Mechanical Engineering Interview Questions with Answers - Fluid Mechanics & Fluid Machinery

Mechanical Engineering Interview Questions with Answers - Fluid Mechanics & Fluid Machinery (Turbine, Boiler, compressor, Pumps, Bearings Etc..): Set 02




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 electro metric 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:
Pressurized 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. One-half 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
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. 86
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. 87
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. 88
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. 89
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. 90
In which part of the steam turbine does corrosion fatigue occur?
Answer:
In the wet stages of the LP cylinder.

Question No. 91
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. 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:
  1. Carbon rings fitted in segments around the shaft and held together by garter or retainer springs.
  2. Labyrinths mated with shaft serrations 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. 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:
  1. The impulse turbine is characterized by the fact that it requires nozzles and that the pressures drop of steam takes place in the nozzles.
  2. 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.
  3. 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
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