burst before leak failure of a hts effluent waste heat boiler

2002 AMMONIA TECHNICAL MANUAL

Burst Before Leak Failure of a HTS Effluent Waste Heat Boiler

F. Bakker, H.Schrijen and J.Keltjens DSM Agro

Introduction

SM Agro operates two ammonia plants on the Geleen site in the Netherlands. Both plants have a design capacity of 1360 mt/d

NH3 (1500 stpd NH3) and both plants use natural gas for feedstock and fuel.

The first plant was designed and constructed by Bechtel and was commissioned in March 1971 (is des-ignated as Plant Number 2). The second plant was de-signed and constructed by Kellogg Continental and was commissioned in August 1984. Following an unplanned outage on the ammonia plant 2 in December 2001, a leak from the shell of the waste heat boiler (H201) was detected during start-up. After visual inspection a full wall crack of 1,2 meters was identified in the 32 mm thick shell of the waste heat boiler (after this named heat exchanger). In this paper the damage, the investigations, the metallurgical phenomenon and the repair of the heat exchanger are described.

Description of the Heat Exchanger, Operating Conditions, History and the Damage

The tube side of the heat exchanger contains process gas at a pressure of ~30 bar. The inlet and outlet gas temperatures are approx. 350 ºC and 250 ºC respec-tively. The shell side of the heat exchanger contains wa-ter and saturated steam circulating through a waste heat boiler steam drum immediately above the heat ex-changer. The waterside operating temperature and pres-

sure are 250 ºC and 40 bars respectively. The tube sheet material in which the failure occurred is 32 mm thick Carbon - 0.5 Mo steel, and the shell material is 27 mm thick Carbon- Mn steel. The exterior of the heat ex-changer is thermally insulated. In appendix 1 a overview of the heat-exchanger and the drum is given.

The heat-exchanger has been in service for approxi-mately 30 years without any repair work or modifications, and has been visually inspected internally, as well as hy-dro tested at 1.3x the design pressure every four years. The exchanger failed on 9 December 2001, during a con-trolled start-up. The gas and water temperature and pres-sure were approx. 50 ºC and 35 bar respectively. As there was no one present near the place of the incident, nobody got hurt. Subsequent visual inspection identified a crack of 1,2 m !, around the 9 o�clock position. The crack ap-peared to have initiated at or near to the toe of a fillet weld joining the inlet tube sheet to the cone section, and was through-thickness (see appendix 2).

During the fabrication of the heat exchanger, the inlet and outlet tubesheets were plastically deformed during the initial hydro test because the tubes had been buckled due to insufficient number of baffle plates. The tubes were replaced and the tubesheets were salvaged.

The deformed tubesheets subsequently have been normalized at 920 ºC and loaded to 6 tons to straighten them. After additional cold straightening the tubesheets were stress relieved at 650 ºC for 1 hour and cooled in air. The new tubes and shell were welded to the tube sheets without post weld heat treatment (PWHT), in accordance with the original de-sign and construction code.

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AMMONIA TECHNICAL MANUAL 2002

Failure Analysis

At different locations boat samples were taken from the inlet and outlet location.

The investigations that have been carried out are: 1. Optical microscopy and scanning electron

microscopy 2. Mechanical tests and chemical composition 3. Fracture Mechanics and finite element

analysis

Optical Microscopy and Scanning Electron Microscopy (Figures 1- 8)

The microstructure shows transgranular cracks and is brittle in nature. Deformation twins were noted adja-cent to the crack path, which are associated with rapid fracture. The fracture faces have been examined in the scanning electron microscope and showed cleavage frac-tures and no other fracture mechanism.

The examination of the samples showed that the di-rection of the cracking appears to be predominantly from the outer surface inwards.

The sample taken at the end of the crack has a shear lip along the inside edge. The reason for the plastic crack-stop could be attributed to the fact that the stresses causing crack propagation were relieved.

The sections from the outlet location indicated the observation of cracking associated with the weld toe, which is likely to be fabrication hydrogen cracking. (Cold Cracking)

Mechanical Tests and Chemical Composition

The results are summarized in table 1 through 7. The recovery of the Charpy toughness to acceptable

levels after heat treatment is worth noting, as it reviels that the embrittlement of the material is reversible and also the toughness from the sample tested at usual oper-ating temperature.

This indicates that during operation the material is ductile, and so the brittle failure is not likely to occur during service.

The brittle fracture from the 9 o�clock position in the tubesheet is not pertinent to the failure. The outlet and inlet Charpy notch toughness of the tubesheet is very low.

The mechanical properties at room temperature, ex-cept for the yield strength, are within the specification. Due to the strain ageing there is an increase of the yield strength of ≈ 30%. The results indicate that the quasi-

static behaviour is totally different from the dynamic behaviour. Charpy notch testing after artificial strain ageing gave the result that the transition temperature brittle to ductile is approx. 70°C

The chemical composition shows that the alumin-ium content is far too less to interact with the nitrogen. In case of fully killed steel the ratio between aluminium and nitrogen must be at least 2.

Following the impact testing the fracture toughness of the tube sheet material has been determined. Both quasi-static and dynamic CTOD values are measured at ambient temperature. The dynamic CTOD is very low (δc = 0,12 mm), the static test show tough behaviour (δc >1 mm).

Finite Element and Fracture Mechanics evaluation (Figure 9 and 10)

Following the mechanical testing an elastic plastic Finite Element Analysis (FEA) has been carried out. In figure 9 the FEA model is shown. The result of the analysis shows that due to pressurization of the gas channel at the initial hydro test severe plastic deforma-tion (max 4%) is introduced at the location of the frac-ture. The red colored areas in figure 10 indicate the re-gions where plasticity occurs. During the first start-up and shutdown cycles some additional plasticity occurred (0.5%). The magnitude of this plastic deformation that is caused by the shape of the vessel dominates plasticity due the welding and/or manufacturing.

The stresses from the FEA have been used in a En-gineering Criticality Assessment (ECA) according to BS 7910. The results indicate that using the dynamic frac-ture toughness the critical defect size is in the same or-der of magnitude as the existing cold cracks. With the quasi-static value a crack over the entire wall thickness is tolerable, hence in that case a leak before break be-havior is present.

The results of the ECA clearly show that a dynamic trigger must have been present prior to final failure of the vessel.

Metallurgical Phenomenon

DSM Techno Partners (Mechanical Plant Services) was responsible for getting the phenomenon clear and to advise the plant on the repair work to be executed. In a short time a lot of work was executed. Discussions have been taken place with experts from TNO (Dutch Organisation for Applied Scientific Research), Gasunie,


BASF, Dillinger, Mannesmann, TWI (The Welding In-stitute) and Technical University Aachen.

Based on the metallurgical and mechanical charac-teristics of the material the embrittlement is related to Strain-ageing. Essential for this phenomena is the presence of plastic deformation of several percent. This plasticity can be introduced during manufacturing, weld-ing and operation. Post Weld Heat Treatment prevents strain-ageing, introduced by the fabrication process.

Depending on the chemical composition and heat treatment the C-Mn and low alloy steels that are semi or Si-killed will be supersaturated with respect to intersti-tial elements carbon and/or nitrogen at room tempera-ture. At room temperature the solubility of nitrogen is very low, about 1 ppm. As the nitrogen content of C-Mn and Mo-steels is in the range of 50-100 ppm, it is clear that in absence of nitride formers almost all of the nitrogen will be in super saturation at room temperature.

These interstitial elements (mainly nitrogen) then segregate to and interact with the strain field of disloca-tions in strained structures. This process of strain ageing will raise the yield strength of ferritic microstructures by pinning of dislocations.

The main problem, however, is the dramatic in-crease of the ductile-brittle transition temperature. Shift-ing of the transition temperature up to of 100°C has been reported.

The effect depends on the level of deformation and the range for strain ageing is from room temperature to about 350 °C. The ageing within a temperature range of about 100-300 °C, occurs simultaneously with straining.

Based on the metallurgical point of view a selection of equipment was performed. It revealed that there were more than 300 vessels and 1400 lines potentially sus-pected . This is seventy five percent of the plant, which was a total shock to everybody involved in the investigation and repair. The problem was potentially immense, but casuistry shows that only a few cases were reported world wide. In our case this was the first fail-ure in fifty years on the entire site, that consists of more than fifty plants. So there should be an additional pa-rameter contributing to this phenomena.

Further investigations based on extensive elastic-plastic FEA showed that plasticity caused by the design of the vessel was dominant for strain ageing. This gave us a additional selection criteria for the roadmap for the selec-tion of the susceptible material and or condition. See ap-pendix 3. With this roadmap the susceptible equipment was reduced to less than 10 vessels and 10 lines.

Repair Work

As the phenomenon and the consequences were not clear at the beginning a reliable and safe temporary re-pair of the heat exchanger was discussed. Two options were available:

1. grinding out the crack, preheating, welding

and a post weld heat treatment (PWHT) 2. welding of a circumferential cover plate

connected to the inlet cone and the shell, without post weld heat treatment.

Execution of option 1 had the potential risk of

cracks in the tubesheet during PWHT and with option 2 there is even bigger risk for new cracks near the T-junction weld.

It was concluded that a reliable and save temporary repair was not possible.

To minimize shutdown time a total repair executed on site had to be performed.

It was decided to replace both tubesheets, the tubes and the inlet and outlet cone bend. It was ensured that the new materials were fully killed with Al, to prevent strain aging.

Stork Industrial Services executed the job in combi-nation with the NEM-Energy Services. The schedule was based on 7 days, 24 hours a day.

The job was finished on 8 of March 2002. Total down time of the plant was 89 days. Total costs of the repair, including costs made for the investigation and re-search, were substantial.

Performed Actions to Ensure the Integrity of the Installations

As indicated by the roadmap, see appendix 3, it was necessary before start-up, to identify the equipment sensi-tive for strain ageing. Design engineers checked all the vessel drawings for non-standard construction details and the possibility if high strain during start up or shut down could occur. From the susceptible equipment only one boiler was found which could have the same problem.

MP testing and finite element analysis of this boiler revealed that there is no risk for strain ageing. No sig-nificant plastic deformation occurs during hydro testing and operation.

The selection of the pipe lines were further refined by adding that the temperature should be above 100 ºC and that it should be a process line (no utility line). All the


lines were checked in the plant for the state of the sup-ports and if the line could expand normally. No configu-rations were discovered that could cause strain ageing.

Conclusions and Recommendations

The failure is a classic case of burst before leakage. It shows that the consequences of strain ageing can be dra-

matic. Internally DSM actions have been taken to screen all process equipment and piping for the sensitivity for the strain-ageing phenomenon. It is recommended that other companies take the same actions as DSM has taken, as the failure due to strain ageing can be dramatic.

Appendix 1: Overview of the Heat-Exchanger (H201) and Boiler (V201)


Appendix 2: Position of the Crack and Overview Boat Samples Taken

See Detail X

Detail X


Appendix 3: Roadmap for the selection of constructions sensitive for strain ageing

Plant built no before 1983

yes

20°°°°C < T < 350 °°°°C no

yes

Material * C-CMn till 15Mo3 no

yes

Plastic deformation

no operation, start-up or pressure testing

yes

Replacement Prevention

- Component assessment

- NDT - Start-up procedure

ok

* These steel types from non- West European countries are still suspected!!


Figure 1. Boat samples from inlet

Figure 2. Boat sample showing cracking

Figure 3. Detail of crack, transgranular in nature

magn. 500

Figure 4. Deformation twins adjacent to main crack

Figure 5. Cleavage fracture

Figure 6. Cold cracking at weld toe


shearlip

Figure 7. Fracture surface at location 4 with shearlip along the inside edge

Figure 8. Fracture surface shearlip

Figure 9. FEA model Figure 10. Plastic Deformation

burst before leak failure of a hts effluent waste heat boiler

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