Synthesis Gas Compressor CouplingCorrosion Fatigue Failures

Failures of gear coupling spacers have completely severed thecompressor drive trains, with catastrophic results. The effects ofcoupling design and lube conditions in the corrosion fatiguemechanism are discussed.

Jan B. SmithApplied Reliability, Inc., Baton Rouge, LA 70816

andGary G. Paulson

Spectra Chem, Inc., Baton Rouge, LA 70809

INTRODUCTION

A. number of corrosion fatigue failureshave been experienced within theindustry on the topping turbine tolow pressure case gear coupling ofthe synthesis gas compressor inKellogg ammonia plants. Of fiveknown failures in three plants, fourwere sudden and unexpected with thedrive train being severed. Demon-strated corrosion tendencies appearto be worsened by centrifugal actionwithin the coupling and designfeatures which trap water and othercorrosives at a high stress area ofthe coupling spacer. The stresseswhich develop and propagate the crackare bending stresses. Lubricationsystem contamination is reviewed withemphasis on sudden and traumaticsources. Corrective measures shouldinclude either the use of a non-lubricated coupling or steps toassure lube system cleanliness. Alsoa conventional gear coupling withspecific features which tend to pro-tect high stress areas from centri-fuged water and contaminants can beused. Minor design modifications tothe conventional gear coupling forimproved reliability are also review-ed.

Four of five known failures re-

sulted from cracking of the spacerflange and one resulted from corro-sion fatigue originating at the boreof the spacer. This paper will re-view the failure mechanism which cul-minates in flange fracture.

This coupling is unique in theKellogg plant in terms of speed andpower transmission at nominally 17MW(23,000 horsepower) and 10,700 rpm.The combination of speed and diametermakes the coupling a rather effectivecentrifuge with sludge accumulationbeing a well known problem, Calistratand Webb (1) . Centrifuged corrosivesappear to produce a highly localized,aggressively corrosive environment.Design features of some conventionalgear couplings inadvertently providetraps for corrosives to collect andfluctuating bending stress andstress concentration at these traps.

CORROSION FATIGUE

The location of the cracking andfracture surface appearance can beseen in Figures 1 to 6.

Radial markings on the fracturesurface seen in Figures 4 and 5 arerächet marks indicative of multipleorigin fatigue fracture. Numerouscrack origins develop in slightly

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different planes, then join togetheras they advance into one crack front.

The large fatigue area in Figure5 relative to the area of final over-load fracture suggests that stressconcentration and corrosion effectsare large in comparison to nominalstress. Also the initial cracks areessentially perpendicular to the XYplane of Figure 2, suggesting thatalternating bending stress, not tor-sional stress, is driving the fatiguemechanism.

Three of the four flanges hadfatigue initiation sites around theentire circumference. One shown inFigure 6, was fatigue cracked arounda portion of the circumference. Thiscrack was found after shutdown due tohigh vibration.

The photomicrograph of Figure 7shows the major crack origin to be ata large pit. A secondary fatiguecrack is located nearby and runsnearly parallel to the major crack.The wide crack mouth is characteristicof corrosion fatigue.

The effect of stress concentra-tion due to a sharp fillet radius isillustrated in Figures 8 and 9 ofunfailed flanges opposite failed ones.Figure 8 shows a very sharp radiuswith a fatigue crack in progress.Figure 9, at the same magnification,shows a slightly larger radius, per-haps accidentally machined, with nofatigue crack yet generated. The de-sign radius was unspecified for thisparticular coupling. In general, theradii appear to be as sharp as normalmachining practice would produce ifno radius was specified.

DESIGN CHANGES

The design of the failed spacer at thelocation of failure is shown in Figure10. Centrifugal force tends to trapwater and other corrosives at thespacer fillet. Some of the failedcouplings were not an anti-sludgedesign, i.e. with sludge escape paths.Those design features would have noeffect, however, on water and corro-sives at the point of failure of thefailed design. The space between the

sleeve and spacer at the fillet willtrap material by centrifugal forceeven with an anti-sludge design.

An interim design used in oneplant is shown in Figure 11. This isa minor modification of a standardcoupling. The coupling is selectedbecause of the tight fit between thespacer and sleeve and absence of thecorrosive trap shown in Figure 10.These features tend to protect thefailure prone location. The stressrelief groove is a modification tothis standard design.

Preventive inspection of the in-terim design of Figure 11 was madetwo weeks after chemical cleaning ofthe entire lubrication system and in-stallation of this new coupling.Figure 12 shows deposits "downstream"of the fillet radius on the spacerflange. Underdeposit corrosion wasevident after removing the deposits.No corrosion had occurred at the fil-let radius. One year later thiscoupling was inspected. Serious cre-vice corrosion was evident at variouslocations between the sleeve and spa-cer tight fit as seen in Figure 13.Some minor deposits with underdepositcorrosion was seen in the filletradius. The tendency for crevicecorrosion is very evident in Figure14. This photograph shows crevicecorrosion between the face of thespacer and the contacting couplinghub.

A final design change was madeto that shown in Figure 15. Whilethe pitch diameter of the couplingteeth was maintained, the diameterof the spacer and sleeve tight fitsurface was reduced. This placed thetroublesome location above the"waterline" established by centrifu-gal force. The coupling is a damlesstype to promote removal of sludge andthe fillet radius was changed to thegeometry shown in Figure 15. Thiscoupling has subsequently undergoneinspection for corrosion and crackingduring outages, with none being ob-served .

CORROSION TESTING

The sludge de.pesifcs ins.idê the coupl-

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ing sleeve in Figure 4 were analyzedby energy dispersive x-ray spectro-scopy and found to contain elementsof copper, sulphur, zinc, calcium andsilicon as well as iron. These sameelements were generally found in thepits and cracks of Figures 7 and 8.Copper and zinc were suspected to befrom admiralty oil cooler tubes. Acorrosion test of admiralty metal inoil with variable concentrations ofammonia and water were conducted withresults shown in Table 1. In general,the corrosion rate increased withammonia and water concentrations.

A corrosion test of the spacermaterial, AISI 4140, was conducted inseveral oil-water mixtures. Resultsare shown in Table 2. There are mod-est increases in corrosion rates atthe liquid-vapor interface and cre-vices versus submerged specimens anda similarly modest increase with cop-per corrosion products added. Theaggressive corrosion environment ofthe coupling is not duplicated, al-though crevice corrosion is obtained.

Field corrosion test of variousmetals in a lubrication system arereported in Table 3. This test wasconducted following the failure shownin Figure 7 and subsequent chemicalcleaning, but before the crevicecorrosion of Figure 13 and 14. Again,the corrosive environment is not dup-licated, but crevice corrosion isobtained. Sludge removed from thetopping turbine to low pressure casecoupling before this test showed mostparticles to be 1 to 25 micrometers inthe largest dimension. Bulk analysisshowed elements of sulphur, copper,zinc, chlorine, silicon, and calciumalong with the predominant elementiron.

LUBRICANT CONTAMINATION

Normal sources of contamination in-clude water through turbine seals,condensation, leaking oil coolers,contaminated new oil and contaminatedseal oil return. Also, in one plantan oil purifier was found to be asource of sudden, massive contamin-ation, utility water was pumpedthrough a venturi nozzle to develop avacuum to which the oil was exposed.

Failure of electrical components al-lowed water to be pumped into thelubrication system in massive quan-tities. Subsequently the purifierwas removed from service. Similarly,utility water to an oil centrifugecontaminated the system as a resultof failure of electrical components.

CONCLUSIONS

Obviously, non-lubricated couplingswould eliminate these corrosion fa-tigue failures. If gear couplingsare used, the lubrication systemsshould be clean. Because of the ap-parently high frequency of thesefailures, their serious consequences,the risk of sudden contamination andabsence of precise understandingabout the corrosion environment, thefollowing additional steps may war-rant consideration if gear couplingsare to be used.

o Inspect periodically for corro-sion and cracking.

o Consider the cracking mechanismin vibration monitoring anddiagnostics.

o Consider the geometry of theconventional coupling in Figure11 with the nonconventionalstress relief groove in lieu ofthat of Figure 10.

o Consider the coupling design ofFigure 15 if an advanced levelof protection is desired, buta non-lubricated coupling isnot considered justifiable.

ACKNOWLEDGEMENT

The authors acknowledge and thankMr. Jack VanSciver for the corrosiontesting.

REFERENCES

1. M.M. Calistrat and S.G. Webb,"Sludge Accumulation in Contin-uously Lubricated Couplings,"presented at the PetroleumMechanical Engineering Confer-ence, ASME, New Orleans (1972).

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,S£E FIGURE 2

Figure 1. Continuously lubricated gear coupling andfailure area.

CRACK OROWTH

FAILUREINITIATIONSITE

Figure 2. Enlargement of failure area from Figure1, placing crack initiation and orientation.Spacer fillet shows crack as perpendicular to XYplane.

Figure 4. Cracked spacer flange severed fromspacer and still bolted to the coupling sleeve. Arrowspoint to racket marks.

Figures. Coupling spacer with left side flange Figure 5. • Flange fracture surface viewed radiallyremoved by cracking. (1 Ox).

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Figure 6. Flange cracked around portion ofcircumference.Arrows = fatigue crack direction; dashed line = limit.

Figure 7. Major crack site at a pit (arrow), andsecondary fatigue crack (left): 200x.

Figure 8. Cracked but unsevered flange oppositefailed flange, Figure 7 (50x).

Figur© 9. Fillet radius opposite an uncracked flangefailure (50x).

O.

OO

Figure 10. Failed spacers with sharp radius and trapfor corrosives.

a.Oo

Figure 11. Coupling with geometry.

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Figure 12. Deposits with underdeposit corrosiondownstream of fillet radius on spacer flange.

14. Crevice corrosion on spacer face whereit contacts coupling hub.

15. Coupling design with failure prone areamoved above waterline.

Figyre 13. Crevice corrosion at spacer-sleeve fitafter one year.

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Exposure

Table 1. AISI4140 vs. oil-water mixtures*.

Liquid Interface Crevice Macro Examination

80% Oil-20% Water

Sludge from Bulk

0.08

0.08

0.39

0.23

0.38

0.39

Pits - Interface& Crevice

Uniform CorrosionStorage

80% Oil-20% Water p.22 0.70 0.42+ 500 PPM Cu ++

(added as CuSO4.5H20H- 500 PPM Cu Dust

* Temp. = 150°F; Exp. Time = 168 h; Vigorous Agitation; Corrosion Rate = mil/yr

Pits - Interface& Crevice

Table 2. Admiralty metal vs. ammonia contaminated lubrication oil*.

% NH3 % H20 % oil mpyMicroscopicExamination

0.01

0.01

0.01

0.1

0.1

0.1

1.0

0.025

1.0

2.0

0.25

1.0

2.0

2.57

Balance

Balance

Balance

Balance

Balance

Balance

Balance

0.36

0.64

0.79

2.. 45

1.34

1.55

3.73

Uniform Corrosion

Uniform Corrosion

Uniform Corrosion

Very Shallowpitting

Uniform Corrosion

Uniform Corrosion

Uniform Corrosion

* Temp. = 125°F^Exp. Time = 168 h; Corrosion Rate = mil/yr

Table 3. Metals vs. recirculated lubricated oil in 103J ammonia compressor system*.

Metal

AISI 4140

AISI 4140

A-265 Grade C

A-265 Grade C

Admiralty Metal

Admiralty Metal

Location

8" Return Line

Floor of Tank

8" Return Line

Floor of Tank

8" Return Line

Flour of Tank

CorrosionRateDuplicate MicroscopicSpecimens Appearance

0.1

0.1

0.15

0.10

0.25

0.20

Slight CreviceAttack

Uniform Corrosion

Slight CreviceAttack

Uniform Corrosion

Very Shallow Pits

Very Shallow Pits

* Temp. = 100-103°F; Exp. Time = 960 h; Corrosion Rate = mil/yr

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Jan B. Smith Gary G. Paulson

DISCUSSIONK. VAN GRIEKEN, DSM Research, Ijmuiden, TheNetherlands: This is not a question, but a generalremark. When the corrosion fatigue crack grows, thestress intensity factor can increase to such a heightthat the corrosion fatigue crack could change to apure mechanical fatigue crack due to the lengths andperhaps their increase as well as perhaps thedecreasing rate of the corrosion fatigue crack. Thenthe corrosion factor is not needed any longer. Andin the end a ductile cracks causing the whole con-struction to be taken apart.

J.B. SMITH, Applied Reliability, Inc., Baton Rouge,LA: I very much agree with you. The corrosion providesthe instigator for the crack, if you will. It helps promotethe fatigue. But once the fatigue crack starts, it cancontinue on its own volition even in the absence ofany further corrosion.

VAN GRIEKEN: Thank you.

J. CURTIN, Irish Fertilizer Industries, Cobh, Ireland:I would like to ask if there is any particular concentrationlevel of water, above which the problem is much morelikely to occur. Is it purely a function of the free waterin the oil, as distinct from any dissolved water thatmight be present?

SMITH: I'm afraid that I can't answer that. Our corrosiontesting is reported in tables in the paper and may shedsome light on the question. In essence, we found thatthe higher the water concentration, the more severethe corrosion. That was also true for the copper inthe ammonia. However, we feel far short of duplicatingin the laboratory the kind of corrosive environmentthat we have in service. So, we really can't commenttoo much on the question. I would have to say thatthe corrosion mechanism—exactly what causes it andunder what circumstances it gets so serious, we reallydon't know.

CURTIN: Thank you.

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