Types and Functions of Centrifugal Pump Impellers

Impellers are the heart of a centrifugal pump, and their design directly determines the pump's performance, efficiency, and suitability for an application. They are classified in three primary ways:

1. Based on Flow Direction (Path of Fluid Through Impeller)

This is the fundamental classification.

• Radial Flow Impeller: The most common type. Fluid enters axially and is discharged radially (90° to the shaft). Generates high head at low flow. Used in most standard centrifugal pumps for water supply, industrial processes, and high-pressure applications.

• Mixed Flow Impeller: Fluid enters axially and is discharged at an angle (between 0° and 90°). Provides a balance of moderate flow and moderate head. Often used in irrigation, drainage, and larger water circulation pumps.

• Axial Flow Impeller (Propeller): Fluid enters and discharges nearly parallel to the shaft (axial direction). Designed for very high flow rates at very low head. Common in stormwater pumping, flood control, and large-scale circulation (e.g., cooling water intakes).

2. Based on Suction Type (Number of Inlets)

• Single Suction Impeller: Fluid enters the impeller from one side only. This creates an unbalanced axial hydraulic thrust that must be managed by bearings. Simpler design, common in standard pumps.

• Double Suction Impeller: Fluid enters the impeller from both sides simultaneously. This design balances the axial thrust, reducing bearing load and shaft deflection. Allows for higher flow rates and is common in larger, more critical service pumps (e.g., main water supply lines).

3. Based on Mechanical Construction / Casing Sidewall Contact

This is the most frequently used classification for standard pumps.

• Closed Impeller:

  • Design: Vanes are shrouded (covered) on both sides by front and back walls (shrouds).

  • Advantages: High efficiency, as clearances between the shroud and casing are small, minimizing internal recirculation losses. Strong structural integrity.

  • Disadvantages: More expensive to manufacture. Can clog with solids or stringy materials.

  • Applications: Clean liquids (water, chemicals, oils). The standard for most industrial and commercial pumps.

• Open Impeller:

  • Design: Vanes are attached to a central hub and are open on both sides (no shrouds).

  • Advantages: Resists clogging, can handle slurries, fibrous materials, and liquids with suspended solids. Easier to clean and inspect.

  • Disadvantages: Lower efficiency due to higher internal leakage (fluid recirculating past the vanes). Requires wear rings in the casing to maintain clearance, which need periodic adjustment/replacement.

  • Applications: Slurries, wastewater, sewage, pulp and paper stock.

• Semi-Open Impeller:

  • Design: Vanes are attached to a hub and have a shroud on only one side (typically the back).

  • Advantages: A compromise between open and closed. Better efficiency than open impellers, better solids-handling capability than closed ones.

  • Disadvantages: Still requires wear rings; can be subject to higher axial thrust.

  • Applications: Viscous fluids, liquids with small solids, some chemical processes.

Specialized Impeller Types

• Vortex Impeller (or Recessed Impeller):

  • Creates a whirlpool (vortex) in the pumping chamber. The impeller itself is recessed and does not directly contact the solid particles.

  • Application: Excellent for handling large solids, stringy materials, and air-entrained fluids without clogging. Common in wastewater and sewage pumps.

• Chopper / Grinder Impeller:

  • Equipped with cutting edges that macerate solids before they pass through the pump.

  • Application: Raw sewage, wastewater with rags and plastics, to prevent clogging in downstream piping.

• Non-Clogging / Channel Impeller:

  • Typically has only 2 or 3 large, curved vanes with wide passages to allow solids to pass through.

  • Application: Municipal and industrial wastewater, plants with heavy solids.

Selection Summary Table

Key Takeaway: The choice of impeller is a critical trade-off between efficiency, head/flow characteristics, and ability to handle solids or specific fluids. Proper selection is essential for reliable and cost-effective pump operation.