Comprehensive analysis of impeller knowledge

The impeller, a key component, plays a vital role in the impulse steam turbine. It not only refers to the wheel disc equipped with moving blades, but also the core part of the impulse steam turbine rotor. In addition, the impeller also covers the combination of the wheel disc and the rotating blades installed on it. Its manufacturing process is diverse, the common ones include casting and welding, and the material selection needs to be determined according to the working medium.

The core function of the impeller is to efficiently convert the mechanical energy of the prime mover into the static pressure energy and dynamic pressure energy of the working fluid. In terms of classification, the impeller can be divided into three types according to the blade form: open, closed and semi-open. Among them, the closed impeller is composed of front and rear covers and blades, and runs smoothly and efficiently; the semi-open impeller is composed of blades and rear covers, with a simple structure and convenient casting, which is suitable for conveying media containing solid particles; while the open impeller only contains blades and part of the rear cover or no rear cover. Its design makes it possible to convey media containing certain solid particles, but attention should be paid to the abrasive effect of solid particles on the flow channel.

It is worth noting that although semi-open and open impellers are easy to cast and suitable for conveying media containing solid particles, the abrasion of solid particles on the flow channel may cause the pump to have a reduced working efficiency. In contrast, although the closed impeller has high operating efficiency and small axial thrust, it is not suitable for conveying sewage media containing large particles or long fibers. Therefore, when selecting the type of impeller, it is necessary to make a comprehensive consideration based on the actual working requirements and the characteristics of the medium.

According to its working mode, the impeller can be further divided into single-suction impellers and double-suction impellers. The design feature of the single-suction impeller is that the liquid is sucked from only one side, while the double-suction impeller can be sucked from both sides at the same time. This design gives it excellent anti-cavitation performance.

According to the structural type, the impeller can be divided into four types: channel type, blade type, spiral centrifugal type and swirl type. Among them, the channel impeller includes two designs: single-channel and double-channel; the blade impeller is divided into closed and open types; and the spiral centrifugal and swirl impellers have their unique working principles and structural characteristics.

Channel impeller:

The channel impeller has a unique design, and its flow channel is curved from the inlet to the outlet. This structure is particularly suitable for conveying liquids containing large particles or long fibers. It has won wide application due to its excellent anti-clogging performance. However, compared with other types of impellers, its anti-cavitation performance is slightly insufficient.

Blade impeller:

Blade impellers include semi-open and open designs. These two types of impellers are easy to cast and relatively easy to maintain and clean. However, it should be noted that during transportation, solid particles may erode the gap between the inner wall of the lower pressure water chamber and the blades, thereby reducing the operating efficiency of the water pump, and may destroy the flow stability of the liquid in the flow channel, causing vibration of the pump body. In addition, this type of impeller is not particularly suitable for conveying media containing large particles or long fibers.

In contrast, the closed impeller has higher operating efficiency, can run smoothly for a long time, and has a smaller axial thrust of the pump. But its disadvantage is that it is easy to be entangled, so it is not suitable for conveying sewage media containing large particles or long fibers.

Spiral centrifugal impeller:

The blades of the spiral centrifugal impeller are twisted and extend axially along the conical hub body to the suction port. Its unique design prevents the conveyed liquid from hitting any part of the pump when it flows through the blades, thus avoiding damage to the pump and the liquid. At the same time, due to the propulsion of the spiral, this type of impeller has excellent permeability for suspended particles. Therefore, pumps using this impeller are very suitable for pumping media containing large particles and long fibers.

Swirl impeller:

The design of the swirl impeller allows the impeller to partially or completely retract from the flow channel of the pressure chamber, and this design also has good anti-clogging performance. During the operation of the pump, the particles move in the water pressure chamber by the vortex generated by the rotation of the impeller. This design prevents the suspended particles from directly contacting the blades during movement, thereby reducing the wear of the blades and the possibility of increasing the gap due to abrasion. Therefore, the swirl impeller is suitable for pumping media containing large particles and long fibers.

The geometric parameters of the impeller, including the impeller inlet diameter (Dj), blade inlet diameter (D1), impeller hub diameter (dh), blade inlet width (b1), blade inlet angle (β1), impeller outer diameter (D2), impeller outlet width (b2), blade outlet angle (β2), blade wrap angle (Φ) and number of blades (Z), have a significant impact on the performance of the pump. Specifically, the geometric parameters of the impeller inlet are crucial to the cavitation performance, while the geometric parameters of the impeller outlet have a direct impact on the performance parameters of the pump (such as head H and flow Q). The reasonable selection and adjustment of these geometric parameters are crucial to optimizing the efficiency of the pump.

During the use of the impeller, problems such as casting defects may be encountered. The occurrence of these problems will not only affect the performance of the impeller, but may also shorten its service life. Therefore, appropriate measures need to be taken to ensure that the impeller can continue to and stably play its due role.

Fault manifestations: inclusions, excess meat, rough surface, sand holes, cracks or shrinkage defects in the flow channel.

Treatment methods: Grinding, repair welding and other measures can be taken for different situations. If the problem is more serious, it needs to be scrapped and recast.

Next, let's discuss another problem that may be encountered during the use of the impeller - abrasion.

Fault manifestation: The impeller of the process pump cast in gray cast iron material suffered severe wear from water containing a small amount of cement foam in just 5 years of use. This abrasion problem is often attributed to improper material selection during design, improper pump type selection, or contamination of the process medium.

Treatment method: For such problems, measures such as changing the impeller material, adjusting the design scheme to select a more suitable pump type, and thoroughly checking the process medium can be taken.

Next, we will continue to explore other problems that may be encountered during the use of the impeller.

Fault manifestation: There are defects in the design scheme, such as the installation position is too high, or the material selection is not suitable, resulting in the phenomenon of mouth ring locking.

Treatment method: Adjust the installation position to reduce the height, select a material with more cavitation resistance, or redesign the pump type to adapt to specific working conditions.

Next, we will further explore the details of the mouth ring locking fault.

Fault manifestation: In the design scheme, the gap of the mouth ring is set too small; the hardness difference between the front and rear mouth rings is less than 50HBW; there are solid particles in the mouth ring gap; the mouth ring is deformed, etc.

Treatment method: appropriately increase the design gap of the mouth ring; replace the mouth ring material to improve its wear resistance; carefully check the process medium and remove the solid particles in it; if the mouth ring is deformed, replace it with a new one.

Next, we will continue to explore other possible faults and their treatment methods.

Cause of failure: It may be caused by cavitation, corrosion or abrasion; it may also be related to poor casting quality; or it may be affected by water hammer effect.

Countermeasures: It is recommended to replace the impeller parts with new ones.

Corrosion problem

Corrosion is another common cause of water pump failure. Since water pumps are in contact with water or humid environment for a long time, they are susceptible to chemical corrosion or electrochemical corrosion, which can damage key components such as impellers and affect the normal operation of water pumps. To deal with corrosion problems, appropriate protective measures need to be taken, such as the use of corrosion-resistant materials, regular inspections and maintenance, etc., to ensure the long-term stable operation of water pumps.

Fault manifestation: Problems caused by improper material selection.

Treatment method: Replace the impeller with a new one, using corrosion-resistant and wear-resistant materials; apply corrosion-resistant coating or spraying to the impeller.