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Fabrication
Figure 6. Thermocouples were installed at internal and external surfaces prior to furnace PWHT.
Improvements To address the recurrence issues, several critical improvements were implemented in the fabrication process of the new unit of Boiler Feed Water Preheater. These improvements were implemented to enhance the structural integrity, welding quality, and long-term reliability of the equipment, with a particular focus on the challenges associated with high-temperature, high-pressure hydrogen service. The improvements began with a thorough review of international standards, particularly API RP-934, especially API RP 934-A for pressure vessels made of 2-1 / 4 Chrome-1 Molybdenum Steel, and API 941 for materials in hydrogen service. Based on these guidelines, a meticulous and comprehensive approach was adopted, addressing key technical aspects such as material and consumable testing, welding techniques, heat treatment methods, and advanced inspection procedures. One significant improvement was the elimination of local PWHT. Instead, furnace PWHT was mandated for all weldments. Thermocouples were placed on both the internal and external surfaces to ensure uniform temperature distribution and effective stress relief and restoring toughness throughout the weldment and heat affected zone( HAZ), as shown in Figure 6 below. This method mitigates the high hardness and residual stress that can lead to crack formation and growth. As for welding procedure, it was mandated that Intermediate Stress Relief( ISR) be used instead of allowing the alternative Dehydrogenation Heat Treatment( DHT), even though DHT in lieu of ISR can be considered for non-restrained joints of conventional or advanced low alloy steel grades, and is commonly allowed for conventional steels on non-restrained welds, as per API RP-934-A. Although a DHT will reduce hydrogen, it will not sufficiently restore toughness, as would an ISR, especially for advanced materials which remain very brittle during pre-PWHT handling. Due to the repetitive crack incidents, even though the material of E-02A / B / C is of only conventional steel grades and the affected joints are not considered as restrained, the authors deemed the ISR would be really contributing and therefore had specified it as a compulsory requirement. The inspection methods were also significantly upgraded to enhance defect detection in the circumferential welds. While methods like radiography and colour contrast dye penetrant testing( DPT) would normally be effective, their limitations in detecting subtle defects in low alloy steels prompted the inclusion of PAUT, and fluorescent penetrant test. PAUT provides high-resolution, real-time imaging, enabling the detection of cracks with greater sensitivity.
Figure 7. Hardness testing was performed pre and post heat treatment. www. heat-exchanger-world. com Heat Exchanger World October 2025
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