CASE STUDY REPORT:
FAILURE ANALYSIS OF CENTRIFUGAL PUMP 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
An oil and gas refinery requested us to analyze the cause of failure of the shaft of centrifugal pump that used to pump petrol mixture so we decided to take up the necessary investigation.
BACKGROUND
The centrifugal pump used to pump the petrol mixture to the separators of refinery; the failure results in a fire of the refinery with estimated total loss of RM200, 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:
.
during the operation in which the life of the shaft was expected to fail after , however it last up to ,so the pump should have failed earlier since the applied cycle stress exceeded the fautige strength for Based on the stress calculation, the stress amplitude was
CHEMICAL DESIGN ANALYSIS The chemical analysis of the broken shaft material show the sha ft 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 for metallographic studies. Also, the hardness test was carried out on the cross section of the shaft.
DISCUSSION
Table 1 shows the chemical analysis of the broken shaft material. The results of analysis show the shaft material is 4140 chromium Molybdenum steel which is quenched and tempered. From the closer visual inspection of the failed shaft, the fracture surface shows some bench marks which is a closure to high cyclic fatigue failure. From the operation data sheet, the pump was tripped and started for 12 times for around 12 hours before the final fractur e occurs. The material was evaluated for any change in general specifications. The microstructure analysis of shaft shows typical tempered martensite material which gives long fatigue life, which is as per design. The microstructure did not show any abnormality. From these results it is evident the material was not directly responsible for this failure. The fatigue analysis shows the shaft has been failed due to the high cyclic fatigue; it exceeds the fatigue life limits.
Failure Mechanism
Since the failure location is at the grooved area nearby the impeller where the seal are constructed The failure mechanism could be divided into three stages namely: Crack initiation. Crack propagation. Shaft sheared.
r ack C
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.
r ack C
propagation
Due to the rotation and vibrations along with cracking, the crack was propagated as a result of the fatigue phenomenon.
h aft S
sheared
The last step of failure occurred due to the huge stresses induced to the shaft. The huge stresses might be attributed to the reverse rotation of the shaft inducted by backflow.
Possibility of r evice C
o rrosion C
The possibility of localized corrosion particularly crevice attack on the shaft may arise due to its relatively poor protection in presence of the chemical flow (petroleum).
CONCLUSIONS
1.
The main contribution to the failure is the life of the shaft exceeded its fatigue limit.
2.
And also the combined action of environment, geometry and stresses could be one of the causes of the crack initiation at the failure location. The propagation stage might be resulted by corrosion fatigue. The final stage resulted by 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.
It is advised that RDC should be assigned to carry out compatibility studies by using duplex stainless steel as stationary parts with austenitic stainless steel as rotary parts.
REFERENCES 1. Handbook of case histories in failure analysis, ASM, volume 2, 1993. 2. Metal handbook, volume 11, Failure analysis and prevention, 9 th edition, 1986.
APPENDIXES -
Chemical analysis
TABLE 1: THE CHEMICAL COMPOSITION OF THE SHAFT MATERIAL
-
Mechanical analysis
Assume the weight of the shaft is infor mally distributed along the shaft Mechanical properties of shaft material
,hardeness Hv=378
-
stress calculation
The loading at the fracture surface are M=73.5 N.m T=834.56 N.m V=87.9 N
- stress concentration factor
-
Fatigue analysis
Assuming to
.
(completely reversed fluctuating stress) since it is small compare
Based the constant and exponent values in Basquin relationship from the table
Since the pump is stopped for 2 hours between 14 hours interval
Since which is more than the fatigue strength for during the long period of operation, the pump shaft should have failed
earlier
would be
The expected life of the pump under
- 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 m
0.137 m
0.4225 m
