Weekly Topic: What are the requirements for material selection in the manufacturing of centrifugal pump impellers? (March 26, 2011)

The material selection for manufacturing centrifugal pump impellers should be considered from two aspects: first, the properties of the medium being transported, namely whether it is corrosive; if it is corrosive, then a corrosion-resistant material such as stainless steel or a higher-grade material should be chosen； Second is the condition of the medium being transported, that is, whether it is at high temperature and pressure. If the medium being transported is at high temperature, materials resistant to high temperatures—such as stainless steel or alloy steel—should be chosen.

It is closely related to the medium used; commonly, materials such as cast iron, cast steel, and stainless steel are chosen for impellers, with cast steel being the most common.

The basic structures of commonly used multi-stage centrifugal pumps are horizontal split-type and segmented, or multi-stage in series, types. The structural feature of the horizontal split-type design is that the upper and lower pump housings are connected at the horizontal section along the axis; the inlet and outlet pipes, part of the volute, and the flow channels are cast in the lower pump housing, which facilitates maintenance. During repairs, it is not necessary to disconnect the pipelines of the pump – the upper housing can be removed directly. The characteristic of this segmented structure is that each stage consists of an impeller located within a diffuser casing; the diffusers are connected to one another using bolts and links, and the various stages are fixed together in series by stationary rods. This design offers high pressure resistance and low leakage rates, but it requires the removal of the inlet pipe during maintenance, making disassembly and assembly processes rather difficult. It is generally believed that horizontal split-case multi-stage pumps have better stiffness and lower vibration levels than segmented multi-stage pumps. For the structure of the suction chamber, horizontal split-case multi-stage pumps generally use a semi-spiral design, while segmented multi-stage pumps mostly employ a circular design. Moreover, in the discharge chambers of each stage of impellers, due to the ease of manufacturing volutes and their high efficiency in converting liquid kinetic energy into pressure energy, horizontal split-case multi-stage pumps generally employ a volute structure ； However, due to the asymmetric shape of the volute, it can cause the shaft to bend; therefore, in segmented multi-stage pumps, volutes are only used in the initial and final stages, while guide vane assemblies are employed in the intermediate stages to facilitate energy transfer between the primary impeller and the secondary impellers. The first-stage impeller of a multi-stage pump is generally designed as a double-suction impeller, while the impellers of the remaining stages are designed as single-suction impellers, especially in cases where the medium is at high temperatures, has a high flow rate, and is prone to cavitation. For pumps under very high pressure, a single-layer pump casing is insufficient to withstand such pressure; therefore, a double-layer pump casing is often used, with the pump body designed in a cylindrical shape. The cylindrical pump body can withstand high pressures, and a horizontally split or segmented rotor is installed inside the cylinder. According to relevant national standards, high-pressure boiler feed pumps should adopt a single-shell segmented or double-shell cylindrical structure; pumps used in generator sets of 300MW and above generally should use a double-shell cylindrical structure. The inner shell of the bivalve has a segmented or horizontally split structure.

There are requirements for the materials used in the manufacture of centrifugal pump impellers: they must possess sufficient strength and stiffness； It has good welding or machining properties ； It has resistance to corrosion by liquid media and cavitation.