# Question Set 12

Question No. 1

Why manholes in vessels are usually elliptical in shape?

Elliptical shape has minimum area of opening and thus plate is weakened the least. Further it is very convenient to insert and take out the cover plate from elliptical opening.

Question No. 2

Which Pump is more Efficient Centrifugal Pump or Reciprocating Pump?

Centrifugal pump. Because of 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.

Question No. 3

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

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.

Question No. 4

Low water in boiler drum is unsafe because it may result in overheating of water tubes in furnace. Why it is unsafe to have high water condition in boiler drum?

High drum level does not allow steam separation to be effective and some water can be carried over with steam which is not desirable for steam turbine.

Question No. 5

What is the difference between Critical Speed and Whirling Speed?

In Solid mechanics, in the field of rotor dynamics, the critical speed is the theoretical angular velocity which excites the natural frequency of a rotating object, such as a shaft, propeller or gear. As the speed of rotation approaches the objects natural frequency, the object begins to resonate which dramatically increases system vibration. The resulting resonance occurs regardless of orientation. Whirling Speed is due to the unbalanced forces acting on a rotating shaft.

Question No. 6

Maximum combustion temperature in gas turbines is of the order of 1100 to 10°C whereas same is around 0°C in I.C. engine? Why?

High temperature in I.C. engine can be tolerated because it lasts for a fraction of second but gas turbines have to face it continuously which metals can’t withstand.

Question No. 7

What is basic difference between impulse turbine and reaction turbine?

(A) In impulse turbine, jet is used to create impulse on blades which rotates the turbine and in reaction turbine, no jet is used pressure energy is converted into kinetic energy.

(B) In impulse turbine fluid enter& leave with same energy, but in reaction turbine fluid enter with pressure energy& leaves with kinetic energy

(C)In impulse turbine all the pressure drops in nozzle only & in reaction turbine pressure drops both fixed & moving blades. The difference is due to blade profiles.

Question No. 8

What is the Difference between a Generator and Inverter?

An inverter is only effective if there is already a source of electrical energy. It cannot generate its own. It can simply convert electrical energy that is already there. On the other hand, a traditional generator cannot make AC current into DC current.

Question No. 9

What are the causes of failure of boiler tubes?

Boiler tubes, usually are made from carbon steel and are subject to

(A) High rates of heat transfer

(B). bending stresses due to uneven heating, especially at expanded or welded joints into headers or drums,

(C) External erosion from burners and flue gas,

(D) Possible corrosion on the boiler side, and

(E) Occasional manufacturing defects.

Failure may occur due to following reasons:

(A) High thermal ratings may lead to rapid failure if the internal fluid flow is reduced for any reason. The resultant overheating leads to a failure by creep, characterized by the bulging of the tube with the eventual development of a longitudinal split.

(B) Fatigue cracking due to bending stresses occurs. These are associated with change of section and/or weld undercut, where tubes are expanded or welded into headers.

(C) Failure may arise due to over-stressing of a reduced section of metal.

(D) Sudden failure of the boiler tube due to corrosion arises from embrittlement of the carbon steel due to interaction between atomic hydrogen from the corrosion process and the iron carbide present in the steel.

(E) Defects in tube manufacture, although far from being a regular occurrence, can be a cause of serious trouble. Lamination in boiler tubes or score marks arising from the cold drawing of tubes give rise to premature failure and may promote corrosion at these regions.

Question No. 10

Why is sound faster in warm air?

The speed of sound in air Cair = 331.3 + (0.66 x T) m/s, where T is the temperature in °C.

The speed of sound is proportional to gas temperature and inversely proportional to its molar mass.

Sound is transferred by collisions of molecules. Therefore sound waves will travel faster on warm air because collision of molecules of air in warm air is greater.