What are Hydraulic Loss, Volumetric Loss and Mechanical Loss of a Centrifugal Pump?

In the energy conversion process of a centrifugal pump, not all input power can be effectively converted into the pressure energy and kinetic energy of the liquid. In actual operation, inevitable energy loss always exists. According to the physical mechanism of energy loss, the loss of a centrifugal pump is usually divided into three categories: Hydraulic Loss, Volumetric Loss and Mechanical Loss. These three types of loss jointly determine the overall efficiency of the pump.

I. Hydraulic Loss

Definition: Hydraulic loss, also known as flow loss, refers to the energy loss generated when the liquid flows through the flow components inside the pump. In terms of results, it is manifested as the difference between the theoretical head and the actual head of the pump. This is the main factor affecting pump efficiency.

Causes: Hydraulic loss is mainly composed of the following three aspects:

1. Shock Loss: When the liquid enters or flows out of the impeller, if its flow direction is inconsistent with the designed direction of the blades or flow passages, impact and sudden change of direction will occur, resulting in shock loss. This situation is particularly prominent when the pump operates away from its Best Efficiency Point (BEP).
2. Friction Loss: The liquid itself has viscosity. When it flows through the rough inner walls of the suction chamber, impeller flow passages, volute and other components, frictional resistance will be generated, and this part of energy will be converted into heat energy and lost. The longer and rougher the flow passage, the greater the friction loss.
3. Eddy Loss: Due to the limited number of impeller blades, it is impossible to guide all the liquid perfectly. Part of the liquid will generate a circulating flow (relative eddy) inside the impeller, resulting in energy consumption. At the same time, the change of flow passage shape will also cause local eddies and lead to losses.

The magnitude of hydraulic loss directly affects the pump head, and we can measure its influence degree by Hydraulic Efficiency (ηh).

II. Volumetric Loss

Definition: Volumetric loss, also known as leakage loss, is the energy loss caused by flow leakage. Specifically, a part of the high-pressure liquid pressurized by the impeller is not effectively delivered to the pump outlet, but leaks back to the low-pressure area (such as the impeller inlet) through various clearances inside the pump.

Causes:

1. Seal Ring Clearance Leakage: This is the main part of volumetric loss. To prevent friction between the high-speed rotating impeller and the stationary pump casing, a clearance (i.e., the wear ring clearance) must be left between them. The high-pressure liquid at the pump outlet will leak back to the inlet through this clearance.
2. Balancing Device Leakage: In multi-stage pumps or some single-stage pumps designed to balance axial force, structures such as balance holes, balance discs or balance pipes will also cause a part of high-pressure liquid to flow back, resulting in losses.
3. Shaft Seal Leakage: A small amount of liquid may also leak from the shaft seal, which, although accounting for a small proportion, is also included in the volumetric loss.

Volumetric loss leads to the actual output flow of the pump being less than its theoretical flow. Its magnitude is measured by Volumetric Efficiency (ηv). As the pump wears, the seal ring clearance will gradually increase, and the volumetric loss will also increase accordingly.

III. Mechanical Loss

Definition: Mechanical loss refers to the energy consumed by the pump shaft to overcome various mechanical frictions during rotation. This part of energy is finally dissipated in the form of heat energy.

Causes:

1. Disk Friction Loss: Severe friction occurs between the outer cover plates (front and rear cover plates) of the high-speed rotating impeller and the liquid in the pump cavity, which is the main part of mechanical loss.
2. Bearing Friction Loss: Rolling bearings or sliding bearings used to support the pump shaft will generate frictional force during operation.
3. Shaft Seal Friction Loss: Whether it is packing seal or mechanical seal, the sealing device will rub against the pump shaft or shaft sleeve, consuming a part of power.

Mechanical loss means that a part of the shaft power transmitted from the motor is consumed before it reaches the impeller to do work on the liquid. Its magnitude is measured by Mechanical Efficiency (ηm).

Conclusion

Understanding the hydraulic loss, volumetric loss and mechanical loss of centrifugal pumps is not only the foundation for professional learning of fluid machinery, but also an important technical means to achieve the "dual carbon" goals and promote energy conservation and consumption reduction in the industrial field. Through scientific design, refined operation and maintenance and intelligent control, we are fully capable of minimizing these "invisible losses" and releasing the maximum potential of the pump system. In the future, Omron Tech Pumps will continue to deepen its research in high-efficiency fluid solutions, help the green upgrading of the industry, and harness every flowing energy together with you.