Practical Questions & Answers for power plant Maintenance Engineers & Fitters

Questions and answers for power plant fitters are categorized into section wise
A-Bearings & Lubrication
1-What are the different types of bearings used in power plants?
1-Journal bearings
2-Rolling contact bearings
Ball bearing (Deep groove, angular contact, self aligning)
Roller bearings (Spherical, cylindrical, taper & needle roller bearings)
2-What are the parts of bearings?
Inner race, outer race, cage, rolling elements
3-How do you measure the bearing clearance?
Bearing clearance is checked by inserting feeler gauge between outer race & rolling elements or between inner race & rolling elements
Bearing clearance is measured by dial gauge
Bearing clearance is measured by putting lead wire between journals & bearing pad
4-What are the acceptable minimum & maximum bearing clearances?
Maximum bearing clearance =0.00254 X Shaft diameter in mm
Minimum bearing clearance = 0.001854 X Shaft diameter in mm
5-What are the possible reasons for bearing vibrations?
Bearing vibrations may be due to
More clearance
Lack of lubrication
Misalignment
Overload
Imbalance in machine
6-How do you decide the use of different bearings for different applications?
Journal bearings: High speed, Heavy loads & forced lubrication
Ball bearing: High speed up to 3000 RPM & moderate or smaller loads
Spherical roller bearings: Medium loads, low speed
Cylindrical roller bearings: Used as thrust bearings, where there is high thrust
Taper roller bearings: Used at high axial loads like Gear boxes
Angular contact ball bearings: Used where there are high thrust & high speeds
7-How do you measure the bearing vibrations?
Bearings vibrations are measured by hand held vibration meter, in terms of displacement (Microns, mils), Velocity (mm/sec) & acceleration (mm/sec2)
8-What are the acceptable bearings vibration limits for Boiler Fans & BFPs?
Acceptable vibration limits
Normal operating value:0.1 to4mm/sec
Alarm value: 4 to 5 mm/sec
Critical value: 5 to 7 mm/sec
Shutdown values :> 7 mm/sec
9-What is the significance of bearings suffix & Prefixes?
Bearings suffix & prefixes indicate the bearing clearances, taper, lubrication holes, type of shields, type of cage etc
10-What is the effects of over lubrication & lack of or less lubrication?
Over lubrication may lead increase in bearing temperature due to not dissipation of heat. Over lubrication also causes higher power consumption due to churning effects
Lack of of lubrication may lead into more friction; more wear & tear eventually bearing will fail
11-What are the operating temperatures of rolling contact bearings?
It is 50 to 70 degree C
12-What is the function of lubricants?
Reduces the friction
Reduces wear & tear of rotating parts
Cools bearings
Reduces power consumption
Reduces noise
Increases equipment life

Power plant Maintenance Calculations

13-What are the different types of lubricants used in power plants equipments
Main types of lubricants
Solid lubricants (Graphite powder)
Semi solid lubricant (Grease)
Liquid lubricant (Oil)
14-What are the contents of lubricants?
Contents:
Base oil: 85 to 95%
Additives: 8-10%
Thickners: 2 to 4%

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15-What are the different types of lubrication?
Film lubrication
Boundary lubrication
Hydrodynamic lubrication
16-What do you mean by forced lubrication?
In forced lubrication there will be pressurized oil flow into the bearing housing, there will not be any leveller at the outlet. This type of lubrication is effective for bearing lubrication and heat removal from the bearing caused by friction and washout of contaminants. If oil supply stops because of any reason bearing failure starts immediately. This type of lubrication system used in Turbine and generators set.
17-What factors affect for bearings lubrication?
Type of bearing & size
Speed of bearing
Contaminant level
Operating temperature
18-How do you decide oil lubrication and grease lubrication for bearings?
This can be judged based on DN value of equipment.
DN Value = Diameter of shaft in mm X RPM of shaft.
Higher DN value need oil lubrication
19-What is the difference between grease & oil lubrication?
SL NO.
GREASE LUBRICATION
OIL LUBRICATION
1
Superior start stop performance i.e. grease remains in bearings even at stop condition
Oil may flow to sump..but flash ring, circulating oil & reserved oil tanks full fill the start & stop conditions
2
Contaminants in grease cannot be removed in running
Contaminants in oil can be removed as oil flash as bearings rotate
3
Less prone to leakage
More prone to leakages
4
Proper grease is to be selected for particular application to avoid oxidation
Oil flows freely, enabling it to conduct and carry away unwanted heat (thermal convection). This keeps base oil viscosity stable and reduces the risk of heat-induced oxidation and additive depletion
5
Due to churning of grease in bearings there will be more power consumption
No churning effect so less power consumption
6
Used only for non critical equipments
Used for non critical as well as critical equipments
7
Used /waste grease disposal is somewhat difficult
Disposal is very easy
8
Replacement of grease requires machine complete dismantle
Oil can be replaced without dismantling the machine
9
Frequent top up of grease required
No frequent top up of oil is required
10
Lesser bearing/gear life
Bearing/gear life will be longer

B-Couplings
1-What is the function of couplings?
Coupling is a Mechanical member which connects two shafts & transmits speed & power
2-What are the different types of couplings used in power plant?
Couplings used in power plants are
Rigid coupling: Flange coupling & Sleeve couplings
Flexible couplings: Pin bush coupling, Gear coupling, Resilient (spring) coupling, Meta flex coupling, Spider coupling, Spacer coupling, Tyre coupling, fluid coupling, Floating coupling etc
3-What are the reasons for coupling failure?
Reasons for coupling failure:
Misalignment
Less or more coupling gap
Jerk loads
4-How do you calculate the torque transmitted by the coupling?
Torque transmitted by the coupling =Power transmitted X 9550 / Speed (RPM)
C-Shaft & Keys
1-What is the function of a shaft?
A shaft is rotating machine element which is used to transmit power & speed from one place to another
2-What material is used for manufacturing of shaft?
Generally for pumps & fans shaft material used is EN-8 & for critical & high temperature exposed shafts are of EN-19 & EN-24 materials
3-What process is employed for manufacturing of shafts?
Shafts are manufactured by hot rolling & finished by cold drawing & machining
4-What should be the hub diameter of  a pulley which has to be fitted on 50 mm shaft?
Hub diameter = 2 X Shaft diameter = 2 X 50 = 100 mm
5-What are the shaft materials used at higher temperature?
Sl No.
Shaft MOC (Material of composition)
Maximum operating temperature in Deg C
1
Mild steel Or EN-8
180
2
EN-19 & EN 24
200-300
3
SS 316
600
4
SS 316
600
5
SS 310
1100
6
Alloy steel
>1100

6-What is the function of shaft key & keyways?
A key and the keyways make up joint to secure the hub and the shaft to prevent relative movement between a power transmitting shaft and an attached component. A key is usually made from steel and is inserted or mounted between the shaft and the hub of the component in an axial direction to prevent relative movement.
6-What are the different types of shaft keys used in power plants?
Sunk key (Square, rectangular, feather), Saddle key, Gib head key, round keys, Wood ruff key
7-What is the width & thickness required for a Sunk key for shaft of diameter 65 mm
Shaft width W = (65 / 4) + 2 mm = 18.25mm, Key Thickness T = 65/6 = 10.8mm
8-What type of forces act on shaft?
Torsion & bending forces
9-What is the allowable run out for pump shaft?
It should be 0.03 to 0.05 mm maximum
D-V Belts
1-What types of belts are used in power plants?
V belt, Flat belt, raw edge belt, conveyor belt
2-What is the MOC of V belts?
V belts are made up of rubber with fabric cords inserted into it to increase the strength
3-What type of V belts generally used in power plants?
A, B, C, D & E type V belts are generally used
4-How do you decide or select the type of belts?
Belts Sections are decided based on Load, pulley groove width & depth
5-A drive end 6” pulley is rotating with 1000 RPM, then what is the Size of NDE side pulley rotating with 1500 RPM?
We have relation that N1/N2 = D2/D1
1000 / 1500 = D2/6”
D2 = 4”
Note: In order to increase the NDE speed need to select smaller size pulley & if speed is to be reduced then select larger pulley.
6-What does it indicate that, if B-56 is written on a belt?
B is the groove section or belt type & 56 is the length of the belt in inches
7-Calculate the belt size for B section V belt, which is to be used drive 150 mm (D1) pulley from 200 mm (D2) pulley. The centre distance between the two pulleys is 700 mm (C).
We have Belt size = 2C+1.57 (D2-D1)2/4C
L = 2 X 700 + 1.57 X (200-150)2/(4X200) =1401.4 mm
Convert into inches, SO belt length L = 1401.4/25.4 =55.17”
So belt size will be B 56 (Select nearest standard size)
8-What do you mean by open & cross belt drives
In open belt drives both drive end & non drive end pulleys rotate in same direction whereas in cross belt drives both rotate in opposite direction.
9-Calculate the speed of V belt if it is transmitting 800 RPM speed & power 7.5 KW from 200 mm DE pulley to 300 mm NDE pulley
We have,
Velocity of the belt, V = Pi X D X N / 60 =3.142 X 200 X 800 / 60000 =8.4 m /sec
10-What are the other types of pulleys used in power plants/industry?
Simplex V belt pulley
Duplex V belt pulley
Triplex V belt pulley
Poly V pulley
Wire rope idler pulley
Flat belt pulley
Idler pulley
Crown pulley
Drum pulley
11-What is the material of composition of pulleys?
Generally pulleys are manufactured by Cast iron.
E-Gear drives & Chain drives
1-What types of gear boxes are generally used in power plants?

Worm and worm wheel, helical and planetary gear boxes are generally used in power plants.
2-Whic is the most efficient gear box among above all gear boxes?
Planetary gear box is more efficient one3-
3-What are the functions of gears?
Gears are generally used for one of four different reasons:
To reverse the direction of rotation
To increase or decrease the speed of rotation
To move rotational motion to a different axis
To keep the rotation of two axis synchronized
4-What are the different types of gears used in power plants
Spur gears, worm gears, Bevel gears, helical gear, rack & pinion gears
5-What type of gears are used in Turbine reduction gear box?
Double helical gear.
6-What is meant by helix angle in helical gears?
Helix angle is a constant angle made by the helices with the axis of rotation.
7-What is the helix angle in single and double helical gears?
In single helical gears helix angle ranges from 20° to 35° and in double helical gears up to 45°.
8-What is the helix angle of helical gears in Turbine RGB?
It is 20 to 22°. It depends on capacity and speed of gear box.
9-What is meant by pressure angle in gear teeth?
It is the angle between the common normal to two gear teeth at the point of contact and the common tangent at the pitch point. The standard pressure angle is 14.5 to 20°.

10-What is meant by Pitch circle and Pitch circle diameter (PCD) in gears?

Pitch circle is an imaginary circle which by pure rolling action would give the same motion as the actual gear.
PCD is the diameter of the pitch circle. The size of the gears is generally specified by PCD.
11-What is meant by back lash and clearance in gears?
Backlash: Difference between the tooth space and the tooth thickness measured on pitch circle diameter.
Clearance: It is the radial distance from the top of the tooth to the bottom of the tooth, in meshing gear.
12- Define the term Module in gears?
Module is the ratio of PCD to the number of teeth.
= PCD/N
13-A 12 teeth spur gear has 55 mm PCD, calculate its module.
Module M= 55/22 = 2.5
14-In a turbine RBG pinion has 39 teeth and gear wheel has 84 teeth, calculate the gear box reduction ratio?
Reduction ratio of gear box 5 Teeth on gear wheel/Teeth on pinion
= 84/39= 2.15:1
15-What are the Rack and pinion types of gears?
A ‘rack and pinion’ gears system looks quite unusual. However, it is still composed of two gears. The ‘pinion’ is the normal round gear and the ‘rack’ is straight or flat. The ‘rack’ has teeth cut in it and they mesh with the teeth of the pinion gear. The pinion rotates and moves the rack in a straight line -another way of describing this is to say ‘rotary motion’ changes to ‘linear motion.’ Rack and pinion gears are used in open/close of doors.

16- A Spur driver gear has 25 teeth and has reduction ratio 3:1, calculate the number of teeth on driven gears?
Given that,
T1 = 25 Nos,
Gear box ratio is 3:1, which means that driven gear rotates one complete round on three revolution of driver gear.
T1:T2 5 1:3
25:T2 5 1:3
Number of teeth on driven gear T2 = 75 Nos.
17-Write a short note on sprocket and chain drive mechanisms.
Sprockets are rotating parts with teeth that are used in conjunction with a chain and, almost always, at least one other sprocket to transmit torque. Sprockets and chain can be used to change the speed, torque, or original direction of a motor.
18-What are the different types of chains used for sprocket and chain drives?
Simplex, duplex and triplex types.
19- How do you specify the sprocket?
Sprocket is specified as:
Number of teeth
Pitch circle diameter
Teeth pitch
20-How do you specify the chain?
Chain is specified based on Type of chain (simplex, duplex etc.) Chain pitch.

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F-Rigging

1-What is rigging technology?
Rigging is the process of using hoists, ropes, pulley, slings, jacks and cranes for lifting, lowering or shifting the heavy objects.
2-Who is rigger?
A person who leads or carries out the rigging process.
3-What are the various rigging elements used in power plant?

  • Synthetic and wire ropes
  • Slings
  • Chain hoist/chain block
  • Pulley block
  • Screw and hydraulic jacks
  • Crane
  • Winch machine
  • EOT cranes
  • Derricks…etc.

4-What is the safe working load of 25 mm Nylon rope?
Safe working load (SWL) of Nylon rope = D2= 25 X 25 = 625 kgs.
Where D is the diameter of rope in mm
5-What is the smallest size of nylon rope used for lifting?
Smallest size of nylon rope used for lifting is 12 mm.
6-What is the material of composition of wire ropes and slings?
MOC of wire ropes and sling is Steel of grade 80.
7-What is the material of composition of wire ropes and slings?
MOC of wire ropes and sling is Steel of grade 80.
8-What is the smallest size of wire rope used for lifting?
Minimum size of steel wire rope for handling loads is 5 mm.

9-How do you calculate the safe working load of steel wire ropes and slings?
SWL of wire ropes slings = 8 D2, Where D is the diameter of sling in inches.
10-What are the pre-use checks to be done on Nylon and wire ropes?

  • Check its manufacturer name plate details and ensure lifting load capacity.
  • Check for strand damages discard if more than 10% of strands are damaged.
  • Check for strand frying.
  • Check for rope knotting.
  • Check for overloaded rope (Rope which was overloaded in previous lifts should not be used for lifting at its rated SWL).
  • Check for reduction in rope diameter, wear out should be less than 10%.
  • Should not have exposed temperature more than 65 and 95 °C for synthetic and wire ropes respectively for long time.
  • Check for corrosion/rusting of wire rope.
  • Check for any scoring or abrasion marks on wire rope.
  • Check for wire end attachments damages.

11-What are the safe checks carried out on chain block?
Following are the inspection checks carried out on chain blocks:

  • Check tightness of block’s nut bolts.
  • Check gearing system is well operating and its lubrication.
  • Check hook of block as per hook check list.
  • Check load chain links for elongation and wear out.
  • Link 10% size less than its original diameter is discarded.
  • Check for unauthorized welding and cutting marks on chain hoist.
  • Check whether chain is in contact with acid or alkali for long time.

12-What is the function of turn buckle?
Turn buckles are used for leveling and distributing the loads.
G-Alignment

1-What is meant by shaft alignment?
Shaft alignment is the process of aligning two or more shafts with each other to within a tolerated margin. Shaft alignment is the act of measuring the relative position of two machines that are coupled and repositioning them so that the rotational axes of the two shafts form a straight line (collinear) when the machines are at normal operating temperatures.
2-What are the effects of misalignment?
  • Excessive vibration and noise
  • Bearings failure
  • Seals failure
  • Excessive heat generation
  • Higher power consumption
  • No equipment reliability

3-What are the possible reasons for misalignment?
  • Loose/broken foundation bolts
  • Loose shim packs/dowel pins
  • Loose coupling bolts
  • Jerk loads due to frequent start/stop

4-Explain the various types of alignment.
Alignment types
1-Axial Alignment:
In axial alignment, the gap between the two couplings is measured. The distance between the two couplings hubs are taken at 12, 3, 6 and 9 o’clock positions.
2-Offset/Parallel/Radial Alignment:
It is the actual radial position of the movable rotational center relative to the stationary center. If the shafts are not parallel, the offset misalignment is different at every axial position.
3-Angular Alignment:

Angular Misalignment is the slope relationship of the two shafts. The slope has a positive value if the offset values are more positive at the rear feet than at the coupling. The shafts are not in the same plane, which causes a difference in measurement between measurements made 180 degrees opposite on the coupling faces.
5-What are the different methods of alignment?
  • Different methods of alignments:
  • Straight edge method
  • Rim and face type (Dial gauge)
  • Reverse alignment
  • Laser alignment 
H-Valves

1-What are the different types of valves used in power plants?
Following different types of valves used in power plants
  • Gate valve
  • Globe valve
  • Angle globe valve
  • Ball valve
  • Diaphragm valve
  • Butterfly valve
  • Non return valve
  • Needle valve

2-What are the various functions of valves?
Valves are used for following applications
  • To stop or allow the flow of fluids in pipe lines or vessels.
  • To prevents excess pressure generation in pipe lines or vessels
  • To prevent back flow of fluid into upstream to avoid damages
  • To regulate the pressure and flow of fluid
  • For blow down.

3-Differentiate between globe valve & Gate valves

Sl No.
Globe valve
Gate valve
1
Time required for complete opening of valve is less.
Time required for complete opening valve is more.
2
For a given size weight of the valve is less.
For a given size weight of the valve is more.
3
For a given size, stem size of globe valve is more.
For a given size, steam size of gate valve is less.



4
Effort required to operate the valve is more.
Effort required to operate the valve is less.
5
More turbulence in valve
No or less turbulence



6
More pressure drop as fluid direction changes twice.
Fluid flow direction is straight, hence no much pressure drop.
7
Accurate flow/pressure control can be achieved.
Not that much accuracy as compared to globe valve.
8
Maintenance is easy for large size valves.
Maintenance is difficult and time consuming for large size valves.





















4-List down the different classes of valves

Sl No.
Valve class
Application
1
150
Water/saturated steam/oil lines up to pressure and temperature 10 kg/cm2 and 150-200 0C
2
300
Water/Steam lines up to pressure and temperature 20 kg/cm2 and 200-250 0C
3
600
Water/ Steam lines up to pressure and temperature 40 kg/cm2 and 300-350 0C
4
800
Water/Primary super heater steam lines up to pressure and temperature 100 kg/cm2 and 350-400 0C
5
900
Super-heated steam lines
6
1500
Feed water lines pressure up to 130-150 kg/cm2 and temperature up to 400 0C
Super-heated steam lines pressure up to 90 kg/cm2 and temperature up to 520 0C
7
2500 
Super-heated steam line pressure up to 120 kg/cm2 and temperature up to 650 0C
5-What are the major parts of valves?
Major components of valves are Body, yoke, bonnet, disc, seat, steam and glands



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