CASE STUDY REPORT:FAILUREANALYSISOF CENTRIFUGALPUMP SHAFT NAME:
YAHYA AHMED AHMED ALDUQRI MATRIC NO: AM073003 PASSPORT NO: 03569372
DATE: NOVEMBER 23, 2010 PROF.DR: NASIR FAILURE ANALYSIS OF
CENTRIFUGAL PUMP SHAFT INTRODUCTION
Anoilandgasrefineryrequestedustoanalyzethecauseoffailureoftheshaftof
centrifugalpumpthatusedtopumppetrolmixturesowedecidedtotakeupthe
necessary investigation. BACKGROUND
Thecentrifugalpumpusedtopumpthepetrolmixturetotheseparatorsofrefinery;
thefailureresultsinafireoftherefinerywithestimatedtotallossofRM200,000.
The pump has been installed on May 1979 VISUAL INSPECTION
OBSERVATIONS 1.The shaft fractured nearby the impeller location at
the threaded/grooved area. 2.The failure appeared like fatigue
fracture. FIGURE 1: THE LOCATION OF THE FAILURE OF THE SHAFT WITH
DIMENSION OBSERVATION FROM OPERATION DATA SHEET 1.The pump is start
up at 2,975 rpm for 14 hours with a 2-hour complete shut-down
interval. 2.The pump normally operates 6 days a week throughout the
year 3.The pump was tripped and started for 12 times for around 12
hours. 4.The failure incident occurred on 30/07/2010 at 8:00 PM.
5.No information about the reason of the trips before the failure
incident. OBSERVATION FROM VIBRATION RECORDS 1.Pump was major
overhauled three times involving seal leaking. 2.From May 1979
until the end of July 2010 the vibration records shows the shaft is
normal. MECHANICAL DESIGN ANALYSIS Assuming the 4140 steel was
quenched and tempered which have the mechanical strength: . Based
on the stress calculation, the stress amplitude was during the
operation in which the life of the shaft was expected to fail after
cycles, however it last up to cycle,so the pump should have failed
earlier since the applied cycle stress exceeded the fautige
strength for cycle CHEMICAL DESIGN ANALYSIS The chemical analysis
of the broken shaft material show the shaft material is 4140 steel
alloy with chromium, Molybdenum and other element as shown in the
table below. TABLE 1: THE CHEMICAL COMPOSITION OF THE SHAFT
MATERIAL METALLOGRAPHIC STUDY The failed shaft was sectioned at
different locations formetallographic studies. Also, the hardness
test was carried out on the cross section of the shaft. DISCUSSION
Table1showsthechemicalanalysisofthebrokenshaftmaterial.Theresultsof
analysisshowtheshaftmaterialis4140chromiumMolybdenumsteelwhichis
quenchedandtempered.Fromthecloservisualinspectionofthefailedshaft,the
fracturesurfaceshowssomebenchmarkswhichisaclosuretohighcyclicfatigue
failure. From the operation data sheet, the pump was tripped and
started for 12 times for around 12 hours before the final fracture
occurs.
Thematerialwasevaluatedforanychangeingeneralspecifications.The
microstructureanalysisofshaftshowstypicaltemperedmartensitematerialwhich
giveslongfatiguelife,whichisasperdesign.Themicrostructuredidnotshowany
abnormality. From these results it is evident the material was not
directly responsible
forthisfailure.Thefatigueanalysisshowstheshafthasbeenfailedduetothehigh
cyclic fatigue; it exceeds the fatigue life limits. Failure
Mechanism Since the failure location is at the grooved area nearby
the impellerwhere the seal are construct edThe failure mechanism
could be divided into three stages namely: Crack initiation. Crack
propagation. Shaft sheared. Crack initiation The location of the
failed area is considered to be suitable for corrosion and cracking
due to the presence of crevices, the pitting initiation could be
started in the beginning followed by crack initiation. Due to the
presence of stress concentration area and rotation stresses, the
crack could be initiated from the pit area. Crack propagation Due
to the rotation and vibrations along with cracking, the crack was
propagated as a result of the fatigue phenomenon. Shaft sheared
Thelaststepoffailureoccurredduetothehugestressesinducedtotheshaft.The
hugestressesmightbeattributedtothereverserotationoftheshaftinductedby
backflow. Possibility of Crevice Corrosion
Thepossibilityoflocalizedcorrosionparticularlycreviceattackon
theshaftmay
ariseduetoitsrelativelypoorprotectioninpresenceofthechemicalflow
(petroleum). CONCLUSIONS
1.Themaincontributiontothefailureisthelifeoftheshaftexceededitsfatigue
limit. 2.Andalsothecombinedactionofenvironment,geometryandstresses
couldbe oneof thecausesof thecrack initiation at
thefailurelocation. Thepropagation
stagemightberesultedbycorrosionfatigue.Thefinalstageresultedby
mechanical stresses. RECOMMENDATIONS 1.Care to be taken by
operation during startup of the pump by ensuring closing of
discharge valve of down pump to avoid back flow from running pump.
2.All pump shafts must be inspected by DPT during routine
maintenance. 3.The pump motors must be internally inspected. 4.Itis
advised thatRDCshouldbe assigned tocarryoutcompatibility studiesby
usingduplexstainlesssteelasstationarypartswithausteniticstainlesssteelas
rotary parts. REFERENCES 1.Handbook of case histories in failure
analysis, ASM, volume 2, 1993. 2.Metal handbook, volume 11, Failure
analysis and prevention, 9th edition, 1986. APPENDIXES-Chemical
analysis TABLE 1: THE CHEMICAL COMPOSITION OF THE SHAFT MATERIAL
-Mechanical analysisAssume the weight of the shaft is informally
distributed along the shaft Mechanical properties of shaft material
,hardeness Hv=378 -stress calculation The loading at the fracture
surface areM=73.5 N.m T=834.56 N.m V=87.9 N - stress concentration
factor
-Fatigue analysisAssuming (completely reversed fluctuating
stress) since it is small compare to . Based the constant and
exponent values in Basquin relationship from the table 0peiating
houis yeais uays houis Since the pump is stopped for 2 hours
between 14 hours interval0peiating houis houis Numbei of cycles
uuiing it opeiating life cyclehouis houis cycle Since which is more
than the fatigue strength for cycleduring the long period of
operation, the pump shaft should have failed earlierThe expected
life of the pump under would be cycles - THE MICROSTRUCTURE OF THE
SHAFT METAL AT THE FAILURE LOCATION -assumption 1- Assume the
weight of the shaft is informally distributed along the shaft
0.4225 m0.4225 m0.137 m
