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
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%
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.
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
|
Super-heated steam
line pressure up to 120 kg/cm2 and temperature up to 650 0C
|
Major components of
valves are Body, yoke, bonnet, disc, seat, steam and glands
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