Pump impellers: Type defines function

The impeller is the pump component which actually imparts energy to the fluid handled, making it the “heart” of the pump. However, the waste water’s composition varies greatly. It may contain solids, wet wipes, sludge or faeces as well as stringy materials or substances forming foam. To achieve the best pumping results, the impeller should be ideally suited to the individual application. The aim is always to prevent clogging and to ensure a long service life and energy-efficient operation.

First step: Dimensioning the pump impeller

Prior to selecting the impeller type the parameters of pump operation must be determined, such as the flow rate and the head. This will help in selecting the best suited pump type as well as the pump size and performance data. Another important factor is the operating mode: Will the pump be running continuously or will it be used for intermittent or short-time operation? This might also decide on the pump’s drive, for which the current Ecodesign Directive (2019/1781/EU) is to be considered.

Second step: Selecting the impeller type

The next step after defining the pump type and size is the selection of an impeller type. KSB offers suitable impeller types for all requirements. The main criterion for selection is the nature of the fluid handled, i.e. the solids content in the waste water. Is it stormwater, raw waste water or pre-treated waste water? Does the waste water contain faeces and wet wipes? These criteria can be “converted” into measurable values. The three measured variables of gas content, sand content and dry matter content, in particular, are decisive for the selection. From a technical point of view, the size of the required free passage and the efficiency are key factors when choosing an impeller.

Open or closed impeller type

This is a brief overview of impeller types and their main applications. Primarily, a distinction is made between open and closed impellers. “Open” refers to a design without an outer (suction-side) shroud. Open impeller types include free-flow impellers, diagonal single-vane impellers, radial multi-vane impellers and cutters. Examples of closed impeller types are single-vane impellers and multi-channel impellers.

Open impeller types

Free-flow impeller

Centrifugal pumps with a free-flow impeller are preferably used for waste water containing solid substances and long fibres, coarse solids as well as entrapped gas or air. Free-flow impellers offer the largest free passage of all available impeller types. This is why they are a good match for waste water with a large gas and sand content. The dry matter content (i.e. the solids content; DS content) should not exceed 7 %; the impeller’s optimum, i.e. its highest efficiency is 59 %.

Diagonal single-vane impeller

The diagonal single-vane impeller – another open impeller design – serves to handle fluids containing solid substances and long fibres, coarse solids as well as entrapped gas or air. It is made to handle raw waste water, combined sewage, recirculated and heating sludge as well as activated, raw and digested sludge with a solids content of up to 8 % as well as high-viscosity fluids. The best efficiency is 81 %. Its free passage is large but slightly smaller than that of a free-flow impeller.

Radial multi-vane impeller

The radial multi-vane impeller – another open impeller design – serves to handle fluids containing solid substances and long fibres, coarse solids as well as entrapped gas or air. It is specially suited to handling raw waste water, combined sewage, recirculated and heating sludge as well as activated, raw and digested sludge with a solids content of up to 8 % as well as high-viscosity fluids. At 84 %, its best efficiency can be compared with the performance of closed multi-channel impellers. The free passage is at least 76 mm and thus meets many local requirements.

Cutter

Centrifugal pumps with a cutter are designed to handle waste water containing faeces when the discharge line diameter is relatively small (DN 32 to DN 65). The best efficiency is 42 %. The maximum free passage is 7 mm.

Closed impeller types

Single-vane impeller

The single-vane impeller in closed design is used for raw waste water containing solid substances and long fibres as well as for recirculated and heating sludge and combined sewage. It is also suitable for handling raw, activated and digested sludge. The maximum dry solids content can be up to 5 %. The best efficiency is 78 %.

Multi-channel impeller

Opting for a multi-channel impeller makes sense when contaminated, solids-laden, muddy non-gaseous fluids which do not contain fibrous, stringy material are to be transported. This impeller type is also suitable for pre-screened waste water, mechanically treated waste water, industrial effluent, landfill waste water as well as stormwater and activated sludge. The maximum dry solids content is 3 %. The best efficiency is 86 %. Depending on the size, impellers with free passages larger than 76 mm (3") are also available. The multi-channel impeller has the highest efficiency of the five impeller types featured. It is primarily used for waste water with a low contamination load or for pre-treated waste water – especially with a large sand content.

Grey cast iron, white cast iron or stainless steel?

Apart from the impeller type, the right material plays an important role. As a rule of thumb, grey cast iron is perfectly suitable for the majority of applications, given that waste water – unlike many other fluids – is not excessively abrasive. If, however, corrosive or chemically aggressive waste water is to be pumped, e.g. industrial waste water, special materials are required.

Hard chrome/nickel alloys are used when pumping abrasive fluids, as they will make the impeller contours as durable as possible, ensuring long-term pump efficiency and operating reliability.

Duplex stainless steel is suitable for pumping chemically aggressive fluids.
This stainless cast steel is resistant to cavitation, has excellent strength characteristics and – in applications involving acidic waste water with a high chloride content, seawater or brackish water – is highly resistant to pitting corrosion. Thanks to its excellent chemical resistance, e.g. to waste water containing phosphorous and sulphuric acid, this material is suitable for further applications in the chemical and process engineering industries

Conclusion: The right impeller for every type of waste water

With its broad pump impeller range KSB can supply the ideal pump for every application.