What are the common reasons for the attenuation of long shaft pump efficiency

Long-shaft pumps are widely used in power plant cooling water systems, municipal drainage projects, petrochemical industry circulating water processes, and large-scale water intake projects. During long-term operation, a common concern for users is efficiency degradation. Decreased efficiency not only increases energy consumption but also shortens equipment life and can even impact overall system stability. Numerous factors contribute to long-shaft pump efficiency degradation, with common causes including hydraulic component wear, impeller scaling and damage, bearing wear, misalignment, cavitation, and operating conditions deviating from the design point.

Hydraulic Component Wear

The main hydraulic components of a long-shaft pump include the impeller, guide vanes, and pump casing. During long-term operation, solid particles, silt, and microscopic impurities in the water flow can cause erosion and wear of these components.

When the impeller blade surface is worn, its geometry changes, causing the blade curvature and hydraulic angle to deviate from the design values, resulting in reduced energy conversion efficiency. Scouring and wear of the guide vanes can cause eddy currents and increased hydraulic losses, further reducing the overall pump efficiency.

Wear is particularly severe in river or seawater intake pumping stations. Over long-term operation, the roughness of the pump's inner surface increases, leading to increased hydraulic losses and a significant decrease in efficiency.

Impeller Scaling and Damage

Impeller scaling is a major factor in the reduced efficiency of long-shaft pumps. In conditions with high water hardness, carbonates in the water easily form scale deposits on the impeller and guide vane surfaces over time.
Scale alters the smoothness of the flow path, hindering smooth water flow and increasing hydraulic friction losses. In severe cases, scale deposits can reduce the flow path cross-section, reducing pump flow and head.

In addition, in some corrosive media, pitting, cracking, or corrosion perforation can develop on the impeller surface. This damage disrupts the impeller's hydraulic structure, causing turbulence and parasitic losses, leading to reduced efficiency.

Bearing Wear

Long-shaft pumps have a long shafting structure and typically require multiple bearings. Over long-term operation, bearings are susceptible to wear due to hydraulic imbalance, vibration, and friction. When bearings wear, the pump shaft becomes less stable, causing wobble and misalignment. This increases the gap between the impeller and guide vanes, leading to greater energy loss. Shaft instability also causes additional frictional losses, further reducing pump efficiency.

Particularly with water-lubricated or rubber bearings, insufficient cooling or lubrication can lead to dry friction or erosion on the bearing surface, accelerating wear and, consequently, efficiency loss.

Misalignment

Long-shaft pumps require high installation precision, and misalignment between the motor and pump shaft can directly impact operating efficiency.

During installation or long-term operation, the coaxiality of the pump and motor shafts can deviate due to factors such as foundation settlement, thermal expansion and contraction, or mechanical shock. This misalignment causes unbalanced coupling operation, increases shaft system energy loss, and accelerates bearing and seal wear.

Misalignment not only increases mechanical energy loss but can also cause the impeller to operate outside optimal hydraulic conditions, leading to a gradual decrease in pump efficiency.

Cavitation

Long-shaft pumps are prone to cavitation if suction conditions do not meet design requirements. Cavitation causes bubbles to form and collapse on the impeller blade surface, generating impact and noise that gradually damages the blade surface.

Honeycomb-like damage or pitting on the blade surface increases the flow path surface roughness, increasing fluid flow resistance and leading to a decrease in hydraulic efficiency.

Additionally, the vibration and noise generated by cavitation can affect the pump's stable operation, increase energy consumption, and significantly reduce efficiency.

Operating conditions deviating from the design point

Long-shaft pumps are typically optimized for specific flow rates and head during the design phase. When operating conditions deviate from the design point for a long period, efficiency can significantly decrease.

When operating at low flow rates, the water flow generates strong backflow and vortexes within the pump, increasing hydraulic losses. When operating at high flow rates, the impeller outlet angle and the guide vane inlet angle do not match, resulting in additional hydraulic losses.

Long-term operation deviating from the design point not only reduces efficiency but also increases wear on the impeller, guide vanes, and bearings, accelerating the process of pump efficiency degradation.

Seal failure

Long-shaft pumps often use packing or mechanical seals. When a seal fails, fluid leakage can cause shaft power loss. The packing seal must maintain a reasonable compression force during long-term operation. Too loose will cause leakage, while too tight will increase friction loss, both of which will lead to reduced efficiency. Once the mechanical seal is worn or poorly lubricated, the friction pair will heat up severely, which will also lead to efficiency loss.