Centrifugal pump impellers primarily come in four forms: closed, semi-open (front-side open), semi-open (rear-side open), and open.

1. Closed impeller: Consists of vanes and both front and rear shrouds. Closed impellers offer high efficiency but are more difficult to manufacture; they are the most widely used type in centrifugal pumps. They are suitable for transporting clean water, solutions, and other low-viscosity, particle-free liquids.

2. Semi-open impeller: Generally exists in two structural forms. The first is the front-side open type, consisting of a rear shroud and vanes; this structure has lower efficiency, requiring an adjustable-clearance sealing ring to improve performance. The second is the rear-side open type, consisting of a front shroud and vanes; because it can utilize the same sealing rings as closed impellers, its efficiency is essentially the same. Additionally, the vanes serve a sealing function-acting like back vanes or an auxiliary impeller-in addition to transporting the liquid. Semi-open impellers are suitable for transporting liquids containing suspended matter such as solid particles or fibers. They are easier and less costly to manufacture and highly adaptable; in recent years, their use in centrifugal pumps for oil refining and chemical processing has increased, extending to the transport of clean water and water-like liquids.

3. Open impeller: Consists only of vanes and reinforcing ribs, with no front or rear shrouds. Open impellers typically have fewer vanes (2–5). They offer low efficiency and have limited applications, being used primarily for transporting high-viscosity or slurry-like liquids.

Centrifugal pump impellers generally feature backward-curved vanes. These vanes can be cylindrical or twisted in shape; twisted vanes reduce vane load, improve suction performance, and enhance cavitation resistance, though they are more difficult and costly to manufacture. For centrifugal pumps used in oil refining and chemical processing, the impeller is required to be a single-piece unit, either cast or fully welded. Welded pump impellers have emerged in recent years and are primarily used in specialized centrifugal pumps for the chemical industry, particularly when the impellers are made from metal materials with poor castability, such as iron and its alloys. Welded impellers offer superior geometric precision and surface finish compared to cast impellers, which helps improve the efficiency of the centrifugal pump.