Alternate thoughts on yankee stress relieving

Alternate Alternate Thoughts Thoughts on on Yankee Yankee Stress RelievingStress RelievingStress RelievingStress Relieving

By:

D K Singhal

[email protected]

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 focus unnecessary, 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.

Alternate Thoughts on Yankee Stress Releiving, August, 2014

Design of Conventional YankeeDesign of Conventional Yankee

TypicalTypical

Yankee.


Overview of ProcessOverview of Process

T• Initial Testing

P• Plate & Dish Ends Bending

P• Plate Bending & Welding

• Dish Ends & Journal WeldingD

• Dish Ends & Journal Welding

R• Ribs Welding

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 500-700 Tons.

� 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 welded � To 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 tension yankee 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 yankeesurface 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 less � At 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 not � Plate 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 easily � Furthermore, 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 without � Is 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 is joint 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 tie these 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 have � Worldwide 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.


Thank You.Thank You.

D K Singhal

[email protected]


Date post:	11-Nov-2014
Category:	Technology
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Alternate thoughts on yankee stress relieving

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