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Mechanical Interview Questions And Answers – Fluid Mechanics

November 12, 2011

Basic Fluid Mechanics Interview Questions And Answers.

 

1.          Why the Centrifugal Pump is called High Discharge pump?

Ans.    Centrifugal pump is a kinetic device. The centrifugal pump uses the centrifugal force to push out the fluid. So the liquid entering the pump receives kinetic energy from the rotating impeller. The centrifugal action of the impeller accelerates the liquid to a high velocity, transferring mechanical (rotational) energy to the liquid. So it discharges the liquid in high rate. It is given in the following formulae:

Centrifugal force F= (M*V2)/R.

Where,

M-Mass

V-Velocity

R-Radius

 

2.          How Cavitation can be eliminated by Pump?

Ans.    Cavitation means bubbles are forming in the liquid.

· To avoid Cavitation, we have to increase the Pump size to One or Two Inch;

To increase the pressure of the Suction Head, or

· Decrease the Pump Speed.

 

3.          Why Cavitation will occur in Centrifugal Pump and not in Displacement Pump?

Ans.     The formation of cavities (or bubbles) is induced by flow separation, or non-uniform flow velocities, inside a pump casing. In centrifugal pumps the eye of the pump impeller is smaller than the flow area of pipe. This decrease in flow area of pump results in increase in flow rate. So pressure drop happened between pump suction and the vanes of the impeller. Here air bubbles or cavities are formed because of liquid vapour due to increase in temperature in impeller. This air bubbles are transmitted to pump which forms cavitation.

 

4.          Which Pump is more Efficient Centrifugal Pump or Reciprocating Pump?

Ans.     Centrifugal pump. Because flow rate is higher compared to reciprocating pump. Flow is smooth and it requires less space to install. Lower initial cost and lower maintenance cost.

5.          Why Centrifugal Pump is not called as a Positive Displacement Type of Pump?

Ans.     The centrifugal has varying flow depending on pressure or head, whereas the Positive Displacement pump has more or less constant flow regardless of pressure.

Likewise viscosity is constant for positive displacement pump where centrifugal pump have up and down value because the higher viscosity liquids fill the clearances of the pump causing a higher volumetric efficiency. When there is a viscosity change in supply there is also greater loss in the system. This means change in pump flow affected by the pressure change.

One more example is, positive displacement pump has more or less constant efficiency, where centrifugal pump has varying efficiency rate.

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