Case Studies in Engineering Failure Analysis 1 (2013) 79–84

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Case Studies in Engineering Failure Analysis

jo ur n al ho m ep ag e: ww w.els evier . c om / lo cat e/c s efa

Case study

Fatigue failure of thermowells in feed gas supply downstream

pipeline at a natural gas production plant

Abdel-Monem El-Batahgy a,*, Gamal Fathy b

a Central Metallurgical R&D Institute, Cairo, Egyptb Natural Gas Production Plant, Cairo, Egypt

A R T I C L E I N F O

Article history:

Received 11 February 2013

Accepted 14 April 2013

Available online 25 April 2013

Keywords:

Thermowell configuration

Stress concentration

Wake frequency

Fatigue damage

1. Introduction

A natural gas production field has been set into operation about six years ago. Natural gas produced from the well isprocessed through different stages before being supplied through pipelines for local market. Temperatures of the gas duringits different processing and supplying stages are being monitored using thermowells made of type 316L austenitic stainlesssteel. In this concern, flanged, straight type thermowells were used for the feed gas supply downstream pipeline (30 in.diameter). This thermowell type consists of a straight tube and a flange. The tube outer diameter, thickness and length are19 mm (0.75 in.), 6.5 mm (0.256 in.), 260 mm (10.24 in.) while the flange diameter and thickness are 155 mm (6.1 in.) and23 mm (0.91 in.) respectively. The thermowells are fixed in a vertical position where its flanges are 200 mm above the top ofthe pipeline. Actual operating conditions are given below. After only one year of operation, several thermowells haveexperienced failure.

2

h

Characteristics

* Corresponding author. Tel.: +20 122 4608265; fax: +202 25010639.

E-mail address: elbatahgy@yahoo.com (A. El-Batahgy).

213-2902 � 2013 Elsevier Ltd.

ttp://dx.doi.org/10.1016/j.csefa.2013.04.001

Open access under CC BY-NC-ND license.

Minimum

Maximum

Flow rate, MMSCFD

700 1100

Pressure, MPa (psi)

7.5 (1071) 8.5 (1214)

Temperature, 8C (8F)

12 (53.6) 28 (82.4)

Gas velocity, m/s

7.6 12.4

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2. Investigations

Failed thermowells were subjected to different non-destructive and destructive tests including visual investigation,dimensions measurement, liquid penetrant test, stereoscopic examination, chemical analysis, optical and scanning electronmicroscopic examinations, and hardness measurements.

2.1. Non-destructive investigation

General and enlarged views of one of several failed thermowells are shown in Fig. 1. One important notice is thatthermowell was circumferentially cracked at the neck of tube with flange. It can be noticed that the outer surface of the failedthermowell is clean and free from deposits or indications for corrosion. Dye penetrant inspection indicated no other surfacecracks either around or away from main crack zone.

Enlarged views of cracked thermowell, during and after separation of the remainder of its circumference, are shown inFig. 2. Crack initiation zone is highlighted with arrows. Visual examination showed that thermowell had cracked along neckbetween flange and tube. The crack extended more than half way around the circumference of the neck. The remainder of thecircumference had been separated gradually with hand. It was not clear whether the crack originally extended more thanhalf way around the circumference, or whether it had been shorter but part of it was extended by mishandling during or afterremoving thermowell outside of the pipeline. However, it is clear that crack was started at outer surface of thermowell neck,which is considered as stress concentration zone.

Stereoscopic photograph of fractured/cracked surface of thermowell is shown in Fig. 3. It is obvious that fracture wasinitiated at outer surface of thermowell tube’ neck where smooth fracture surface can be seen. Approximately 80% of thefracture surface appeared relatively smooth and associated with beachmarks of a propagating ductile crack. The remaining20% of the surface had a rough texture associated with the final brittle fracture of the thermowell. Beachmarks, crackinitiation sites on outer surface and propagation directions are highlighted with arrows. Generally, internal surface ofthermowell tube showed smooth surface with no indications for internal corrosion. In other words, no thinning or variationin tube wall thickness was observed where uniform wall thickness (6.5 mm) was obtained at both fractured and non-fractured zones.

2.2. Destructive investigation

Specimens from both failed and non-failed zones of thermowell were cut out and prepared for chemical analysis,metallographic examination, and hardness measurements. Results of chemical analysis of failed thermowell together withthe specified chemical composition range of type 316 stainless steel are shown in Table 1. It is obvious that chemicalcomposition of the used thermowell is a typical for austenitic stainless steel type 316.

Fig. 1. General (a) and enlarged (b) views of one of several failed thermowells. Note that crack had occurred at the neck between flange and tube.

Fig. 2. Enlarged views of cracked thermowell during (a) and after (b) separation of the remainder of its circumference. Crack initiation zone is highlighted

with arrows.

Fig. 3. Stereoscopic photograph of fractured/cracked surface of thermowell. Note beachmarks at crack initiation zone.

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Optical micrographs of etched cross sections taken from both fractured and non-fractured zones of thermowell are shownin Fig. 4a and b, respectively. Typical austenitic microstructure was obtained for both fractured and non-fractured zones. Nomicro-cracks or other internal defects were found in either fractured or non-fractured zones.

Survey of hardness measurements indicated almost same hardness values for both fractured and non-fractured zoneswhere average hardness values of 187HV and 179HV were obtained for fractured and non-fractured zones, respectively.

Table 1

Results of chemical analysis (wt%) of the used thermowell together with the specified chemical composition range for 316 stainless steel.

Material C Si Mn S P Cr Ni Mo

Failed thermowell 0.03 0.38 1.99 0.004 0.026 18.00 10.04 2.1

316 stainless steel �0.08 �1.0 �2.0 �0.03 �0.04 16.0–19.0 9.0–12.0 2.0–3.0

Fig. 4. Etched optical micrographs of cross sections taken from fractured (a) and non-fractured (b) zones of failed thermowell showing normal austenitic

structure.

Fig. 5. Scanning electron microscopic photographs with different magnifications of fracture suspected initiation zone showing fatigue striations.

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A. El-Batahgy, G. Fathy / Case Studies in Engineering Failure Analysis 1 (2013) 79–84 83

In order to help in identification of failure mechanism, cracked/fractured surface including suspected initiation zonewas investigated using scanning electron microscope. It is confirmed that crack initiation zone are confined tothermowell tube outer surface just at thermowell neck where stress concentration sites were existed. Scanning electronmicrographs with different magnifications of fracture suspected initiation sites at neck outer surface are shown in Fig. 5.

Crack initiation sites can be seen on outer surface of thermowell neck where a single macroscopic direction of crackpropagation is impossible to be defined. This is because crack size is still in the microcrack zone, where multiple cracksform at the surface, initiating at different locations and with different orientations. The important notice is the fatiguestriations at fracture initiation zone on neck outer surface. Brittle fracture was observed away from fracture initiationzone.

3. Discussion

Chemical analysis and metallurgical examinations revealed that the failed thermowell material is conformed tospecifications of 316L austenitic stainless steel. Visual and macroscopic examinations of the failed thermowells showedthat the fractured zone was confined only to thermowell neck where cracks were propagated from outer surface intoinner surface in two opposite directions along tube circumference. No indications for corrosion attack were observed. Itwas not clear whether the crack originally extended more than half way around the circumference of thermowell neck,or whether it had been shorter but part of it was extended by mishandling during or after removing the thermowelloutside of the pipeline. However, it is clear that crack was started at outer surface of thermowell neck that is consideredas stress raiser zone.

Stereoscopic examination of fracture surface of failed thermowell indicated beachmarks at crack suspected initiationzone. Scanning electron microscopic investigation of fracture surface showed fatigue striations at fracture initiation zone.These findings support fatigue damage as a failure mechanism [1–5].

Fatigue failure is the phenomenon leading to fracture under repeated or fluctuating stresses that are less thanthe tensile strength of the material. Fatigue fractures are progressive, beginning as minute cracks that grow underthe action of fluctuating stress. There are three stages of fatigue failure: initiation, propagation, and final fracture.The initiation site is minute, never extending for more than 2–5 grains around the origin. The location of the initiation isat a stress concentration [6–9]. It is believed that thermowell neck that acts as local stress raiser played a remarkablerole in fatigue failure. In other words, thermowell neck worked as site for initiation of fatigue damage on its outersurface.

A cyclic stress could have been applied due to pressure surges, pressure pulses, and/or overpressure stresses.Generally, thermowells are subjected to more than just the static forces from the medium going past. They can also havevibrations induced from the medium vortices in the wakes created by the interaction between them and the medium.This is most significant in the realm of highly energetic flows. The induced vibrations are very critical when theirfrequency corresponds to the resonance frequency of a thermowell. Under such conditions, not only the temperaturesensors can literally be pounded to pieces but also thermowells themselves can rupture in extreme cases. In otherwords, medium flow past a thermowell causes vortices to be shed at a frequency, termed the wake frequency,proportional to the flow velocity. If the wake frequency is at or near the natural frequency of thermowell, a resonancecondition may occur where massive amounts of energy are absorbed by the thermowell, resulting in very high stressesand possible failure.

After fatigue crack is formed, it becomes an extremely sharp stress concentration that tends to drive the crack ever deeperinto the metal with each repeating of the stress. The local stress at the tip of the crack is extremely high because of the sharp‘‘notch,’’ and with each crack opening, the depth of the crack advances by one ‘‘striation’’. Striations are very tiny, closelyspaced ridges that identify the tip of the crack at some point in time.

Whenever there is an interruption in the propagation of a fatigue fracture a unique feature of macroscopically visiblemarks or ridges may be found. These marks are described as ‘‘beachmarks’’ or ‘‘growth rings’’. Fig. 3 shows an example ofbeachmarks in the subject fatigue failure. Beachmarks must not be confused with striations, although they frequently arepresent on the same fracture surface; there may be many thousands of microscopic striations between each pair ofmacroscopic beachmarks. As the propagation of the fatigue crack continues, gradually reducing the cross-sectional area, iteventually weakens the material so greatly that final, complete fracture occurs.

4. Conclusion

Based on the results obtained in this investigation, it can be concluded that the subject premature failure of the subjectthermowells is attributed to fatigue damage, mainly due to improper selection of thermowell design for the concernedoperation condition. The used flanged, straight type thermowell configuration has played a remarkable role in the initiationof fatigue damage due to higher stress concentration at its sharp neck. High stress concentration at thermowell neck andflow-induced vibration, both have shortens the lifetime of the used flanged, straight type thermowell.

It is believed that high flow velocity of the pipeline medium has increased the wake frequency to be at or near the naturalfrequency of the thermowell that in turn resulted in a resonance condition where massive amounts of energy were absorbedby the thermowell, resulting in very high stresses and possible failure.

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5. Action taken

In order to avoid such failure in future, the failed thermowells were replaced with new ones having a modifiedconfiguration to minimize both stress concentration and resonance. In this regard, thermowell having a continuous low-gradient slope (truncated conical-type thermowell) was used since it is structurally stronger, having a higher naturalfrequency and a lower stress concentration in comparison with the failed one.

References

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