»
Fabrication
Preventing weld cracks in BFW preheaters for high-temp hydrogen service
In a high-pressure, high-temperature hydrogen service, the integrity of pressure equipment is critical to the safe and efficient operation of the plant. This is especially true in facilities where long-term exposure, material selections, and demanding operation converge to challenge design assumptions and inspection practices.
By M Norhisyam Awang, Principal Mechanical Engineer, and Nur Athirah Ishar, Staff Mechanical Engineer, PETRONAS
This paper presents a case study from an ammonia complex that has been operational since the late 1990s, focusing on recurrent cracks found in its Boiler Feed Water( BFW) Preheaters. These preheaters, constructed from 2 1 / 4Cr-1Mo low alloy material, are operating in high-temperature and high-pressure hydrogen service. Despite adhering strictly to the material selection and specifications outlined in API RP 934 and API RP 941, which align with the Nelson Curve operating limits, these units have consistently exhibited cracks in their thick-walled circumferential weldments. The cracks originated from the internal surface and, in some cases, propagated through the full thickness of the material. These recurring incidents have necessitated the removal of the affected equipment, underscoring the severity of the issue and the irreparable nature of the damage. In response to the incidents, enhanced replacement strategies were developed with stricter fabrication and heat treatment protocols aligned with API RP 934-A standards. Advanced inspection techniques such as 100 % Phased Array Ultrasonic Testing( PAUT) at all circumferential welds were implemented. This case highlights the importance of proactive assessments and stringent fabrication requirement and quality controls in maintaining plant reliability and efficiency.
Background A high-pressure, high-temperature hydrogen production facility operates with multiple heat exchanger systems, including a set of three vertically stacked Boiler Feed Water( BFW) Preheaters in the ammonia synthesis section. These units, shown in Figure 1— referred to as E-02A, E-02B and E-02C— are designed to transfer heat from synthesis gas to boiler feed water, significantly improving thermal efficiency of the process. On the tube side, synthesis gas containing hydrogen enters at a temperature of 340 ° C and pressure of 190 barG. Simultaneously, boiler feed water enters the shell side at around 216 ° C and 130 barG. The exchangers’ function is to recover heat from the hot synthesis gas stream to preheat the feedwater for a boiler, thus recovering thermal energy and optimising process efficiency.
Figure 1. E-02A / B / C installation at site.
Each of these heat exchangers is constructed using low alloy material SA-336 F-22 Cl. 3( 2¼ % Chromium, 1 % Molybdenum) for the tube side, while the shell side is fabricated from low alloy of same specifications at the front and carbon steel SA-516 Gr. 70 towards the rear side. The tubesheet is also made from SA-336 F-22 Cl. 3 with an Inconel 600 overlay of 6 mm thickness for enhanced resistance against High Temperature Hydrogen Attack( HTHA). The detailed construction is illustrated in Figure 2. The circumferential weldments discussed for the heat exchangers are as follows:
• CS-01 refers to the weldment between the channel’ s flat head( item 1) to the channel head’ s barrel( item 2), both fabricated from low alloy material.
• CS-02 refers to the weldment between the channel head’ s barrel( item 2) and the tubesheet( item 3), also of the same low alloy.
• CS-03 refers to the weldment between the tubesheet( item 3) and the shell( item 4), which are also of the same low alloy.
• CS-04 refers to weldments between the shell courses( item 4 and 5), and are of different materials— low alloy steel and carbon steel, respectively— requiring careful consideration of dissimilar metal welding.
• CS-05 pertains to the weldments between the shell( item 5) to the rear head( item 6), both
20 Heat Exchanger World October 2025 www. heat-exchanger-world. com