Scholarly article on topic 'Leg weld fatigue cracks in anhydrous ammonia nurse tanks'

Leg weld fatigue cracks in anhydrous ammonia nurse tanks Academic research paper on "Materials engineering"

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Abstract of research paper on Materials engineering, author of scientific article — A.M. Russell, A.T. Becker, L.S. Chumbley, D.A. Enyart

Abstract In an accident in southwest Iowa, USA in 2012, an anhydrous ammonia nurse tank vented its entire cargo of 5500L (1500gallons) of liquid ammonia to the atmosphere. Follow-up study of the failed tank revealed a through-crack along a weld used to connect the tank to its running gear. Side-angle ultrasound examinations were performed on 532 used anhydrous ammonia nurse tanks to measure the locations, sizes, and orientations of flaw indications. The tanks examined had manufacture dates ranging from 1952 to 2011. A total of 83 indications were found in or near the leg welds of 50 of these 532 tanks. Several factors suggest that these indications are fatigue cracks, not the stress corrosion cracks more commonly detected in nurse tanks. These findings suggest that roughly 9% of the 200,000 nurse tanks in the U.S. nurse tank fleet may contain leg-weld fatigue cracks. Nurse tanks are the only large, pressurized packages for hazardous cargo that do not contain manways; thus, their interior walls cannot be inspected for flaws with magnetic particle or fluorescent dye penetrant methods. Since the tank interior is inaccessible, side-angle ultrasound is the only detection method capable of detecting cracks in nurse tanks initiating at both interior and exterior tank surfaces. For this reason, the authors recommend that side-angle ultrasound be considered for use in periodic nurse tank inspections.

Academic research paper on topic "Leg weld fatigue cracks in anhydrous ammonia nurse tanks"

CASE STUDIES IN ENGINEERING FAILURE ANALYSIS

ELSEVIER

Case study

Leg weld fatigue cracks in anhydrous ammonia nurse tanks

A.M. Russell3,*, A.T. Beckera, L.S. Chumbleya, D.A. Enyartb

a Department of Materials Science and Engineering, 2220 Hoover Hall, Iowa State University, Ames, IA 50011, USA b Center for Nondestructive Evaluation, 275 ASCII, Iowa State University, Ames, IA 50011, USA

ARTICLE INFO ABSTRACT

In an accident in southwest Iowa, USA in 2012, an anhydrous ammonia nurse tank vented its entire cargo of 5500 L (1500 gallons) of liquid ammonia to the atmosphere. Follow-up study of the failed tank revealed a through-crack along a weld used to connect the tank to its running gear. Side-angle ultrasound examinations were performed on 532 used anhydrous ammonia nurse tanks to measure the locations, sizes, and orientations of flaw indications. The tanks examined had manufacture dates ranging from 1952 to 2011. A total of 83 indications were found in or near the leg welds of 50 of these 532 tanks. Several factors suggest that these indications are fatigue cracks, not the stress corrosion cracks more commonly detected in nurse tanks. These findings suggest that roughly 9% of the 200,000 nurse tanks in the U.S. nurse tank fleet may contain leg-weld fatigue cracks. Nurse tanks are the only large, pressurized packages for hazardous cargo that do not contain manways; thus, their interior walls cannot be inspected for flaws with magnetic particle or fluorescent dye penetrant methods. Since the tank interior is inaccessible, side-angle ultrasound is the only detection method capable of detecting cracks in nurse tanks initiating at both interior and exterior tank surfaces. For this reason, the authors recommend that side-angle ultrasound be considered for use in periodic nurse tank inspections.

© 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Contents lists available at ScienceDirect

Case Studies in Engineering Failure Analysis

journal homepage www.elsevier.com/locate/csefa

CrossMark

Article history:

Received 24 February 2015

Received in revised form 11 March 2015

Accepted 12 March 2015

Available online 20 March 2015

Keywords: Fatigue failure Non-destructive inspection Hazardous cargo transport Residual stress

1. Introduction

Nurse tanks (Fig. 1) are welded steel pressure vessels used to transport anhydrous ammonia fertilizer from vendor sites to farm fields. There are hundreds of thousands of ammonia nurse tanks in use worldwide; some have been in service for more than 60 years. The steels used to manufacture nurse tanks are all low-carbon steels with mixed ferrite-pearlite microstructures (e.g., ASTM A285, ASTM A455, and ASTM A516 grade 70).

Anhydrous ammonia (NH3) is among the most dangerous chemicals used in agriculture. Accidental NH3 releases have caused deaths, severe injuries, and extensive property damage. NH3 damages skin, lung, eye, and mucous membrane tissue; causes frostbite; and suffocates victims. At one atmosphere pressure, NH3 boils at -33 °C; thus, it must be stored under pressure to remain liquid at ambient temperatures. The possibility of failure of the pressurized vessel adds explosion hazard to the other dangers. The dangers posed by either slow or explosive NH3 releases make the safe storage and transport of anhydrous ammonia an important concern for both agricultural workers and the general public [1-3].

* Corresponding author at: 2220K Hoover Hall, Iowa State University, Ames, IA 50011, USA. Tel.: +1 515 294 3204; fax: +1 515 294 5444. E-mail addresses: russell@iastate.edu (A.M. Russell), beckerandy@gmail.com (A.T. Becker), chumbley@iastate.edu (L.S. Chumbley), denyart@iastate.edu (D.A. Enyart).

http://dx.doi.org/10.1016Zj.csefa.2015.03.002

2213-2902/© 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/ 4.0/).

Fig. 1. Anhydrous ammonia nurse tank mounted on running gear. Inset shows that the leg attachment brackets are welded directly to the tank body to connect the tank to the running gear.

Nurse tank failures most often result from stress corrosion cracking (SCC) or welding defects introduced during manufacture or repair [4-8]. In one incident [7], an exploding tank rocketed across a farmyard and struck a tractor, severing the rear wheels and cab from the engine and front wheels. In another incident [6], a weld failed while the tank was being filled, killing one worker and inflicting permanently disabling lung injuries on another.

In a 2012 incident near Casey, Iowa, USA, a nurse tank rapidly vented its entire contents from a fracture initiated at a crack on a leg weld. A nearby worker escaped injury by running to his truck and driving upwind away from the expanding ammonia cloud, which destroyed more than a hectare of corn plants. That incident occurred while the authors were performing an extensive study of SCC in nurse tanks [9-11] that included examination by side-angle ultrasound of all welds on each of 532 used nurse tanks. The great majority of the 3326 indications detected by examining those tanks appeared in or near the longitudinal and circumferential welds used to fabricate the tanks from plate (Fig. 2), but the measurements also revealed that more than 9% of the tanks showed indications in the heat-affected zones near the tanks' leg welds (Fig. 1 inset). These leg weld indications are the subject of this study.

2. Materials and methods for measuring flaw size, location, and orientation

During May-August, 2012, 532 tanks owned by farm cooperative companies in central Iowa, USA were examined by ultrasound. Only areas near welds were examined, generally in a band approximately 200 mm wide centered on the weld. Ultrasound cannot discriminate perfectly between cracks and other defects in tank steel. In recognition of this fact, the term indications is generally used to describe ultrasound reflections that reveal a discontinuity in the metal. Indications are usually

Circumferential Welds

Longitudinal Welds

Fig. 2. Locations of circumferential and longitudinal welds in nurse tanks.

cracks, but other types of discontinuities can also generate indications [9]. A mixture of 3800-L (1000-gallon) and 5500-L (1450-gallon) tanks was inspected. Only tanks with legible data plates (which indicate the year of manufacture) were inspected.

Ultrasound examination of the nurse tanks was generally performed in accordance with the ASME Boiler and Pressure Vessel Code 2011a Section V Article 4: Ultrasonic Examination Methods for Welds. The details of these methods are described elsewhere [9]. Nurse tanks do not contain manways, so only external examination is possible. A tank would have to be cut open to reveal its interior. Since the tanks were in active commercial service (and contained pressurized ammonia during the inspections), cutting tanks to obtain access to their interior walls was not feasible.

3. Results and discussion

3.1. The relation between tank age and the number of leg weld indications

Fig. 3 displays the numbers and years of manufacture for the tanks inspected. It shows that most tanks bought by farm cooperatives in central Iowa manufactured before the mid-1980s had a 3800-L capacity; tanks manufactured more recently usually had a 5500-L capacity.

Newer tanks generally had more indications in the ultrasound examinations of their leg welds than older tanks. Fig. 4 shows the distribution of leg indications as a function of year of manufacture. Changes in ASME specifications allowed tanks manufactured after 1998 to be built with thinner steel if the manufacturer followed a 100% longitudinal weld radiographic inspection regimen. This thinner steel must carry the same loads as the thicker sections previously used so the stress in the thinner metal is greater. The data do not show a sharp transition in the numbers of indications before and after the 1998 ASME specification change. More numerous leg weld indications more nearly correlate to the transition from 3800-L to 5500-L tanks that occurred in the mid-1980s.

3.2. External visibility and orientation of leg weld indications

In some cases, ultrasound indications around leg welds corresponded to hairline cracks visible from the tank exterior (Fig. 5). This contrast with the much more numerous indications found near the long circumferential and longitudinal welds (Fig. 2) where no cracks were externally visible at the locations indicated by ultrasound.

T3 25 о

■B 10

□ □ □

— ♦ 3800-liter tanks □ 5500-liter tanks

□ □

♦ * ♦ ♦ ♦ □ □

♦ * ♦ ♦ □ ♦ □ □

* □ ♦ -I II ll lll 1ТГШ ♦ □ □ □ 1 1 1 1 1 1 П—1 ♦ □ □ ♦ П □ i—a—m □ n + I □ □ i ♦

1970 1990

Year of Manufacture

Fig. 3. Scatter plot of the tanks inspected as a function of year of manufacture.

a> 4 .c

▼ t f ▼

MM* ♦-

1950 1960 1970 1980 1990 2000 2010 2020 Year of Manufacture

(ft (A

§)= 40% iS a) c ® o> o i— <0 £L

0%< 1950 1960

♦ ♦

♦ ♦

1970 1980 1990 2000 Year of Manufacture

Fig. 4. Nurse tanks with indications at leg welds (upper plot) absolute number of tanks, (lower plot) as a percentage of total tanks tested.

The leg indications also differed from the indications near circumferential and longitudinal welds because 96% of the leg indications were parallel to the weld (an orientation more commonly caused by fatigue than the perpendicular case), while only 15% of the indications near circumferential and longitudinal welds lay parallel to the weld.

Several factors suggest that the leg weld indications are probably fatigue cracks, not stress corrosion cracks. These include:

• The fusion zones of leg welds (Figs. 1 and 5) do not penetrate to the nurse tank interior. Therefore, the residual tensile stresses resulting from differential thermal contraction in the metal around the fusion zone would be substantially smaller on the tank interior than they would be for circumferential and longitudinal welds on the main tank body, which do penetrate to the tank interior [10]. Moreover, cracks at leg welds were sometimes visible on the tanks' exterior surface, but in most cases they were not accompanied by any detectable release of NH3 vapor even though all the tanks examined were holding full loads of pressurized NH3. If those cracks had initiated at the tank wall interior surface where NH3 drives SCC, they would have to have been through-cracks to be visible on the tank exterior. A through-crack on a pressurized nurse tank will leak NH3, and the sharp odor of NH3 would be quickly detected for even a small leak. Thus, it seems unlikely that the leg weld indications were cracks that initiated at the tank interior as do cracks from SCC.

• For the indications detected in the 532 nurse tanks that were thought to be stress corrosion cracks, there was a large reduction in the numbers of indications in tanks that had been given a full stress-relief anneal after their welding had been completed during manufacture. (Such annealing treatments are sometimes called post-weld heat treatment.) Of the 532 tanks examined, 104 had been given full-body, stress-relief anneals, and those stress-relieved tanks averaged only 0.77 indications per tank, while the tanks without a stress-relief anneal averaged 7.55 indications per tank [9]. Most of the stress-relieved tanks were manufactured during the 1990s. However, the leg indication data show no reduction of

Fig. 5. Three photographs of a leg weld crack from the 2012 Casey, Iowa incident nurse tank that penetrated the entire wall thickness of the tank, causing rapid NH3 venting. The region showing a hairline crack is circled on the exterior view (upper image). The interior view (middle image) shows fluorescent dye penetrant highlighting the crack under ultraviolet illumination. A cross-section view of the crack (lower image) indicates that the crack initiated at the weld bead/tank shell interface on the exterior surface.

LEFT PROFILE VIEW FRONT VIEW

Fig. 6. Orthographic views of a nurse tank leg weld assembly utilizing reinforcing pads (highlighted in blue) welded to both the tank wall and the support bracket. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

indications for tanks manufactured during the 1990s, a sharp departure from the low incidence of SCC in those tanks. This suggests that the leg indications may have a cause other than the SCC that was thought to cause the great majority of the 3326 indications in the tanks. The leg weld carries a substantial load from the tank to the running gear, and tanks are typically towed over rough, uneven ground in farm fields. This causes cyclic loading of the welds of the sort that can lead to fatigue cracking.

Assuming the leg weld indications are indeed fatigue cracks, their detection and remediation will require different strategies than those appropriate for the much larger numbers of SCC flaws found near circumferential and longitudinal welds. Side-angle ultrasound is not routinely used to inspect nurse tanks, but these findings suggest that it can be a useful tool to identify cracks before they become through cracks that leak NH3 or critical-sized cracks that cause tank explosions.

ASME offers a number of examples of guidance for how running gear feet may be attached to the tank shell, but none constitutes a specification. The risk of fatigue cracks at leg support welds could be reduced by modifying the conventional leg supports (Fig. 1) to include a steel reinforcing pad that is welded to the pressure vessel; the leg support is then welded to the pad, as shown in Fig. 6. This configuration reduces the stress along the weld on the pressure vessel wall [12] and makes it more likely that any fatigue failure that might occur will be located in the pad/leg weld, which poses no threat of anhydrous ammonia escape. One U.S. tank manufacturer recently adopted this design change for its nurse tanks.

4. Conclusions

Several conclusions can be drawn from these findings:

1. More than 9% of 532 nurse tanks examined by side-angle ultrasound contained indications near the leg welds; a total of 83 indications were detected in 50 of the 532 nurse tanks inspected.

2. Nearly all (80 of 83) leg weld indications lay parallel to the weld.

3. Several factors strongly suggest that leg weld indications are fatigue cracks rather than stress corrosion cracks.

4. On average newer tanks had more leg-weld indications than older tanks.

5. No clear correlation was observed between full-body stress-relief annealing of a tank and its incidence of leg weld indications. This contrasts with observations of stress corrosion indications near circumferential and longitudinal welds, which appear about 10 times less frequently in tanks given full-body stress-relief annealing.

6. Assuming the leg weld indications are fatigue cracks, their inspection and remediation will require different strategies than those used for SCC flaws near circumferential and longitudinal welds.

If the incidence rate observed in this study were extrapolated to the 200,000 nurse tanks in use in the United States, it implies that the U.S. nurse tank fleet contains roughly 18,000 tanks with one or more leg-weld fatigue cracks. Thus, many thousands of nurse tanks containing fatigue cracks in the pressurized tank wall are being used to transport pressurized, toxic ammonia over roadways and agricultural fields. Nurse tanks are the only large, pressurized packages for hazardous cargo that do not contain manways; thus, their interior walls cannot be inspected for flaws with magnetic particle or fluorescent dye penetrant methods. The authors recommend that side-angle ultrasound be considered for use in periodic nurse tank inspections.

Acknowledgments

The authors gratefully acknowledge the financial and technical support of the United States Federal Motor Carrier Safety Administration, contract DTMC75-07-D-00006 Task order DTMC75-08-J-00017. The authors also acknowledge the extensive assistance received from D. Goettee and A. Fleener of the U.S. Department of Transportation, Tom Mowrer and his

staff at West Central Coop, Boone, IA, and G. McRae of Trinity Industries. In addition we acknowledge the consultation of all 23 members of the nurse tank safety project peer review group who provided guidance throughout the project.

References

[1] Blanken JM, Adams L, Brown R, Cracknell A, Guild J, Krishner AS, et al. Plant/Oper. Progr. 1983;2:247.

[2] HSEES 1997 Annual Report, U.S. Department of Health and Human Services.

[3] Welch A. Exposing the dangers of anhydrous ammonia. Nurse Pract 2006;31:40-5.

[4] Dawson TJ. Behavior of welded pressure vessels in agricultural ammonia service. Weld J 1956;35:568-74.

[5] Anhydrous ammonia nurse tank rupture kills agricultural cooperative worker. Iowa Case Report: 03IA027, Report Date: June 24, 2005

[6] Nurse tank failure with release of hazardous materials. Hazardous materials accident report NtSB/HZM-04/01, PB2004-917001. National Transportation Safety Board: Washington, DC; Apr 2003.

[7] Testing and recommended practices to improve nurse tank safety, phase I, federal motor carrier safety administration report No. FMCSA-RRR-13-032, October 2013.

[8] Packer engineering. Metallurgical evaluation of a cracked head. Final Report, DAS-438. Office of Hazardous Materials Technology, U.S. Department of Transportation; April 2008.

[9] Russell AM, Chumbley LS, Becker AT. Anhydrous ammonia nurse tank safety. Saarbrücken, Germany: Scholars' Press, OmniScriptum GmbH & Co. KG; 2014, ISBN 978-3-639-71848-5, 276 p..

[10] Becker AT, Chumbley LS, Goettee D, Russell AM. Neutron diffraction analysis of residual stresses near welds in anhydrous ammonia nurse tanks. J Agric Saf Health 2014;20:3-13.

[11] Testing and recommended practices to improve nurse tank safety: phase, II report, FMCSA-RRR-13-055. U.S. Dept. of Transportation; 2013 p. 43-52.

[12] Blachet J, Magnucki K. Strength, stability, and optimization of pressure vessels: a review of selected problems. Appl. Mech. Rev. 2008;61:1-33.