New research links off-design flow to shaft fatigue, exposing its impact on hydraulic pump energy efficiency and reliability.
Fatigue doesn’t just break shafts. It breaks efficiency. New research into a multi-stage centrifugal pump has revealed how pressure-induced strain on the shaft can quietly erode hydraulic pump energy efficiency. The study, led by researchers from Jimma University and the University of Stavanger, uses fluid–structure interaction (FSI) analysis to show how off-design flow conditions accelerate fatigue through asymmetric pressure forces. The result is a real-world performance penalty many engineers may be underestimating.
Where flow strays, shaft fatigue follows
Centrifugal pumps are built to operate around their best efficiency point, but this study demonstrates what happens when they don’t. By modelling flow conditions from 60 to 140 per cent of the design rate, the researchers found that fatigue stress rose sharply outside optimal conditions.
“Fatigue stress increased significantly when the pump operated away from its design point,” lead author Leta Yadeta said. “Radial loads caused by uneven pressure distribution were the dominant factor.”
At low flow (0.6Qd), stress peaked at 141.19 MPa. At high flow (1.4Qd), it reached 142.88 MPa. Both figures were higher than the 137.58 MPa observed at the BEP. The effect was most apparent in impeller stages two and three, where unbalanced pressure led to concentrated shaft stress and higher deformation.
Design efficiency isn’t just hydraulic. It’s structural
The FSI model showed how pressure asymmetry, not torque alone, drives shaft fatigue. Even though motor load remained consistent, asymmetric fluid pressure created localised deformation and cyclical bending. Maximum shaft displacement hit 0.507 mm at low flow, exposing the structural cost of operating outside the design range.
“Maximum shaft deformation occurred at the lowest flow rate,” co-author Hirpa G. Lemu said. “This was driven by greater pressure asymmetry and flow-induced vibration.”
While structural-only simulations showed a stress of 122.6 MPa, FSI modelling revealed more than a 10 per cent increase under realistic fluid loading. Analytical models alone missed this hidden source of stress, pointing to a gap in conventional pump fatigue estimation methods.
Why conventional models miss real-world fatigue
To verify the accuracy of their approach, the researchers compared their results with analytical fatigue assessments using torsion and bending theory. While those calculations came close, predicting 120.6 MPa, they failed to capture the additional forces introduced by fluid behaviour at non-ideal flow rates.
“The small variation between the numerical and analytical results validated the model,” Yadeta said. “But only the FSI approach revealed how off-design flow raises stress well beyond static estimates.”
For pump designers and asset managers, the takeaway is clear: if fluid dynamics are excluded from fatigue assessment, the resulting design may be vulnerable to unexpected wear, failure, or energy loss. The study underscores that mechanical strength alone is not enough when flow instability adds load with every cycle.
Designing for strain: efficiency through resilience
The shaft analysed was made of X5CrNiMo17-12-2 stainless steel, a high-performance material widely used in corrosive and high-load environments. Yet even this alloy approached fatigue thresholds under pressure conditions that are common in real-world operations. The authors argue for broader adoption of FSI-based analysis in pump and shaft design, especially for systems with wide duty ranges or frequent flow variation.
“Running pumps outside the optimal range adds more than inefficiency. It adds risk,” Lemu said. “Fatigue accumulates faster, and long-term energy efficiency suffers.”
This shift in perspective encourages engineers to account for fatigue behaviour earlier in the design process, not just during failure analysis. It also reinforces the value of predictive maintenance, pressure monitoring, and system tuning to stay within safe operating envelopes.
Looking ahead: building efficiency into every cycle
This research makes the case that true hydraulic pump energy efficiency goes beyond input and output. It must consider how internal dynamics, such as pressure, flow rate, and structural response, interact over time. Although the current model excludes cavitation and long-term corrosion, it sets a clear foundation for integrating fatigue-aware design into everyday engineering workflows.
As the Australian pump industry faces growing pressure to improve reliability, reduce lifecycle costs, and meet energy targets, shaft fatigue modelling may offer a new pathway to long-term performance. Efficiency, after all, is only sustainable if the system can endure the strain.
