How to Prevent Failures due to Shaft Damage
Shafts are an integral part to centrifugal pumps. Shafts create rotation for the motor, couplings, and the impeller, altogether generating centrifugal force to pump liquids. Therefore, any issues with a pump’s shaft will inevitably lead to not only unnecessary downtimes, but also the potential for more components of the pump, i.e. the impeller or bearings to become damaged. There are three likely causes for a pump’s shaft to become damaged, namely: excessive vibrations, impeller imbalances and interruption of flow to the pump.
The primary cause for a damaged shaft is due to excessive vibrations. Sometimes, “Pump bearings begin wear, allowing the shaft to move laterally, causing the shaft to flex and eventually fail. The vibration harmonics also puts extra stress on the pump shaft.”1 Of course, the shaft may not be the only part damaged, for excessive vibrations can also damage the mechanical seal2 and cause leaks. The best practice to prevent excessive vibrations is by proper vibration analysis on installation, followed by monitoring vibrations whenever inspections are performed on a pump. Ideally, a pump should be monitored daily.
On the subject of impellers, the second common issue for a damaged shaft can be caused by and imbalanced impeller or rotor. An unbalanced shaft can lead to major issues for a shaft, in what’s known as “shaft whip”: “An unbalanced impeller will create ‘shaft whip’ while in operation. The effect is the same as if the shaft was bent and/or deflected, even though the shaft would measure straight if you stopped the pump and checked the shaft…The number of bending cycles in a given time frame is reduced, but the amplitude (strain) of displacement (due to the imbalance) remains in the same range as the higher-speed factors”3 In short, an imbalanced impeller can lead to deformation of a shaft, which will lead to lesser efficiency of the pump, as well as a need for a new shaft. Besides improper installation of an impeller, an improperly installed packing seal can also badly damage a shaft (see image). The best way to prevent impeller imbalances, simply, is to properly install the impeller and to also take into account any manufacturer specifications otherwise.
The third and last common cause of damages to a pump’s shaft is due to interruption of the liquid flow to a pump. “Interruption to water flow can affect the impeller and cause the pump shaft to break and wear away shaft bearings.4 The other problem regarding an interruption of liquid flow is what is known as “hydraulic shock”: “Serious damage can be caused by hydraulic forces. For example, when a check valve slams shut, interrupting the flow of fluid, a massive shock wave results. This shock wave reverses flow and travels back downstream. When the shock wave collides with a pump…the shaft could bend or break instantly or over time.”1 In order to prevent interruption of flow in pumps, it is advisable to apply the proper impeller and observe that the level of clearance is appropriate. Otherwise, if absolutely necessary, a surge tank installed on a pumping system can help stop interruptions by preventing pumping surges.
Though it is not always feasible to turn off a pump in order to observe that the shaft is still in working condition, it is recommended to check for issues which can cause a shaft to become damaged when inspecting a pump. Simply stated: watch for signs that might indicate potential future damages for pumps — especially excessive vibrations.
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