Inservice Waste Heat Boiler Failure

Engineering Failure Analysis 13 (2006) 683–697

www.elsevier.com/locate/engfailanal

Failure analysis and acoustic emission tests of anin-service waste heat boiler

Yu-Lin Han a,*, Shi-Qiang Zhang b, Shu-Jun Zheng b, Wen-Guang Zhu b,Bin Zhang b

a College of Civil Engineering, Southeast University, Nanjing City 210096, PR Chinab Jingxi Natural Gas Chemical Corporation, Huludao City 125001, Niaoling Province, PR China

Received 10 October 2004; accepted 11 October 2004

Available online 16 June 2005

Abstract

A series of failure phenomena of an in-service waste heat boiler is presented and the failure is analyzed. After the

failed parts of the boiler were repaired, the boiler was tested by acoustic emission. The repaired and tested boiler

had operated safely for three months by July 2004.

� 2005 Elsevier Ltd. All rights reserved.

Keywords: Failure analysis; Boiler tube; Buckling; High temperature fatigue; Acoustic emission

1. Introduction

Jingxi Natural Gas Chemical Corporation (JNGCC) has an ammonia synthesis plant operating by theBraun Purifier Process. The throughput of JNGCC is 300,000,000 kg ammonia per year. A waste heat boi-

ler, a thin tube-sheet heat exchanger, is a key part of the ammonia synthesis plant. A waste heat boiler of

JNGCC, site number 100E8, is designed to produce high pressure (13.29 MPa, 331 �C) steam by heat ex-

change between boiler water (331 �C, shell process) and process gas (869 �C, tube process) from the second-

ary reformer. If 100E8 cannot work then the whole plant must be shut down.

Main design and operation parameters of 100E8 are listed in Table 1. Main materials of 100E8 are listed

in Table 2. Composition of process gas from secondary reformer is listed in Table 3.

1350-6307/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.engfailanal.2004.10.020

* Corresponding author. Tel./fax: +86 25 83791812.

E-mail address: [email protected] (Y.-L. Han).

mailto:[email protected]

Table 1

Main design and operation parameters of 100E8

Shell process Tube process

Material Boiler water High-temperature process gas from outlet of second reformer

Flux of inlet material, kg/h 113,708 159,592

Temperature (inlet/outlet), �C 331/331 (water/steam) 869/479 (temperature of process gas)

Operation temperature, �C 370 (tube-sheet) 374 (tubes)

Operation pressure, MPa 13.029 (tube-sheet) 3.06 (tubes)

Design pressure, MPa 13.53 (tube-sheet) 3.5 (tubes)

Design temperature, �C 346 500 (tubes)

Table 2

Main materials of 100E8

Tube B 44.5 · 5.8 · 4950, 404 pieces, ASME SA213 T 11

Tube-sheet B 1885 · 27, 2 sheets, ASME SA387 Gr 11 Class 2

By-pass pipe B 273 · 31, ASME SA335 P 11

Inner pipe (of by-pass pipe) B 192 · 3 (measured from failure pipe), Incoloy800. Inner pipe protects the by-pass pipe

from damage of high-temperature process gas

Tube box B 2004 · 32, ASME SA387 Gr 11 Class 2

Shell B 1891 · 32, ASME SA302 Gr B

Table 3

Composition of process gas from secondary reformer

Composition CO2 CO H2 N2 CH4 Ar

Volume, % 8.3 12.6 56.2 22.3 0.3 0.3

684 Y.-L. Han et al. / Engineering Failure Analysis 13 (2006) 683–697

Ammonia synthesis system of JNGCC began to produce ammonia in 1992. There was no leak from 100E8

from 1992 to 2003, until the outlet operating temperature of 100E8 tube process gas fell from 479 to 440 �C in

January 12th 2004 and could not be modified to the normal temperature of 479 �C by the process method.

From January 12th 2004 to March 23rd 2004, while 100E8 was shut down, the outlet operating temperature

of 100E8 tube process gas fell to 385 �C, and the steam output of 100E8 reduced to about 13,000 kg/h. 100E8

was opened-up in March 26th 2004 and it was found that the outlet tube-sheet had leaked.

In this paper, the following will be discussed.

� First, the failure phenomena of 100E8 in March 26th 2004 are listed.

� Second, failure analyses of 100E8 are given. It is concluded that the bell mouth of the 100E8 tube-sheet

leaked first, then high-pressure water leaked from shell process to tube process. Thin-wall inner pipe of

100E8 was pressed to buckling by the high-pressure water which leaked from the shell process. After

buckling, the cross-section shape of the thin-wall inner pipe changed from a regular circle to an irregular

quadrangle. When process gas flowed through those irregular quadrangle sections at high speed, the pro-

cess gas flow became turbulent and applied a cyclic load on the inner pipe wall. Finally, the inner pipe

wall broke because of cyclic load and high temperature.� Third, acoustic emission test procedures and test conclusions are presented in this paper. Acoustic emis-

sion (AE) technology [1–3] is a passive listening technique which is extremely sensitive and can detect

small crack growth events. AE can thus provide early information on defects or deformation in any

material or structure. If atomic bonds break during an integrity test, the energy released propagates

through the material according to the laws of acoustics. Very sensitive transducers detect the propagat-

ing wave, and the detected waveform can then be subjected to a series of analytical techniques, which can

Y.-L. Han et al. / Engineering Failure Analysis 13 (2006) 683–697 685

be used to detect, locate and identify defects activated by the test program. AE techniques can provide a

most sophisticated monitoring test and can generally be done with the plant/pressure equipment oper-

ating at or near normal conditions [1]. After the failed parts of the 100E8 boiler were repaired, the boiler

was tested to verify its integrity by using AE technology.

2. Failure phenomena of 100E8 boiler on March 26th 2004

The 100E8 waste heat boiler was opened-up on March 26th 2004 and it was found that the boiler had

leaked in the bell-mouth of the outlet end tube-sheet (Fig. 3). Failure phenomena of the 100E8 are listed as

follows:

� Once 100E8 was opened, it was found that half of the tube box was filled with water. After the water was

pumped away, a water mark on the outlet part of the inner pipe was shown (see Fig. 1, and a water mark

on the fire-resistant material of the inlet tube sheet of 100E8 is shown in Fig. 2. The pipe in the center of

Fig. 2 is the so-called inner pipe in this paper.

Fig. 1. Water mark on inner pipe.

Fig. 2. Water mark on fire-resistant material of inlet tube-sheet.

Fig. 3. Sketch map of the outlet end of 100E8 boiler.


� The outlet end of the by-pass pipe is in bell-mouthed in shape (Fig. 3). A half-circle long through-wall

crack was found on the top half part of the bell-mouthed section of by-pass pipe (Figs. 4 and 5). The

through-wall crack is wide enough to be seen by eye (Figs. 4 and 5). The state of the through-wall crack,

polished before repair, is shown in Fig. 6.

� The left end of the inner pipe (Fig. 3), 200-mm long is broken from the inner pipe. The shorter brokenpart of the inner pipe is shown in Fig. 7. The longer part of the inner pipe is shown in Figs. 4 and 8. Fig. 9

shows enlarged parts of Fig. 8.

� Visible buckling deformation is found in the outlet part of the inner pipe (left part of inner pipe of Fig.

3). The buckled length of inner pipe is 2.5 m (Figs. 1 and 10). A through-wall crack, 0.85 m from the

outlet tube-sheet, is found in the buckling zone of the inner pipe (Fig. 11).

Fig. 4. Through-wall crack.

Fig. 5. A through-wall crack.

Fig. 6. Polished through-wall crack.

Fig. 7. Broken down part of inner pipe.


Fig. 8. Rest of inner pipe.

Fig. 9. (a) Enlarged view of Fig. 8. (b) View from right side of Fig. 8. (c) Part of (b) with typical high-temperature fatigue crack.


Fig. 10. Visible buckling deformation of inner pipe.

Fig. 11. Buckling deformation and a crack (pointed by arrow) of bore.

Fig. 12. Seriously damaged flange of the short pipe.


Fig. 13. Original state of broken part of inner pipe and flange of short pipe.


� The flange of the short pipe (at the right of Fig. 3) is damaged seriously (Fig. 12). The original state of

the short pipe and the shorter broken part of the inner pipe are shown in Fig. 13. Obvious buckling

deformation of the shorter broken part of the inner pipe can be seen in Fig. 13.

3. Failure analysis of 100E8 boiler

3.1. Observations

A. There is a long through-wall crack in the bell mouth of the outlet tube-sheet of the boiler (Figs. 3–6).

B. Fire-resistant material, that should have been there between the by-pass pipe and the inner pipe

according to the design, could not be found (Fig. 4). Fire-resistant material, between the short pipeand the inner pipe, could be found too (Fig. 13).

C. Serious buckling deformation is found in part of the inner pipe, a little away from the outlet tube-

sheet (Figs. 3, 10 and 11).

D. A through-wall crack, 0.85 m from the tube-sheet (Fig. 3), is found in the buckling zone of the inner

pipe (Fig. 11).

E. No visible buckling deformation of the inner pipe is found in the part near the bell mouth of the tube-

sheet in two directions along the inner pipe (Figs. 3–6 and 8).

F. Serious buckling deformation of the inner pipe is found in the part a little away from the bell mouthand near the flange of the short pipe (Fig. 3, 7, 8 and 13).

G. Many significant high-temperature fatigue cracks are found in the remnant part of the inner pipe (Fig.

9); Obvious high-temperature fatigue cracks are also found in the broken part, linked originally with

the above remnant part (Fig. 7).

H. Most of the flange of the short pipe (Fig. 3) broke and disappeared; a long crack was found along

the root circumference of the flange of the short pipe (Figs. 12 and 13). The connected length

between the flange and the short pipe is shorter than 1/8 of the perimeter of the short pipe

(Fig. 14).I. The ammonia synthesis system of JNGCC began to produce ammonia in 1992. No leak was found in

the 100E8 boiler from 1992 to 2003, until the outlet operating temperature of the 100E8 tube process

gas fell from 479 to 440 �C in January 12th 2004 and could not be modified to the normal temperature

Fig. 14. A long circumferential crack along flange root of short pipe.


of 479 �C by the process method. From January 12th 2004 to the shutting down of the whole system

in March 23rd 2004, the outlet operating temperature of the 100E8 tube process gas fell to 385 �C,and the steam output of the 100E8 boiler reduced to about 13,000 kg/h.

3.2. Failure analysis

The authors deduced that the failure of the 1000E8 boiler occurred in the following way.

� First, a crack initiated, propagated and finally became a through-wall crack. As soon as the crack grew to a

through-wall one, the boiler water (331 �C, 13 MPa) was ejected out (corresponding to observation A).

� Second, the ejected high-temperature and high-pressure water began to erode the fire-resistant seal mate-rial between the bell mouth and the short pipe (Fig. 3).

� Third, the through-wall crack grew. The longer the crack size, the bigger was the flux of the water ejected

from the shell-process of the 100E8 boiler.

� Fourth, after the fire-resistant seal material between the bell mouth and the short pipe was eroded away,

the boiler water began to erode the fire-resistant material between the by-pass pipe and the inner pipe.

The ejected boiler water also began to erode the fire-resistant material between the short pipe and the

inner pipe (corresponding to observation B).

� Fifth, after the fire-resistant material between the inner pipe outer-wall and the by-pass pipe inner-wallwas shattered, boiler water (300 �C) could get to the space that was originally occupied by the fire-resis-

tant material and was heated to boil away rapidly by the process gas (700–800 �C) in the inner pipe. The

boiled water expanded rapidly and brought high-pressure to bear on the outer-wall of the thin-wall inner

pipe (Fig. 3). The thin-wall inner pipe was pressed to lose stability by high-pressure vapor and boiler

water. The part near the bell mouth of the tube-sheet (Fig. 3) did not lose stability, because this part

was directly opened to the outlet end of the boiler (in which the pressure of 3 MPa was equal to the inter-

nal pressure of the inner pipe). The boiled water filled in the space that was a little away from the bell

mouth in two directions along the inner pipe (Fig. 3), and supplied high pressure to the outer-wall of thethin-wall inner pipe because the vapor could not escape from the narrow space in time. Observations C,

D and E correspond to the above sequence.


� Sixth, reasons similar to the fifth resulted in the observation F.

� Seventh, after the buckling deformation of the inner pipe, the process gas flow changed from regular

circular to irregular wavy (Figs. 10 and 13). Because of the leak of the tube-sheet, relatively low-

temperature boiler water (331 �C) was mixed with high-temperature process gas (479 �C) and lowered

the temperature (outlet end temperature of 100E8 boiler) below the plant system control temperatureof 479 �C. In order to increase the admixture temperature to 479 �C, as requested by the next pro-

duction process, the plant operating manger had to increase the flux of the process gas flowing in the

inner pipe. The bigger the leak was, the larger the flux was. When the leak of the tube-sheet was too

large, it was impossible to increase the outlet end temperature of the 100E8 to the normal produc-

tion process setting temperature, 479 �C (corresponding to observation I). As process gas flowed

through the irregular inner pipe at higher speed, the flow became turbulent. High-temperature tur-

bulent flow would supply a cycle pressure load on the buckling wall of the inner pipe and induced

high-temperature fatigue cracks. The ring welding line on the outlet end of the inner pipe (right ofFig. 3) was a weak section, and the inner pipe broke into two parts in this position (corresponding

to observation G).

� Eighth, under normal conditions, both the inner-wall and the outer-wall of the short pipe endured the

pressure of the process gas and the pressure difference of the inner pipe wall was small. After the leakage

of the tube-sheet, the short pipe had to endure a cyclic high pressure and cracks initiated at the stress

concentration at the root of the short pipe flange. The crack grew and caused observation H.

3.3. Failure analysis of bell mouth crack

In the overhaul, a long through-wall crack was found on the bell mouth. Because one is not permitted to

cut down the bell mouth to do a failure analyses, the present paper will only give inferential analyses

according to the existing information. The existing information is listed as follows:

� Through-wall crack not in welding line position.

� Water quality of 100E8 conformed to quality standard.� No temperature excursion was recorded.

� Thickness of bell mouth at the crack position was thinner than designed.

� Failure appearance of bell mouth is demonstrated by Figs. 4–6.

From the above information, the following conclusions can be inferred.

� The through-wall crack at the bell mouth is not generated by corrosion or over-heating of the boiler. If

the crack is a corrosion crack, it should occur at the welding line.

� The through-wall crack at the bell mouth might be a small defect that was generated in the forging pro-

cess of the bell mouth. In 12 years of service of 100E8 boiler, the small defect grew and resulted in theleak of the bell mouth.

� The above conclusions are only deductions. The actual failure reason of the bell mouth cannot be deter-

mined. The bell mouth is still a latent failure source, and should be inspected periodically.

� In order to check the safety of 100E8 tube-sheet and bell mouth, an acoustic emission test was used.

4. Acoustic emission test of 100E8 tube-sheet at March 30th 2004

Acoustic emission (AE) is a good method to assess the integrity of an in-service pressure vessel. When an

in-service pressure vessel is tested by water pressure, defects, already there, become active and can emit

Fig. 15. Four AE sensors on tube-sheet near bell mouth.

Fig. 16. Positions of AE events before 9 MPa (numbers are number of AE sensors).


acoustic waves as the water pressure is increased. Acoustic waves can be received by AE sensors. The AE

sensors transform acoustic waves into electrical signals that can be recorded by computer and analyzed by

software. The value of water pressure is recorded in the AE test by computer too. AE signals, such as


strength, times and history of AE events that happen with the changing of water pressure applied to the

pressure vessel, are important to assess the safety of the inspected pressure vessel.

4.1. Test conditions

� Multi-channel AE equipment is used in the AE test process.

� Water pressure method is selected in the AE test.

� Four AE sensors are used to inspect the 100E8 outlet end tube-sheet (Fig. 15).

� Test threshold is 40 dB. Frequency range is 100–400 kHz.

� The highest water pressure applied on the AE tested tube-sheet is 10.2 MPa.

4.2. AE test results

� The setting of four AE sensors is described in Fig. 15.

� From Figs. 16 and 17, it can be concluded that AE events happened mainly at a water pressure lower

than 9 MPa in the whole AE test loading process and AE events chiefly happened in the re-welded area

of the bell mouth, the triangle zone of Sensor Nos. 1, 2 and 3 (Figs. 15 and 16).

Fig. 17. Position of AE event, amplitude (dB), hit counts and pressure (MPa) vs. time (s).

Fig. 18. Amplitude (dB) of AE events, pressure (MPa) vs. time (s).

Fig. 19. AE hit counts vs. time (s) when pressure lower than 9 MPa.


Fig. 20. AE average signal level (ASL, dB), pressure (MPa) vs. time (s) when pressure lower than 9 MPa.


� From Figs. 17–20, AE events were mainly recorded when the pressure was lower than 6.5 MPa; times of

recorded AE events decreased as the pressure went up and there were no AE events recorded during

pressure reducing and pressure holding at 10.2 MPa. The above can be explained as AE events primarily

introduced by welding stress releasing from the re-welded zone of the bell mouth. The most important

thing was that there was no record of unstable crack propagation.� According to the AE test information and Chinese standard JB/T 7667-1995 [4], the repaired 100E8 out-

let end tube-sheet is suitable for service in the next production year.

5. Conclusions

� First, the failure phenomena, of 100E8 in March 26th 2004, are listed.� Second, failure analyses of 100E8 are given. It is concluded that the bell mouth of the 100E8 tube-sheet

leaked first, then high-pressure water leaked from shell process to tube process. The thin-wall inner pipe

of 100E8 was pressed to buckling by the high-pressure water which leaked from the shell process. After

buckling, the cross-section shape of the thin-wall inner pipe changed from a regular circle to an irregular

quadrangle. When process gas flowed through those irregular quadrangular sections at high speed, the

process gas flow became turbulent and applied a cycle load to the inner pipe wall. Finally, the inner pipe

wall broke because of cyclic load and high temperature.


� Third, acoustic emission test procedures and test conclusions are presented. After the failed parts of the

100E8 boiler were repaired, the boiler was tested to verify its integrity by acoustic emission technology.

The 100E8 boiler has now operated normally for 3 months to date.

References

[1] Wood BRA, Harris RW. Structural integrity and remnant life evaluation of pressure equipment from acoustic emission

monitoring. Int J Pres Ves Pip 2000;77:125–32.

[2] Barthelemy H. Periodic inspection of compressed gas cylinders and tubes – flaws detection using acoustic emission testing. J Pres

Ves Technol 1988;110:161–7.

[3] Pellionisz P, Szucs P. Acoustic emission monitoring of pressure vessels. Int J Pres Ves Pip 1993;55:287–94.

[4] Chinese Standard. JB/T 7667-1995. Acoustic emission test evaluation method of in-service pressure vessel. Mechanical Industry

Standard of PR China; 1995.

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Inservice Waste Heat Boiler Failure