80-Interview questions & answers for fresher Mechanical Engineers

1.What do you mean by Statics?

It is the branch of Engineering Mechanics which deals with the forces & their effects, while acting upon a bodies at rest

2.What do you mean by Dynamics?

It is the branch of Engineering Mechanics which deals with the forces & their effects, while acting upon a bodies in motion

3.What are the types of Dynamics?

Kinetics: Branch of science which deals with bodies in motion due to applied forces

Kinematics: Branch of science which deals with bodies in motion without considering the applied forces

4.Define the term ‘Force’

It is an agent which produces or tends to produce, destroy or tends to destroy the motion of the body.

5.What is meant by Resultant of Force?

It is a single force which produces the same effect of all the forces acting on a body

6.What does Lami’s Theorem says?

It states that if three coplanar forces acting at a point be in equilibrium, then each force is proportional to sine of the angle between other two forces

7. What do you mean by couple?

The two equal & opposite forces, whose lines of action are different

8. What do you mean by Centre of Gravity (CG)?

The point through which the whole mass of the body acts irrespective of the position of the body is called as CG

9. Define Moment of inertia

It is the moment of moment or second moment of mass or area of a body

10. Define the term ‘Friction’

A force acting in the opposite direction to the motion of the body is called friction force

11. What are the types of friction?

Static friction: Friction experienced by a body, when it is under rest

Dynamic friction: Friction experienced by a body, when it is in motion

Dynamic frictions are classified into sliding friction & Rolling friction

12.What do you mean by coefficient of Friction?

It is the ratio of Limiting friction to the normal reaction friction

13. Define Newton’s three laws of motion

1st Law: It states that everybody continues in the state of rest or of uniform motion, in a straight line, unless it is acted upon by some extent force

2^nd law: It states that rate of change of momentum is directly proportional to the impressed force & takes place in the same direction, in which the force acts.

3^rd Law: Action & reactions are equal & opposite

14. How do you calculate moment of Inertia?

M = Mass X Velocity

15. How do you calculate the angular velocity of a body if it is rotating at speed N?

Angular velocity  = 2N/60……Rad/sec

16. How do you calculate the linear velocity of a body if it is rotating at speed N & having diameter D?

Linear velocity V = N / 60….m/sec

17. What do you mean by Amplitude?

It is the maximum displacement of a body from its mean position

18. How do you define the work?

When a force of F acts on a Body & the bodyundergoes displacement x in the direction of the Force, then the work is said to be done

Work, W = Force F X Displacement x

19. How do you define the term Power?

Power is rate of doing work

20.What is meant by Energy?

Energy is the capacity to do work, expressed in J or calories

21. What do you mean by Potential energy?

Energy possessed by a body by virtue of its position.

A body kept at height ‘h’ & having mass ‘m’ has potential energy E = mgh (g = 9.81 m/sec)

22. What do you mean by Kinetic energy?

Energy possessed by a body by virtue of its mass & velocity

E= mv²/2

23. What is meant by strain energy?

It is the potential energy stored in an elastic body when deformed

24. Convert 1 kg of Force into Newton

1 kg = 9.81 N

25. What is reversible Machine?

Machine having efficiency < 50% are called Reversible machines

26. How do you calculate the velocity of freely falling body?

V = SQRT(2gh)

27. What do you mean by retardation?

Retardation is the negative acceleration

28. Does a freely falling body possesses weight?

No

29. The force applied on a body of mass 10 Kg to produce an acceleration 2 m/s2 is…….

F = ma = 10 X 2 = 20 kg m/s2 = 20 N

30. What do you mean by Stress?

Stress is force per unit area, Stress = Force / A

31. Define the strain?

The deformation of a body per unit length is called strain

32. What are the units of stress & strain?

Stress N/mm² or MPa & Strain: Unit less

What is meant by Young’s Modulus?

Young’s Modulus = Stress / Strain

33. What is Poisson’s Ratio?

The ratio of lateral strain to linear strain is called Poisson’s ratio

34. What are the thick and thin cylinders?

Thick Cylinder: Cylinder thickness is greater than 1/15 to 1/10 of its diameter.

Thin Cylinder: Cylinder thickness is less than 1/15 to 1/10 of its diameter

35. What are the different types of Beams used in construction?

Cantilever beam

Simply supported beam

Overhanging beam

Fixed beam

 Continuous beam

36. What are the different types of loading?

Concentrated or point load

Uniformly distributed load

Uniformly varying load

37. What do you mean by spring?

A spring is a device whose function is to start when loaded and to recover its original shape when the load is removed.

38. What are the different types of stresses developed in cylinder?

Circumferential stress or hoop stress & Longitudinal stress

What do you mean by strut?

It is structural member, subjected to an axial compressive force is called strut. It may be horizontal or inclined or even vertical.

39. Write a short note on properties of materials used in engineering of power plant.

Strength: Ability to resist externally applied forces.

Stiffness: Ability to resist deformation under stress.

Elasticity: Ability of materials to regain its original shape after deformation when the external forces are removed.

Plasticity: Ability of a material which retains deformation produced under load permanently.

Ductility: Property of metal to become a wire/thin plate after applying external forces.

Brittleness: Breaking property of material after applying a little or more load.

Malleability: Property of a material to enable it to roll or hammer it into thin sheets.

Toughness: Ability to resist fracture due to high impact load.

Resilience: Ability to absorb energy & to resist impact load.

Creep: Ability of a metal to undergo permanent deformation under constant stress under high temperatures.

Fatigue: Ability to resist repeated stress.

Hardness: Resistance to wear, scratch deformation

40. What do you mean by column?

It is vertical structural member, subjected to axial compressive force.

41. What are short and long columns?

The columns which have lengths less than 8 times of their diameter are called short columns. And columns which have lengths greater than 8 times of their diameter are called short columns

42. What is meant by continuous beam?

A beam supported on more than two supports

43. What is meant by truss?

A truss is a structure that consists of two-force members only, where the members are organized so that the assemblage as a whole behaves as a single object. A two-force member is a structural component where force is applied to only two points

44. Define the term ultimate stress

It is the ratio of ultimate load to the original cross sectional area

45. Is Hooks law holds good for Elastic limit?

Yes

46. State the Young’s law?

It is the ratio of linear stress to linear strain

47. Define the term density?

Density is the mass per unit volume of a liquid at standard pressure & temperature. It is expressed in kg/m3.

48. Define the term specific volume

Volume per unit mass is called as specific volume

49. What do you mean by Specific gravity?

It is defined as the ratio of specific heat of liquid to the specific weight of pure water at standard temperature (4 Deg C).

50. What do you mean by Viscosity?

It is the property of liquid which resists flow of one layer of liquid over another

51. What are the two types of viscosity?

Kinematic viscosity & Dynamic Viscosity

52. What is meant by surface tension?

It is the property of a liquid which enables it to resist tensile stress

53. What does Pascal’s law state?

The intensity of the pressure at any point in a fluid at rest is same in all directions

54. What do you mean by absolute pressure?

It is the sum of Atmospheric pressure & Gauge pressure

55. What do you mean by Buoyancy?

It is the tendency of a liquid to uplift an immersed body, because of the upward thrust of the liquid.

56. What are the different types of fluid flow?

Uniform flow, on uniform flow, Streamline flow, Turbulent flow, Steady flow, unsteady flow, compressible flow, incompressible flow, rotation flow, irrotational flow etc

57. What do you mean by Ideal fluid?

Fluid which has no viscosity

58. What do you mean by real fluid?

Fluid which has viscosity

59. What do you mean by Reynolds’s number?

It is the ratio of Inertia force to the Viscous force

60. What do you mean by Mach number?

It is the ratio of velocity of fluid in an undistributed stream to the velocity of sound wave

61. Define the term Thermodynamics

It is the branch of Engineering science which deals with the energies possessed by gases & vapors

62. What are the different types of Thermodynamics system?

Closed system

Open system

Isolated system

63. State the Zeorth law of thermodynamics

The law states that, when two bodies are in thermal equilibrium with a third body, they are also in thermal equilibrium with each other.

64. State the first law of Thermodynamics

Energy can neither be created nor be destroyed, however it can be transformed from one form to another

65. State Charles law, Boyle’s law & Gay-Lussa law

Charles law: The volume of a given mass of perfect gas is directly proportional to absolute temperature, when the absolute pressure remains constant

Boyle’s law: The absolute pressure of gas is inversely proportional to the volume of gas at constant temperature

Gay-Lussac law: The absolute pressure of a given mass of a perfect gas varies directly as its absolute temperature, when the volume remains constant

66. Define the specific heat of a substance

It is the amount of heat required to raise the temperature of its unit mass by 1 deg C

67. What are the different types of Thermodynamic processes?

Isochoric process-Constant volume process

Isobaric process-Constant pressure process

Hyperbolic process

Isothermal process-Constant temperature process

Adiabatic process

68. What are the various types of solid fuels used in power plants?

Coal, wood, bagasse, briquettes etc

69. What are the various types of gaseous fuels used in power plants?

Natural gas, biogas

70. What are the various types of liquid fuels used in power plants?

Diesel, HSD, furnace oil, spent wash etc

71. Define the term calorific value of a fuel

Amount of heat released on complete combustion of 1 kg of fuel

Atmospheric pressure goes on decreasing as altitude goes on increases.

It is 1.033 kg/cm2 at sea level & goes on decreasing at higher elevation from sea level.

It is calculated as below;

Atmospheric pressure at particular elevation = Atmospheric pressure at sea level X (1–2.255 X (10^-5) X Elevation in meter)^5.25585

Note: Pressure is in Kg/cm2

Elevation is in meters.

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Boiler refractory dry out procedure

 

Why do you carryout refractory dry out for Boilers?

It is done to ensure proper drying & curing of refractory in furnace & other areas where refractory is applied. The refractory under goes chemical changes during initial heating. While heating there must be free air flow over the refractory to ensure complete removal of moisture.

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What are the Prechecks carried out before refractory dry out?

Pre checks:

• Ensure Boiler erection work is completed with all respect
• Ensure Boiler official Hydraulic test is done
• Ensure insulation work is completed
• Ensure abundant quantity feed water is available
• Ensure required quantity wood logs are available
• Ensure sufficient & qualified operation staff is available
• Ensure thermal expansion pointers are fitted at all required locations
• Ensure steam drum & super heater vents are opened

Write down the standard procedure for refractory dry out

Refractory dry out is done as per OEM recommendation, too rapid heating of refractory may collapse the refractory material due to development thermal stresses. So it is recommended to heat the refractory for longer time at low temperature.

• Cover all air & coal nozzles with bed materials, this will avoid the damage to the nozzles (This is not applicable for Travelling grate & dumping grate Boilers) while throwing wood logs into furnace
• Select the required size wood logs generally 2 to 3” diameter & 2 to 3 feet length wood sizes are preferred. Ensure wood logs do not have nails, packing strip
• Ensure wood logs have optimum moisture. Too dry or too wet woods are not good for dry out
• Ensure refractory dry out is done on natural daft, no fans are necessary. Ensure all the suction & discharge dampers of fans are kept open
• Put the woods on bed materials, spray the small amount of diesel & then fire the woods
• Initially temperature raising should be slow at the rate of 25 deg c per hour for 3 to 4 hours
• Then raise the boiler outlet flue gas temperature up to 100 deg C & hold for 8 to 10 hours (as recommended by OEM) for soaking the refractory inside the Boiler
• Then raise the boiler outlet flue gas temperature up to 250 deg C & hold for 6 to 8 hours (as recommended by OEM) for soaking the refractory inside the Boiler
• Finally raise the boiler outlet flue gas temperature up to 350 deg C & hold for 8 to 10 hours (as recommended by OEM) for soaking the refractory inside the Boiler
• After completing the above process, firing is stopped & Boiler is allowed to cool naturally
• After cooling down, Boiler must inspected for refractory damage/crack etc
• Minor cracks formed during dry out procedure should be rectified with same quality refractory material

 

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What do you mean by Turbine supervisory system???

 

What do you mean by Turbovisory or Turbine supervisory system?

Turbovisory or Turbine supervisory system is the monitoring of a Turbine. It supervises the condition of turbine and informs to an operating person. It also ensures the parameters do not exceed maximum allowable limit.

What is the significance of Turbine supervisory system?

Turbines are heavy & high speed machines, failures of machine lead into unrecoverable losses. So in case of any abnormality turbine should stop automatically. This is taken care by Turbine supervisory system.

What are the equipments or systems used in Turbine supervisory system?

Vibration probes: These probes are used for sensing shaft or bearing casing vibrations. Generally two probes are fitted at 90⁰ apart at X & Y-directions to measure the shaft vibrations. These vibrations are measured in microns, mills or mm/sec.

Speed probes: Speed sensors are generally Magnetic Pick up unit (MPU) type. These are fitted at teethed portion of turbine shaft for measurement. Generally 2 MPUs are used to measure the shaft speed, one for speed sensing & other for controlling.

Axial shift probes: This probe is fitted at the turbine front end to measure the axial displacement of shaft. Axial displacement probe of the shaft is generally set between +/- 0.4 to +/- 0.6 mm.

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Bearing temperature measuring sensors: These are used to sense the bearing metal temperature or bearing drain oil temperature. For measuring bearing metal temperature, RTD is inserted into drilled hole & touches the white metal & to measure the oil temperature RTD sensor touches the drain oil only.

Differential expansion probes: During abnormal operation conditions like quick start up, wrong SOP & uninform temperature distribution in casing & turbine rotor there could be the possibility of rubbing among turbine internals. To measure & monitor these gaps differential expansion probes are used & trips the turbine if these gaps increase beyond set values.

Casing temperature measuring sensors: Generally turbine casing thickness is very large around 150 mm &it depends on turbine operating parameters. So it is very much necessary of uniform distribution of temperature throughout the casing thickness.

And also the difference between top & bottom casing temperature of turbine should be very less.

Casing expansion measurement probes/sensors: These sensors are used to measure absolute expansion of casing. During startup &subsequent loading conditions turbine casing thermal expansion is must. Generally LVDTs are used for casing expansion measurement.

ESV opening indication: This is used to indicate the actual position of valves.

Eccentricity: This is very important supervision system. This is used to measure the mechanical bow. This may happen due to sudden trip of turbine & unavailability of barring device. Standstill position of turbine rotor for long time

Why do vacuum breaker valves are provided on steam condensers?

During tripping of turbine due to any of the above reasons like bearing vibration, temperature, axial shift, differential expansion etc, the vacuum breaker valve opens to bring down the turbine rotor speed to zero at the earliest time

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 Turbine oil flushing procedure

 

Questions & Answers on Coal analysis & related calculations

 

1-What are the various coal analysis are carried out in plant lab?

• Surface moisture
• Inherent moisture
• Total moisture
• Volatile matter
• Ash
• Fixed carbon
• Unburned carbon
• Moisture in ash
• GCV by Bomb calorimeter & by calculation
• Sieve analysis

2-How do you calculate the surface moisture (SM) of coal?

Take 100 gram of coal sample whose size is < 12.5 mm & keep this sample in lab for 24 hours at atmospheric condition.

Then calculate the surface moisture = Loss in weight X 100 / Weight of the sample (100 gram)

For example: After drying the 100 gram coal in lab for 24 hours its total becomes 92 gram then,

Surface moisture = (100-92) X 100 / 100 = 8%

3- How do you calculate the Inherent moisture (IM) of coal?

After analysis of surface moisture, take 5 gram of coal sample & powder it to 212 microns or 0.212 mm & keep it in oven at temperature 108 deg C for 1 Hr

Sample is then cooled & is weighed & IM is calculated as

Inherent Moisture = = Loss in weight X 100 / Weight of the sample (5 gram)

4-What do you mean by Total moisture (TM) of coal? & how do you calculate the Total Moisture of coal?

Coal containing free visible & non visible water is called total moisture of coal. The total moisture of coal is due to rain or coal in contact with water.

TM is calculation:

Take 100 gram of coal sample of size < 12.5mm & keep it in oven for 1 hour at 108 deg C

Sample is then cooled & is weighed & TM is calculated as

Total Moisture = Loss in weight X 100 / Weight of the sample (100 gram)

Total moisture is the sum of IM & SM

5-How do you calculate Volatile matter present in coal?

Take 2 gram of moisture free coal sample & keep it in muffle furnace for 7 minutes at 900 deg C temperature. Then cool it in desiccator for 15 minutes. Then sample is again weighed for final weight.

Volatile matter = Loss in weight X 100 / Weight of the sample (2 gram)

6-How do you calculate the percentage of ash present in coal?

Take 1 gram of coal sample & keep it in muffle furnace for 1 Hr at 900 deg C temperature. Then cool it in desiccator for 15 minutes. Then sample is again weighed for final weight.

Ash % = Residual weight X 100 / Total weight of the sample (1 gram)

7-Write down the formula for calculating Fixed Carbon (FC)

Fixed carbon FC = 100-(TM+VM+Ash)

8-How do you calculate the unburned carbon in ash sample?

Take 1 gram of ash sample & burn it in muffle furnace for 1 hr at temperature 900 deg c. Then cool it in desiccator for 15 minutes. Then sample is again weighed for final weight.

% of unburned carbon = Loss in weight X 100 / Weight of the sample (1 gram)

9-How do you convert GCV of coal from Air dried basis (ADB) to As received basis (ARB) & Vice versa?

GCV ADB to ARB =GCVADB X (100-TM) / (100-IM)

GCV ARB to ADB =GCVARB X (100-IM) / (100-TM)

10-How do you convert coal GCV from ADB to Dried basis?

ADB to DB = GCVADB X 100 / (100-IM)

11-What are the two different types of coal analysis?

Proximate Analysis & Ultimate analysis

12-What parameters of coal are analysed in proximate analysis of coal?

Moisture, Volatile matter, Ash & Fixed carbon

13- What parameters of coal are analysed in Ultimate analysis of coal?

Total Carbon, Hydrogen, Nitrogen, Oxygen & Sulphur

14-How do you convert Higher Calorific value of coal into Lower calorific Value coal?

LCV = HCV - (9 X H2 X 586)

15-A coal sample of HCV 5100 kcal/kg having 3.5% of hydrogen in it, then calculate the LCV

LCV = HCV - (9 X H2 X 586) = 5100 – (9 X 3.5% X 586) = 4915.41 kcal/kg

16-A coal sample of GCV 4800 kcal/kg having total moisture 18%, then calculate the Net calorific value (NCV) of coal

NCV = GCV-(10.02 X Moisture) = 4800 – (10.02 X 18) = 4619.64 kcal/kg

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