yankee pressure test tappi

8/10/2019 Yankee Pressure Test TAPPI

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Vol. 78, No. 9 Tappi Journal 97

Yankee Dryers

The hydrostatic pressure test isperformed by filling the dryerwith water and increasing the pres-

sure to 1.52.0 times the design pres-sure, depending on the specificpressure-vessel code. The dryer is

filled and pressurized with waterbecause the potential for explosivefailure of a cracking cylinder during

this test is greatly reduced with theincompressible medium.

Hydrostatic testing is a fairly rel-evant testing procedure for a pres-

sure vessel that will only be exposedto an internal pressure. A Yankeedryer, on the other hand, has a much

more complicated loading situation.The dryer is pressurized with hot

steam and cooled on the outside ofthe shell by the wet sheet. This in-

duces large thermal stresses in theshell. The Yankee dryer is also arotating pressure vessel that is me-

chanically loaded from the outside

with the linear load from one or twopress rolls. In Fig. 1, the relative

magnitude of these loads can be seen.

In this case, the load from internalpressure is only 23% of the total load.

These facts reduce the relevancy

of the hydrostatic testing of a Yan-kee dryer; in some cases, it maycause more harm than good.

Dryer deformation

Figure 2illustrates the deformationof the Yankee dryer during the hy-

drostatic test, which means defor-mation from internal pressure only.

This load causes a bending momentin the shell and head flange areathat attempts to increase the angle

between these parts at the flangeinterface.

Figure 3illustrates the deforma-tion of the Yankee dryer during op-

erating conditions. The significantdeformation here is the thermal de-

formation caused by the wet sheeton the shell surface. This reduces

the diameter and length of the shelland causes a bending moment in theflange corner that decreases the

angle between the shell and head atthe flange interface.

Stress distribution

Figures 4 and 5provide two plots ofthe maximum principal stress in the

flange area, with the hydrostatic testcondition shown in Fig. 4 and therunning condition in Fig. 5. The dark

areas in these plots indicate highstresses.

These plots clearly show that thestress distribution is quite different

for these two scenarios. During hy-drostatic testing, the high stressesare located on the inside of the shell

flange and extend into a regionaround the first grooves for a ribbed

dryer. For the operating condition,the stresses in this area are very low

and mostly compressive. The hightensile stresses are, in this case,found on the outside of the shell.

This is an effect of the thermal stress,caused by the temperature differ-

ence through the shell wall, that cre-

ates a bending stress in the shellwith compressive stresses on the in-side and tensile stresses on the out-side.

The hydrostatic test requirementmakes the design of the flange area

more difficult, as two different loadscenarios must be considered. For

running conditions, the currentbending forces tend to open a gapbetween the head and shell at the

Hydrostatic testing of Yankee dryersBengt Unneberg

ABSTRACT:The hydrostatic pressure testing of a Yankee dryeris performed as a fitness-for-service test made before delivery ofthe Yankee. In some countries, there are also statutoryrequirements for on-site hydrostatic testing after the delivery, aswell as periodic testing of the cylinder during its lifetime.Hydrostatic pressure testing is discussed, and the particular case

of a Yankee dryer failure during on-site hydrostatic testing isdescribed.

KEYWORDS:Drum driers (drum dryers), failure, hazards,hydraulic pressure, pressure, test methods, yankee driers (Yankeedryers).

Unneberg is tissue machinery application engineer, Valmet-Karlstad AB, Box 1014,S-65115 Karlstad, Sweden.


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98 September 1995 Tappi Journal

Yankee Dryersouter head radius. This will increasethe risk for crevice corrosion betweenthe shell and head flange.

The loads during hydrostatic test-ing are just the opposite of those for

running conditions, so a design com-promise must be developed that

keeps the deformations within ac-ceptable limits for both. If this is notdone, a gap may develop in the spigot

fit area on the inside of the flangeconnection at the shell-flange inner

radius.This gap will only be present dur-

ing the hydrostatic test, but it maycause permanent deformation thatcould cause problems in the future

steam leaks, for instance.The hydrostatic test is examining

the dryer and demanding design con-siderations for a completely differ-

ent load scenario than what the dryerwill be exposed to during its operat-ing lifetime. The primary justifica-

tion for a hydrostatic examination isto establish that no major flaws exist

in the materials of construction andto inspect the dryer for eventual

leaks.

On-site hydrostatic testing

The previous discussion describedhydrostatic testing that is routinely

performed by the manufacturer be-fore delivery of the Yankee dryer.

Some countries, such as Italy andRussia, also require an additional

hydrostatic test after delivery of theYankee to the mill. The purpose ofthis test is to determine whether any

damage was inflicted during trans-portation to the mill site.

Some countries also require a hy-drostatic test at regular intervals

during the lifetime of the dryer. Oth-ers require a hydrostatic test evenafter relatively minor repair work

on the dryer.These on-site hydrostatic tests in-

troduce some new concerns aboutthis procedure, since mills normally

do not have the same control overthe testing process as the manufac-

turer, which does this procedure rou-tinely.

These new concerns include: The enormous weight of water

upwards of 300,000 lb in large Yan-

keescan require structuralanalysis and additional structuralsupport for the vessel.

The Yankee cannot sustain theadditional water weight through

its bearings and, therefore, ar-rangements must be made to sup-

port this load through the heads.Without proper support of the

Yankee, it can be irreparably dam-aged.

Care must be taken to ensure thatthe water used does not carry or

create deposits that may cause anonuniform barrier to heat trans-fer.

The monetary costs and downtime

to perform the structural analy-sis, provide adequate building andYankee support, perform the hy-

drostatic test, and internally clean

the Yankee are considerable.

Case study

Lets look at a case study from aYankee dryer in a Russian mill as an

example of the potential danger in-volved with these on-site hydrostatictests.

On this dryer, the mill had prob-lems with two minor steam leaks be-

tween the head and shell flanges andwas injecting sealant through pre-

drilled holes in the head. After thisrepair, the Russian authorities re-

quired a new hydrostatic test.The Yankee was manufactured in

1987. The diameter was 5000 mm (195in.), and the shell length was 4850mm (191 in.). The design pressure

was 900 kPa (130 psi). This dryerwas hydrostatically tested the first

time at 1350 kPa (196 psi), which is1.5 times design pressure, before de-

livery. Then, according to Russianrules, it was hydrostatically tested a

second time after arrival in the mill.This was done in December 1988. InJanuary 1993, steam leaks were

sealed, and the dryer was hydrostati-cally tested a third time.

Before filling it with water, theYankee was unloaded from its bear-

ings and supported with cradles thatwere positioned underneath the shellin the flange area on the front and

back sides. The water pump that was

used to fill the dryer had a capacityof 730 L/min at 2100 kPa (190 gal/min at 305 psig) or 420 L/min at 3750

kPa (111 gal/min at 554 psig).This means a pressure capacity

was possible of up to 4.2 times the

maximum allowable working pres-sure of the dryer. A manual pump

was also installed in the system that,after filling, would be used to increase

the pressure to its final value.

1.Yankee loads


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Vol. 78, No. 9 Tappi Journal 99

The dryer was made ready for thehydrostatic test but was not filled

with water because it was gettinglate in the day. Plans were made for

the testing to be done the next day,and preparations would continue thenext morning. However, during the

evening, the plant employees began

to fill the dryer with water to savetime the next day.The filling was initially made

through the open manhole, which waslocated in the 12 oclock position. Ataround 10 p.m., the manhole cover

was closed and the filling continued.The air evacuation in this stage was

done through a 10-mm-diameter holein one of the head-to-shell bolts.

Abo ut an hou r later, the dryer

cracked. No personnel were near thedryer at the time.

It was not known at what volumeor pressure the pump was operating

when the failure occurred, but a rapidpressure increase is expected whenthe dryer is filled and water starts to

be evacuated through the 10-mm air-

venting hole, or if this hole becomesplugged by some foreign material.Later calculations indicated that at

about 2500 kPa (363 psi) the stressesin the dryer reach a critical level,which is also the pressure level that

was expected to be created by thepump.

Here is a list of the resultingcracks in the dryer:

2.Deformation during hydrostatic test 3.Deformation during operation

Drive-side journal cracked at in-ner flange radius

Six journal-to-head bolts brokenon tending side

Radial cracks at Bolt 135 on tend-ing-side head

Crack across shell at Bolt 135

Circumferential crack in tendingside at second groove

Circumferential crack in drive sideat first groove.

This list is written in the probableorder in which the cracks developed.

It suggests that the initial crack prob-ably occurred in the center flange,

followed by the heads being pushed

4.Stresses during hydrostatic test 5.Stresses during operation


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100 September 1995 Tappi Journal

Yankee Dryers

out and the breaking of the journal-to-head bolts. From this point, ra-

dial cracks progressed in the headout to the shell flange, where theaxial crack started and progressed

over the full width of the cylinder.

From this axial crack, the two cir-cumferential cracks were initiated.These cracks started close to the

outer grooves in the shell that is, asshown earlier, one of the areas of

highest stress during a hydrostatictest.Figure 6 shows the position of

the crack in the center flange, also avery highly stressed area during a

hydrostatic test, and the cracks inthe tending-side head.

6.Cracks in drive-side journal and tending-side head

Six boltscracked

Tending-side head

Drive-side journal

7.Tending-side cracks in shell

Second groove

Tending-side shell

8.Drive-side cracks in shell

First groove

Drive-side shell

Figures 7 and 8show the crackpattern in the tending side and the

drive side of the shell.

Summary

The case study clearly establishesthe danger inherent in on-site hy-drostatic testing. When we combine

that danger with the costs associ-ated with hydrostatic testing and

compare them with the informationgained by performing the tests, therisk and costs are far too high.

With improved calculation meth-ods and material investigation pro-

cedures, the value of the on-sitehydrostatic test has changed over

the years. Global acoustic emissionexamination and a variety of mod-ern and sophisticated local nonde-

structive testing procedures providegood alternatives to hydrostatic test-

ing. These examinations should beperformed by experienced, qualifiedtechnicians using a detailed written

procedure.

For all these reasons, the YankeeDryer Safety Committee stronglyfeels that in any case where a re-

quest for an on-site hydrostatic testis made, a discussion should be initi-

ated with the involved authorities totry to substitute the hydrostatic test-ing with a safer and more appropri-

ate testing method. TJ

Received for review July 11, 1994.

Accepted Nov. 11, 1994.

Presented at the TAPPI 1994 Engineering Con-ference.

yankee pressure test tappi

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