20 TOP FLUID MECHANICS VIVA Questions Answers

20 TOP FLUID MECHANICS VIVA Questions Answers

1. How Cavitation can be eliminated in a 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.

2. 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.

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

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

b.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.

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

4. 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

5. Why cavitation will occur in Centrifugal Pumps and not in Displacement pumps?

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.

6.What is a radial-flow turbine?

Ans: In a radial-flow turbine, steam flows outward from the shaft to the casing. The unit is usually a reaction unit, having both fixed and moving blades.

7.What are four types of turbine seals?

Ans: Carbon rings fitted in segments around the shaft and held together by garter or retainer springs.
Labyrinth mated with shaft serration’s or shaft seal strips.
Water seals where a shaft runner acts as a pump to create a ring of water around the shaft. Use only treated water to avoid shaft pitting.
Stuffing box using woven or soft packing rings that are compressed with a gland to prevent leakage along the shaft.

8.What is the function of a thrust bearing?

Ans:  Thrust bearings keep the rotor in its correct axial position.

9.What are some conditions that may prevent a turbine from developing full power?

Ans: The machine is overloaded.
The initial steam pressure and temperature are not up to design conditions.
The exhaust pressure is too high.
The governor is set too low.
The steam strainer is clogged.
Turbine nozzles are clogged with deposits.
Internal wear on nozzles and blades.

10.What is a stage in a steam turbine?

Ans: In an impulse turbine, the stage is a set of moving blades behind the nozzle. In a reaction turbine, each row of blades is called a “stage”. A single Curtis stage may consist of two or more rows of moving blades.

11.What is a diaphragm?

Ans: Partitions between pressure stages in a turbine’s casing are called diaphragms. They hold the vane-shaped nozzles and seals between the stages. Usually labyrinth-type seals are used. One-half of the diaphragm is fitted into the top of the casing, the other half into the bottom.

12.What are the two basic types of steam turbines?

Ans: 1.Impulse type. 2.Reaction type.

13.What are topping and superposed turbines?

Ans: Topping and superposed turbines arc high-pressure, non-condensing units that can be added to an older, moderate-pressure plant. Topping turbines receive high-pressure steam from new high-pressure boilers. The exhaust steam of the new turbine has the same pressure as the old boilers and is used to supply the old turbines.

14.What is a combination thrust and radial bearing?

Ans: This unit has the ends of the Babbitt bearing extended radically over the end of the shell. Collars on the rotor face these thrust pads, and the journal is supported in the bearing between the thrust collars.