What are the consequences of running the pump below rated flow for a long time?

Generally speaking, it is reasonable for a pump to operate under rated conditions. However, sometimes, due to certain reasons, long-term operation below the rated flow rate can bring some negative effects. In this article, HNYB PUMP will take you to understand the consequences of long-term operation below the rated flow rate of the pump and the main manifestations.

• Pump body temperature rises:

Because the actual pump flow rate is extremely low, meaning the pump performs very little useful work, most of the shaft power is converted into heat energy and transferred to the liquid within the pump, causing the entire pump housing to heat up. This increases radial thrust. Continuous operation at extremely low flow rates can lead to excessive shaft deflection, rapid wear of bearing rings, and even fracture due to excessive shaft fatigue. During long-term operation at low flow rates, the flow rate and pump outlet pressure will exhibit regular, cyclical variations, a phenomenon known as surge. Surge generates vibration and noise, adversely affecting the pump.

• The internal reflux of the pump increases significantly:

The internal heat accumulation increases, causing the temperature of the liquid in the pump to rise, resulting in heating of the pump body, affecting the mechanical properties of the pump components, and at the same time deteriorating the cavitation performance of the pump, further affecting the suction conditions of the pump.

• The radial force of the pump increases and the stress condition of the pump rotor worsens:

When the pump is working in a small flow area, it deviates from the design operating point, and the flow rate of the liquid in the volute decreases. However, according to the velocity triangle analysis, the outflow speed of the liquid in the impeller increases instead, resulting in the inability of the liquid to converge, forming an impact, and the pressure continues to rise, generating radial force.

• Reduced efficiency and increased power consumption:

Pumps are typically designed for peak efficiency near rated operating conditions. If a high-flow pump is operated at low flow, its efficiency will drop rapidly. Generally speaking, the lower the flow rate, the lower the efficiency of the same pump. Therefore, operating at low flow rates is highly uneconomical. Generally, a suitable smaller pump is required.

• Increased vibration and noise, causing environmental pollution, damage to pump components, and shortening the pump's service life:

At the design operating point, because the liquid flow direction aligns with the blade direction, flow separation loss, impact loss, and eddy current loss are relatively small, close to zero. However, when the pump operates in the low flow range, due to deviation from the design point, flow separation loss, impact loss, and eddy current loss in the pump flow path components increase further. When these losses occur, they are accompanied by significant hydraulic noise and mechanical vibration.

To ensure reliable operation, high-flow pumps require inspection before each use, specifically:

• Check the pump's rotating parts for signs of binding or collision, and the nuts on the pump base and joints for looseness.
• Check the coaxiality and parallelism of the pump shaft and the motor main shaft, and check for leaks in the piping. If any leaks are present, correct them.
• Check that the fluid level in the pump casing is above the upper edge of the impeller. If the fluid level is insufficient, inject fluid directly into the pump body through the inlet port on the pump casing. Never start the pump if the fluid level is insufficient, as this will prevent proper operation.
• Lubricate with calcium-based grease or No. 10 engine oil, depending on operating conditions. For grease-lubricated pumps, regularly add grease to the bearing housing. For oil-lubricated pumps, top up the oil level if it is insufficient.