Investigation of an Impeller Failure in aProcess Air Compressor

A fatigue failure of an ammonia plant process air compressor impeller occurred in1989 in Iraq. This paper describes the incident, as well as subsequent investigation to

identify its causes and corrective actions,

Rudolph C. Frey and Leonard E. SchaiderThe M.W. Kellogg Company, Houston, TX 77210

Introduction

Seidac's Fertilizer Plant #4 is a 1000 t/dammonia plant with a 1750 t/d urea plant. Theplant is located in Baiji, Iraq, which is about 200km (124 mi.) northwest of Baghdad. Theprocess for the ammonia plant is an M. W.Kellogg design, and the plant was engineeredand constructed by a joint venture consisting ofM. W. Kellogg, Ltd. in the U.K. and HitachiZosen in Japan.

The plant was commissioned in late 1988 inpreparation for start-up in early 1989.

This paper describes an impeller failure inthe process air compressor, identifies probablecauses of the failure, and summarizes the stepsinvolved to investigate causes. It also covers thesteps taken to repair the damage to thecompressor and corrective actions to avoidrecurrence.

Description of the Compressor

The air compressor, Item 101-J, consists oflow pressure and high pressure casingsconnected through a speed increasing gearboxand driven by a condensing steam turbine. (SeeFigure 1) The compressor bodies are both

horizontally split designs which weremanufactured by Hitachi, Ltd. The low pressure(LP) casing is a model 2MCL805, and the highpressure (HP) casing is a model 2MCL457. Themachine compresses 53200 m3/h (31300 CFM)at the compressor inlet from ambient conditionsto 38 kg/cm^a (540 psia). Rated speeds are7500 rpm for the LP compressor and 11183 rpmfor the HP body.

101-J was the first compressorcommissioned in the plant, being needed duringthe commissioning procedures. It was firstoperated in September, 1988 to clear plantpiping and vessels by blowing air through theequipment. It was later used to circulatenitrogen to purge air from the plant and toprepare the plant for start-up and was put intonormal process service early in 1989.

History of Operation

Prior to the failure, the compressor hadaccumulated approximately 1680 hours ofoperating time. This included 960 hours duringplant commissioning from September 14,1988 toOctober 29,1988, and 720 hours during normaloperation from December 6,1988, to February9,1989.

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During the 5 month period that precededthe failure, the plant was in various stages ofcommissioning and startup, and a number oftransient operating conditions were experienced.Among these were 16 start/stop cycles, half ofwhich were planned and half were initiated byemergency shutdown systems. Three shutdownsoccurred during the period December 22-23,1988, one of which was preceded by a prolongedsurge that lasted about ten minutes.

From the time this machine was initiallycommissioned, the radial vibration readings weregenerally in the 8 to 10 jam (0.3 - 0.4 mil) rangeand never exceeded 17 fim (0.7 mil), the latteroccurring only during the first few days ofoperation.

On February 9,1989, a sudden increase invibration from 8 to 43 ;u,m (0.3 -1.7 mils)occurred at the turbine end bearing of the LPcompressor. The machine stabilized at thisreading and continued as such until February 14.

On February 14, the vibrations at thisbearing increased suddenly to 50/xm (2.0 mils),but once again settled to a steady value at thatlevel. No overt sign of any serious problemexisted (other than the elevated vibration).

On February 17, there was another suddenincrease in vibration at this bearing to 83 jam (3.3mils) at 7170 rpm. At this level of vibration itwas necessary to shut the machine down toinvestigate the source of the elevated vibrations.

This compressor vibration history issummarized in Figure 2.

Initially, coupling alignment and bearingconditions were examined, however, theseprovided no insight to the cause of the highvibrations. It was reasoned, therefore, that theproblem might be internal to the machine. Assuch, it was decided to open the low pressurecase. Upon opening the casing, the 4th stageimpeller was found severely damaged and verylikely the cause of the elevated vibrationreadings.

Initial Observation of Failure

Visual examination of the LP compressorrotor revealed that pieces of the 4th stageimpeller disc were missing in 3 separate placesaround the periphery. It was surmised that atotal of 5 broken pieces were removed, threefrom one place and the other two from otherlocations. (See Figure 3)

Each break showed a similar "scalloped"shape at or near the impeller vanes, with theexception of one damaged area roughlyequidistant between vanes. No damage wasfound on any of the other impellers except forsome impact marks on the inlet of the fifth stageimpeller which were not considered serious.These were presumably caused by debris fromthe fourth impeller.

Deformation was observed, some of itsevere, at the inlet section of the 4th stagediffuser, again being secondary damage fromdebris from the 4th stage impeller. Severeimpact damage had also occurred at the 5thimpeller inlet guide vanes due to broken piecesfrom the 4th impeller. Some minor impactmarks were seen on the casing in the region ofthe 4th and 5th stage impellers. No damage wasobserved on the bearings or seals.

Immediate Steps to Return 101-Jto Service

Full inspection, including dye-penetrantchecking for cracks, was carried out on all of theLP compressor impellers and diaphragms.Impact damage in various locations was cleanedup by grinding. With the compressormanufacturer's concurrence, the damageddiffuser vanes at the 4th stage were totallyremoved. The manufacturer determined thatthe rotor would provide almost full service as aninterim spare without the 4th impeller and the4th stage diffuser vanes, therefore, it wasdecided to remove the damaged 4th impeller.(The 4th stage diffuser vanes had already beenremoved.)

After careful cutting and preservation ofsamples from broken parts of the 4th impeller,the remains of the impeller were precision-machined off the rotor. The remaining impellerswere fully checked for runout, their dimensionsinspected, and the rotor dynamically balanced.The inspections had confirmed the rotor to befully serviceable, and it was preserved and storedas an interim spare. It was determined that afterinvestigation of the failure was concluded, areplacement spare rotor would be necessary.

The compressor, utilizing the spare rotor,was then put back into full service by the end ofFebruary, 1989.

Failure Analysis

The criticallity of 101-J to the processdemanded that a rigorous investigation beconducted to assure that root causes of the

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failure were identified and eliminated.Extensive analysis and testing by themanufacturer (1X2)(3) and independentexamination of failed parts were carried out toavoid further mishaps and to identify whethermodifications would be required to compressorparts that required replacement.

Fault tree analysis

A detailed list was made of all of the itemsthat could have potentially contributed to thefailure of the 101-J fourth stage impeller. Eachlisted item was thoroughly examined todetermine if it could have been a possible causeof the failure or have made a significantcontribution to the failure. By the process ofelimination, the probable causes of the failurewere brought into focus. At the same time, thisanalytical technique pointed to the studies,examinations and additional testing required toprovide the data needed to eliminate some itemsand to support the indictment of the culprititems.

The Fault Tree Analysis, coupled with thetests, examinations and studies performed insupport of the analysis, clearly eliminated all ofthe items considered except for the two that arediscussed later under sections entitled Internallyexcited frequencies and Externally excitedfrequencies. There was no evidence of damagedue to foreign materials, and no foreignmaterials were found in the compressor.Chemical and mechanical tests of material fromthe failed impeller demonstrated that thematerial used to fabricate the impeller met all ofthe required specifications. The design stresseswere reviewed and found to be appropriate forthe materials used in the impeller. A review ofthe manufacturing and assembly of the machinefound nothing that could be the cause of thefailure.(l) A review of the operating recordsindicated that the compressor was not subjectedto overloads due to overspeed. It was alsoestablished, by shop tests of a similar compressorwith instrumentation to measure impellerstresses during operation, that the normalvibratory stresses are far below the level thatwould have a significant effect on this failure.(2)Although not considered to have contributeddirectly to the failure, it was noted that atreduced compressor speed, the operatingconditions for the machine are closer thandesired to the surge line; that a minor amount ofammonia and sulfur compounds were found inthe compressor drain water (not unusual); andthat the fillet radius at the impeller disc to vaneconnection was marginally below the designvalue. Clearly, an abnormal situation was

required to cause the impeller to fail.

The investigation thus focused on theunusual items that could have generated thehigh stresses needed to cause material failure.Two suspect items were found, each of whichcan lead to resonant vibration of the impellerand the very high stresses that can occur duringsuch vibration, although they arise from differentmechanisms. One involved the prolonged surgeexperienced in December, 1988, coupled withinternally developed excitation related to thediffuser blade passing frequency and gasbackflow. Surge alone could not have causedstresses high enough to cause the failure. Theother involved long term exposure to externallydeveloped acoustic excitation generated in theexternal interstage piping and equipmentbetween the two sections of the low pressurecompressor case.

The two specific probable causes aresummarized as:

(1 ) A diffuser vane passing frequencyexcitation, at the compressor speed during theprolonged surge period (6000 rpm), thatcoincided with the third diameter mode naturalfrequency of the failed impeller. The stressesdue to such a resonance condition, added to thenormal operation stress and possible additionalstresses due to surge, could have caused thefailure. Without surge and resulting backflow,the effects of the diffuser vane passing frequencyare too low in energy to cause the failure.

(2) A high level noise excitation, generated inthe piping and equipment between the first andsecond sections of the 101-J LP case, coincidedwith the first diameter mode natural frequencyand was a sub-harmonic of the third modenatural frequency of the failed impeller. Thestresses due to resonance at one or both of thesefrequencies, when added to the normaloperating stresses, could have caused the failure.

It is not known which of these two actuallycaused the initiation of the fatigue cracks andthe subsequent crack propagation and failure. Itis even possible that both contributed to thefailure, since the external excitation was alsopresent during the prolonged surge period.

Stress analysis

The specified minimum yield strength forthe fourth stage impeller material is 85 kg/mm2

(121000 psi) and the specified minimum tensilestrength is lOO kg/mm2 (142000 psi). The actualmeasured values from the material test

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certificates are 94 kg/mm2 and 106 kg/mm2

(134000 and 151000 psi) respectively. Nosituation, or combination of situations werefound to have possibly occurred that could leadto stresses high enough to fail this material, oreven to cause yielding of the material. Thisevaluation, coupled with the nature of thefracture surface, pointed to a fatigue type failure.Fatigue failure generally occurs at stress levelswell below the yield strength of the material,with the actual value of the fatigue failurestrength varying with the cyclic life; the greaterthe number of cycles, the lower the fatiguefailure stress until the endurance limit isreached. At the endurance limit stress level, thecyclic life is essentially limitless.

Impeller stresses due to normal operatingconditions, including noise and other vibratoryeffects, could not be expected to be significant.Stresses on the order of 1.0 to 2.0 kg/mm2 (1420- 2840 psi) are normally the level experienced ina machine of this type. However, should thenoise frequency coincide with a natural vibrationmode of an impeller, or be a sub-harmonic ofsuch a mode, the impeller can be excited by thenoise to undergo unusually high vibrationamplitudes and, thus, experience unusually highstresses. The extent to which an excitation at thenatural frequency will cause highly amplifiedvibrations depends on the damping of the partexcited. Inherent damping, that due to theproperties of the material, is quite low for steel(about five percent of critical damping). Thus, asteel impeller, when excited at a naturalfrequency mode, can experience high vibrationseven from a small strength source. For acomponent with many natural frequency modes,the excitation of the lower modes is of greatestconcern since more energy can generally beabsorbed at lower modes than at higher modes,leading to higher vibratory stresses at the lowermodes.

Metallurgical examinations showed that thefractures started in at least three differentlocations.(S) Each fracture was initiated at ajunction of the impeller disc with an impellervane. These are the locations that experiencethe highest stresses during normal operation.Further, should the impeller experienceresonant vibration, these would be the locationsof the highest vibratory stresses.

Internally excited frequencies

Initially, compressor data was examined todetermine if any of the frequencies which couldoccur due to the combination of compressorspeed, fixed and rotating vanes and other

machine components could have excited any ofthe natural frequency modes of the impeller. Nosignificant coincidence of excitation frequencyand impeller natural frequency modes werefound at the normal operating speeds of thecompressor.

However, at the compressor speed duringthe extended surge period (6000 RPM) theproduct of the number of stationary diffuservanes and the compressor speed indicated apotential critical internal frequency of 1000 Hz.The third diameter mode natural frequency ofthe fourth impeller was effectively 1000 Hz at6000 rpm. Thus, there was the potential ofexciting the impeller into third mode vibrations.Microscopic examination of the failure surfacein the vicinity of the crack initiation point,coupled with a fracture mechanics analysis of thedata, showed that the alternating stresses thatcaused the fatigue failure were approximately +_12 kg/mm2 (17100 psi). At the 6000 RPM speedof the compressor the mean stress in theimpeller disc due to rotation is 26.6 kg/mm2

(37800 psi).(3)

Thus, at the points of crack initiation, astress of 26.6 ± 12 kg/mm2 (37800 + 17100 psi)could have been present. The GodcTmandiagram in Figure 4 shows the allowablealternating stress as a function of the mean stressfor a life of 300,000 cycles, the number of cyclesthat could have occurred during the ten minutesurge. Examination of this diagram shows thestress of 26.6 + 12 kg/mm2 (37800 ± 17100 psi)to be just at tfie fatigue limit and capable ofcausing crack initiation.(3) Also, the mean stressduring surge may have been higher than duringnormal operation due to surge effects, furtherindicating this to be a possible cause of thefailure.

Externally excited frequencies

In order to examine all possibilities forcausing resonant vibration of the impeller, a siteinvestigation was conducted to determine theenvironmental noise and vibration frequenciesaround the 101-J compressor.

This investigation found that there weresignificant noise levels in the area, some ofwhich were found to originate in the piping andequipment between the first and second sectionsof the LP case of the 101-J compressor. A highlevel noise was found to be present atapproximately 600 Hz, close to the first modenatural frequency and a sub-harmonic of thethird mode natural frequency of the fourthimpeller. This excitation appears to have been

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present during the entire period the compressorwas in operation, and could have caused naturalfrequency vibration of the fourth impeller ateither the first mode, third mode or both naturalfrequency modes.

The 101-J compressor operated for manydays at speeds of 6800 RPM and higher. At 6800RPM the stress of the imneller-vane junction isapproximately 34 Kg/mm2 (48000 psi). TheGoodman diagram in Figure 5 for 10,000,000cycles, long term exposure to the fluctuatingstress, shows that the fatigue limit stress is about+_ 3 kg/mm^ (4300 psi).(3) Thus, only a modestincrease in the normally operating fluctuatingstresses of about 1.0 to 2.0 kg/mm2 (1420 - 2840psi) would be needed for crack initiation.Forced resonance of the impeller due to externalnoise could easily provide this amplification ofthe normal fluctuating stresses.

Outside consultant firms specializing in thisarea of technology (Southwest ResearchInstitute in the United States and Institute ofSound and Vibration Research in England)advised that the external noise represented apotential cause of the failure.(4)(5)

Conclusions

The cause of the failure, based on thisinvestigation together with supportingexaminations and tests, was found to be fatigueof the impeller material. This was due toresonance of the fourth impeller causing highadditional fluctuating stresses which, whenadded to the normal operating stresses, or to thesurge induced stresses, resulted in a summationof stresses that exceeded the fatigue strength ofthe material for the number of cyclesexperienced.

The possible causes of the failure wereinvestigated in a methodical and comprehensivemanner, and all factors, such as those included inthe broad categories of material properties,design and construction of the compressor, theoperating conditions and the operatingenvironment - including factors external to themachine - were carefully examined. This analysisrequired considerable supporting informationand original work to provide the detailsnecessary to conclude this investigation. Itincluded operational tests by the manufacturerof a similar rotor instrumented to permit themeasurement of impeller stresses duringoperation in addition to the normal instrumentsrequired to measure speed, vibration andpressure. Also required were noise

measurements around the compressor, vibrationmeasurements on the compressor casing and itsassociated piping and equipment, tests of thechemical composition and physical properties ofthe impeller material and detailed metallurgicaland microscopic examination of the fracturesurfaces.

The fault tree analysis identified twopossible causes under the category of "operatingconditions" that could result in forced resonanceof the fourth stage impeller (one of which alsoincluded coincident surging). These were foundto be probable causes of the failure, since all theother possible causes were eliminated. Thesepossible causes include a diffuser vane passingfrequency coincident with the third diametermode natural frequency of the failed impellerand a high level noise excitation generated byinterstage piping between the first and secondsections of the LP compressor coincident withthe first diameter mode natural frequency. Bothof these conditions could generate alternatingstresses high enough, when coupled with thenormal operating stresses, to cause a fatiguecrack to develop. The crack propagation andultimate failure that would follow the crackinitiation would cause a failure consistent withthe type of failure that occurred.

Therefore, corrective measures were takento ensure that both conditions leading to thistype of failure are prevented from occurring inthe future.

Impeller failures of this type are rare, andalthough there are reports in the literature ofsuch failures,(6) there is little useful dataavailable concerning avoidance of such failures.Tests carried out to investigate this specificfailure conducted with an instrumented, similarcompressor demonstrated that the normalalternating stresses, due to the randomvibrations and broad band random noise that arepresent at low levels in all rotating machinery,are very low and of no significance. The testsalso demonstrated that it is very difficult to causean impeller to resonate at a high amplitude; firstdiameter mode vibration stresses were detected,but their amplitudes never exceeded theminimum detectable level. We know of nopractical way of reasonably predicting when suchresonances may have significant effects in orderto take measures to prevent them from occurringbefore they happen. However, having nowidentified the factors that can lead to resonanceof the fourth impeller in the 101-J compressor,actions have been taken to prevent recurrence.

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Corrective Actions Literature Cited

Based on the conclusions reached as to thenature and cause of the failure, the followingcorrective actions have been taken to preventthe recurrence:

(1) In order to improve the chances of avoidingextended surging, the existing anti-surge controlsystem was supplemented by the addition ofinstruments and controls to fully automate theblow-off system on the LP case of the 101-Jcompressor. This increased the sensitivity andreaction speed of the system. The schematicdetails of these additions are given in Figure 6.

(2) The setting of the minimum speed at whichthe governor will permit operations of thecompressor was raised. This will provide morestable compressor operation and preventoperation at the low speeds where the marginagainst surge is lower than in normal operation.

(3) An in-line noise attenuator was added tothe interstage piping between the two sections ofthe low pressure case of the 101-J compressor toacoustically de-couple the interstage piping andequipment from the compressor. The noiselevel at frequencies that coincide or are sub-harmonics of the fourth stage impeller naturalvibration modes that are generated in theexternal piping and equipment will be stronglyattenuated before they enter 101-J, therebyreducing the energy of the noise to a harmlesslevel.

(4) A new rotor was manufactured to replacethe original rotor that experienced the impellerfailure. This rotor was manufactured asoriginally designed, since the failure was notcaused by any deficiency in the design or in themanufacturing of the rotor.

Closing Comments

Actions were taken to remove the known

Eotential causes of the impeller failure, and nojrther difficulties of this nature were expected

with the 101-J air compressor.

The compressor operated without similarproblems for over a year following thecompletion of all corrective actions, however,since late 1990, information from the plant hasnot been available. The current operationalstatus of 101-J is not known.

1. Air Compressor Fracture TroubleInvestigation Report, Hitachi Ltd. technicalreport, April 8,1989

2. Investigation Report for Measurement ofImpeller Stress, Hitachi Ltd. technical report,July 12,1989

3. Air Compressor Fracture TroubleInvestigation Report, Hitachi Ltd. technicalreport, June 7,1989

4. Blodgett, L. E., and Bates, C. L., PipingAcoustical Resonance Analysis Report,Southwest Research Institute, July 31,1989

5. Pinder, J. N., Noise Frequency Analysis,Institute of Sound and Vibration Research, June15,1989

6. Van Laningham, Fred L. and Wood, DaveE., Fatigue Failures of Compressor Impellersand Resonance Excitation Testing, Proceedingsof the 8th Turbomachinery Symposium, TexasA&M Gas Turbine Laboratories, 1979, pgs. 1-9

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DISCUSSION

Gopal Ramaswami, National ChemicalFertilizers, Saudi Arabia: Did you do any vibrationanalysis during startup and shutdown to analyzethe failure?Schaider: At the time of the failure, vibrationanalysis was done using data available fromconventional monitors. There was no equipment atthe job site to carry out a detailed frequencyanalysis, and this limited the ability to accuratelydiagnose and troubleshoot the elevated vibration.Ramaswami: Was there any indication of whatcaused the probable surging?Schaider: The surging was real and was

recognized by the operators. It was caused by acontrol valve in the compressor discharge whichinadvertently closed during plant startup.T. S. Hariharan, Fertil, Abu Dhabi: If yoususpected that the machine was surging, why was itnot stopped?Schaider: The compressor should have beenstopped or the surging conditions promptlycorrected. Attempts were made to correct thesurging condition so that the startup couldcontinue, however, these were unsuccessful. Thecompressor ran for about 10 minutes in surgebefore it was shut down.

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FAILED IMPELLERMODEL

2 MCL 805MODEL

2 MCL 457

SECTION SECTION2 1

SECTION SECTION4 3

STEAM TURBINE LP COMPRESSOR GEAR

Figure 1. Air compressor train

arrangement.

HP COMPRESSOR

VIB

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N A

MP

LIT

UD

E (

MIC

RO

ME

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RS

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M

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11

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43

8 (0.3 MIL)

83 (3.3 MILS)

50 (2.0 MILS)

(1.7 MILS) I

14 SEP 88 9FEB

Figure 2. LP air compressor vibration.

Figure 3. Sketch of failed 4th stage

impeller.

130

U)enwDC

ü

O

1210 -

FAILURE LINE

10 20 26'6 30 60 70 80

STEADY STATE STRESS (Kg/mm 2)

Figure 4. Goodman diagram of the fractured4th stage impeller (300,000 cycles).

30 -i

EE

t zo

U)U)UIcc

10 -FAILURE LINE

(48000 PSI)

10 20

STEADY STATE STRESS (Kg/mm 2)

Figure 5. Goodman diagram of the fractured4th stage impeller (10,000,000 cycles).

Rudolph C. Frey

Figure 6. Anti-surge control schematic. Leonard E. Schaider

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