V. LENDE, M. G. CHOUDHURY, R. NARODIA, S. S. DARGAN and A. ASHESH, KBR, Haryana, India
In the process and power industries, high-temperature applications often require the use of different materials for pipes. For example, stainless-steel (SS) tubes might be specified for furnace coils due to their superior temperature and corrosion resistance, while carbon steel (CS) is preferred for outside the furnace because of its cost-effectiveness. Sometimes, dissimilar joints are necessary due to licensor/client specifications, advancements in metallurgy from technology upgrades, refractory-lined cold walls to non-refractory-lined hot walls, etc. However, joining dissimilar pipe materials presents significant challenges, particularly in high-temperature operating environments. These challenges stem from differences in thermal expansion, mechanical properties and wall thicknesses at the interface. Transition pieces, designed to bridge the gap between such dissimilar materials, are crucial to ensure system integrity and performance in these demanding environments. In this article, the authors explore a few transition piece usage situations and attempt to elucidate the challenges and approaches adopted.
Transition piece usage example cases. In this article, three case studies on transition piece usages are detailed below.
Case A: ASME SA213 TP316H to alloy ASME SA335 P91. In one project, there was a requirement of change of material usage from SS 316H to Alloy P91. A piping and instrumentation diagram (P&ID) sketch of the transition piece is detailed in FIG. 1.
In this scenario, the decision was to use an Inconel 625 material as a transition piece with a 13.5-mm thick pipe piece. The questions that arise first are: Why was a transition piece required at all? If a transition piece was necessary, could an Alloy P91 transition piece be used in this case?
The challenge was that this was a high-temperature and high-pressure system, and Alloy P91 had a lower thickness of 11 mm vs. the 13.5 mm thickness of SS 316H. Any reduction in thickness for SS 316H at the joint junction would have resulted in a code violation of the thickness requirement. Therefore, 13.5 mm had to be maintained for the strength of SS 316H.
Could Alloy P91 been used in this case (FIG. 2)? Possibly, yes. The welding process for SS 316H to Alloy P91 is well-established. It involves applying a buttering layer with a nickel (Ni)-based alloy filler material, performing post-weld heat treatment (PWHT) on the buttered weld deposit and the heat-affected zone (HAZ) of Alloy P91, and subsequently welding to the SS pipe using the same Ni-based alloy filler.
One point to note is that the Alloy P91 transition piece would be considerably cheaper than the Inconel 625 transition piece that was ultimately used.
As shown in TABLE 1, values taken from ASME B31.3, at 530°C, the thermal expansion coefficient of SS 316H is nearly 30%–35% higher than that of Alloy P91, whereas Inconel 625 has an expansion coefficient value between Alloy P91 and SS 316H. Therefore, the Inconel 625 transition piece was the best-suited material.
This choice significantly decreased the thermal discontinuity stress at the joint. While a high Ni-filler (ErNiCr-3) buttered layer could have helped smooth the thermal expansion difference, the behavior of a proper transition piece is more reliable than that of the buttered layer. Therefore, Inconel 625 was selected.
Case B: Alloy P91 to Alloy P22. During the revamp of old thermal power stations, a common issue arose when welding was required at tie-in joints, such as an existing header extension and the installation of new steam generators. The new side may be made of Alloy P91, while the existing side may be Alloy P22. These joints can be executed using a similar type of transition piece made of Alloy P91 material. The Alloy P91 side will have a substantially lower thickness than the Alloy P22 side. The transition piece will have the thicker side facing Alloy P22 and the thinner side facing Alloy P91. This is shown in FIGS. 2 and 3.
Properly approved welding procedure specifications (WPSs) with PWHT requirements must be followed. Since the welding involves new Alloy P91 (CSEF material) materials with existing Alloy P22, which may have been in use for several years and undergone creep, it is advisable to keep the calculated stress at the joint area on the lower side, ideally < 50% of the allowable stress.
Case C: Hot-walled to cold-walled transition joint. In very hot plants like fluid catalytic cracking units (FCCUs), there is sometimes a need for hot-to-cold walled joints. This means one side will be refractory-lined with a CS (cold-walled) shell, while the other side will be without refractory (hot-walled) and made of a high-temperature resistant alloy steel, such as SS 304H.
FIG. 4 shows a typical correct arrangement of the insulation around the hot-cold walled pipe junction. External insulation is required to extend over the transition cone so the pipe metal temperature is gradually reduced from the internal fluid temperature to the cold pipe temperature when exposed to the ambient environment. High-temperature resistant alloy steel or SS 304H is used at the cone connection and occasionally extends a few inches beyond the cone junction to ensure that the dissimilar metal weld is in a stabilized cold temperature region. Because the cone is surrounded by both internal insulation (refractory) and external insulation, its metal temperature gradually and asymptotically reduces to the cold pipe temperature as it emerges and is exposed to the ambient environment, as shown in FIG. 4.
Specific issues with transition pieces include:
Thermal expansion differences: Metals expand when heated. Dissimilar materials may have different coefficients of thermal expansion, leading to thermal discontinuity stress and potential failure at the joint if not properly managed.
Corrosion resistance: In high-temperature processes, corrosion and oxidation can be severe. Transition materials must be selected with corrosion resistance in mind to avoid premature degradation.
Mechanical strength: The mechanical properties of dissimilar metals, such as tensile strength and ductility, may vary significantly at elevated temperatures. The transition piece must maintain system strength and handle operational thermal stresses. These strength differences result in variations in calculated thickness, and the differences in the thickness of both sides of the joints must be managed.
Welding compatibility: Not all metals can be directly welded due to differences in melting points, thermal conductivity, expansion coefficients and metallurgical incompatibility. When welding dissimilar metals, special techniques or filler materials may be required to prevent issues like cracking or weak joints. Using the right filler material can bridge the gap between dissimilar metals, ensuring a strong and durable weld. For example, when welding SS to CS, a Ni-based filler can be used. Considering these factors, specialized WPSs and procedure qualification records may be required to form a reliable joint in the shop or onsite. PWHT requirements are often an issue. For example, for an Alloy P22 to Alloy P91 connection using a transition piece of Alloy P91 with the thicker side facing Alloy P22, care should be taken during PWHT to ensure the soaking temperature does not exceed the lower critical transformation temperature of the Alloy P22 material.
Transition materials: In cases where direct welding of dissimilar metals is infeasible, using transition materials can help create a gradual transition between the dissimilar metals. Due to code requirements like differences in thickness, this transition piece cannot be avoided. A properly designed transition piece aids in reducing abrupt changes in properties and potential failure points.
Takeaway. Transition pieces are essential components in high-temperature piping systems within the process industries. The selection of a transition piece must consider the unique demands of the application. By carefully choosing materials and methods that account for thermal expansion, corrosion, mechanical strength and metallurgical properties, process, piping and metallurgical engineers can ensure reliable and long-lasting joints between dissimilar material pipes. These solutions are critical for maintaining the efficiency, safety and durability of industrial operations in harsh environments. HP
REFERENCES
American Society of Mechanical Engineers (ASME), “ASME B31.3—Process piping,” ASME, 2024, online: https://www.asme.org/codes-standards/find-codes-standards/b31-3-process-piping
Peng, L. C. and T. L. Peng, Pipe Stress Engineering, ASME, 2009.
Vikrant Lende is an engineering professional with 24 yr of experience in piping engineering, from basic engineering to detailed engineering in the process, refining, petrochemicals, inorganic chemicals and fertilizer industries. He has a proven track record of successfully leading multidisciplinary teams in the design, analysis and implementation of complex piping systems, ensuring compliance with safety and quality standards. Lende works in KBR’s technology centre in Gurgaon, India. Previously, he has worked for Fluor Daniel India Private Ltd., IOTL and Stone & Webster Mumbai.
Mrinmoy Ghosh Choudhury is an engineering professional with extensive experience in process plant engineering, with special emphasis on concept layout, stress and support of materials. He has been involved in numerous problem-solving activities related to plant startups—vibration, rotary equipment alignment, piping/equipment system failures, etc. Choudhury has contributed numerous papers to respected engineering journals like Hydrocarbon Processing. In his professional life, he has been involved with Reliance Engineering, Toyo Engineering India, Chemtex Engineering India, Engineers India Ltd. DCPL and KBR Technology India.
Rikeen Narodia is an engineering professional with 12 yr of experience in piping layout and design engineering for process plants, refineries and hydrogen plants, among others. He works for KBR Technology in India. Previously, Narodia has worked at Reliance Industries Ltd. in Jamnagar and Prodair Air Products India Pvt. Ltd. in Vadodara.
Sukhjinder Singh Dargan is an engineering professional with 18 yr of experience in piping stress analysis for process plants, refineries and fertilizers, among others. He works at KBR in Gurgaon, India. Previously, Dargan worked for L&T Chiyoda and Fluor Daniel.
Avijit Ashesh works as a piping engineer at KBR, Gurgaon, India. He has more than 15 yr of experience in the design, engineering and troubleshooting of piping systems for the fertilizer, refining and petrochemical industries. Prior to joining KBR, Ashesh has been associated with PDIL, Samsung Engineering and Fluor Corp.