Centrifugal Pump

What are the parts of a Centrifugal Pump?

This pump is primarily made up of an outer Pump casing or housing, Impeller, Mechanical Seals, Pump Shaft & Bearings and Motor.

The pump casing is the major stationary component of the pump and provides two major functions:

• Converts velocity head from the impeller into pressure head, and guides the flow to the discharge connection.
• Serves as the pressure boundary for the liquid pumped.

The impeller is the rotating component of a pump, usually made of iron, steel, aluminium or plastic, which transfers energy from the motor that drives the pump to the fluid being pumped by forcing the fluid outwards from the centre of rotation.

Impeller designs are grouped as either radial flow, mixed flow, or axial flow depending on their hydraulic geometry.

Pumps are fitted with some kind of sealing device to ensure a proper seal at the area where the pump shaft enters the pump casing.

What are the factors considered for a good pump selection?

Firstly, you need to select a pump type most appropriate for the application, for example, Centrifugal or Positive displacement pump. Assuming you have identified that centrifugal is the most suitable, how do you select a model to deliver the required performance?

Every engineer must fully understand and determine what are the basic criteria of the application in question.

Such as the flow rate, the total dynamic head or pressure required, the specific gravity (or density) of the fluid, other properties i.e. viscosity, corrosive or aerated, the suction conditions i.e. lift or low NPSH available (NPSH is the net pressure suction head needed to avoid damaging cavitation), the nature & size of any solids present and finally the single or multiple duty point (dynamic system)

Once we know this data, we then need to consider whether a safety margin been added to the calculated system pressure drop?

As the pressure drop is based on a calculation using a software, it is, in reality, only a ‘best guess’. Therefore, a safety margin should be added so that the pump is guaranteed to give the desired flow rate.

This is where care needs to be exercised. If too much of a margin is added, the pump will always try to find the intersection of the system curve (pressure drop vs. flow), even if this is detrimental to the efficiency and reliability of the pump.

See diagrams below of a good centrifugal pump selection vs a poor pump selection to fully illustrate this point: