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Pump Impeller Types Explained.

Home » Pump Impeller Types Explained.

Closed Single Channel Impellers

Closed, or shrouded, single channel impellers with high head, high efficiency capabilities. The wide spherical clearance through the impeller can pass soft compressible solids up to 80% (and sometimes 90%) of the discharge diameter.

Most closed impellers have balancing vanes on the rear of the back shroud to help reduce unbalanced axial forces that can occur due to the back shrouds larger surface area. These small vanes also help keep the sealing area free from debris.

Hydraulic efficiency is dependent on reasonably tight clearances between the impeller peripheries and the volute, particularly where the eye protrudes through the suction port and often incorporate renewable wear rings to enhance pump longevity.

Closed Multi Channel Impellers

Closed multi-channel impellers have all the same attributes of the single channel impeller but can accommodate far greater flows and hence larger branch connections.

Open and Semi-open Impellers

Technically true open impellers consist simply of vanes attached to a hub that is mounted on a pump shaft. But to overcome their structural weakness they are strengthened with a rear shroud, as shown above, making them semi-open impellers. The illustrations are single and multi-vane variants.

Semi-open impellers generate greater unbalanced axial forces than closed ones due to the lack of a front shroud (albeit the rear shroud covers the entire vane structure). Consequently, they employ balancing holes and back vanes to prevent premature bearing failure, they are also more prone to wear on the leading edge of the vane(s) and require regular adjustment to maintain hydraulic
efficiency.
The single vane impeller allows for the largest possible free passage but there is a consequence in that it is inherently out of balance due to a lack of symmetry plus it cannot be trimmed like the multi-vane and must be replaced if a reduction in performance is necessary.

Vortex Impellers (Recessed into Pump Body)

The vortex impeller is rapidly becoming the preferred impeller style for solids handling wastewater pumps due to their ability to pass solids up to 100% of the inlet diameter.

On “Recessed Impeller” versions, as above, the liquid is carried through the pump via the vortex created in the volute (only about 15% comes in contact with the impeller), which is a distinct advantage when handling fibrous solids. It also means less wear on the impeller and unbalanced forces are minimal. However there is a price to pay for these features and that is hydraulic efficiencies are seldom above 50% simply because a proportion of the energy imparted in to the liquid is consumed creating and maintaining the vortex.

Screw or Spiral Impeller

Centrifugal screw impellers are a hybrid that combines characteristics of a centrifugal pump and a positive displacement screw. They are the least common of the group and are the preferred choice for waterworks sludge application requiring a smooth pumping action with minimal shear. Hydraulic efficiencies are relatively high and free passage (of solids) good however the head\flow curve is quite steep. Like the single and double channel impellers the portion that sits in the inlet port is prone to wear and is protected by replaceable wear rings. A major disadvantage of this design is they tend to be expensive when compared to others in this group.

Open and Semi-open Impellers

Technically true open impellers consist simply of vanes attached to a hub that is mounted on a pump shaft. But to overcome their structural weakness they are strengthened with a rear shroud, as shown above, making them semi-open impellers. The illustrations are single and multi-vane variants.

Macerator/Grinders/Cutter Pumps

Centrifugal macerator pumps are possibly the most misunderstood, misapplied pumps in the pumping industry. They were originally designed to pump small quantities over long distances through small-bore pipes (typically 32mm to 50mm bore) specifically for properties located hundreds of meters from the main sewers. Consequently, most 2900rpm motors to ensure critical cutter speed and are therefore capable of achieving high differential heads. In theory this high speed cutter assembly produces fine slurry that is almost all water hence there is no requirement for a large free passage through the impeller. However, in the writer’s experience, they are prone to blockages within the cutter and impeller due to tight clearances in the volute.

Care should be taken when selecting macerator pumps that firstly the duty parameters are perfectly matched to the hydraulic performance of the pump and latterly the over-current and over-temperature controls are properly set prior to use – particularly on single phase installations where premature motor and capacitor failures are fairly commonplace.

Summary

Unfortunately, there is not one impeller that can do everything, a fact recognised by most leading centrifugal sewage pump manufacturers since they offer different impeller options for their popular ranges. With that said the vortex impeller is a good all-rounder for lower flow applications that require the passage of large solids. However, it is acknowledged that this style of impeller consumes approximately 30-40% more power than a shrouded single or twin channel impeller but given that the pump may run for less than one hour per day this amount is not so significant compared to the costs of unblocking and repairing pumps that are less capable of handling solids. Consequently I would suggest reliability takes precedence over efficiency in this case.