Question

Describe the principle of operation of both the positive displacement and roto-
dynamic pumps.

b) List three (3) characteristics each of the above pumps .

c) Sketch a gear pump and label it.

Answer 1

Hi, thanks for the question.

a) The principle of operation of a rotodynamic pump is providing kinetic energy to a fluid via rotating parts like an impeller, propeller, or rotor. Pumps like centrifugal pumps are examples of rotodynamic pumps that increase the fluid velocity or pressure depending on requirement.

The principle of operation of a positive displacement pump is that a fluid movement is performed by trapping a fixed amount (volume) of fluid and then forcing (or displacing) that trapped volume through the discharge pipe. These are in essentiality constant flow machines.

In crude terms, the suction side of the pump expands as the liquid flows into the cavity, and the cavity collapses as the liquid flows out. So, for a given cycle, the volume will always remain constant.

b) Characteristic of rotodynamic pumps:

i. They use rotating parts to impart kinetic energy.
ii. Their head flow can vary with each cycle.
iii. They can operate at variable speeds for each cycle.

Characteristic of positive displacement pumps:

i. They maintain a constant volume for each cycle
ii. They operate at fixed speeds for a fixed cycle
iii. Their flow rate remains constant

c) This is the basic diagram of a gear pump.

This is a prototype gear pump that is used in real case scenarios.