Stress Corrosion Cracking of AmmoniaStorage Spheres: Survey

and Panel Discussion

Can damage done by stress corrosion become detectable yearsafter the process conditions have stopped to be favoring stresscorrosion cracking? Is there an optimum water content?

Jan M. BlankenUKF, Umuiden, Holland

INTRODUCTION;

Some of you may remember that I was the Session Chair-man of the Panel Discussion on Stress Corrosion Crack-Ing of Ammonia Storage Spheres at the Symposium In LosAngeles last year.

During the discussion so much Information was exchangedbetween experts In the field that an outsider may havebecome confused.

In this Paper I w i l l try to summarise the results ofthe Survey and the discussion.

The subject Is controversial and therefore, when tryingto be objective, difficult to explain systematically. Iw i l l not try to be objective but w i l l give you my ownpersonal views.

The Paper w i l l be limited to stress corrosion crackingas experienced In ammonia storage spheres.

WHEN DO YOU GET STRESS CORROSION CRACKING?

Figure No. 1 shows when you may get stress corrosioncracking. As the name Implies, you require:

- tensile stresses which are sufficiently high -stress,

- oxygen and water In certain concentrations - corro-sion,

- the temperature has to be above -33"C (-28T).

Stress Corrosion Temperature

Stress H20ppm

risk of stres:corrosioncracking

»Temp.

Figure 1. Stress corrosion cracking.

Now let's look at the requirements Item by Item:It has been found that with low tensile strength steela stress of yield point magnitude or higher Isrequired. For higher strength steels higher stressesare required to initiate cracking, although most likelycrack propagation can proceed at lower stresses.

Overall, higher strength steels are more vulnerable forstress corrosion cracking than low strength steels; itseems as if the crack propagation rate is higher.To try to illustrate this point, in Europe we now findcracks in non-stress relieved ammonia pressure vessels

140

Not to scale

Not to scale

Stress-^stress required to get stress

corrosioncracking

high tensileI strength steel

low tensilestrength steel

Strain

Crack propagation rateRisk of stress corosion cracking

Ultimate tensile strength

Figure 2. Effect of stress and material properties.

built 30 years ago from low strength steel - and wefind cracks In non-stress relieved ammonia pressurevessels made of high strength steel less than 10 yearsago, but this may be an over-simplification.

UKF condemned 12 rail-tankers made of high strengthsteel which had been carrying ammonia without water andwhich had experienced stress corrosion cracking becausethey were not properly stress relieved.

All laboratory experiments have shown that. If the li-quid ammonia contPlns 0.2? water, this ammonia w i l l notcause stress corrosion cracking with the steels usedfor building spheres and oxygen contents normally ex-perienced.There are Indications that It could be that you do notexperience stress corrosion cracking with very low wa-ter contents either, say In the ppm range.

The next figure shows the Influence of oxygen and watercontent on the risk of stress corrosion cracking:

log0.2%2OOOppm

Tigh tensile strength steelow tensile strength steel

Risk of stress-corrosion-cracking

\

'~t*u.t

••- log 02

Figure 3. Effect of oxygen and water content.

As you need oxygen to get stress corrosion crackingthere w i l l be a lower limit for the oxygen content fi-gures of 2.5 ppm and 1 ppm have been mentioned forlow and high strength steel respectively.

Whether the envelopes for low strength and highstrength material are really as shown Is unknown.

Figure No. 4 shows the Influence of temperature:

%

1OOStress corrosion cracking

Ultimatetensilestrength

Not to scale

-30 -2O -1O O 1O 2O 3O°CTemperature

Figure 4. Effect of temperature.

141

The result of the survey has shown that with pressuresof the sphere 0-3 bar g on average -14°C, only 1 sphereout of 16 experienced stress corrosion cracking, where-as from 3-6 bar g, 20 out of 38 experienced stress cor-rosion cracking; above 6 bar g, 15 out of 17 expe-rienced stress corrosion cracking. These figures havebeen plotted.

Also plotted Is nil percent for the atmospheric pres-sure temperature of -33°C (-28°FÎ.

I do not know of any stress corrosion cracking beingfound In an atmospheric storage tank.It should be realised that for a given contamination ofthe vapour In ppm wt 02 the contamination of the liquiddecreases considerably at lower temperature because ofthe lower solubility and partial pressure.

Also plotted Is the percentage of spheres that exportthe vapour from the sphere. The graph shows that atlower pressures nearly all spheres export the vapour.

One could therefore argue that the fact that hardly anystress corrosion cracking Is experienced with lowerpressure spheres Is not caused by the lowerpressure/temperature, but by the fact that vapour Isexported. Thereby removing any oxygen.

The experience within my Company Is In line with this.We have Inspected four atmospheric storage tanks about10 years after commissioning and did not find anystress corrosion cracking.

We operate four ammonia storage spheres - three ambientwithout vapour export and one at low pressure with va-pour export. We found stress corrosion cracking In thethree ambient temperature spheres and no stress corro-sion cracking In the low temperature/ pressure sphere.

Plotted also Is the ultimate tensile strength of thematerial of construction of the sphere.

Because of the fact that there Is a tendency for thespheres In the U.S. to be designed for low pressure andmade of low strength material - and In Europe and Japanto be designed for higher pressure and made of higherstrength material - one could also argue that the lowerrisk at lower temperature Is caused by the lowerstrength materI a I.

Also taking Into account laboratory experience I per-sonally think that all three, viz low temperature, va-pour export and low strength material, contribute.

WHERE WE CRACKS - CAUSED BY STRESS CORROSION CRACKING'NORMALLY FOUND7

Stress corrosion cracks are normally found where thesum of design stress i- residual welding stress Is high-

est - that means In the weld metal or In the heataffected zone, or where welding cleats have been.

HOW CAN YOU FIND CRACKS CAUSED BY STRESS CORROSION?

I have learnt that the only way to find most of thecracks caused by stress corrosion Is to Inspect thesphere from the Inside by a magnetic particle method.

Do not ask me whether you should use D.C. or A.C. mag-netic yokes; white paint or fluorescent Ink, because Ido not know, and I do understand that opinions vary.

There are metallurgists who strongly believe that youcan find cracks caused by stress corrosion cracking(which may lead to leakage or failure) by other techni-ques such as acoustic emission.

Those who have never Inspected their spheres should doso at the first possible opportunity.

WHAT MIGHT BE DONE TO REDUCE THE RISK OF STRESS CORRO-SION CRACKING?

Keep Tensile Stresses In the Material Down;

By constructing new spheres of a low strength carbonsteel using a low strength weld rod, and check the weldfor hardness.

By using welding cleats on the outside of the sphereonly.

By decreasIng the res I du a I we IdIng stresses by stressrelieving crown plates, leg column plates and bottomplates In a furnace, and use preheat when making theother welds,or: by stress relieving the whole sphere.

More than a hundred spheres have been stress relievedafter construction of which more than 10 spheres areused for storing ammonia.

Figure No. 5 shows how this Is done:

Top manholeInsulation

Controlroorn Burner Fan

Figure 5. Whole body annealing.

142

Tensilestrength

Compr.stress

-Surface of steelNot to scale Not to scale

Depth Q5mmO.02inch

Figure 6. Shot peening inside a sphere.

The sphere Is Insulated on the outside and a burner IsInstalled In the bottom manhole.

The temperature Is controlled by controlling the amountof excess air of the burner.

Temperature distribution dur Ins this type of stressrelieving has been found to be very good.

By decreasing the residual welding stresses bypeening all welds and heat affected zones.

Figure No. 6 shows the result of shot-peening the In-side of a sphere.

CompressIve stresses are Introduced In a 0.5-1 mm(0.02-0.04") layer of the shell of the sphere.

These compressIve stresses compensate the tensilestresses caused by design and welding.

A disadvantage of shot-peenlng compared with stressrelieving Is that the stresses are decreased only In a0.5-1 ran thlcR protective layer Instead of throughoutthe whole shelt.Sho-t-peenîng therefore has to be done on smooth surfa-ces to make sure there Is no crack In the protectiveIayer.

Stay Outside the Envelope In which there Is the Risk ofStress Corrosion Cracking;

By opérâttng the sphere In Area A where the oxygen con-

log

!. B

Risk of stresscorrosion-.cracking

log O2

Figure 7. Stay outside the envelope in which thereis the risk.

tent Is too low to get stress corrosion cracking.

This requires that:

- sphere Is purged carefully when commissioning,- transport vessels are purged carefully when commis-

sioning,- the pressure at the suction nozzle of refrigeration

and holding compressor Is positive,If possible, vapour Is exported from a semt-refrlgerated sphere.If possible, also the vapour space of an ambienttemperature sphere Is purged.

By operating the sphere In Area B where the water con-tent Is too high to get stress corrosion cracking - say0.2?.

This operating practice Is frequently used In the U.S.

This can be combined with A - that means low oxygencontent and high water content - Area A/B.By operating the sphere In Area C, where the water con-tent may be too low to get stress corrosion cracking,say a few ppm.

This Is Impossible for storing most of the ammonia pro-duced In the world as the water content of the ammoniaas produced Is too high.

Keep Temperature of the Ammonia Down;

If the pressure/temperature can be decreased this w i l l

143

help In avoiding stress corrosion cracking.

IF WE THINK VE KNOW ALL THIS, VXAT THEN IS THE PROBLEM?

The fact that we do not know answers to al I questionsyet Is perhaps best Illustrated by the following exam-ple:

A Company operated an ammonia storage sphere since1968. They Inspected the sphere In 1981 and found568 cracks caused by stress corrosion and had toreplace 12 n? (130 sq.ft.) of the shell of the sphe-re. They started adding water and hydrazlne and whenthey Inspected the sphere again In April this year,they found 565 cracks, and had to replace 7 nr (70sq.ft.) of the shell of the sphere.

Now why Is that?

Let's look systematically at some areas which are dif-ficult to understand.

Problems with Decreasing Stresses;

Effect of Stress Relief;

It Is generally accepted that stress relieving an ammo-nia storage vessel - made of a steel which does nothave too high a strength - In a furnace, avoids stresscorrosion cracking.

However, the survey showed that cracks have been foundin spheres which were stress relieved after stress cor-rosion cracking had been experienced.

The next figure shows the result of the survey:

0

1234

1 6S 7

i 8

i 9

* 10B« ng «" 13

M151617

92

SE0x0

0x0

107

SR

0x0

138

sa

no Secfoundafter16 yearsofopera-

tion

•n?« v** HJ u^pui wi wj\.tv3 in unit

Sphere number139 1 27 76

IbeforePTatterPTsa

no Secfoundafter16 yearsofopera-

tion430x3

34x1SR2x

0x0

0x0

0x0300x5

67x3

60x2

5x2

1000x20SB

x5x5

77

x16x22

SRsg*3-

29x11x 7

x5l25x 2

x16x 5

x19.

xll x14x9!300xx 8 1200x16x8|

78

e--9CC

SR

x3

200 x150x6

SecSR0x0

140

SR afterrepairweldingbecauseof Sec.SRdidnot stopcracking

SR = stress relief PT= pressure test

This shows that no cracks have been found, as yet, Inthe two spheres which were stress relieved Immediatelyafter construction, but severe cracking has been foundIn some of the spheres that were stress relieved afterstress corrosion cracking had been experienced.

Effect of ShotHPeenlng;

In spheres that had been shot-peened both before andafter stress corrosion cracking was experienced, crackscaused by stress corrosion were found.

In 1965 two spheres were shot-peened Immediately afterconstruction. Cracks caused by stress corrosion werefound In these spheres every time they were Inspectedby magnetic particle method except once.When the welds In the bottom part of one of the sphereswere ground smooth and then shot-peened, considerablyless cracks were found.

This sphere was shot-peened In 1976 after stress corro-sion cracking had been experienced, before shot-peenlngall welds were ground flush.

In 1980 some minor cracks were found which were groundaway: these areas were not shot-peened again. After theInspection the sphere was pressure tested at 1.5 x thedesign pressure and not Inspected again after the pres-sure test.

In 1982 cracks were found In the shot-peened areas - nocracks were found In the areas where the cracks hadbeen ground away In 1980.

Possible Explanations;

Now let's look at some possible explanations:

The site stress relief of the sphere was not up tothe stan-dards requI red.

Original shot peenedlayer 1976cracks found19821

Layer taken away198Ono cracks found1982

Figure 8. Effect of stress relief.

Pressure tested1.5 xafter inspection198O

Figure 9. Effect of shot peening.

144

Sphere No. 27 was stress relieved at a temperatureof 620°C which was maintained for one hour.

The temperature was measured by 67 thermocouples andthe temperature spread was In the order t 5°C,

The material of the sphere has an ultimate tensilestrength of 520-600 Mpa; the wall thickness Is 11.5to 17.5 mm.

The cracks found after stress relieving were In thehorizontal and vertical welds below the equator.

From talking to metallurgists In our Company I havelearnt that they would accept the kind of stress re-lief as used for sphere 27 without hesitation forammonia rail tankers stress relieved In a furnace,and In rail tankers made of this type of materialwith about the same wall thickness stress relievedI n a furnace at about the same temperature for aboutthe same time never have we found stress corrosioncracking.

It Is therefore difficult to explain the cracksfound In sphere No. 27 by assuming that the stressrelief was not up to the standards required.

The spheres - 76, 77, 78 - were stress relieved at atemperature of 575°C during an unknown length oftime. The material Is SI, Mn, Al-flne grain steelwith an ultimate tensile strength of 500-580 Mpa;the wall thickness Is 22-28 mm.

The cracks found after stress relieving were foundpredominantly In the upper part of the spheres.A temperature of 575°C has been used by my Companyfor a furnace stress relief of an ammonia roadtanker made of the same type of material.

The damage was done before stress relieving but thecracks appeared aftar stress relief.

This figure shows the results of the 8th and 9th In-spection of sphere No. 27.

Both these Inspections were carried out by the samepersonnel using the same technique, I.e., A.C.magnetic yokes with fluorescent Ink and ultra-violetlight Inspection lamps.

I have to warn you that what I am going to say now Idid not learn last year, .but deduced from the factswhilst writing this report.

This figure could show that, If you find cracks cau-sed by stress corrosion there wl 11 also be areaswhere you w i l l not find cracks, but where thematerial has been damaged to such an extent thatcracks w i l l form when pressure testing or later Inthe course of time.

In the next figure I have developed this Idea fur-ther:

Part 'a» of the figure shows the maximum depth ofcracks found before and after stress relieving. Itseems that the maximum depths are about the same be-fore and after stress relieving. If this Is notcoincidence, but fundamental. It would confirm thehypothesis.

Part 'b' shows the general Idea before sphere No's27 and 76 were stress rel leved - cracks 3 ma and 20ram deep were found: these were ground away. At otherplaces however the material had been damaged to thesame depth and cracks developed there after stress•-a I lev Ing to about the same depth.

AECI Sphere

PRESSURE TEST ONLY

/ /TMaterialdamaged?Damage notdetectable

No cracks I

9th Inspection \Cracks foundwith moxdepthof 2.5mm'////////c 2.5mm > ///

r S S f ' f S S

AECI Sphereafter pressuretest

Figure 10. A.C. magnetic yokes and fluorescent inkused in both inspections.

SphereNo

92107138139

27767778

140

Maximum depht of cracks in mmBefore stress

relief00003

2022

unknownsevere Sec

After stressrelief

00003

19U6

severe crackingFigure 11 a. Why are cracks being found after stressrelief?

145

Sphere No 27 Sphere No 76

-\ j- Material damagedy Material damaged but crack not

yet detectable=*• Crack detectable

Number of detectable cracks

Conditions favouring stress corrosionYes No

Incu- With pressurebation testperiod { V |—

H\^l

! >A.Withoutpressure test

time years

Figure 11 b and c. Why are cracks being found afterstress relief?

1234S7

1718192021232729303132336972737«7576777879909193

103

105117118119120

1

OXX

XoXXXXX

XXXXXXXXXX0X

X

XX

2

X

3

xlx

4

o xX X 0

o

X

XKXXXXX

o

X

XX

xx

xx

X

XX

xx

xx

5

X

X

XXXXXX

6

X

XX

xx

xx

7

X

XXo

6

X

XX

9

X

K>

X

11

X

12 13

'

Xo

u 15

1i

cracks foundno cracks found

Number of spheresinspected again 25 spheresCracks found again 75 timesNo cracks found 8 timesTotal number ofinspections 83 timesNo cracks found duringlatest inspection 2 spheres

:

i

; , 1

Figure 11d. Inspection after first cracking detection.

Part 'c' then shows what could happen. After stresscorrosion has been Initiated there could be an Incu-bation period before the first crack can bedetected, then In a few years perhaps more and morecracks become detectable until finally the lastcrack has been formed.

Could It be that the length of the Incubation periodvaries with the stress level?

Pressure testing would then bring about a stepchangeIn the appearance of detectable cracks.

For sphere 76 the period that cracks developed wouldthen have to be 10 years which Is very long.

On the otherhand no other methods which were triedto prevent stress corrosion cracking In this periodsuch as careful purging, addition of water andhydrazlne, were successful either.

If this were true, would It Influence the methodsused for calculating whether a sphere leaks beforefracture?

Figure No. 11 d shows the results of the Inspectionsof the spheres after the first time stress corrosioncracking was found.The figure shows that of the 37 spheres that expe-rienced stress corrosion cracking 25 spheres wereInspected again after the first time stresscorrosion cracking was found.The total number of these Inspections was 83, In 75cases cracks were found again and only 3 times nocracks were found. In only 2 spheres no cracks werefound during the latest Inspection.This figure confirms what one panelist said lastyear, namely that trying to cure the stresscorrosion cracking "disease" Is nearly alwaysunsuccesfu 11.

The shot-peenlng of the spheres Immediately afterconstruction was not up to the standards requiredThis shot-peenlng was done on welds which had notbeen ground smooth and this could perhaps explainwhy stress corrosion cracking was experienced.When the welds were ground smooth and then shot-peened again, considerably less cracks were found.

For the cracks found after shot-peenlng, afterstress corrosion cracking had been experienced, onlythe second explanation, given under stress relief,seems to be applicable.

Figure No. 12 shows the experience with sphere No.27 and what could, perhaps, be expected when shot-peenlng and then pressure testing a sphere which hasexperienced stress corrosion cracking.

146

Damage notdetectable

PRESSURE TEST

Sphere Mo. 17

Commissionned: 1970

After shot peening

Either

Damage not/detectable

Cracks not - /detectable but

Sec doesdoes not'////. continue/I

Cracksdetectable;

Sec does/ / /, continue

Figure 12. Experience AECI.

Problems with the Addition of Water;

Although all laboratory exper'ments have shown that«hen the ammonia contains 0.2? water or more, stresscorrosion cracking w i l l not take place, all surveyshave shown that In practice, stress corrosion crackingIs being experienced In spheres containing ammonia with0.2$ water.

Two surveys have shown the stress corrosion cracking tobe predominantly In the vapour area when the' ammoniacontains 0.2? water.

One survey has shown the stress corrosion cracking tobe predominantly in the liquid area when the ammoniacontains less than 0.2? water.

Figure No. 13 shows typical distribution of cracks withlow and high water content.

Table 1 attached shows the distribution of cracks re-ported In the survey.

The few laboratory experiments done In the vapour areahave shown stress corrosion cracking with 0.2? water Inthe liquid.Stress corrosion cracking was first found in the sphe-res my Company operate when the water content of theammonia was low.We Increased the water content to 0.2? or more about

Walercontent: 0.05%Inspected: 1976magn.part. Insp. 100%

Sphere No. 122Commissionned: 1971

Watercontent: 0.3%Inspected: 1974magn.part. insp. 100%

Number of cracks

1 V. 1 crack(s)

37% 32 -

62% 53 "

Number of cracks

24% 127 cracks

58% 307 -

Figure 13. Typical distribution of cracks.

five years ago,three spheres.

but are still finding cracks In all

Possible explanations are:

When stress corrosion cracking was experienced with lowwater content, the water content was Increased andcracky were stl I I being found;This could perhaps be explained by assuming that thedamage was done before the water content was Increasedas explained under stress relief.

The envelop In which there Is risk of stress corrosioncracking Is not as shown until now but as follows:

This would mean that In area A more then 0.2? H^O wouldbe required to avoid stress corrosion cracking«This is more than found In laboratory experiments, thelaboratory experiments having been done say In area Bat a higher oxygen content to keep the time to failuredown«

Not to scale

log

O.2%2OOOppm

H20

£ Risk of \|/stress- \f corrosion-l0^f cracking

-" • B

•— log O2

Figure 14. Envelope in which there is risk of cracking.

147

Not to scale

log H2Oo\ water in bulk of liquid

High waterF7777? *777777y777*777777~/77\

If *K

¥water in condensate Risk of stress-

cor rosion-cracking

L "Ï9

Miter in bulk of liquid JÎ

Low water A

water in condensate

•log 02

Figure 15. Distribution of cracks.

The distribution of cracks with low and high watercould perhaps.be explained as shown In figure No. 15.

The equilibrium water content of the vapour expressedIn ppm (wt) is about 1/5000 of the water content of theliquid also In ppm (wt) at atmospheric pressure. Theequilibrium content Is about 1/500 of the water contentof the liquid in ppm (wt) at a pressure of 7 bar (100psla). This means that the vapour above liquid ammoniaunder pressure containing low water w i l l contain about0,5 ppm (wt), and the vapour above liquid ammonia con-taining 0,2? water w i l l contain about 4 ppm (wt).

Figure No. 15 shows the temperature distribution of anambient temperature sphere, when the temperature diffe-rence between noon and mid-night Is 5°C ~ 10"F.

The figure shows that with the ambient temperature chan-ging 5°C the temperature of the liquid In the spherechanges by less than 1°C.

This msans that during night-time vaporous ammonia w i l lcondense In the top part of the sphere.

Whilst at equilibrium this condensate would have thesame water content as the bulk liquid, because of slowrates of diffusion and the large distances over whichdiffusion w i l l have to occur, the condensate may have avery low water content.

With 2000 ppm of water In the liquid phase and 4 ppm inthe vapour, it Is possible that the condensate may con-tain as little as say 10 ppm of water. And with 250 ppmof water In the liquid phase and 0.5 ppm In the vapouras little as 1 ppm and this could perhaps explain thedifference.

The oxygen content of the vapour expressed In ppm (wt)Is about 10000 times higher than the oxygen content ofthe liquid, also in ppm (wt) at atmospheric pressureand about 500 times higher than the oxygen content ofthe liquid In ppm (wt) at bar (100 psla). Thereforethe oxygen content of the condensate w i l l ten dot behigher than the oxygen content of the bulk of the li-quid.

I do realise that there are a number of loopholes Inthis explanation, but I do not know any better expla-nation: If you think you have a better explanation, youare welcome to mention this during the discussion afterthis Paper.

In one of our spheres a bucket was Installed to samplethe condensate running down the wall of the top part ofthe sphere.Table 2 shows the first preliminary results.Although we underestimated the complexity of the pro-blem and therefore the figures are not yet consistent,the results seem to show that:

- the water content of the ammonia vapour Is low- with sufficient water In the bulk of the liquid to.Inhibit against stress corosion cracking the conden-sate _does_jiot_oojita£n_ sufficient water to inhibitagainst stress corrosion cracking.

Noon 12.5 °C

iheretempérature

Midnight

Figure t6. Effect of temperature change.

SUMMARY

In summary it could be argued that most of the problemsexperienced can be explained by:

- accepting that the stress corrosion cracking"disease" is difficult to cure. This means that onceyou have experienced stress corrosion cracking youw i l l nearly always find cracks with later I-nspec- sv*t Ions, whatever you do.

- accepting that the addition of 0.2% water Ihlbitsagainst stress corrosion cracking but that with 0.2?water in the bulk of the liquid there can be areas inthe sphere such as in vapour area or In hte meniscus

148

where the liquid may not contain sufficient water toInhibit against stress corrosion cracking.

RECOMMENDATIONS:

I think that, as the ammonia Industry, we should spon-sor further research to answer questions such as:

- can damage done by stress corrosion become detectableyears after the process conditions have stopped to befavouring stress corrosion cracking?

- Is the theory as developed to explain cracks In theliquid and vapour area with low and high water con-tent, right or wrong?

- Is there an optimum water content?

As an Industry we have to know how to design, operateand maintain our spheres such that they are safe.

The fol lowing recommendations can, I Think, be derivedfrom the Panel Discussion:

1. If not yet inspected by magnetic particle method.do

this as soon as possible. If operated with 0.2%water

and not recently inspected in the vapour area do this

as soon as possible.

Inspect regularly

Time between inspections to depend on:

material of construction

treatment after construction

operating conditions and

extent sec has been experienced

2. -Purge sphere as carefully as possible when commissioning.

- Purge transport vessels as carefully as possible whencommissioning.

- Check that the pressure at the suction nozzle ofrefrigeration and holding compressor is positive.

— Preferably run the vapour from a semi refrigerated

sphere to a consumer

— Also t y to purge the vapour space of an ambient

temperature sphere.

3. Consider individually whether changing the water

content of the ammonia is appropriate.

4 If it cannot be guaranteed that the ammonia will always

be oxygen free, shot peen or stress relief sphere

This before sec has been experienced.

Figure 17. Recommendations for as-welded existingspheres.

1 Do not use welding cleats or use them onthe outside only.

2 Stress relieve crown plates.legcolumn platesand bottom plates. Use preheat for welding.

3 Do not use either plate material or electrodeswith a high yield strength, check weld tor hardness.

U Use the recommendations as mentionedunder existing spheres.

Figure 18. Recommendations for new spheres.

I think I learned a lot last year and I would like tothank my teachers - the Panelists of the Panel Discus-sion - for the excellent job I think they have done.

REFERENCES;

Panel Discussion of Stress Corrosion Cracking In Am-monia Storage Spheres;

Plant/Operations Progress

Panelists;

Dr. Alan Cracknel I

Fred. F. Lyle Jr.

LIv Lunde

Larry Adams

A.S. Krlsher

Jeff Morgan

Jim Guild

Prof.Dr. Heinz Spahn

Robert S. Brown

J.L. Smith

I.C.I. England.

South West Research In-stitute, U.S.A.

Institute for EnergyTechnology, Norway.

Monsanto Co., U.S.A.

Monsanto Co., U.S.A.

Cooperheat,U.S.A./England.

A.E.C.I. Limited, SouthAfrica.

BASF, Germany.

Agrlco Chemical Co.,U.S.A.

Grace & Co., U.S.A.

A summary of the discussion Is given.

Complete Proceedings of the Panel Discussion areaval(able from:

149

- A.I.Ch.E. Manuscript Center345 E 47th StreetNew YorkNY 10017U.S.A.

- Jan M. BlankenU.K.F. b.v.P.O. Box 463UmuldenHolland

- Logl now A.W. and Phelps E.H.

Stress Corrosion Cracking of Steels In AgriculturalAmmonia:

Corrosion - National Association of Corrosion En-gineers,Vol. 18, August 1962.

Hutch Ings G., Sanderson G., Davles O.G.S., DeweyM.A.P.

Stress Corrosion of Steels In Anhydrous Ammonia;

A.I.Ch.E., Vol. 14, 1971, Page 102-108.

Phelps E.H.

Causes of Stress Corrosion Cracking of Steel InAmmonia:

A.I.Ch.E., Vol. 16, 1973, Page 32-37.

Zeis L.A. Stress Corrosion Prevention;

An Engineering Approach to Stress Corrosion Crack-Ing of Austenltlc Stainless Steel - How It can Oc-cur and Steps to Prevent Its Occurrence;

A.I.Ch.E., Vol 17, 1974, Page 56-60.

Class I. and Gering K., Zur Frage der SpannungsrlszKorrosion durch flüssiges Ammoniak

Werkstoffe und Korrosion 25. Jahrg., Heft 5/1974.

Van Grieken, K.A.,

Cracking In Storage Spheres;

A.I.Ch.E., Vol. 18, 1975, Page 15-18.

• Lyle, Fred F. Jr.,

A Study of Stress Corrosion Phenomena Resultingfrom Transportation of Anhydrous Ammonia In Quen-ched and Tempered Steel Cargo Tanks;

U.S. Department of Transportation Report No»

DOT-FH-11-8568, February 1976.

- Clark W.D. and Cracknel! A.,

Avoiding Stress Corrosion Cracking;

A.I.Ch.E., Vol. 19, 1976, Page 69-72.- Arup H.,

Stress Corrosion Cracking In Ammonia Storage:

A.I.Ch.E., Vol. 19, 1976, Page 73-76.

- Ludwlgsen P.B. and Arup H.,

Stress Corrosion Cracking of Mild Steel In AmmoniaVapour above Liquid Ammonia;

Corrosion Nace, Vol. 32, No. 11, November 1976.

- Cracknel I A.,

Stress Corrosion Cracking of Steels In Ammonia;

Institute of Chem. Eng. Symposium, Series No. 50,1977.

- Jones O.A., Kim C.D., Wilde B.E.,

The Electrochemistry and Mechanism of Stress Corro-sion Cracking of Constructional Steels In LiquidAmmonia;

Corroslon-Nace, Vol. 33, No. 2, February 1977.

- Tetsuya Takegawa,

Preventing of Stress Corrosion Cracking In CarbonDioxide Absorber of Ammonia Plants:

A.I.Ch.E., Vol. 22, 1979, Page 170-177.

- Cracknel I A.,

Stress Corrosion Cracking In Ammonia Storage;

A.I.Ch.E., Vol. 22, 1979, Page 63-68.

- Brown R.S.,

Damage Detection, Repair and Prevention In an Am-monia Storage Sphere;

A.t.Ch.E., Pre-Prlnt, 1981.

- Wilde B.E.,

Stress Corrosion Cracking of ASTM A517 Steel InLiquid Ammonia ; Environmental Factors;

Corroslon-Nace, Vol. 37, No. 3, March 1981.

150

Spähn H. and Steinhoff U.,Bemerkungen zur Zerstörungsfreien Prüfung vonstandortgefer-tl gen KugeIdruckbehaItern ;

VGB, Kraftwerkstechnik 62. Jahrgang, Heft 1, Januar1982, Selten 61 - 66.

Chol H., Beck F.H., Szklarska-Smlalowska Z.,MacDonald D.D.

The Effect of Fluid Flow on the Stress Corrosion

Cracking of ASTM A508 CI 2 Steel and AISI Type 304Stainless Steel In High Temperature Water;

Corroston-Nace, Vol. 38, No. 2, February 1982.

Cracknel I A.,

Stress Corrosion Cracking In Steels In Ammonia;

Session 1981-82, Institute of Refrigeration at theInstitute of Marine Eng., May 6th, 1982.

Low water

Low water—»High water

High water

BLANKEN, J.M.

Table 1. Survey on crack distribution.Mo 2

4.0

-,.CP

2

1

BP

Nov81

M.I.O

OlO

lil.O

tl.I.O

0>0

,0,,,.,

iis.i.o0.0

Ȕ.2.0

Nol

31

'CP

3

2

1

BP

3.5-005

M0.3

1.3

IS»1

S5i3

™

™àIOf.1

lt.!4.3

Whl

No 3. t. 5

t.1 30.2

I.tro*

la. 1

f™".z».^fl

No7

5.7

CP

5

4

3

2

1

BP

*0.0(

1974n.j.i

0.0

0.0

0*0

0.0

0(0

0.0

ISO.3.1

- IM.M

370.*. 0

" 110.*. 1

I3«4.*

- 110.3.0

IIB...S

No 17

S005Dec76

i.».*CP °"°

« - 0.0

5 0.0., * 0.0

4 irf.*s - 0.0

3 *"•»« - ' 2».*

2 «M . ! IliS.Ï

1 '•'k. ». 37,).?

BP] ,.,

15.1.0

' NO 19

7.5!009 tt25 025

'1973 11979 19B1, „.. : ,.3.1 „...v

1

-j- ' iO*

2

1

BP

1 0.0 fj,f) 0.0

0.0 0.0 0.0

, Ort i.î.a s.3.0

<h» 2îi3.l ; J«3.*

•• ' Brt t,i | s»j g

<M> Ort Irt.S

1

j 1U3 30*3.1 ÏS.S.O

No 122

700.3

1974

gf

P

3

2

1

p-*—

127

3Q7

S«

W

'"

No 18

152006F«br66t. ii

CP 0.0

- . 1.1?3 G>0

* - I.I

2 ! I.I

- » 0.0

1 ».1

* - Î.I

BP 0.0

_1_

Oct72

tS.3

0.0

10.3

*>o.s1.2. i

ISil

It.S

Uiï.a•*.».*.B.i

IÏ.1 MIO.S

June75*.-

OiO

«0.0

0.0

Ort

23.

il.10•Hl

•viS

x.l0

No 901Ï

CP

3•w

2

1^

BP

No 27 Introduction No 5

2.5

1975 ] 1977

051978

; IM.t.O II.I.O 3i!.?

. ' iCP 0.0 ort o.o

* - 2rt.S 0.0 0.0

3 Jrt.S 0,0 1.2.2

- - 73rt.S 0*0 Ort

021980i.i.i

DiO

OtO

OiO

i.ï.s

« - ,0*1.0 art M«j.oim:1 SSiZ.5 Î.O.Ï ISil.O ' Ibl- - trt.S Ort Tiï.O '. tiïP Ort Ort Ort i Ort

I7*.î.î' l*.),0 i iïiî.O MI i

sss

s

0.1

1980«.*.!

Ort

4.3.1

Ort

ï.*.ï

«tÏ.O

10.4.4

02.2.1

SI. .110.2.1

301. 4.4

No 9116

T*CP- -1A -

BP- .

0,1

1979ilil.J

2iO.

l«l.

Uil.

3il.

i.'l.

31.1,1

No 33

CP

*f.

3

2

1

BP

60021977S.lB.t

0.0

Ort

Ort

Ort

tolB.B

fî.î.l.O

-. fï.i.oert

" IM

Ort

j s>ia.i

02

1980lil.O

8rt

Ort

0rt

Ort

0<9lrt.9

' lit.l

e>6-Ort

9rt

Ort

i«..

No93

5.3

C?

5M

4

3*l

2

1

BP

0.1

197141.1.0

Ort '

Ort

• 111.«Sil.l

- 111. JMiI.O

• »il.OIO.Î.2

. llil.l

lliï.Z

Ort

Zltil.1

0.1

19»1M.1.9

Brt

lrt.7

OtO

Irt.S

llil.I

IKil.

17*1.

M.2.

i.f.

ïrt.

0.0

»Oil.«

No 72 See not« 14.14

C?" "

4

3

2

1*• -

BP

0011978114(1.1

M

» lOSal.S

IBfal

•9,1

uI7i!liso.1100x1

Ort

, OtO

Ort

lllil

021980«.j

et«Ort

(,j,iMd

4it

till

1.1

a.i>i0.0

S5tJ

02

1982

11.1

M

Ort

lil

'ie.8

'.H5.1

ai10x1

e.00.0

»

No69 See not« 2 No 73 - 79

CP•U

3M

2

1

BP

6.5 5

'

0051974«N3.Î

0.0

11.1.0

lrt.lMil.l

SlxJ.4

1.1.2

11.3.0

0.0

7Bil.4

Example

Note 1 Number of cracks 'indicated as % weld lengthNote 2 Spraynozzle in vapour area

aos976111. 7

Ort

lil.O

Ort

Oil.?

Ort

Î.Î.Ï

Oi-

Jrt

J.O

32-0301977

12-Q3J02-Û3|Q2-Q3? - 0.05-Û3978 1 1979 1960 1982

H*t.9 l f 14- II.}.) îi,3.(

M ; o.- Ort ui .• tfrtiisOrt u. S.J.3 3lî j ioi|«tiii*t

tut.* ' i 7,.

W.1.S 1 l,-

IS t 1.1 • *<-

txi.i2*1. S

0.0

OiO

14il.l

No 2

4.0

CP

2

1

BP!j .

Nov81

M.J.O

0.0

til.O

»> .0

0.

«H. 0

US. 0

Qi

MM..

liU.0 ' J.3.S ^«,*tH,j

»»Ï.B 1, .6 8***r»

15.- 14.3.0 ii, ,| f««

*S»- ' 3.0.0 12. .1 p"r!f inh.r»

S' 2.0.4 U, .«

Ort I>- 1. Ï

".- . 29.3.0 j S2.3.0

loifc.r »t tpfetr*

trttttrt tir }

C,«M«.

.Cawi* 1

C«w»t 1

a«tt« f ut*

•Itir ctatrat S

0«» .f iMJMCtll*

•mb«r tf cr»t*t

; -*""

l«o «r «r tritki i MI. liltli

ItdAir af trvckt

I M(. 4.th .f

tria* biln

151

Table 2. Water content of liquid in a UKF's storage spheres: preliminary result.

To nitric Acid plants

DATA 1383

TIME

AMBIENT TEMP *C

WtTERCONTENT ppm wt

VftPOVH

CONOCMSATE

•in.» or LIQUID

24-5

I4.OO

11

3OO

17OO

21.5

«45

11

SCO

leoo

28-3

»3O

1O

280

»00

2«J

I1.3O

N>

17O

I5OO

27.5

•JO

1O

31O

1«OO

18.«

tOO

11.9

34O

I«-O

7-tO

12

54O

Z8OO

5-7

»5

2100

8.7

83

NC

3000

7-7

•

CONI

3OOO

23.»

I3-3O

23

aa

ENSAT

26OO

so.e

tooo

17

«1

ON

20OO

si. e

IOOO

20

a*

1300

1.*

laoo

1&5

55

—

31OO

To consumers androi HonkersFrom ammonia ploot

The situation is complex. It is possible that vapour coming from the evaporator containing about 2000ppm water mixes with vapour from the ammonia plant containing hardly any water and passes throughthe vapour area of the sphere on its way to the gasholder.

The vapour therefore should not be considered to have a water concentration in equilibrium with theliquid.

Although the water content of the vapour in general is higher than the equilibrium content, the watercontent of the condensate is not high enough to inhibit against stress corrosion cracking.

152

DISCUSSION

ALAN CRACKNELL, ICI: I would like to congratulate youon your excellent survey of S.C.C, in ammonia, but think itwould be a pity if you persuaded people to inspect theirammonia spheres using the techniques that may notreveal the cracks. Your paper does not differentiatebetween magnetic particle inspections using a D.C. or anA.C. magnetic yoke, yet in the United Kingdom noevidence of S.C.C. was found in spheres when the D.C.yoke was used. Only when the A.C. yoke came into usewere cracks found showing that it would be a waste oftime and money for anyone to do an inspection now usinga D.C. yoke.

JAN BLANKEN, UKF Holland: Before this symposium Isent you a draft copy of this paper, and you recommendedusing A.C. electrodes because you consider using D.C.

electrodes is a technique that is virtually useless indetecting these particular cracks. Being employed byUKF I asked the advice of Ron Dye, our UK metallurgist,and he said that I should leave it as it is. So I left it as it is.

What I learned is that if you do not have any equipmentand want to spend some money, buy A.C. equipment. Butif you do have D.C. equipment, send Ron Dye a letter firstbefore you spend money to buy A.C. equipment.- As Iunderstand it, cracks have been found with D.C. equip-ment, Ron found the cracks in his railtankers with D.C.equipment, but you find more cracks with A.C. equipment.But you also have to be careful not to find things that willnever cause a problem. Most of us I assume have beenthrough a stage starting up a plant when we wanted tokick out all experts to be able to produce ammonia.

153