Formalized Failure Reporting as a Teaching Tool

1. Overview - this is the first paragraph or subsection in a formal failure report. The overview could also be called a ‘management summary’. As this term implies, it contains a synopsis of the entire investigation and answers in a few well-chosen words or sentences the basic questions of what happened, when-where-how, and possibly why.

2. Background information and/or summary of events – a formal failure analysis report will probably be read by management and staff personnel

overview

background information and/or summary of events

failure analysisconclusions

recommendations

whose familiarity with the equipment, process unit, operation etc. will vary greatly. It is thus appropriate to include pertinent background information in the failure analysis report so as to make it a ‘stand alone’ document.

3. Failure Analysis – this is normally the next major heading in a formal report. Detailed data collected and observation made are usually documented under this heading. This is where a competent researcher will put the solid evidence and will prove the soundness of his reasoning. Both of our demonstration reports contain this heading and lay the evidence on the table.

4. Conclusions – the word ‘conclusion’ has several dictionary definitions; the close or termination of something, the last part of a discourse or report, a judgment or decision reached after deliberation. The conclusion of a good failure analysis should be none of the above. Instead, it should contain the evident and logical outcome of the result of the observations, and thought processes documented up to this point. Both of the demonstration reports contain partial restatements of what probably caused the failure, as well as what was judge not to have caused the failure.

5. Recommendations – these are made last, their ultimate aim is of course a prevent recurrence of the problem failure. However recommendations can sometimes be separated into near term and long term requirements. Also they can be supplemented with an action plan, or simply a notation of action assignments and action status.

FAILURE ANALYSIS OF SHEARED SHAFT OF A BRINERECYCLE PUMP1

Saleh A. Al-Fozan, Anees U. Malik and Mohammad Al-Hajri

Research and Development CenterSaline Water Conversion Corporation (SWCC)P.O.Box 8328, Al-Jubail 31951, Saudi Arabia.

E-mail: rdc@swcc.gov.sa

INTRODUCTION

The Manager, Al-Jubail Plants in his letter No. 3226/1648 dated 6/12/2004 and

3221/1/4-560 dated 7/12/2004 addressed to the Manager, R&D Center, Al-

Jubail requested R&D Center to analyze the cause of shearing of brine recycle

pump shaft of C-3 unit # 15. The R&D Center decided to take up the necessary

investigation.

BACKGROUND

Al-Jubail plants Phase 2 A&B consist of 40 desal units. Each unit operated with

two brine recycle pumps. Pump columns and elbows are fabricated by Ni-resist

D2 construction material in all four areas. In C-2 and C-3 the brine recycle

pump columns and elbow were failed by stress corrosion cracking (SCC), while

C4 and C-5 brine recycle pumps were free from damage. The results of

investigation had clearly shown that the .cause of failure was originated from

material. Therefore, the columns and elbows were replaced by 2205 duplex

stainless steel, which has better corrosion resistance properties.

The upper and lower columns and elbow of brine recycle pump (A) of desal #

15 were replaced in 1997. At this time, the pump was overhauled. During the

period between last overhauled and failure, two new rewind motors were

installed.

VISUAL INSPECTION OBSERVATIONS

1. Pump was sheared at two locations.

2. Many cracks were found in key area.

3. The failure appeared like fatigue fracture. 4. Sleeve was found broken to many pieces.

1 Issued as Troubleshooting Technical Report No. TSR 3804/05005 in August 2005.

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 µ 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 µ).

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.

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.

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

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

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, 9 th edition,

1986.

Table 1. Shaft Material Analysis

Elements Chromium Nickel Molybdenum Manganese Silica Iron(Cr)% (Ni)% (Mo)% (Mn)% (Si)% (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

Top view

Side view

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

Figure 2. Cracks location on shaft key area

Bearing Bearin

g

Figure 3. Broken shaft located in bearing area

Figure 4. Closer view of first rupture in the shaft

Figure 5. Pitting corrosion on the shaft surface

cps

50 F

e

40

30

20

Fe

Cr

10

OSi

ClFeC

A Ca Cr N

l i

MgMo

0

Na

0 5 10 15 20Energy

(keV)

Figure 6. EDX profile of pit on the shaft surface

A

B

C

Figure 7. Closer view of one face of shaft fracture