Heat exchanger failure investigation report

List of Contents

Abbreviations

1. Introduction
1.1 Methodology

2. Failure Investigation
2.1 Material description
2.2 Service condition
2.3 Localized corrosion
2.4 Effect of rolling expansion
2.5 Stress corrosion
2.6 Corrosion fatigue
2.7 Effects of temperature

3. Discussion
3.1 Causes of crevice corrosion
3.2 Crevice corrosion mechanism
3.3 Stream temperature effects
3.4 Causes of pitting
3.5 Crack initiation

4. Conclusion

5. Recommendation to mitigate failure
5.1 Tube assembly
5.2 Residual stresses
5.1 Temperature and baffle design

6. References

7. Appendices

LIST OF IMAGES

Figure 1 – Localized corrosion

Figure 2 – Rolling expansion

Figure 3 – Deterioration of the strength

Figure 4 – Crack initiation

Figure 5 – Incipient crevice corrosion

Figure 6 – Localized crevice corrosion

Figure 7 – Iron oxide deposit

Figure 8 – Internal pitting

Figure 9 – Tube crack

List of Tables

Table 1 – Hastelloy C22 composition 6

Table 2 – Stream temperatures 7

ABBREVIATIONS

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1. Introduction

A HP gas (CH ₄) cooler heat exchanger of the shell and tube type located in an offshore platform suffered a tube leak failure after six months short service period since it was last repaired from the previous failures. The heat exchanger have experienced from the past ten years several materials upgrade to contain the failures. A defect assessment of the tube bundle was carried out by eddy current, several sketches and photographs were provided to aid describing the nature, reasons and the general circumstances of the failure.

1.1. Methodology

The methodology adopted to carry out this failure investigation was based on the available information supplied by the client. It is worth to point out that there was a lack of some useful information needed to aid carrying out this failure investigation. Therefore, all the information not readily available by the client was hypothesized to fit the evidence seen from the photographs.

2. Failure Investigation

A detailed description of this failure investigation is briefly explained in the following subsections.

2.1. Heat Exchanger material description

The material used for the tube bundle was a highly corrosion resistant nickel-chromium-molybdenum alloy C22. Such austenitic alloys rely on the stability of a thin chromium oxide film for protection against corrosion (Mon, Gordon and Rebak, 2005). The chemical composition of C22 alloy is listed in table 1. The material used for shell and tube plate was a low alloy steel.

Table 1 – Hastelloy C22 composition

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2.2. Service condition

The table 2 below refers to the design and operating temperature of the heat exchanger.

Table 2 – Stream temperatures

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2.3 Localized corrosion

Seen from the photographs the onset of tube failure was due to localized corrosion. The localized corrosion occurred at location of the joined metal to metal surface between the tube and tube plate exposed to high temperature. At that location there was a possible dead zone with limited flow of coolant fluid, producing a poor heat transfer, between the shell side fluid and the tube side. Within the metal to metal surface there was a gap with restricted geometry which supported the development of crevice chemistry (Shan and Payer 2008).

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Figure 1 – Localized crevice corrosion row 19 – tube 1 r.h.s

2.4 Effect of rolling expansion

The process of de-stubbing and re-tubing was an operation performed several times during the service life of the heat exchanger.

The mechanical roller expansion method induces a radial force inside the tubes. This increasing force moves the tube material outwards until it contacts the internal diameter of the tube plate hole and continues until the tube plate material is just below its yield point (Bloodworth).

The process of re-tubing into the tube plate holes, may result in deterioration in the strength of tube to tube-plate joint having large initial clearance (see figure 3) as well as in the strain hardening and thinning of the expanded tubes and their surrounded ligaments (Merah et al., 2012).

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Figure 2 – Rolling expansion

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Figure 3 – Deterioration of the strength in the tube to tube-plate joint.

2.5 Stress corrosion

Stress corrosion cracking is the combined influence of tensile stresses when subjected into a corrosive environment. The impact of stress corrosion induces the material to a crack. The required tensile stresses are in the form of residual stresses. The residual stresses occur due to inelastic deformations and heat treatment. When rolling expanding the tubes, the differential displacement of the inside diameter surface of the tubes in the transition zone from the fully expanded to unexpanded zones creates both a tensile and compressive residual stresses (Merah et al., 2012)

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