What Is an Impeller?

Impeller refers not only to the rotor disc with moving blades, but also the component part of the impulse steam turbine rotor ^[1] , and it can also refer to the collective name of the rotor disc and the rotating blades mounted on it. Impellers can be classified according to their shape and opening and closing conditions.

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GB / T7021-1986

The centrifugal pump impeller mainly has the following 4 forms, (a) closed type; (b) front half-open type; (c) rear half-open type; (d) open type

1impeller; 2rear cover; 3hub; 4front cover; 5impeller seal ring; 6stiffener

Closed impeller

It consists of blades and front and rear covers. Closed impellers have higher efficiency and are more difficult to manufacture. They are most commonly used in centrifugal pumps. Suitable for conveying clean liquids with small viscosity, such as clean water and solution, without particles.

Semi-open impeller

Generally, there are two types of structure: one is the front half-open type, which is composed of the rear cover and the blade. This structure has low efficiency of the impeller. To improve the efficiency, a seal ring with adjustable clearance is required. It consists of a front cover and a blade. Since the same sealing ring as the closed impeller can be applied, the efficiency is basically the same as that of the closed impeller, and in addition to conveying liquid, the blade also has a sealing effect (back blade or auxiliary impeller). The semi-open impeller is suitable for conveying liquid containing solid particles, fibers and other suspended matter. The semi-open impeller is less difficult to manufacture, lower in cost, and highly adaptable. It is increasingly used in centrifugal pumps for refining and chemical industries, and is used to transport clear water and liquids that are approximately clear water.

Open impeller

Impellers with only blades and blade stiffeners, without front and rear cover plates (less 2-5 blades for open impellers). The impeller has low efficiency and few applications. It is mainly used for conveying liquids with higher viscosity and slurry liquid.

Centrifugal pump impeller blades are generally backward curved blades. There are two types of blades: cylindrical and twisted. The use of twisted blades can reduce the load on the blades, improve the suction performance of the centrifugal pump, and improve the anti-cavitation capability.

Centrifugal pumps for refining and chemical industry require impellers to be cast or full-welded integral impellers. Welding impellers have been developed and are mostly used in chemical special centrifugal pumps made of metal materials with poor casting properties (such as iron and its alloys). Geometric accuracy and surface of the welded impeller

Centrifugal impeller

The blade shapes of centrifugal fan impellers are veneer type, arc type and wing type. Airfoil blades have good aerodynamic characteristics, high efficiency, good strength and high rigidity. The disadvantages are that the manufacturing process is complicated, and when the gas containing a high concentration of dust is conveyed, the blade is easy to wear. After the blade is worn through, impurities enter the inside of the blade, causing the impeller to lose balance and generate vibration. The flat straight blade is simple to manufacture, but the flow characteristics are poor. Compared with the airfoil blade, the flat curve backward blade has lower efficiency near the highest efficiency point, and the efficiency at other operating conditions is quite close.

Forward impeller

The forward impellers generally use arc-shaped blades. In the backward-direction impellers, large-scale fans mostly use airfoil blades. For coal-fired boiler induced draft fans with low dust removal efficiency, circular or flat blades can be used. There are many centrifugal fans using flat blades.

Also known as

The wear of the fan impeller is related to the composition, particle size, concentration, shape, impact speed, impact angle, chemical composition, properties, temperature and humidity of the abrasive. The non-uniformity of gas flow inside the impeller accelerates wear. As a measure to prevent impeller wear: First, reduce the dust and corrosive gas entering the fan. To this end, the fan operating system must be modified; second, try to make the local wear tend to uniform wear, which needs to improve the wear resistance of the impeller. . If the wear resistance of the impeller is improved, a material with high hardness and good wear resistance can be used. This will not only cause difficulties to the impeller manufacturing process, but also be unreasonable from an economic point of view. Therefore, it is an economical and reasonable solution to improve the surface quality of the fan impeller, overlay or spray-weld (spray) abrasion resistant layers on the abraded parts of the impeller and add a layer of lining on the blade to achieve wear resistance.

Apply anti-wear coating

The main anti-wear coating has resin

The corrosion and abrasion of the fan impeller is mainly the corrosion and stress corrosion caused by the corrosive gas contained in the conveying medium of the fan. The types of corrosion caused by different transport media are also different. In order to prevent the occurrence of corrosion and impeller corrosion and wear, the following measures are usually adopted: stainless steel with strong corrosion resistance is used as the impeller material; the surface of the impeller is coated or sprayed with anticorrosive layer and the surface quality of the impeller is improved to solve the problem of corrosion and wear.

Corrosion-resistant wear materials

The so-called auxiliary impeller hydrodynamic seal refers to an open impeller installed coaxially in the reverse direction near the back of the impeller rear cover of the pump. When the pump is working, the auxiliary impeller rotates with the pump main shaft, and the liquid in the auxiliary impeller also rotates together. The rotating liquid will generate an outward centrifugal force. This centrifugal force on the one hand stands against the liquid flowing to the mechanical seal, which reduces the mechanical Pressure at the seal. On the other hand, the solid particles in the medium are prevented from entering the friction pair of the mechanical seal, reducing the wear of the mechanical seal abrasive block and extending its service life.

In addition to the sealing function of the auxiliary impeller, it can also reduce the axial force. In the submersible sewage pump, the axial force is mainly composed of the differential pressure of the liquid on the impeller and the gravity of the entire rotating part. The two forces act in the same direction, and the resultant force is the sum of the two forces. It can be seen that under the condition that the performance parameters are completely the same, the axial force of the submersible sewage pump is greater than that of the ordinary horizontal pump, and the difficulty of balancing is more difficult than that of the vertical pump. Therefore, in submersible sewage pumps, the reason that the bearings are easily damaged is also related to the large axial force.

If the auxiliary impeller is installed, the direction of the differential pressure of the liquid acting on the auxiliary impeller is opposite to the combined force of the above two forces, which can offset a part of the axial force, which also extends the bearing life. However, there is also a disadvantage of using the auxiliary impeller sealing system, that is, a part of energy is consumed on the auxiliary impeller, which is generally about 3%, but as long as the design is reasonable, this part of the loss can be minimized.