The twisting, the bending, and finally the snap. I'm not talking about the end result of my all-too-infrequent workouts, I'm talking about centrifugal pump shaft breakage.
To find out what causes centrifugal pump shafts to break, I took a walk to our Service Center and spoke with one of our longest tenured and most experienced service technicians. He shared 7 ways this happens.
Metallurgical/Manufacturing Process Flaws
Occasionally there are flaws in the metal or manufacturing process that can cause shafts to fail prematurely.
When a pump shaft breaks, the natural reaction is to blame the manufacturer. But in reality, shaft breakage is more often a direct result of the pump's operating conditions. Only a very small percentage of shaft failures are due to manufacturing flaws.
Vibration is most commonly caused by cavitation, critical speed, passing vane frequency, and operating outside the pump's best efficiency point. 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.
Imbalance creates problems while the pump is running, though the shaft will measure straight if stopped. It's a source of vibration and will reduce machine, bearing, and mechanical seal life.
Impeller imbalance is caused by:
- New, never balanced impeller
- Trimmed and not balanced impeller
- Foreign object stuck in vanes
- Vanes bent or out of plane
- Balance holes plugged
- Product build up on impeller
Note, in a misalignment scenario, the pump's bearings are more likely to fail before the shaft does. Nonetheless, because misalignment creates bending moments for the shaft, weakening it over time, we've included it here.
When a pump is selected to move lower viscosity fluids, but subjected to higher viscosity fluids, likelihood of shaft breakage is higher. Be mindful of fluids that experience changes in viscosity due to temperature, or being at rest. Fluids/slurries that dewater as they sit, such as paper stock, can also be problematic.
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, assuming the shock is strong enough, the shaft could bend or break instantly or over time. Read more about water hammer in our post, "What Is Water Hammer?"
What happens when reverse flow causes a pump's impeller and shaft to turn backwards and the pump suddenly kicks on? You have a stressed or broken shaft. This scenario occurs when check valves upstream are partially or completely plugged, or otherwise not functioning properly.
Is there a pump in your facility that frequently breaks shafts? Something sinister may be at play. Consult an engineer in your area well versed in pumps and fluid hydraulics.
Busted shaft? Ask us about it! We gladly provide technical assistance to businesses and municipalities in Wisconsin and upper Michigan.