Alternate thoughts on yankee stress relieving

AlternateAlternateThoughtsThoughts
ononYankeeYankee
Stress RelievingStress RelievingStress RelievingStress Relieving
By:
D K Singhal
deveshksinghal@gmail.com

ObjectiveObjective
During manufacturing, some steps are
taken to improve the performance of
yankee over its lifetime.
However, some of the steps may be
unnecessary, some requiring more focusunnecessary, some requiring more focus
while some are really useless, and
occasionally harm more than their
benefits.
The present work is aimed to discuss
some of these.
AlternateThoughts onYankee Stress
Releiving,August, 2014

Design of ConventionalYankeeDesign of ConventionalYankee
TypicalTypical
Yankee.

Overview of ProcessOverview of Process
T
• InitialTesting
P
• Plate & Dish Ends Bending
P
• Plate Bending &Welding
• Dish Ends & JournalWeldingD
• Dish Ends & JournalWelding
R
• RibsWelding
T
• Tie Rods Mounting
S
• Stress Relieving
G • Grinding

Stress RelievingStress Relieving
The major problem with stress relieving is
that though it is done to remove
unwanted stresses, yet, useful stresses are
also removed in the SR process.also removed in the SR process.
Let us see how-

Force on Dish EndForce on Dish End
Steam pressure inside yankee imparts
great forces on both dish ends.
The force can be calculated as a
multiplication of area and steam pressure.multiplication of area and steam pressure.
Typical values exceed 500-700Tons.
As a result, dish ends tend to expand.

Expansion of Dish EndsExpansion of Dish Ends
Due to the geometry, expansion tends to
increase diameter of dish ends.
As a result, yankee ends tend to swell.
To counter this process, ribs are weldedTo counter this process, ribs are welded
on dish ends.
Furthermore tie rods are used to connect
(tie) both dish ends.

Tie RodsTie Rods
Again, the tie rods expand with increase
in temperature during operation.
That is why it becomes important to
mount these tie rods under tension.mount these tie rods under tension.
For this tie rods are stretched by using
cap nuts at both ends.After the tie rods
are mounted properly, nuts are welded
with tie rod and dish ends for safety.

Tie Rods Under TensionTie Rods Under Tension
This way, tie rods remain under tension
for long duration.
Due to increased temperature inside
yankee during operation, this tensionyankee during operation, this tension
keeps on reducing slightly, however, the
behavior is very slow at operating
temperatures.

Tie Rod Tension and SRTie Rod Tension and SR
The problem appears with stress relieving
after yankee is fabricated completely.
At 620-630˚C, almost all (>90%) the
stresses are removed.stresses are removed.
This way, the benefits of mounting tie
rods under tension are lost completely.

Partial SRPartial SR
Some yankee suppliers have initiated a
shortcut for SR.
They insulate yankee from outside and
fire it from inside.fire it from inside.
This being a crude method does not give
adequate results in terms of yankee
surface de-stressing.
That is why, I have given the heading as
“Partial SR”.

Internal HeatingInternal Heating
Requirement of heat can be calculated for
raising the temperature of a yankee cylinder
from ambient to SR condition.
For the same, quantity of fuel can also be
calculated.
For this amount of fuel, requirement of
calculated.
For this amount of fuel, requirement of
stoichiometric air can be calculated.
It can be shown that the calculated quantity
of air cannot be fed to yankee through
manhole, specially, if the exhaust has also
being expected to come from the same
manhole.

Effect of Internal HeatingEffect of Internal Heating
In fact, in most of the cases, temperature
does not increase more than 300-320˚C
by internal heating.
At these temperature levels, only lessAt these temperature levels, only less
than 10-15% stresses are removed.

So,Which SR is Better?So,Which SR is Better?
Normally, one has to choose between the
two options. One does more harm than
benefits, and the other does nothing
except wasting money.except wasting money.
I think, it becomes necessity to list our
priorities.

Why SR?Why SR?
Remove Surface Stresses
Reduce Welding DefectsReduce Welding Defects
Maintain Tie Rod Stresses

Surface StressesSurface Stresses
Surface stresses develop during the
bending of plates and plate to plate joint
welding.
Plate to dish end welding stresses do notPlate to dish end welding stresses do not
result in significant problems.
This way, priority is surface to surface
welding joint, and the surface to dish end
welding joint becomes secondary.

Welding DefectsWelding Defects
As far as welding defects are concerned,
today good welding techniques ensure a
defect welding joint.
Furthermore, quality of weld can be easilyFurthermore, quality of weld can be easily
checked by UT (ultrasonic testing) and
MPT (magnetic particle testing).These
tests do not cost much and can be done
easily.

Tie Rod StressesTie Rod Stresses
Protection of tie rod stresses remains
problem with conventional SR.
Is it possible, if we do the SR withoutIs it possible, if we do the SR without
having tie rods mounted?
Or, if we do the SR with only a few tie
rods mounted?

SR Without Tie RodsSR Without Tie Rods
Only the area
in the vicinity
of welding
joint isjoint is
subjected to
SR.

SR with Few Tie RodsSR with Few Tie Rods
Another approach is to mount only a few
say 4, tie rods mounted, and carrying out
SR as usual.After SR is done completely,
these tie rods are removed and all the tiethese tie rods are removed and all the tie
rods are mounted in position.
In the next sheet, a comparison between
the two is given.

ComparisonComparison
SR of Shell
Only
Treats shell
SR with few
Tie Rods
Treats shell as
Treats shell
only.
Uses lower
cost and time
Treats shell as
well as shell to
dish end joints.
More cost and
time is
required.

InvitationInvitation
Since long, yankee cylinders have been made
of cast iron. However, in India, there are
many manufacturers who are making carbon
steel fabricated yankee cylinders.
Worldwide also, some new companies haveWorldwide also, some new companies have
emerged over the past couple of decades
who have gained a good reputation in
supplying good quality and better results.
Through this presentation, I invite all
concerned professionals to develop a good
SR approach for yankee cylinder.

ThankYou.ThankYou.
D K Singhal
deveshksinghal@gmail.com

Alternate thoughts on yankee stress relieving

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