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Consider the following statements pertaining to centrifugal pumps: 1. The inlet to the impeller of a centrifugal pump is always axial, while the outlet may be radial or inclined. 2. The impeller may be shrouded on both sides with an eye at the center and vanes curved backward. 3. Impeller of double-entry type has a balanced radial thrust. 4. Un-shrouded and part-shrouded impeller are used only where high efficiency is unimportant. Which of these statements are correct?

Consider the following statements pertaining to centrifugal pumps: 1. The inlet to the impeller of a centrifugal pump is always axial, while the outlet may be radial or inclined. 2. The impeller may be shrouded on both sides with an eye at the center and vanes curved backward. 3. Impeller of double-entry type has a balanced radial thrust. 4. Un-shrouded and part-shrouded impeller are used only where high efficiency is unimportant. Which of these statements are correct? Correct Answer 1, 2 and 4 only

Concept:

Centrifugal or radial pumps are simple and versatile constructions with a wide range of impeller designs. The motion of water in it is from the center towards the periphery. The motion is caused by the centrifugal force created in the pump as a result of the revolving motion of the working wheel.

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Different type of impellers:

The different types of impellers used include:

Radial flow impellers:

The liquid enters the impeller axially and discharges radially, changing the direction of the flow 90°. These impellers can be single suction (where the fluid enters the impeller from one side) or double suction (where fluid enters from both sides).

Axial flow impellers

The liquid both enters and discharges from the impeller axially (i.e. the flow does not change direction). Single suction only.

Mixed flow impellers

The liquid enters the impeller axially and discharges in both radial and axial directions. Can be single or double suction..

So, statement 1 is true.

Various types of impellers:

 

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Closed impellers (Two-sides shrouded):

  • In the closed or shrouded impellers, the vanes are covered with shrouds (side plates) on both sides
  • The back shroud is mounted into the shaft and the front shroud is coupled by the vanes
  • This ensures full capacity operation with high efficiency for a prolonged running period
  • This type of impeller is meant to pump only clear water, hot water, and acids


Semi-open impeller (One-side shrouded):

  • It has a plate (shroud) only on the backside
  • The design is adapted to industrial pump problems which require a rugged pump to handle liquids containing fibrous material such as paper pulp, sugar molasses, and sewage water, etc.


Open impeller:

  • In open impeller, no shroud or plate is provided on either side i.e. the vanes are open on both sides
  • Such pumps are used where the pump has a very rough duty to perform i.e. to handle abrasive liquids such as a mixture of water, sand, pebbles, muds and clay, wherein the solid contents may be as high as 25%.


Both the Un-shrouded and part-shrouded impeller are used only where high efficiency is unimportant and where the working fluid plays an important role.

Statements 2 and 4 are correct.

Statement 3:

The combined unbalanced forces within a pump that act on the impeller and shaft end in the axial direction are known as axial thrust. 

The most common way to combat axial thrust involves the use of a ‘hydraulically balanced’ impeller. The Impeller of double entry type has balanced axial thrust. So, statement 3 is incorrect.

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