3. Why this E-book?
3
What happens when you get stuck in a problem in
the plant? Initially, it seems a difficult problem, you
apply your full capacity, wisdom and knowledge to
solve it. Finally, you solve it using the simple bits of
knowledge combined together.
Let’s have an example.
4. Empty Soap Box Problem
4
You must have already heard about this problem.
A soap manufacturing factory received a complaint
that one of the soap box reached to the customer
with no soap in it.
They installed a conveyor, on which each soap box
moved, under an X-ray scanner, which was closely
observed by supervisors and empty boxes could be
separated away before dispatch.
5. Empty Soap Box Problem
5
Another mill took a different approach.
In place of X-Ray scanner, they mounted a small
fan.
Empty box can move away due to the strong force
of air.
No supervisors needed.
No costly X-Ray scanner.
Simple system.
6. Empty Soap Box
After you have read the story, would you not think twice
before installing an X-Ray scanner for soap packing?
Suppose you need to pack something else in place of
soap?
Think outside the box.
For example, consider the case for a shampoo bottle.
6
7. Knowledge Vs. Experience
7
Knowledge is small
bits of information,
facts, theorems etc.
By experience, you
learn to gather all
these bits together to
use for your purpose.
8. Case Studies
8
Yes! Case studies give us more food for thought to
work on any problem, and are more useful in the
modern day complex world.
This E-book is an effort to bring out several such
case studies related to mechanical engineering.
You may consider it as a virtual experience!
11. Pneumatic Cylinder Loading
11
In a pneumatic cylinder, you apply load on either
side.
The force exerted by the cylinder is equal to
multiplication of pressure and area.
At the piston rod side, as the area is less, cylinder
exerts lesser force even at same pressure.
Let us see-
14. What If….
14
Imagine a cylinder is loaded with the same pressure
(say 5 kg/cm2) at both sides.
What would be the resultant force on the piston in
such a case?
16. Calculation Procedure…
16
If you calculate force separately on both sides, and
then calculate resulting force by subtraction, you get
a definite non zero force exerted by the rod.
If you subtract pressure on both sides, you get zero
resultant pressure, and hence no net force acting by
the cylinder.
What is the correct way to calculate?
17. Correct Answer
17
Zero.
When a cylinder was connected to same pressure
supply at the both ends, the rod remained
unaffected.
However, when his query was posted on different
social networking sites relevant groups, got mixed
opinions.
Don’t you think it could be a good interview
question?
18. Thinking?
18
The objective of the book is to ignite your thinking
process to evaluate process conditions in a faster
way, and solve various problems.
20. Fan Pump
In paper mills, we use a pump called Fan Pump.
It is named because the flow rate is much higher,
while the pressure required is relatively low.
The present one is a real case which baffled the
minds of the mill maintenance team.
20
22. Pump Tripping
22
The plant was running for years, but once there was
a problem of electrical tripping of pump motor.
On checking, pump was found jammed.
Obviously, the pump was opened up.
It was found that the bearings were broken.
23. Pump Checking
Bearing problem! Replace the bearings.
Yes. The mill did the same. But after a couple of
days, the new bearings broke again.
Maybe, there was some problem from bearing side.
23
24. Bearing Quality
24
The mill ordered bearings from other supplier.
By the time new bearings were received, the
running one bearing failed again.
Anyway, new bearings reached in plant in time, and
these were mounted.
Everyone hoped that these bearings were of
reputed make, so there should be no problem in
future.
25. Better Bearings
New bearings could run for 3-4 days.
Should I say, 50-70% better life?
Fed up with the problem, the mill decided to
procure bearings from a reputed brand.
As there is a risk of getting fake bearings, the mill
procured two sets from two different vendors,
hoping at least one supplier will give genuine ones.
These failed again within 3-4 days.
25
26. Failure Review
26
Bearing: 6314
Operating Load: Less than normal.
Pump Sound: OK
Operating Vibrations: OK
27. Bearing Failure
27
Actions Taken:
Check mounting procedure.
Replace pump shaft.
Replace supplier.
Switch to original bearing.
Nothing worked, so-
Replaced the complete pump with a new one.
28. Pump Puzzle
Plant parameters were normal.
The new fan pump was running at same pressure
and flow rate as earlier.
The motor load was also normal.
But bearings were failing so fast that the mill
increased inventory level for that bearing from 2
earlier to 8.
Still everybody was clueless.
28
29. Got Clue
29
After a couple of days while the pump was stopped
for the same problem, some ceramic tiles pieces
were observed in it.
A ceramic tile piece can damage the pump easily.
On inspection, the pieces of tiles were found
separated from the floor and walls of the storage
chest.
30. Reason Appeared Suddenly!
Breakdowns are not always bad. Some breakdowns
also give clue about the problem.
In this particular failure, the mill observed small
pieces of ceramic tiles in the pump volute.
How could that come into system?
Oh!. Ceramic tiled detached from the chest and
entered into the pump along with pulp stock.
No one could see these earlier.
30
31. Back to Operation
31
The civil work was taken to replace all the ceramic
tiles in the chest, and the same pump operated for
several years without any problem.
32. Pump
There could be many reasons for bearing failure-
Improper mounting
Inadequate lubrication
Substandard bearing
Bad or corrosive environment
But, sometimes,
You need to find out like a detective!
32
33. Lesson:
33
Don’t just stop relying on your equipment (bearing,
in this case) after a couple of breakdowns!
If you don’t get a clue after a breakdown, never
ignore the need to find a reason for it.
35. Press Section
35
In paper mills, wet web of paper is pressed
between two rolls.
These rolls are called “Press Rolls”.
The objective of pressing is to squeeze out water
from the wet web so that paper can be dried easily
in the dryer section.
41. Can a Roll Journal Shift?
41
A roll is made up of a shell, generally of mild steel,
with cast iron hubs press fitted on both ends.
In the hub, journals are press fitted.
Thus, it seems impossible in the normal case that
journal had shifted.
Journal bending was also not appearing to be
there in this case.
What else could be the cause, then?
43. Investigation
43
For a roll to rotate perfectly, we need to review a
simple rule-
Roll rotates around the bearing centerline, while
to our eyes, roll surface must seem to rotate
around roll centerline.
In case the bearing center is different from the roll
center, problem will appear.
44. What Could be the Problem?
44
It may result a haste decision to replace a roll
journal, resulting a significant amount of time,
money and downtime.
The roll had been taken out by then, but there was
no sign of any shifting of journal.
The journal as measured against roll surface, and it
was found OK.
45. The Clue
45
After investigation, it was thought that possibly the
sleeve was not properly fitted and the same was
tilted.
This caused the bearing center moved away from
the roll journal center and hence the problem
started.
To recheck, the same roll was mounted again, and
this time, this showed no problem at all.
47. Lesson Learnt
47
Mounting bearings need special care, as a
carelessness may result in severe losses also.
Had the mill not taken decision to evaluate the real
cause of the problem, probably the roll was sent to
the supplier for journal replacement.
49. Three Bricks
49
There are three bricks, each having
a size of 225mmX100mmX75mm.
Each packed is a gift wrap, so you
cannot see the color, surface
texture etc. These are-
IS:6 Refractory Brick
70% Alumina Brick
Insulation Brick
How can you identify, without
opening the packing?
51. Open the Packs
51
Well, even if you open the cover, two bricks look
almost the same, while the third is different.
Any idea, now?
52. Insulation Brick
52
Insulation brick is the lightest. It is typically white in
color, and you must be able to identify this just after
lifting it in your hand.
53. 70% Alumina Brick
53
The 70% Alumina brick is generally used for high
furnace temperature applications, and is nearly 5-
5.5kg in weight.
The other brick, maybe around 3 kg is general IS:6
brick, used for general low pressure boilers.
54. Why Bricks?
54
Many fresh mechanical engineering graduates fail
to identify the difference. This was just a way to
explain, what is generally not taught in the classes.
56. Press Roll
56
About the press roll, we have already discussed in
earlier section.
This is the case where the press roll bearing failure
became too frequent.
58. Press Roll Bearing
58
The press roll bearing used is a conventional
23240K bearing.
This is a taper roller bearing.
Weight: 56.7kg.
ID: 180mm
OD: 360mm
Width: 128mm
Mounted with: AH3240 sleeve
59. Failure
59
Bearing inner race broken.
You may observe the crack
mark on the top side of the
inner race.
Before failure, the bearing
was running properly.
The shift fitter had checked
the same just an hour
before and found OK.
60. Repeated Failure
60
Bearing replaced with a
new one.
After few weeks, broken
again.
Got bearings from
different suppliers.
Total 7 bearing failures
within 5 months.
61. Bearing Make
61
Bearings were of different make.
Possibly a few of these were duplicate.
Option of trying bearing from reputed suppliers
was considered, but dropped due to price
constraints.
These bearings had been procured at price 25-
30% of new genuine branded ones.
62. Discussions About History
62
After long discussions, it was revealed that some
time back while mounting the bearing, a shift fitter
left the sleeve loose by mistake.
As a result, sleeve turned over the roll journal, and
the journal got damaged.
The mill had to get the journal replaced.
Now the fitters started getting extra careful (rather
to say over careful) while mounting bearings.
63. Over Tightened Sleeves
63
The over tight sleeves resulted in excess pressure on
to the inner race of the bearing.
As the inner race could not bear the load, it
cracked.
Everyone thought that the problem was from
bearing quality only.
64. Verification
64
The bearing inner race had broken.
Under normal operation, load is compressive.
Over tight sleeve creates tension in inner race.
65. Proper Mounting
65
Before mounting the clearance between the rollers
of bearing and outer race should be checked.
While fitting, the same should again be checked.
The reduction in clearance MUST be as specified by
the supplier.
69. Steam Line of Boiler
69
Imagine a hypothetical case-
There is a mill having a boiler of 5 TPH capacity.
The steam is being supplied to indirect contact
dryer at 8-9 kg/cm2 (g) pressure.
The pipeline for the same is 4”NB, and the dryer is
installed 125m away from the boiler.
But, the pressure inside dryer is just 3.0 kg/cm2 (g)
only (being controlled by a throttling valve).
70. Steam Line of Boiler
70
Well, as steam requirement is just 3 bar, supplying
steam at 7-8 bar means we are supplying it at
higher temperature.
That means higher heat losses from pipeline, even if
the same is insulated.
Someone suggests you to operate the boiler at 4-5
bar.
What will you do? Agree? Disagree?
71. Boiler Operating Pressure
71
High pressure operation results in increased flue gas
temperature.
That means high percent losses or lower boiler
efficiency.
Thus operating boiler at reduced pressure means
better efficiency, reduced fuel consumption.
72. A Win-Win Situation
72
In this way, by reducing boiler pressure, you get
better boiler efficiency as well as reduced heat
losses from pipeline.
Or, is there something that must also be considered?
73. Lower Boiler Pressure
73
Will there be any adverse effect on process?
Will you face any operational problem?
Should one immediately go for it? It is very simple.
Just put a lower pressure set-point in boiler control
system?
75. Detailed Analysis
75
Boiler capacity is 5 TPH.
What is the steam demand?
Does steam demand make any effect on our
decisions?
Is steam demand fluctuating?
Does that make any effect on our decisions?
If yes, how?
76. Detailed Analysis
76
Let us assume steam demand as 3.0 TPH.
Now consider both cases for a 4”NB pipeline size.
Parameter Unit 7.5 bar(g) 4.5 bar(g)
Steam Flow Kg/hr 3000 3000
Specific Volume m3/kg 0.232 0.349
Total Volume m3/hr 696 1047
Flow velocity m/s 24.6 37.0
77. Pipeline Size
77
Well, you have an option to increase the pipeline
size to 5” or 6”NB.
But, with increased pipe diameter, the surface area
of insulated pipeline increases further, and hence
heat losses.
Can you calculate heat loss from an insulated
pipeline?
78. Assignment
78
Consider both cases. One with smaller pipe
diameter with higher steam pressure and other with
more pipe diameter with lower steam pressure.
Find out heat losses in both the cases.
Find equivalent fuel saving in better option.
Compute payback period.
79. Further Analysis
79
Let’s review both cases for steam opening from
boiler drum.
What happens if we need more steam, say 4.0 TPH
for a shorter time?
Parameter Unit 7.5 bar(g) 4.5 bar(g)
Steam Flow Kg/hr 3000 3000
Specific Volume m3/kg 0.232 0.349
Total Volume m3/hr 696 1047
Flow velocity m/s 24.6 37.0
80. Boiler Main Steam Nozzle
80
Normally main steam nozzle is suitably size for
operating pressure and flow rates.
A reduced pressure operation results in increased
velocity through main steam valve and hence there
could be a possibility of water carryover with
steam.
So, we need to miss this opportunity of fuel saving
by reducing boiler pressure.
If we increase nozzle size?
81. IBR
81
IBR stands for Indian Boiler Regulation Act.
According to this, you cannot make any changes in
the boiler without permission from the concerned
authority.
You need to obtain permissions before ordering,
installing, erection and commissioning and steam
generation.
The boiler condition is also checked annually by the
competent authority.
82. Permissions Required
82
While ordering boiler
Boiler drawings are made
After boiler fabrication
After boiler is received
Boiler commissioning
Hydraulic Test
Steam Test
Annual Inspection
83. Changing Main Steam Nozzle
83
This way, replacing the main steam nozzle of the
boiler is just NOT ALLOWED.
This indicates that to maintain desired flow velocity
(below the design value), you must operate it at a
minimum pressure.
84. Lesson Learnt
84
Many energy conservation projects look lucrative on
paper.
Think well, decide well.
85. MG Gearbox Failure
85
The MG paper machine was being driven by a line
shaft.
There were four sections-
Wire Paper
Press Part
MG
Pope Reel
88. The Paper Machine
88
Paper is formed from pulp in the wire part, by
dewatering water through a wire mesh.
In the press part, paper web is squeezed further.
On MG cylinder, paper is dried.
Finally, it is rolled on a roll at the pope reel.
89. MG
89
MG stands for Mono Glaze.
This is a large drying cylinder on which the paper is
dried.
This is a large cylinder, so the rotational speed is
low.
For the same a gearbox followed with a gear
pinion was installed.
The gear box was a helical U:1200 model one.
93. Drive Arrangement
93
You may see the cone pulley at the left side of gear
box in the next slide.
The flat belt drives this cone pulley, while the drive
cone pulley is mounted on the line shaft (Shown in
another slide).
96. Failure
On a day, joint shaft broken.
Immediately, a new shaft was arranged and
mounted in its place.
The machine was started, and production continued.
After a couple of days, this shaft broke again.
96
97. Failure Continued
97
The reason for shaft breakage was originally
considered as due to the weakness of the shaft.
It was decided to increase the shaft diameter from
100mm to 150mm.
To use same coupling and pinion, the shaft diameter
was higher in the center (where breakages were
occurring) and same 100mm at both ends.
Everyone was sure about the shaft now.
98. Failure Shifts
98
After a few days, the gearbox broken.
Well, another one was arranged and mounted.
A few days later, the shaft broke.
This started becoming a routine.
Shaft-shaft-gearbox-shaft-gearbox…
Finally 4 shafts and 3 gearboxes failed, within a
couple of months.
99. Why?
Why all this started?
What could have been done?
Was it an operational problem?
Was it a design problem? Were the gearbox or
shaft undersized?
Supplier said he used to give U-1000, but gave a
bigger U1200 just as a factor of safety.
The mill ordered a bigger gearbox. U-1600
against U-1200 running. Was this decision right?
99
100. Problem Identification
Later, in a brainstorming meeting the possibility of
backlash appeared.
The question was- “How to check for it?”
Another question was, “Why this problem started
after nearly an year from startup?”
100
101. Backlash
The problem with the machine was that there are
two coupled rolls running in the machine with the
same drive.
Press roll was being driven by one cone pulley.
MG was also been driven by one cone pulley.
Both the cone pulleys were being driven from the
same line shaft.
In such cases, any speed difference can create
backlash.
101
102. Mechanical Connection
102
After the press section, the paper goes with the felt
and passes through another nip to get wrapped on
MG cylinder.
You may notice the felt (looks like a blanket) in the
next slide, behind the yellow color staircase railing.
104. Backlash
In such a case, the line shaft drives two different cone
pulleys, to their matching cone pulleys, to the different
gearboxes, to their joint shafts and finally to two
different rolls.
But, the rolls nips are connected mechanically through
the felt.
A speed difference WILL create backlash in any of the
gearbox.
Most gearboxes are not designed to operate with
backlash.
How can one check backlash?
104
106. Checking Backlash
106
As shown in the figure in the previous slide, the
coupling bolts were removed.
A chalk mark was made on both sides of coupling
(As shown in yellow line).
Now, the both sections were started, the left part of
coupling rotates at a speed proportional to MG
drive.
The right part of coupling rotates at a speed
proportional to press drive.
107. Checking Backlash
107
By this, it is possible to practically ‘see’ the
backlash.
In the above case, one side of coupling took 10
turns, while the other took 9 turns before the chalk
marks coincided with each other again.
Thus there was more than 10% speed difference.
108. Problem Solved
After you know the problem, you can always take
corrective actions.
There has been no such failure reported after the
problem was solved in past more than 10 years.
108
109. Origin
109
After the problem was solved, the next ‘food for
thought’ was to identify the reason for the same.
After discussions with operators, it was found that
the running flat belt broke one day, and the mill
had no stock.
As an alternate, conventional belt generally used
for agricultural purpose was mounted, after
improper jointing.
110. 110
This belt was loose, so it was slipping on the pulleys.
The position of the belt was adjusted in order to
compensate for the slippage.
After a few days, when the new belt arrived, it was
mounted on the same position without checking the
speed difference.
A little negligence resulted in heavy loss to the mill.
111. Lesson:
111
Ensure proper mountings of all accessories.
Try to evaluate the root cause of the problem rather
than blindly changing equipment to higher size.
114. Problem
114
The case is of a paper drying cylinder installed in a
paper mill.
The mill complained that after the startup from a
shut of a few hours or more, the paper was not
drying properly.
The steam pressure in the cylinder (yankee cylinder)
was normal.
115. The Process
115
Live steam enters the dryer through thermo-
compressor and the condensate is sent to a pressure
vessel.
The flash vapors generated from this pressure vessel
are again taken by the same thermo-compressor.
The condensate is sent to the condensate tank
through a pump.
116. The Process
116
Simultaneously, a part of live steam is used in
hood’s live steam coils to heat up the air, which is
impinged on to the surface of paper for faster
drying & hence faster production rates.
The condensate from these is also sent to the same
condensate tank.
From condensate tank, it goes to boiler feed water
tank.
118. Back to Problem
118
The production for some time remains low even at
the operating steam pressure.
After say 10-20 hours, slowly and slowly it
increases to normal.
In case of frequent web breaks, the production rate
comes to normal relatively faster.
In case there is a web break, the operators need to
vent out a part of steam from the drying cylinder.
Are the above two points interconnected?
119. Operator Observation
119
Considering the experience, the operators started
using the knowledge gained to vent out a part of
steam for some time in case the production rate is
not normal.
This practice resulted in maintaining the production
rate.
But, what was the cause of it?
120. Non Condensable
120
Presence of non condensable reduces the heat
transfer from the steam side drastically.
When the plant is shut for a long time, the steam
inside the system condenses and thus a vacuum is
created inside the yankee dryer (or MG Cylinder).
As a result air enters through gland packing etc.
into the system.
Cold gland packing allows steam to enter the
system easily.
121. Steam Venting
121
While some steam is vented, non condensable gases
also escape out from the system, resulting the
system to work normally.
But excess venting also means steam wastage.
As a solution, a part of the flash steam was used in
direct heating applications. This way, the excess air
got released soon, and the problem was solved.
122. Steam Venting
122
In fact, the similar problem was also faced in a
large paper machine, where the system and
condensate system was being operated in
cascading mode.
It was noticed that by installing the proper steam
venting system, the production could be increased
by more than 10%.
124. Bearing Housing Bolt Breaking
124
The paper machine was running well.
At 2:55am, a bolt of MG bearing housing broken,
and jumped off falling onto the machine frame.
This resulted in severe damage to two machine
clothing, two rolls, and more than 24 hours of
downtime.
127. ….. and Damage!
127
One of the bolt of the bearing housing (yellow
circle, but at other side) broke.
The head part of bolt jumped up and fell down on
the machine clothing, thus breaking two rolls and
damaging two felts.
129. The Bolt
129
The size of the bolt was M24 X 150mm.
The bolt was of high strength type.
However, the break was nearly 2” away from head.
All the bolts had been replaced just 6 months back.
Earlier bolts of the same type and make had been
running for past 8-10 years.
130. The Change
130
The MG cylinder is a huge cylinder with a weight of
40-42 MT.
With surface wear due to operation, the weight
reduces but it was more than approximately 35 MT
when the accident took place.
This roll is pushed upwards by a press roll
(commonly called as TOUCH ROLL).
The mill, had installed second touch roll also on the
machine one year back.
132. Two Touch Rolls
132
These rolls impart an upward force on MG cylinder.
Maybe the total of these was higher than the
weight of MG.
Detailed calculation indicated that this load was
nearly 55MT, or a net force equal to 20 MT (acting
upwards) was acting of MG cylinder.
133. Analysis
133
So, in all, all the eight bolts needed to take care of
this force.
That means one bolt had to effectively handle a
load of 2.5MT only.
Why did one bolt brake?
134. Why…
134
The major questions were-
Why did only one bolt brake?
Why the head of bolt jumped
so high that severe damage
took place? Remember, the
bolt had broken around 2”
below the head.
135. Analysis
135
All remaining bolts of the same side (3) were
checked thoroughly.
The bolts were perfectly OK.
There was no indication of any damage, thread
shearing etc.
All four bolts of the other side were also in good
shape.
Decided to replace all 8 bolts, precautionary.
136. Fitter Confessions!
136
Later on, a fitter indicated the possible root cause in
his confessions.
While mounting the bolts few months back,
accidentally, one spring washer fell down in drain.
Ignoring the importance, he just tightened the bolt in
place without a spring washer.
Can a missing spring washer could be a cause of
this major loss?
137. How Does a Bolt Break?
137
To understand it further, we must recall typical stress
strain diagram.
With increased tension or load, the bolt elongates
slightly.
The elongation is more at higher load.
After a particular load, elongation increases
suddenly and the bolt breaks.
The same is shown in next graph.
140. Bolts in Parallel
140
When several bolts share a load, it is unlikely that
the initial load will be really equal on all.
Addition of spring washer allows a slight movement
of tightened elements so that the load distribution
becomes uniform.
When a bolt faces load, it may elongate slightly
thus transferring load to other one and so and so
forth.
141. Tightening the bolts
141
When bolts are not mounted with spring washer,
and are tightened, it is unlikely that all bolts are
tightened with equal tension.
When the load is gradually applied, first the
tightest bolt bears the load, and elongate slightly.
To ensure uniform load distribution, it is always
recommended to tighten all bolts slightly and
retighten again and again.
You must have noticed this practice being followed
while the tyres of your car are being changed.
142. Spring Washer
142
Furthermore, by addition of a spring washer, this
elongation of bolts is taken care off by the same.
Thus the remaining load is shifted onto other bolts.
143. Cost of a Spring Washer
143
What do you think, is the cost of a simple spring
washer?
To that mill, that missing spring washer resulted in a
loss of more than a million rupees and a production
loss for a couple of days.
144. Lesson
144
That particular mill learnt the true cost of a spring
washer after making a big loss.
Another hidden culprit was the negligence of the
fitter who did not report the authorities about the
washer felling down and getting lost somewhere.
However, by the confessing of the fitter even after
the accident, the mill could be able to take future
precautions.
145. Remarks
145
Sometimes the fitters, electricians and other workers
hide small incidences which may result in huge loss in
future.
Every management must ensure creating an
atmosphere where the workers are able to convey
their individual failures, negligence, or problems
caused by their carelessness, without any fear.
If any worker admits his mistakes, be assured that
he is really willing to not repeat it in future.
147. DG Alternator Flashing
This seems an electrical issue?
Why discuss it here?
Electrical department has its own team for heavy
motors and alternators.
Anyway, let’s see what they faced.
147
148. DG Alternator
The unit had 5 DG sets, each having 380kVA rating.
Nameplate data indicated-
Rating: 380kVA
RPM: 1500
PF: 0.8
Frequency: 50Hz
Voltage: 415V
Current: 504A
Operating conditions were-
Current: 380-450A (75-90%)
Fairly acceptable load; no severe load fluctuations.
148
150. DG Alternator Flashing
One day, alternator of DG 3 flashed.
DG operator immediately shut the engine off, and
called electrical people.
To run the set, it was decided to mount another
spare alternator (which was kept aside), and to
send the flashed one for repair.
The same was done, and the plant resumed
production.
150
151. Flashing Repeated
151
Within 3-4 days, the next alternator flashed.
Fortunately, DG no. 5 was not in operation, so it was
decided to use the alternator from DG-5 for DG-3.
This again could survive for 2-3 days only.
Three failures, within a week can disturb anyone.
152. Flashing Again
On discussion with many, it was told that alternator
may fail as during operation, the winding insulation
might have weakened.
But, spare alternator had run only for 3-4 months
earlier, long back.
Was its insulation also weak?
What about the alternator from DG-5?
152
153. DG Alternator Flashing
All the three DG operators were asked for the
possible cause.
Obviously, no one had any idea or any clue.
One thing they all were sure-
“The DG set was running much under load.”
153
154. Reason?
154
Can under-load be the reason for failure?
Or, reason was something else?
And the clue appeared after long discussions.
155. Approach
155
Unable to find a solution, it was decided to ask the
operators for any abnormality faced by them
during recent past.
There were long discussions with operators, one by
one, simply asking them to recall any abnormality
faced.
Initially, they could not tell, but after several such
sessions one operator gave a clue.
156. DG Frequency Reduction
Often there was frequency reduction in DG supply.
The reduced frequency condition survived only for a
few seconds, and repeated 3-4 times a shift, even
at the lower operating load condition.
The operators did not mention the same in log book,
as they were noting hourly values, like 7:00, 8:00,
9:00 hrs. or so.
156
158. Low Frequency
Yes! Low frequency caused resulted in an increase in
field current.
The frequency depends on RPM of engine, which is
governed by the supply of fuel to engine.
On further investigation, it was found that the fuel
pump had suffered severe internal wear.
Fuel pump was a gear pump, and it was badly
worn out internally.
158
160. Fuel Quality
The issue was discussed with DG supplier, who
indicated that the problem could have been
appeared due to poor quality of fuel.
The mill had changed HSD supplier some time back,
and was procuring HSD at a lower price by nearly
7-10%.
Some experts indicated that HSD from that
particular supplier lacked lubrication, and hence
could be a reason for DG fuel pump (gear pump)
wear.
160
161. DG Alternator or Anything
161
Quite often, you need to look into minute details, to
identify the source of the problem.
There are cases when you ignore such details and
remain trapped with the problems.
162. Lesson:
162
The breakdown analysis needs a long chain of
knowledge to understand. Here, the chain was-
Alternator flashing
Due to high field current
Due to low frequency
Due to low engine rpm
Due to inadequate fuel supply
Due to fuel pump problem
Due to pump wear
Due to poor quality of fuel oil
164. Supercalender Uneven Loading
In coated paper plants,
coated paper is passed
through supercalender
to achieve gloss.
A photograph on the
right gives a general
idea about the
equipment.
164
165. Supercalender
165
In the supercalender, paper is repeatedly pressed
between two rolls- one made of cotton (soft) and
other of high hardness chilled steel.
The object is to smooth the paper surface and make
it glossy.
The whole action on paper is similar to ironing of
the clothes.
167. Supercalender Loading
Uniform load across full width of paper is essential
to get uniform paper properties.
But, in case, the load is different at one side
compared to the other, problems start.
You may get a paper that is not at all saleable.
167
168. Procuring Supercalender
A mill decided to procure a second hand supercalender
from a mill that was shut three years back.
After the deal was closed, and the mill personnel went
to dismantle the same, someone told that the mill’s
decision was not good; as the operator used to face the
problem of uneven loading with that equipment.
According to him-
“At one side paper was hot while other side it was cold”.
168
169. The Problem
Well, this was shocking for the mill.
On further inquiry to different people, it was
informed that there was some problem with the
equipment foundation, for which the earlier mill had
broken the old foundation, and casted a huge
foundation.
The cement used for the second foundation was
more than 350 bags, nearly twice compared to the
first one.
169
170. The Options…
170
The earlier mill had already spent a huge sum of
money and nearly 12-15 months on this equipment,
before it got shut due to heavy losses.
Now, mill could have decided to cancel the deal,
and bear the loss of nearly 15-20% of the price
paid as advance.
Alternatively, the mill could decide to buy the
equipment, obviously with the risk involved.
171. Calculated Risk
Anyway, the mill decided to honor the deal and
continued the dismantling work.
The equipment was then transported back to new
mill.
After the same reached at mill site, the erection
work started.
The mill took enough precautions in making
foundations also.
All the rolls were sent for regrinding and dynamic
balancing.
171
172. Hydraulic Cylinders Checking
So far, no problem seemed to appear with the
machine.
But, before installation of hydraulic cylinders, the
maintenance team decided to open up and check
the hydraulic cylinder internals.
The primary aim, however, was to make a drawing
of internals seal kits, so that the order for spare
seals can be placed.
172
173. Hydraulic Cylinders
173
Basically, each different supplier of hydraulic
cylinder prefers to use own design of seals.
This way, to get a new set of seals, you need to
have its drawings.
After drawings are ready, the seals can be made,
but the whole process needs a few days.
Mill maintenance team wanted to have a set of
seals as spare to avoid future downtime increase.
174. Cause Appeared
174
while checking the hydraulic cylinders, they
observed that the seals of one cylinder were
cracked badly.
The liners of this cylinder were also worn out.
The reason behind the problem faced earlier mill
was now clear.
175. Some Defects Make You Happy
Everybody concerned was next to happy to see the
condition of hydraulic cylinder.
After all, they got the root cause of the problem.
Finally, the mill procured new cylinders, and the
supercalender was started with the new cylinders
only.
175
176. Uniform Loading
176
While the supercalender was started, it ran trouble
free, and even the first real produced was of
acceptable quality.
177. Lesson:
177
The earlier mill was probably too overconfident on
hydraulic cylinders, and did not check the cause of
the problem properly.
Unable to identify the problem, the mill decided to
make a new foundation thus investing lot of money
was well as downtime.
In all it suffered significant losses and even got
permanently closed.
179. About MG Cylinder
179
A large drying cylinder for a paper mill needs to
be replaced. Its dimensions are-
Face: 3200mm
Diameter: 4267mm
Shell Thickness: 75mm
Dish End Thickness: 65mm
Tie Rods: 80mm X 12 Nos.
Ribs: 8 nos. each side X 42mm thick
Journal Diameter: 320mm
Weight: Approx. 42-45MT
181. MG
181
It is a carbon steel fabricated cylinder.
As the diameter is large, the shell is fabricated by
welding two steel plates together.
The typical location of welding joints can be seen in
the next sketch.
183. Joints on Surface
183
Welding on the surface results in uneven drying on
paper due to operational deformation of shell near
welding joints.
Hence, stress relieving is done to remove stresses
developed during the plate welding process.
184. Stress Relieving
184
For stress relieving, the temperature of the whole
cylinder needs to be increased gradually to 620⁰C,
and then reduced slowly.
For this you need a big furnace in which such a
huge equipment can be put.
Let’s review it as a practical assignment problem for
students.
185. A Practical Case
185
Your mill has placed the order to the manufacturer in
Ahmedabad, but, such huge furnace is available at
Vadodara.
Ahmedabad & Vadodara are two cities around 350km
apart.
The process flow sheet looks like-
Fabricate
Transport from Ahmedabad to Vadodara for SR
Get SR Done
Transport back to Ahmedabad
Get it machined
Packing & dispatch
186. Costs Involved
186
This way, the additional costs involved include-
Loading & Unloading at both places.
Transportation to and from
SR Charges
Typically, these aggregate to nearly 10-15% of
landed equipment cost.
187. A Cheaper Alternate
187
While you are busy with negotiating with transporters,
the manufacturer comes up with a simple alternate-
The cylinder shall be covered with glass wool blanket.
From the manhole, HSD and combustion air shall be injected
and fired inside the cylinder.
Exhaust gases will come out from the same manhole and the
journal steam entry opening.
Cost for SR alone shall be nearly the half. No
transportation.
However, in place of reaching a temperature of 620⁰C, it
could be around 500⁰C or so.
188. Market Trends
188
As per the supplier, only a few mills opt for Vadodara
option, as in paper mills, the cylinder has to operate at
a maximum temperature of 170⁰C -180⁰C only.
Most mills follow the same path and save money.
A few mills in the past had decided not to get the SR
done, and use the cylinder after fabrication and
machining only. Their cylinders are also running well.
Now, you have to decide!
189. The Choices are-
189
Get SR done at Vadodara.
4.0-8.0 L (Incl. transportation)
Get SR done at Ahmedabad only.
1.0-2.0 L
Go ahead without SR.
Free!
Your answer? Ahmedabad or Vadodara
Why?
190. Clues
190
As you want to decide, you may ask the supplier
about the quantity of HSD expected to be
consumed for SR in both process.
An In-Situ process reportedly needs around 300-
400 lit., while in furnace, it may need around 1000-
1200 lit., as the furnace is really big enough.
Does HSD consumption figure help you in deciding?
191. How to Check Temperature Achieved?
191
In both cases, the person doing SR will mount three
sensors on the surface of cylinder, and the readings
shall be recorded on a plotter.
He is ready to show you the calibration certificate
also.
Still, if he can manage to display higher readings
on plotter, can you check it? How?
192. Temperature Cross Checks
192
Color of the job
At temperatures above 400⁰C, the color of object can
be used to roughly estimate the temperature.
Use of magnet
Magnet loses magnetism at temperatures beyond
300⁰C. This property can be used to roughly cross
check the rate of temperature increase.
193. Temperature Cross Checks
193
TEMPINDIC Crayons
Various temperature indicative crayons are available in
the market. Just mark something on the metal with a
crayon of suitable range, and after SR, the smearing of
mark will indicate that the desired temperature was
reached.
Fuel Consumed
Combustion calculations may be used to evaluate the
theoretical fuel consumption to achieve the specified
temperature.
194. Combustion Calculations
194
The weight of the object is known.
As the temperature of the object has to be
increased rapidly and for practically smaller
duration, the only a part of refractory will get
heated. The same may be considered accordingly.
Now, using mass multiplied by specific heat
multiplied by temperature rise, one can calculate
total heat required.
195. Combustion Calculations
195
As the combustion gases exit the system at
temperature higher than the object temperature, the
overall heating efficiency needs to be taken into
account.
With these, ne can find total heat input required.
Using the calorific value of fuel used, one can
calculate the theoretical quantity of fuel required.
In case the actual fuel consumption is much less, one
may doubt about the temperature actually
achieved.
196. Additional Points
196
In case of internal firing, you need to supply
combustion air, fuel to the equipment, while the
combustion products must escape out the cylinder
simultaneously.
You need nearly four hours to increase temperature
@100-150⁰C per hour.
The cylinder has two openings, first the manhole,
and second the 125mm bore inside one of the
journal (for steam entry).
197. Internal Firing
197
Are these two openings sufficient enough to
accommodate the air supply and exhaust gases
release from the cylinder?
It is not important what the mill did. It is more
important that you understand the process of
making decisions.
198. Lesson
198
Thorough evaluation helps making your decisions
faster and firmly.
Making decisions based on proper calculations
saves you from choosing a wrong alternate.
200. Feedback
200
Feedback is an important aspect for updating
ourselves.
If you liked this e-book, please do share your views
with your colleagues and other industry persons,
who might get benefitted with this.
If you wish to share any comment, further
improvement suggestion for the book etc., just feel
free to write the author at-
deveshksinghal@gmail.com
201. About the Author
201
Born in 1968, D K Singhal is B.E., M.E. (Pulp & Paper, 1993) from Deptt. of
Paper Technology, University of Roorkee (now IIT, Roorkee). He was
certified as Certified Energy Auditor in 2004 with Bureau of Energy
Efficiency. In addition to practical experience in papermaking, project
planning, research and development, consultancy, automation etc.; energy
conservation and cost reduction have been his main areas of concern.
With nearly 100 publications, he has emphasized on development of low
cost technologies and management practices for quality and profitability
improvement. With publications on energymanagertraining.com and
paperonweb.com, he has been constantly contributing to IPPTA. He is also
serving IPPTA as a member of Editorial Board and has served as a member
of its Executive Committee in the past.
He can be contacted at deveshksinghal@gmail.com
D K Singhal
Digitally signed by D K
Singhal
Date: 2018.01.28 17:20:31
+05'30'
