Centrifugal pump impellers mainly come in the following four forms: (a) closed type; (b) front semi-open type; (c) rear semi-open type; (d) open type.
1-Impeller; 2-Rear cover plate; 3-Hub; 4-Front cover plate; 5-Impeller sealing ring; 6-Reinforcing rib
Closed Impeller: Composed of blades and front and rear cover plates. Closed impellers have higher efficiency but are more difficult to manufacture and are the most widely used in centrifugal pumps. Suitable for conveying clean liquids with low viscosity and no particles, such as clean water and solutions.
Semi-open Impeller: Generally has two structures: one is the front semi-open type, composed of a rear cover plate and blades. This structure has lower impeller efficiency, and an adjustable clearance sealing ring is needed to improve efficiency; the other is the rear semi-open type, composed of a front cover plate and blades. Since the same sealing ring as the closed impeller can be used, the efficiency is basically the same as the closed impeller, and the blades, in addition to conveying liquid, also have a sealing function (back blades or auxiliary impeller). Semi-open impellers are suitable for conveying liquids containing suspended solids such as particles and fibers. Semi-open impellers are easier to manufacture, lower in cost, and more adaptable, leading to their increasing application in centrifugal pumps for oil refining and chemical industries, and also for conveying clean water and near-clean water liquids.
Open impellers consist only of blades and blade reinforcing ribs, without front or rear cover plates (open impellers typically have 2-5 blades). They have lower efficiency and are less commonly used, mainly for conveying high-viscosity liquids and slurry-like liquids.
Centrifugal pump impeller blades are generally backward-curved. Blades come in cylindrical and twisted shapes. Twisted blades reduce blade load and improve the pump's suction performance and cavitation resistance, but are more difficult and expensive to manufacture.
Centrifugal pumps for oil refining and chemical industries require impellers to be cast or integrally welded. Welded impellers have been developed and are mostly used in special chemical centrifugal pumps made from metals with poor casting properties (such as iron and its alloys). Welded impellers have better geometric precision and surface finish than cast impellers, which is beneficial for improving the efficiency of centrifugal pumps.