TYPES OF IMPELLERS

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 TYPES OF IMPELLERS

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There are three types of construction seen in an impeller. These are based on the presence or absence of the impeller covers and shrouds. 

1. Closed 

2. Semi-open 

3. Open. 

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Closed impellers :

• The closed impeller consists of radial vanes (typically 3–7 in number), which are enclosed from both sides by two discs termed ‘shrouds’. These have a wear ring on the suction eye and may or may not have one on the back shroud. Impellers that do not have a wear ring at the back typically have back vanes. Pumps with closed type impellers and wear rings on both sides have a higher efficiency. 

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Semi-open impellers 

• The semi-open type impellers are more efficient due to the elimination of disk friction from the front shroud and are preferred when the liquid used may contain suspended particles or fibers. The axial thrust generated in semi-open impellers is usually higher than closed impellers. 

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Open impellers 

• There are three types of back shroud configurations. The first one is a fully scalloped open impeller. 

• The back shroud is almost taken out and thus the axial thrust caused by the hydraulic pressure is almost eliminated. 

• The second type is known as the partially scalloped open type of impeller. It experiences a greater axial thrust than the fully scalloped open impeller. 

However, this has higher efficiency and head characteristics. 

• The third type is known as the fully back shroud open impeller. where there is an open impeller with a full back shroud. It normally has almost 5% higher efficiency than a fully scalloped impeller, though it has diminished head generation capabilities. 

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• The fully shrouded open impellers experience the maximum axial thrust among the open impeller types. To reduce this effect, back vanes are provided to relieve the hydraulic pressure that generates the axial thrust. 

• The vortex or non-clog impellers are the fully shrouded open type of impellers. These are used in applications where the suspended solid’s size maybe large or the solid’s maybe of crystals and fibers type. The vortex impeller does not impart energy directly to the liquid. Instead it creates a whirlpool, best described as a vortex.

• The vortex in turn imparts energy to the liquid or pumpage. The location of the impeller is usually above the volute, so it experiences hardly any radial forces. This allows extended operation of the pump even at closed discharge conditions

• Some of the other non-clogging designs of impellers in the closed and semi-open types. In general, most of the open impellers are of the partially scalloped and fully shroud types. Fully open impellers are rarely used because of its lower efficiency and the bending load on the vanes.