failure analysis of sheared shaft of a brine recycle pump

8/12/2019 Failure Analysis of Sheared Shaft of a Brine Recycle Pump...

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4. Sleeve was found broken to many pieces.

OBSERVATION FROM OPERATION DATA SHEET

1. Unit was tripped or shutdown on 30/10/2004 between 12:00 and 16:00 hours.

2. Unit was restarted between 16:00 and 20:00 hours with one pump only (Pump B)

on the same day.

3. On 31/ 10/2004, between 16:00 and 20:00 hours, the pump B trip again.

4. No information about the reason of second trip was available.

5. Pump A was running without any indication of problems as per plant staff.

6. Pump was not returned to service after the first trip due to the failure.

OBSERVATION FROM VIBRATION RECORDS

1. Pump was overhauled on 1997.

2. From year 1997 until the end of Jan 2000 the vibration trend was excellent.

3. On Aug 19, 2000 new rewound motor was installed.

4. The vibrations of new motor were considered to be high.

5. The vibration of coupled motor and pump was slightly high but in acceptable

range.

6. The pump was kept in operation.

7. On 8/9/2002, abnormal sound from water cooling line was recorded.

8. On 1/11/2003 abnormal sound was heard at discharge.

9. Vibrations were in the same range (104 at 1x).

10. On 26/1/2004 new rewound motor was installed.

11. Solo ran vibrations were high.

12. Balancing of motor was carried out.

13. After coupling vibrations of pump was kept as they were before motor

replacement (slightly higher than 102 ).

14. Last vibration reading recorded before incident was on 8/8/2004.

15. Startup with present of T&I was carried out on 31/10/2004, one day after unit

trip. Pump was found noisy and not taken load during test run.

16. Motor was transferred to desal 19 (P-3B), solo run was carried out on 6/11/04 as

per T&I vibration records.


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17. A metal piece was found inside motor.

18. T&I recommended internal inspection of the pump by WR#1888619 dated

18/12/2004.

DPT RESULTS

1. Cracks were found at many locations in key position at both sides.

2. One crack was found in groove in sleeve area.

Figure 1 shows the cracks location on the shaft.

SAMPLE PREPARATION FOR 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.

CRACKS IN KEY POSITION AREA

Many cracks were found in the key position. These cracks were concentrated in one

edge area. Figure 2a shows the locations of these cracks at key position area. Figure 2b

shows closer view of the cracks on the shaft.

Two other brine recycle pump shafts were tested by DPT (D-1 P3-B & D-33 P3-B) and

no crack was found as per T&I DPT report.

DISCUSSION

Table 1 shows the chemical analysis of the broken shaft material. The results of

analysis show the shaft material is Austenitic 316 SS. From the closer visual inspection

of the failed parts, many cracks were found in different locations especially in key area.

Also, some pits were found on the shaft surface, which indicate corrosion activities on

the shaft. From the operation data sheet, it appeared that the final stage of failure was

occurred during the start-up of the unit after the trip.


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Figure 3 shows the location of shearing in bearing area. The accumulation of corrosion

product can be clearly seen in the sheared area. This corrosion initiated in sheared area

due to the non-availability of protective film due to the shearing. Figure 4 shows the

failed shaft after removal from its location.

The material was evaluated for any change in general specifications. The chemical

analysis of shaft shows typical Austenitic 316 SS material, which is as per design. The

microstructure did not show any abnormality. Also, the hardness of material does not

show any deterioration of material as shown in table (2). From these results it is evident

the material was not directly responsible for this failure.

From the visual inspection, some pitting was observed on the shaft surface. Figure 5

shows these pits. The EDX analysis was carried out to investigate the origin of the pits.

Figure 6 shows the results of the EDX analysis, the chloride ions were found in the pits.

This indicates the initiation of corrosion process in the shaft area. The corrosion would

have initiated during the operation and/or during long or short shut down. Figure 7a &

b show the closer view of the one face of the failed fracture. The fracture shows typical

fatigue failure. The crack initiation was located in bearing key area. This area could be

classified as crevice area.

Failure Mechanism

The 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


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Due to the rotation and vibrations along with cracking, the crack was propagated as a

result of the fatigue phenomenon.

Shaft sheared

The last step of failure occurred due to the huge stresses induced to the shaft. The huge

stresses could be attributed to the reverse rotation of the shaft inducted by backflow .

Cracks on key area

Many cracks were found in the keys area of BRP P3-A. Some of these cracks were

found deeply penetrated. The dye penetration test (DPT) of the two pump shaft of desal#1 & 33 was carried out to investigate the extent of problem in other pumps. As per the

attached DPT result sheets, no crack was found in both the shafts.

As per design the key should fit well in the groove. If there is any looseness then the

key will hit the key groove specially during startup and shutdown which appear to be

the main reason responsible for initiation of these cracks.

Possibility of Crevice Corrosion

The possibility of localized corrosion particularly crevice attack on 316L shaft may

arise due to its relatively poor protection in presence of duplex stainless steel discharge

column pipe which is more cathodic. This case is reverse to the previously used pumps

where Ni-Resist casing used to provide cathodic protection to 316L shaft.

CONCLUSIONS

1. The combined action of environment, geometry and stresses caused the crack

initiation. The propagation stage resulted by corrosion fatigue. The final stage

resulted by mechanical stresses.

2. The cracks on the key area could be due to the looseness of key installation in key

groove.

RECOMMENDATIONS


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1. Care to be taken by operation during startup of brine recycle pump by ensuring

closing of discharge valve of down pump to avoid back flow from running pump.

2. All brine recycle 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, 9thedition, 1986.

Table 1. Shaft Material Analysis

Elements Chromium

(Cr)%

Nickel

(Ni)%

Molybdenum

(Mo)%

Manganese

(Mn)%

Silica

(Si)%

Iron

(Fe)%

Compositions 17.7 11.5 2.1 1.6 0.3 bal

Standard 16-18 10-14 2-3 2 1 bal

Table 2. Hardness Values of Broken Shaft

Location Hardness Value (BHN)

From Edge to shaft center 200 158 150 140 145

Near from key area 165 165 162 165 169


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Figure 1. Cracks Location on Shaft Surface as detected by visual examination

Top view

Side view


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Figure 2. Cracks location on shaft key area

BearingBearing


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Figure 3. Broken shaft located in bearing area

Figure 4. Closer view of first rupture in the shaft


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Figure 5. Pitting corrosion on the shaft surface


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Figure 6. EDX profile of pit on the shaft surface

0 5 10 15 20Energy (keV)

0

10

20

30

40

50

cps

COFe

NaMg

AlSi

Mo

Cl

Ca

Cr

Cr

Fe

Fe

Ni


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Figure 7. Closer view of one face of shaft fracture

A

BCrack initiation

failure analysis of sheared shaft of a brine recycle pump

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