominated by 1X RPM and blade pass frequencies. Pressure gauges at the pump outlet will show fluctuating readings. If equipped with a swing check valve, the valve arm and counterweight may oscillate, indicating unstable flow.

• Bent Pump Shaft

A bent shaft often produces high axial vibration, with axial phase differences near 180° along the rotor. If the bend is near the center of the shaft, vibration peaks at 1X RPM; if near the coupling, peaks occur at 2X RPM. Bent shafts are most common at or near the coupling and can be confirmed using a dial gauge.

• Unbalanced Pump Impeller

An unbalanced impeller increases bearing loads, reducing bearing life significantly. In center-hung impellers, force imbalance usually dominates, causing high radial vibration at 1X RPM. Blocked impellers or cantilevered designs can result in excessive axial and radial vibration. Balancing may require removing the rotor and using a two-plane balancing machine.

• Shaft Misalignment

Misalignment occurs when the centerlines of coupled shafts do not match. Parallel misalignment causes offset but parallel centerlines, producing vibration at 1X, 2X, and higher harmonics. Angular misalignment results in high axial 1X vibration, some 2X and 3X, with 180° phase differences across the coupling.

• Bearing Issues

Bearing defects generate peaks at non-synchronous frequencies. Common causes include excessive loads, poor lubrication, or high operating temperatures. Failure frequencies for specific bearing components can be identified using manufacturer data or predictive maintenance software.

Conclusion

Understanding the common causes of split case pump vibration is critical for maintaining reliable operation and preventing costly downtime. Regular monitoring, proper installation, and preventive maintenance can help detect and resolve these issues early, ensuring longer service life and higher efficiency for your split case pump.