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Centrifugal Compressor Failure Analysis
Mr. Cliff Knight, PEPresident and Chief Engineer
Dr. Carlos R CorletoDirector of Materials Lab & Field Services
KnightHawk Engineering, Inc.17625 El Camino Real, Suite 412,
Houston, TX 77058, USA
1st Middle East Turbomachinery Symposium, Doha, Qatar, Feb 13-16, 2011
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Outline
• Introduction & Background
• Site Visits • Root Cause Failure Analysis▫ Failure Site Assessment
▫ Mechanical, Metallurgical, Compressor Performance
▫ Failure Scenario Assessment
• Conclusions• Recommendations
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Introduction & Background• Compressor failure at refining facility in Gulf
Coast of USA
▫ 6000 HP two stage process gas compressor in catalytic cracking process
▫ Catastrophic failure occurred prior to shutdown for schedule turnaround
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Overall View
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Site Visits
• KHE conducted 4 site visits ▫ Observed failed compressor – documentation▫ Gathered relevant data – process/mechanical▫ Interview plant personnel – chain of events▫ Inspect failed components - metallography▫ Directed removal of compressor – preservation of
evidence▫ Discuss analyses results conducted – Failure scenarios
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Failure Site Assessment
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Southwest View
North View
South View
• Impellers still on rotor• Sudden case rupture evidence
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Impeller Rub Damage
Bearing Housing Damage Mechanical Seal Damage
• Evidence of rubbing
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Damaged Compressor
Small damage parts
Recreation of Failed Compressor
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• Damaged casing separated by brittle fracture under sudden overload ▫ Most of the case broke into large pieces▫ No sign of fatigue, corrosion or impact damage▫ Fracture origin difficult to detect due to brittle nature of
cast iron
• Inlet vanes showed signs of thinning and pitting due to corrosion but had no effect on the failure
• Corrosion on pipe and cast fittings ▫ Not related to failure
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Root Cause Failure AnalysisMetallurgical Assessment
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• No significant changes in measured radial and axial vibration levels before failure (maybe due to slow sampling time)▫ However, Stage 2 thrust bearing shows significant activity even though failure
occurred in Stage 1 Maybe due to coupling of stages or clearances favor loading in Stage 2
• Lower bearing temps suggest bearings loads decreased• Power trend shows unloading of compressor (decreasing process
load) at 3:30 am▫ After 6:50 am power signals visibly unstable
• Motor and compressor power imbalance possible due to:▫ Increased flow through balance line▫ Measured pressure ratios lower than actual▫ Measurement error in flow rate at low flow▫ Recirculation through Stage 1.
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Root Cause Failure AnalysisMechanical Assessment - Vibrations, Bearings, Power
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• Case thermal stresses below case strength
• Suction and discharge pipe stresses within acceptable design stress levels
• Discharge nozzle stresses acceptable
Root Cause Failure AnalysisMechanical Assessment – Thermal, nozzle design, piping
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• 50 lbs projectile piece due to explosion not discharge pressure
Root Cause Failure AnalysisMechanical Assessment – Projectile Analysis
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Root Cause Failure AnalysisCompressor Performance Review – Stage I
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• Surging evidence ▫ Data from 3:00 AM until failure around 7 PM
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Root Cause Failure AnalysisCompressor Performance Review – Stage II
• Evidence of surging evidence ▫ Data from 3:00 AM until failure around 7 PM
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• Surge control with spillback valve for both stages▫ Inadequate, response time too slow▫ Separate surge control systems with short response
time needed for each stage
Root Cause Failure AnalysisSurge Control Evaluation
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Root Cause Failure AnalysisFailure Scenario Assessment
• At 3:30 am periodic surges started; before 4:00 am a more significant surge took place▫ Compressor more unstable Flow readings, amps readings of motor drive and thrust bearing
temps (no protection)
• After initial surging, erratic DSC trend data• Another major surge at ~ 6:50 am after reactor shutoff▫ Possible damage to balance piston or labyrinth seals of 1 Stage ▫ Compressor became highly inefficient ▫ 1 stage discharge temp increase while suction temp decreased Probably due to recirculation via internal leak from component
damage, incipient surge, full surge or gas through balance line
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• At 7:14 pm another possible significant surge might have cause the following failure scenario:▫ Thrust rotor towards discharge end (no protection)▫ Impeller contact with stationary components Rub damage on backside of third impeller possibly caused upper and
lower sections compressor failure, allowing air to enter compressor Ignition source as well
• Analysis of all data provided strongly suggested the root cause of the failure is the surging of the compressor without adequate surge protection.
• Without surging , failure would not have occurred.
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Root Cause Failure AnalysisFailure Scenario Assessment
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Conclusions• Analysis of all data suggest that surging of the
compressor without adequate surge and thrust bearing (axial displacement and temperature) protection were main causal factors.
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Recommendations• Modification of compressor surge control
▫ Provide appropriate surge protection for both stages
▫ Surge control with only Stage 2 spillback should be reviewed
• Verify material imbalance around shutdown
▫ Review up and downstream process flows
• Use performance curves during operation
• Install polytrophic/measured discharge temp alarm
• Install surge alarm
• Install thrust movement and vibration protection systems
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Recommendations• Install on-line gas analyzer ▫ Account molecular weight variations for suction flow measurements▫ Improved compressor control
• Install leak detection monitors
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Recommendations were implemented and the replacement compressor ran well for several years until the plant was
shut down and dismantled