I. Improving Pump Efficiency
1. Extending and thinning the blades toward the suction inlet allows the liquid to be acted upon earlier by the blades. This reduces the impeller's outer diameter and increases the length of the flow path within the blade passages, thereby minimizing relative diffusion. However, extension must be appropriate. Excessive forward projection reduces the inlet area, shrinks the wall angle where the blade inlet meets the blade cover plate, and consequently increases hydraulic friction losses. It also constricts the inlet flow passage, adversely affecting both cavitation and efficiency.
2. Maintain the ratio of outlet to inlet area between adjacent blades within 1.0–1.3 to minimize diffusion losses. If this ratio exceeds 1.3, severe channel diffusion occurs, reducing efficiency.
3. A larger hydraulic radius is preferable. Strive to make the inlet cross-section of the blade as close to square as possible to reduce friction losses. In hydraulics, the ratio of the flow cross-sectional area to the wet perimeter is called the hydraulic radius, i.e., hydraulic radius = flow cross-sectional area / wet perimeter. A larger wetted perimeter effectively increases the liquid-to-wall contact area. When the flow channel cross-section changes from a near-square to an elongated rectangle, the liquid essentially flows through a narrow gap within the elongated section, inevitably increasing resistance.
4. Due to significant hydraulic losses in sharply curved diffusers, most modern designs employ slightly curved, near-straight diffusion sections. For guide vanes, their inlet angle and circumferential position should be determined based on fluid flow characteristics within the diffuser section. The principle is to form a continuous flow path, avoiding excessively narrow inlet cross-sections at guide vane entrances. Otherwise, vortex formation and impact losses will occur at the guide vane inlet.
5. For multistage pumps, adding pre-swirl at the impeller inlet (with guide vane outlet angle <90°) reduces relative velocity at the inlet while minimizing relative velocity diffusion. When the guide vane outlet angle is selected <90°, pre-swirl is generated before water enters the impeller, allowing 1 ≠ 0.
6. The pre-swirl generated by the guide vane outlet angle significantly affects the characteristics of the next stage impeller. To eliminate the “1Vul” term in the theoretical head formula Ht = U²Vu² - 1Vul during design, the guide vane outlet angle should ideally be set to 90°. This approach can eliminate rotational components for the final stage guide vane. However, experiments demonstrate that this approach is detrimental to both efficiency and obtaining stable performance curves, particularly for low-specific-speed pumps. To achieve a downward-sloping performance curve, the outlet angle of the guide vanes should be selected to be less than 90°, typically between 60° and 80°. The ends of the blades should be thinner to prevent impact and vortex losses.
7. Increase the impeller outlet width to reduce the absolute velocity at the outlet, thereby minimizing hydraulic losses in the pressure chamber.
8. Bevel the impeller outlet, reduce the length difference between forward and backward streamlines, or assign different outlet angles to distinct streamlines. This minimizes pressure differentials between forward and backward casing streamlines, reducing secondary flow at the outlet.
9. Expand the throat area of the pressure chamber. When the original design area is insufficient, this prevents flow obstruction.
II. Reducing Mechanical and Friction Losses
1. Mechanical friction losses caused by bearings and packing are generally minor and have little impact on efficiency. Mechanical friction losses in packing seals are greater than those in mechanical seals.
2. Improve the surface finish of impeller and guide vane flow passages. If possible, use hand-held grinding tools to polish the flow passage surfaces, which will significantly reduce hydraulic friction losses.
3. Disc friction losses between the front/rear casing surfaces and the fluid are proportional to the fifth power of the impeller's outer diameter. Selecting a larger blade outlet angle reduces the impeller's outer diameter, thereby decreasing disc friction losses. Disc friction losses are highly dependent on surface roughness; the outer surfaces of impeller casings should be as smooth as possible. Appropriately reducing the clearance between the impeller casing and guide vanes can also lower disc friction losses.
III. Reducing Leakage
1. Appropriately reduce clearances between components, extend seal gaps, or employ labyrinth seals to increase leakage resistance and minimize volumetric losses. Leakage points within pumps occur at the impeller-seal ring interface, between stages in multistage pumps, and in axial force balancing devices. Improving pump efficiency
2. Optimize the piping system to minimize resistance. Keep pipeline lengths as short and straight as possible to reduce flow velocity and minimize friction head loss. Reduce the number of components such as gate valves, foot valves, elbows, and orifices to decrease local head loss.
3. Reduce excess discharge pressure to precisely meet pipeline system requirements. Excessive pressure forces throttling measures like valve closure, wasting power. In such cases, pump retrofitting is essential. Remove primary or secondary impellers based on system pressure requirements. If excess pressure is minimal, reduce pressure by machining the impeller. This ensures pumps operate near their optimal efficiency point within the system (piping) setup, avoiding operation at low or high flow rates (where efficiency is reduced).
