Fluid Mechanics & Fluid Machinery Interview Questions

Question No. 01
What is the difference between streamline body and bluff body?
Answer: In streamline body the shape is such that separation in flow occurs past the near most part
of the body so that wake formed is small and thus friction drag is much greater than pressure drag.
In bluff body the flow gets separated much ahead of its rear resulting in large wake and thus
pressure drag is much greater than the friction drag.


Question No. 02
Define hydraulically efficient channel cross section.
Answer: The shape of such section is that which produces minimum wetted perimeter for a given
area of flow and carries maximum flow.



Question No. 03
What is the difference between Center of Mass and Centroid?
Answer: If the material composing a body is uniform or homogeneous, the density or specific weight
will be constant throughout the body, and then the Centroid is the same as the centre of gravity or
centre of mass.
Centroid: Centroid is the point, where the whole area of plane is going to be act. It is valid only for
2D problems like plane figures, square plate etc. The centre of mass is a point that acts as if all the
mass was centered there (the mass on one side of the point is equal to the mass on the OPPOSITE
side). If supported at the centre of mass, an object will be balanced under the influence of gravity.


Question No. 04
On what factors does the pressure at a point as a static mass of liquid depends upon?
Answer: Specific weight of liquid and the depth below the free liquid surface.

Question No. 05
When maximum discharge is obtained in nozzle?
Answer: At the critical pressure ratio.

Question No. 06
Under what condition the work done in reciprocating compressor will be least?
Answer: It is least when compression process approaches isothermal. For this purpose, attempts are
made to cool the air during compression.


Question No. 07
What is the difference between stalling and surging in rotary compressions?
Answer: Stalling is a local phenomenon and it occurs when How breaks away from the blades.
Surging causes complete breakdown of flow and as such it affects the whole machine.


Question No. 08
State Archimedes principle.
Answer: Any weight, floating or immersed in a liquid, is acted upon by a buoyant force equal to the
weight of the liquid displaced. This force acts through the centre of buoyancy, i.e. the e.g. of the
displaced liquid.

Question No. 09
What do you understand by centre of buoyancy?
Answer: Centre of buoyancy is the centre of gravity of the displaced liquid and buoyant force acts
through it.

Question No. 10
Why the Centrifugal Pump is called High Discharge pump?
Answer: 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= (MV²)/R.
Where, M-Mass, V-Velocity, R-Radius


Question No. 11
Why the electric motor of a fan with backward curved blades is never got overloaded under any
condition?
Answer: The maximum power is consumed at about 70% of maximum flow in case of fan with
backward blades. For higher flow, power consumption gets lower.


Question No. 12
Why the work per kg of air flow in axial flow compressor is less compared to centrifugal
compressor for same pressure ratio?
Answer: Isentropic efficiency of axial flow compressor is higher.

Question No. 13
What is pitting? How it is caused?
Answer: Non uniform corrosion over the entire metal surface, but occurring only in small pits is
called pitting. It is caused by lack of uniformity in metal.

Question No. 14
How Cavitation can be eliminated by Pump?
Answer: 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.


Question No. 15
Which impurities form hard scale and which impurities soft scale?
Answer: Sulphates and chlorides of lime and magnesium form hard scale, and carbonates of lime
and magnesium form soft scale.

Question No. 16
What is the difference between hard water and soft water?
Answer: Hard water contains excess of scale forming impurities and soft water contains very little or
no scale forming substances.


Question No. 17
Which two elements in feed water can cause corrosion of tubes and plates in boiler?
Answer: Acid and oxygen in feed water lead to corrosion.

Question No. 18
Why Cavitation will occur in Centrifugal Pump and not in Displacement Pump?
Answer: 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.


Question No. 19
Why large boilers are water tube type?
Answer: Water tube boilers raise steam fast because of large heat transfer area and positive water
circulation. Thus they respond faster to fluctuations in demand. Further single tube failure does not
lead to catastrophy.


Question No. 20
What type of boiler does not need a steam drum?
Answer: Supercritical pressure boiler.

Question No. 21
Why manholes in vessels are usually elliptical in shape?
Answer: 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. 22
Which Pump is more Efficient Centrifugal Pump or Reciprocating Pump?
Answer: 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. 23
Why Centrifugal Pump is not called as a Positive Displacement Type of Pump?
Answer: 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. 24
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?
Answer: 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. 25
What is the difference between Critical Speed and Whirling Speed?
Answer: 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. 26
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?
Answer: 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. 27
What is basic difference between impulse turbine and reaction turbine?
Answer:
(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) (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. 28
What are the causes of failure of superheater tubes?
Answer: Superheater tubes are subjected to the most severe combination of stress, temperature
and corrosive environment. In addition to high temperature strength, resistance to corrosion is also
important. For example, low alloy ferrite steel such as 1% Cr, 1% Mo would not be used at metal
temperatures above 580°C because of inadequate resistance to corrosion and oxidation over a full
service life of 100,000/150,000 hr.
Failures in superheater tubes may arise from:
(A) Prior fabrication history
(B) Faulty heat treatment
(C) Consequences of welding
(D) Overheating of the tube metal
(E) Gas-side corrosion
(F) Stress corrosion (austenitic steels).

Question No. 29
Out of electric heater and heat pump, which is economical in operation?
Answer: Heat pump.


Question No. 30
What is the Difference between a Generator and Inverter?
Answer: 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. 31
Why is sound faster in warm air?
Answer: 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.


Question No. 32
What is a Newtonian fluid?
Answer: A Newtonian fluid possesses a linear stress strain relationship curve and it passes through
the origin. The fluid properties of a Newtonian fluid do not change when any force acts upon it.


Question No. 33
What are the causes of failure of boiler tubes?
Answer: Boiler tubes, usually are made from carbon steel and are subject to
(A) High rates of heat transfer
(B) (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, characterised 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 overstressing 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. 34
Why supercritical boilers use less amount of steel compared to non supercritical boilers?
Answer: Supercritical boilers do not head heavy drum for separation of steam from mixture of water
and steam.

Question No. 35
What does a pump develops? Flow or Pressure?
Answer: A pump does not create pressure, it only creates flow. Pressure is a measurement of the
resistance to flow.


Question No. 36
What is a Hydrostatic system?
Answer:
Hydrostatics is the study of fluid bodies that are
(A) At rest
(B) Moving sufficiently slowly so there is no relative motion between adjacent parts of the body
For hydrostatic situations
(A) There are no shear stresses.
(B) There are only pressure forces that act perpendicular to any surface.
It’s a closed loop hydraulic systems. It comprises of motor and pump. Here pump supplies energy to
motor and motor gives return energy to pump supply.


Question No. 37
What is the difference between Blower and Fan?
Answer: Fan is an air pushing device. Either Axial or Centrifugal type systems are used to move the
air in low pressure. It is rotated by a motor separately.
When the fan is a housing of blades and motor, then it called as Blower. It directs the air in a single
path with high pressure.

Question No. 38
What is Hydrodynamic Cavitation?
Answer: Hydrodynamic Cavitation describes the process of vaporization in a constrained channel at
a specific velocity. Bubble generation and Bubble implosion which occurs in a flowing liquid as a
result of a decrease and subsequent increase in pressure.


Question No. 39
Is gate valve used for Throttling?
Answer: Gate valves are not suitable for throttling because the control of flow is difficult for the
valve’s design, and the flow of fluid slapping against a partially open gate can cause extensive
damage to the valve.


Question No. 40
Where Multi-stage pump used
Answer: Pressure washing of Aircraft, Trains, Boats and Road vehicles as well as Spray washing of
industrial parts and Electronic components.

Question No. 41
Which furnace burns low ash fusion coal and retains most of the coal ash in the slag?
Answer: Cyclone furnace.

Question No. 42
What is the effect of friction on flow of steam through a nozzle?
Answer: To decrease both mass flow rate and wetness of steam.


Question No. 43
How the thickness of thermal boundary layer and thickness of hydrodynamic boundary layer
related?
Answer: Ratio of their thickness = (Prandtl number)-1/3.

Question No. 44
What is the function of Scoop in BFP (Boiler Feed Water Pump) in Thermal Power Station?
Answer: The Function of Scoop tube is regulating the varying amount of oil level in the coupling
during operation of infinite variable speed.

Question No. 45
In the Thermal Power Plant why Deaerator (D/A) placed on height?
Answer: To build a Very high pressure and the temperature for a boiler feed water pump and it
discharge high pressure water to the boiler. And to provide the required Net Positive Suction Head
(NPSH) for the BFW pump and to serve as a storage tank to ensure a continuous supply of feed
water during rapid changes in BFP.


Question No. 46
In orifice why the Pressure and Temperature are decreases?
Answer: Orifice is a small hole like a nozzle. When a high pressure fluid passes through the orifice,
Pressure gets reduced suddenly and the velocity of the fluid gets increased. Also the heat transfer
rate increases. We know that Heat transfer rate is directly proportional to the difference in
temperature, Area and the Heat transfer coefficient. Heat transfer Coefficient remains constant for a
fluid at a particular temperature.
Q= KA (T1-T2)
If the heat transfer rate increases, it seems the difference in temperature gets increased. There is no
way in reduction of inlet temperature of the orifice. As a result, the outlet temperature of the orifice
gets reduced. Hence the pressure and temperature gets reduced when it passes through orifice


Question No. 47
Why gas turbine power plant needs efficient compressor?
Answer: Because a large portion of turbine work is eaten away by compressor and its inefficiency
will affect net power output and cost of generation.

Question No. 48
Why rockets using liquid hydrogen have higher specific impulse compared to liquid hydrocarbon?
Answer: Liquid hydrogen has higher burning velocity.

Question No. 49
Why axial flow compressor is preferred for gas turbines for aeroplanes?
Answer: Because it has low frontal area.


Question No. 50
What is the difference between gas turbine and a steam turbine?
Answer: Gas turbine works on Brayton cycle where as steam turbine works Rankine cycle.
Construction, operation of a gas turbine is entirely different to steam turbine. Gas turbine has a
compressor to compress the combustion air, a combustion chamber to burn the fuel and a turbine
section to extract the work for burning fuel. Steam turbine is just has a turbine section to extract the
work from steam.

Question No. 51
What is operating pressure?
Answer: The amount of pressure nearest the point of performing work at the output end of a
pneumatic system. The system operating pressure is used to specify the capability of valves and
actuators.


Question No. 52
What is the effect of inter cooling in gas turbines?
Answer: It decreases thermal efficiency but increases net output.

Question No. 53
What are the safety valves? How many on each boiler?
Answer: A valve opening automatically to relieve excessive pressure, especially in a boiler.
There are normally two to six safety valves provided in the drum depending upon the capacity.
The super heater outlet will have one to three safety valves on either side of the boiler. There will be
an electrometric relief valve on the super heater pipe in addition. This valve will be set at lower
pressure than the lowest set safety valve on the super heater. The reheater pipes both at the inlet
and outlet side will also have safety valves which can range from two to eight both in the inlet and
outlet of the reheater put together.


Question No. 54
What is a sentinel valve?
Answer: Sentinel valves are simply small relief valves installed in some systems to warn of
impending over pressurization. Sentinel valves do not relieve the pressure of the system. If the
situation causing the sentinel valve to lift is not corrected, a relief valve (if installed) will lift to
protect the system or component. If a relief valve is not installed, action must be taken quickly to
secure the piece of equipment or system to reduce the pressure.


Question No. 55
In which reactor the coolant and moderator are the same?
Answer: Pressurised water reactor.

Question No. 56
Which reactor has no moderator?
Answer: Fast breeder reactor.

Question No. 57
What are thermal neutrons?
Answer: Thermal neutrons are slow neutrons (having energy below 1 eV) which are in thermal
equilibrium with their surroundings.

Question No. 58
What is the function of Hydrogen seals work on a generator?
Answer: Provide a seal between the generator housings and rotor shaft to maintain the pressurized
hydrogen gas inside the generator. Also, provides a trap-vent system to prevent the release of
hydrogen into the turbine generator lube oil system and building atmosphere.


Question No. 59
How is the excess discharge pressure prevented?
Answer: Pressure relief valves on the discharge side of each seal oil pump relieve back to seal oil
system.

Question No. 60
Which two seal oil pumps are driven from the same motor?
Answer: Main seal oil pump and re-circular seal oil pump; both are driven by Main seal oil pump
motor.

Question No. 61
Which heating value is indicated by a calorimeter and why?
Answer: Gross heating value because steam is condensed and heat of vapour formed is recovered.


Question No. 62
When does Emergency Seal Oil Pump automatically start?
Answer: When its pressure switch senses Main Seal Oil Pump discharge pressure reduced to 78 PSI.

Question No. 63
What is the consequence of not maintaining hydrogen (or air) pressure in generator casing at a
value above atmospheric pressure when seal oil system is in service?
Answer: Failure to do so will cause excessive seal oil to be drawn into the generator.


Question No. 64
What is a radial-flow turbine?
Answer: 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.

Question No. 65
What are four types of turbine seals?
Answer:
1. Carbon rings fitted in segments around the shaft and held together by garter or retainer
springs.
2. Labyrinth mated with shaft serration’s or shaft seal strips.
3. 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.
4. Stuffing box using woven or soft packing rings that are compressed with a gland to prevent
leakage along the shaft.


Question No. 66
What are two types of clearance in a turbine?
Answer:
Radial – clearance at the tips of the rotor and casing.
Axial – the fore-and-aft clearance, at the sides of the rotor and the casing.

Question No. 67
Which reactor uses natural uranium as fuel?
Answer: Gas cooled reactors.

Question No. 68
Which reactor uses heavy water as moderator?
Answer: CANDU.

Question No. 69
Which reactor requires no moderator?
Answer: Breeder reactor.

Question No. 70
Which reactor uses primary coolant as fluoride salts of lithium, beryllium, thorium and uranium?
Answer: Molten salt breeder reactor.


Question No. 71
Why an increase in area is required to produce an increase of velocity in case of supersonic flow?
Answer: Increase in area for increase in velocity for supersonic flow is required because the density
decreases faster than velocity increases at supersonic speeds and to maintain continuity of mass,
area must increase.

Question No. 72
Under what circumstances would there be an increase in pressure in a divergent nozzle?
Answer: For subsonic flow at inlet section of a diffuser a lower velocity and higher pressure will exist
at the exit section. For supersonic isentropic flow at the inlet section a higher velocity and lower
pressure will exist at the exit but if a shock wave occurs in the diffuser then a higher pressure will
exist at the exit.


Question No. 73
What are some conditions that may prevent a turbine from developing full power?
Answer:
1. The machine is overloaded.
2. The initial steam pressure and temperature are not up to design conditions.
3. The exhaust pressure is too high.
4. The governor is set too low.
5. The steam strainer is clogged.
6. Turbine nozzles are clogged with deposits.
7. Internal wear on nozzles and blades.

Question No. 74
What is a stage in a steam turbine?
Answer: 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.


Question No. 75
What is a diaphragm?
Answer: 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. Onehalf
of the diaphragms is fitted into the top of the casing, the other half into the bottom.

Question No. 76
What are the two basic types of steam turbines?
Answer:
1. Impulse type.
2. Reaction type.


Question No. 77
What are topping and superposed turbines?
Answer: 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.

Question No. 78
What is a combination thrust and radial bearing?
Answer: 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.


Question No. 79
What is important to remember about radial bearings?
Answer: A turbine rotor is supported by two radial bearings, one on each end of the steam cylinder.
These bearings must be accurately aligned to maintain the close clearance between the shaft and
the shaft seals, and between the rotor and the casing. If excessive bearing wear lowers the he rotor,
great harm can be done to the turbine.


Question No. 80
How many governors are needed for safe turbine operation? Why?
Answer: Two independent governors are needed for safe turbine operation. One is an over speed or
emergency trip that shuts off the steam at 10 percent above running speed (maximum speed). The
second, or main governor, usually controls speed at a constant rate; however, many applications
have variable speed control.


Question No. 81
How is a fly ball governor used with a hydraulic control?
Answer: As the turbine speeds up, the weights are moved outward by centrifugal force, causing
linkage to open a pilot valve that admits and releases oil on either side of a piston or on one side of a
spring-loaded piston. The movement of the piston controls the steam valves.

Question No. 82
What is meant by critical speed?
Answer: It is the speed at which the machine vibrates most violently. It is due to many causes, such
as imbalance or harmonic vibrations set up by the entire machine. To minimize damage, the turbine
should be hurried through the known critical speed as rapidly as possible. (Caution, be sure the
vibration is caused by critical speed and not by some other trouble).


Question No. 83
How is oil pressure maintained when starting or stopping a medium-sized turbine?
Answer: An auxiliary pump is provided to maintain oil pressure. Some auxiliary pumps are turned by
a hand crank; others are motor-driven. This pump is used when the integral pump is running too
slowly to provide pressure, as when starting or securing a medium-sized turbine.


Question No. 84
Besides lubrication, what are two functions of lubricating oil in some turbines?
Answer: In larger units, lube oil cools the bearings by carrying off heat to the oil coolers. Lube oil in
some turbines also acts as a hydraulic fluid to operate the governor speed-control system.

Question No. 85
In which part of the steam turbine does stress corrosion cracking (SCC) occur?
Answer: In the wet stages of the low-pressure turbine.


Question No. 86
Do you stop cooling-water flow through a steam condenser as soon as the turbine is slopped?
Answer: You should keep the cooling water circulating for about 15 miles or more so that the
condenser has a chance to cool down gradually and evenly. Be sure to have cooling water flowing
through the condenser before starting up in order to prevent live steam from entering the
condenser unless it is cooled. Overheating can cause severe leaks and other headaches.


Question No. 87
How can the deposits be removed?
Answer:
1. Water soluble deposits may be washed off with condensate or wet steam.
2. Water insoluble deposits are removed mechanically after dismantling the turbine.
3. Experience shows that water soluble deposits are embedded in layers of water-insoluble
deposits. And when the washing process is carried out, water soluble parts of the deposit
dissolve away leaving a loose, friable skeleton of water insoluble deposits which then break
loose and wash away.


Question No. 88
How can the fatigue damage on high pressure blades be corrected?
Answer: Fatigue-damage on high-pressure blades arises due to vibration induced by partial-arc
admission. This can be corrected by switching over to full arc admission technique.


Question No. 89
How many governors are needed for safe turbine operation? Why?
Answer: Two independent governors are needed for safe turbine operation:
1. One is an over speed or emergency trip that shuts off the steam at 10 percent above running
speed (maximum speed).
2. The second, or main governor, usually controls speed at a constant rate; however, many
applications have variable speed control.


Question No. 90
How will you detect that misalignment is the probable cause of excessive vibration?
Answer:
1. Coupling to the driven machine is to be disconnected.
2. The turbine is to be run alone.
3. If the turbine runs smoothly, misalignment, worn coupling or the driven equipment is the
cause of the trouble.

Question No. 91
In which part of the steam turbine does corrosion fatigue occur?
Answer: In the wet stages of the LP cylinder.


Question No. 92
In which zone of steam turbines has temperature-creep rupture been observed?
Answer: Damage due to creep is encountered in high temperature (exceeding 455°C) zones. That is,
it has been found to occur in the control stages of the high-pressure and intermediate-pressure
turbines where steam temperature sometimes exceed 540°C. In the reheat stage, it has been
observed that creep has caused complete lifting of the blade shroud bands.

Question No. 93
What are the types of thrust bearings?
Answer:
1. Babbitt-faced collar bearings
2. Tilting pivotal pads
3. Tapered land bearings
4. Rolling-contact (roller or ball) bearings

Question No. 94
What are the types of turbine seals?
Answer:
(A) Carbon rings fitted in segments around the shaft and held together by garter or retainer
springs.
(B) Labyrinths mated with shaft serrations or shaft seal strips.
(C) 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.
(D) Stuffing box using woven or soft packing rings that are compressed with a gland to prevent
leakage along the shaft.


Question No. 95
What are the basic causes of the problem of rotor failure?
Answer:
(A) Normal wear.
(B) Fatigue failure due to high stress.
(C) Design deficiency.
(D) Aggressive operating environment

Question No. 96
What are the differences between impulse and reaction turbines?
Answer:
(A) The impulse turbine is characterized by the fact that it requires nozzles and that the
pressures drop of steam takes place in the nozzles.
(B) The reaction turbine, unlike the impulse turbines has no nozzles, as such. It consists of a row
of blades mounted on a drum. The drum blades are separated by rows of fixed blades
mounted in the turbine casing. These fixed blades serve as nozzles as well as the means of
correcting the direction of steam onto the moving blades.
(C) (C)In the case of reaction turbines, the pressure drop of steam takes place over the blades.
This pressure drop produces a reaction and hence causes the motion of the rotor.


Question No. 97
What is the function of EGR VALVE?
Answer: EGR means Exhaust Gas Recirculation. The main function of EGR is to control NOx emission
from the engine. At elevated temperature (during long run & full condition)if due to some reasons
engine gets heated up beyond certain level… it produces NOx emission. In order to avoid this
pollution, we have to control the temperature rise inside the cylinder. This can be achieved by
mixing some amount of exhaust gas with intake air. By doing this the temperature inside the
combustion chamber is reduced. And NOx is controlled.


Question No. 98
What is the difference between Critical speed & Whirling speed?
Answer: 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. 99
What is specific speed of turbine?
Answer: The specific speed is defined as the speed of the geometric turbine which can produce unit
power at unit head

Question No. 100
Material of Aircraft turbine blades
Answer: Ni, Cr, Al, Traces of C