In centrifugal pumps, impeller design is one of the most important factors affecting performance, efficiency, maintenance, and service life. The type of impeller selected depends on the fluid being handled, whether the application requires low or high pressure, and whether the pump is designed to be self-priming or to handle entrained gas.
Care must be taken during selection to achieve the correct balance between the desired process outcome and maintenance intervals, while still obtaining maximum process efficiency.
Impeller Types and Applications
There are ten main types of centrifugal impeller, each with its own advantages and disadvantages depending on the application.
In general, the closer the radial vanes are together, the tighter the tolerances and the more enclosed the impeller design, the higher the efficiency. However, this also reduces solids-handling capacity because the free passage through the impeller becomes more limited.
The ten common impeller types are described below, together with their typical advantages and the fluids they are suitable for.
1. Closed or Enclosed Impeller
Typical applications: Centrifugal pumps, multistage pumps, and the first stage of side channel pumps.
The closed impeller is designed for clean fluids containing only small particles. It offers the highest efficiency of all impeller types because its clearances against the internal casing are very tight. It usually includes wear rings to help maintain efficiency.
Solids-handling capability is determined by the space between the front and rear channels of the impeller. In small units, this clearance may be measured in microns.
Fluid is drawn into the impeller eye at the centre. Centrifugal force then directs the fluid through the blades and towards the side of the pump casing, from where it is expelled through the outlet.
2. Double Suction Impeller
Typical applications: Split casing pumps and double suction pump designs.
This design allows the pump to draw liquid through both sides of the impeller simultaneously. It is typically used for clean liquids without solids, at high flow rates and relatively low heads. A double volute casing feeds both sides at the same time.
The double suction impeller delivers the highest possible flow rate within a single casing.
3. Flexible Impeller
Typical applications: Self-priming pumps handling clean, viscous, or solid-laden liquids.
A flexible impeller consists of curved rubber vanes that remain in contact with the pump casing, allowing the pump to be self-priming. It is designed to handle clean, viscous, and solid-laden liquids. Solid particles fit between the blades, so the impeller can handle suspended solids such as fruit chunks up to approximately 25 mm in diameter.
This design produces low shear because rotational speed is usually below 1,400 rpm. The lower speed is necessary because the impeller makes contact with the casing.
4. Open Impeller
Typical applications: Handling large solids and applications requiring easy cleaning.
In an open impeller, only the vanes are visible. This allows the pump to handle large solids and makes the impeller easy to clean. However, efficiency is lower than with other designs because the free passage area is large.
An open impeller struggles to generate high pressures because it lacks side walls, but it can accommodate large solid particles.
5. Semi-Open Impeller
Typical applications: Centrifugal pumps and trash pumps.
The semi-open impeller has wide channels that allow large solid particles to pass without clogging. It is designed for large capacities and low pressures, making it suitable for sewage handling and aggressive industrial process applications.
6. Sinusoidal Impeller
Typical applications: Gentle handling of soft or damage-prone products.
The sinusoidal impeller has an appearance similar to a sinusoidal sound wave, which makes it unlike any other impeller design. It is designed to scoop pieces and transfer them gently towards the outlet. A scraper at the outlet helps prevent fluid from recirculating within the pump head. The scraper moves in and out in line with the wave shape of the impeller.
Unlike other low-shear pump designs, the sinusoidal impeller does not have a tip. This means it can transfer soft or damage-prone products without breakage. For example, fruit pieces can be handled because they do not pass through the impeller.
7. Vortex Impeller
Typical applications: Submersible pumps and process pumps.
In a traditional impeller, liquid flows through the impeller before being expelled radially towards the casing outlet. In a vortex impeller, however, a vortex is created within the casing. This allows fibrous materials or solid particles to pass through, often without coming into contact with the impeller. Only a small percentage of particles make contact with the rotating part, which is why this design is also referred to as non-clogging.
Vortex impellers are predominantly used for low-head applications. They can be recessed within the pump casing to allow larger suspended solids to pass without contacting the pumped solids.
8. Mixed Flow Impeller
Typical applications: Vertical turbine pumps and immersion designs.
The mixed flow, or semi-axial, impeller transports flow diagonally. Because it generates strong vertical currents, it helps prevent settling at the bottom when immersed within a tank.
9. Side Channel Impeller
Typical applications: Side channel pumps combined with a closed impeller.
This multiple radial design enables the pump to handle entrained gas. It fits tightly between the suction and discharge casings to maintain efficiency.
The side channel impeller cannot handle solids. Instead, it separates gas or air, allowing both liquid and gas to be pumped without vapour locking. This also makes the pump self-priming.
Ten Common Impeller Problems
1. Clogging
Clogging occurs when the pumped fluid collects within the casing, reducing clearances and pump efficiency. This leads to a reduction in flow rate and pressure.
2. Chemical Attack
Chemical attack occurs when liquids or chemicals are not compatible with the material of the pump. This can lead to corrosion and premature failure of pump parts.
3. Cavitation
Cavitation occurs when there is insufficient suction pressure. Bubbles form in the suction area and then collapse or implode on the impeller surface. This erodes large areas of the impeller quickly and can lead to failure.
4. Imbalance
All impellers are balanced during manufacturing. If an impeller is not properly balanced, unequal radial forces can occur, causing accelerated component wear through heavy vibration. This directly affects the lifespan of the mechanical seal and bearings, leading to premature pump failure.
5. Water Hammer
Water hammer is the sudden stopping of a fluid, causing the momentum in the liquid to reverse flow. The flow then travels back and forth rapidly, creating vibration. The resulting forces can crack impellers and casings.
6. Loosening
Impellers can become loose when vibration occurs within the unit. This may cause the impeller to come into contact with the wear rings or pump casing at speed, potentially destroying the pump.
7. Resonance
With variable speed drives, it may be assumed that a pump can be operated at any rpm. However, some speeds can create resonance within pump components, leading to vibration.
8. Fouling
Fouling can reduce the free-flowing path of liquids and lower efficiency. If pumps have been standing for some time, growth can occur not only within the pump but also within suction pipework. In marine environments, this is known as biofouling.
Fouling can lead to cavitation within the unit. If closed impellers are selected and then encounter solid particles, the internal passages can clog.
Some liquids contain dissolved solids that separate once the fluid stops or dries. These are often found in scrubber applications and can lead to calcification or limescale build-up, also reducing efficiency.
9. Turbulent Flow
When flow is turbulent rather than laminar, separation of the pumped fluid can occur. It also creates the ideal conditions for cavitation.
10. Overpressure
High inlet pressures can force impellers against wear rings or stage casings, drastically reducing service life.
Impeller Trimming
Impellers are often trimmed so that a pump can meet a specific duty. Typically, pumps are manufactured with impellers at full size. When used with an inverter, the rotational speed can be reduced to lower the flow and pressure to the required duty point. By keeping the impeller at full diameter, the clearances between the rotating part and the casing remain small, helping to maintain efficiency.
Trimming the impeller is another way to make a pump meet a duty point. However, the more material that is removed, the greater the internal tolerances within the casing become, which reduces efficiency.
The speed of rotation and the diameter of the rotating part are used as predictors of performance. These relationships are known as the affinity laws.
Centrifugal Affinity Laws
Pump affinity laws are a set of formulas used to determine a unit’s performance when a change is made to speed or impeller diameter. They can predict the resulting flow and pressure with a high degree of accuracy.
There are three main affinity laws:
1. Flow Is Proportional to Shaft Speed or Impeller Diameter
As shaft speed or impeller diameter is altered, flow changes by the same proportion. If the speed of a pump is reduced by 20%, the flow at the same head will also decrease by 20%.
2. Pressure Is Proportional to the Square of Shaft Speed or Impeller Diameter
When impeller diameter or shaft speed is changed, pressure changes in proportion to the square of that change. If shaft speed increases by 10%, the pressure at the same flow will increase by approximately 21%.
3. Power Is Proportional to the Cube of Shaft Speed or Impeller Diameter
If shaft speed increases by 10%, power increases in proportion to the cube of the speed change. This results in an increase of approximately 33.1%.
Wear Rings
Wear rings sit on either side of the impeller and act as sacrificial components. They help prevent casing wear and maintain efficiency within the casing.
A doubling of wear ring clearances can increase the required NPSH by as much as 50%.
Conclusion
Centrifugal impeller design has a direct impact on pump efficiency, solids-handling capability, maintenance requirements, and overall reliability. With ten common impeller types available—ranging from closed and double suction designs to vortex, sinusoidal, and side channel impellers—selecting the right design is essential for matching the pump to the fluid, duty point, and operating conditions.
Understanding common impeller problems, trimming principles, affinity laws, and wear ring maintenance also helps engineers and operators extend pump life, reduce downtime, and maintain efficient operation.
