Why do pumps fail more often at low flow than at high flow?

There is a common misconception that ‘running a pump easily’ (i.e. low flow) extends a pump’s life. For centrifugal pumps, operating far to the left of the Best Efficiency Point (BEP) can be more destructive than running at high flow.

Why less is sometimes worse

When a pump operates at low flow (often called “Part-Load Operation”), the energy that should be moving fluid through the system is instead converted into heat and mechanical stress.

1. Increased Radial Thrust

At the BEP, the pressure distribution around the impeller is relatively uniform. However, as you move toward low flow, the pressure becomes unbalanced. This creates significant Radial Thrust—a sideways force that acts on the shaft.

The Result: Excessive shaft deflection, which leads to premature bearing failure and mechanical seal leaks.

2. Internal Recirculation & Erosion

When the flow is restricted, the fluid cannot exit the impeller smoothly. Instead, it begins to “recirculate” at the suction and discharge eyes.

The Result: This creates high-velocity vortices and “back-flow” that cause localised pitting and erosion on the impeller vanes, looking remarkably similar to cavitation damage.

3. Thermal Temperature Rise

Pumps rely on the pumped fluid to carry away the heat generated by the motor and friction. At very low flows, there isn’t enough fluid movement to dissipate this heat.

The Result: The liquid can actually boil inside the casing, leading to “vapour locking,” seal face distortion, and the potential for the pump to seize.

4. Discharge Recirculation Cavitation

Even if your NPSH (suction head) is perfect, low-flow conditions can trigger Discharge Recirculation. This creates shear forces and pressure drops within the pump casing itself, causing vapour bubbles to form and implode.

Top tip

There is a dangerous temptation to ‘oversize’ a pump, buying more capacity than you need, ‘just in case’. If you need low flow, specify it rather than using a larger, unnecessary pump.