Case Study: UAE Gas Plant Eliminates HX Tube Failure with Super Duplex 2507 Tubing

Customer Background

In early 2023, we received an urgent inquiry from a midstream gas processing facility located in the Habshan area of Abu Dhabi, UAE. The plant processes approximately 1.2 billion standard cubic feet per day (BSCFD) of raw sour gas with H₂S concentrations ranging from 3% to 8% by volume, alongside CO₂ levels of 4% to 6%. The facility operates multiple shell-and-tube heat exchangers serving as lean/rich amine interchangers and overhead condensers in the acid gas removal unit (AGRU).

The procurement manager — a chemical engineer with 18 years in gas processing — had been dealing with recurring tube bundle failures across three identical TEMA-type BEM shell-and-tube exchangers. Each exchanger contained 640 tubes, 19.05 mm OD * 1.65 mm wall thickness, 6,000 mm tube length, in a two-pass configuration. The original tube material was ASTM A213 TP316L, selected during the plant's FEED phase in 2013 based on standard sour service assumptions.

By late 2022, the maintenance logs showed something alarming: tubes were being replaced on a 14-to-18-month cycle, far below the 8-year design life specified in the original mechanical datasheet. In one particularly bad quarter — Q3 2022 — the plant logged 11 unplanned shutdowns directly attributed to tube leaks in the AGRU exchanger train. Each shutdown cost the operator an estimated $380,000 in lost throughput and emergency repair labor.

Customer Pain Points

When we conducted a failure analysis on the returned tube samples, the damage pattern was clear — and it went well beyond what standard 316L could handle:

• Chloride-induced stress corrosion cracking (Cl-SCC): Despite the amine solution being nominally low-chloride, chloride concentration had built up in dead zones and under-deposit areas to levels exceeding 800 ppm. The tubes operated at 135°C on the hot side — right in the critical temperature window for Cl-SCC in austenitic 316L.
• Pitting corrosion under scale deposits: Iron sulfide scale from trace H₂S carryover created localized crevices where the passive layer of 316L broke down. PREN (Pitting Resistance Equivalent Number) for 316L sits at 24-26 — insufficient for this environment.
• Crevice corrosion at tube-to-tubesheet joints: The rolled and seal-welded joints showed preferential attack, with corrosion rates measured at 0.15-0.25 mm/year in the crevice zone versus 0.02 mm/year on the free tube surface.
• Hydrogen-induced cracking (HIC) in weld HAZ: Micro-hardness mapping revealed localized hardness values above 300 HV in the seal weld heat-affected zone, making it susceptible to HIC in the wet H₂S environment.

"We were essentially running a material that wasn't designed for the actual operating envelope. The original FEED assumptions about chloride levels were wrong — and we were paying for it every 16 months." — Plant Maintenance Director

Why Customer Chose Us

The customer had approached three suppliers: a European mill, an Indian re-roller, and us. The decision came down to four factors:

1. Dual-grade proposal: We were the only supplier who proposed a graded approach — Super Duplex UNS S32750 (2507) for the hot-end tube bundles (operating above 100°C) and Alloy 625 (UNS N06625) for the tubesheet-facing sections and U-bend regions where residual stresses were highest. The European mill proposed 254SMO across all positions, which would have addressed Cl-SCC but left HIC risk unresolved.
2. Mill test report (MTR) transparency: We provided full EN 10204 Type 3.1 certification with actual heat analysis — not just typical values. The customer's metallurgist specifically noted that our Cr, Mo, and N values for the 2507 heats were consistently in the upper third of the ASTM A790 specification range, which directly affects PREN.
3. Delivery confidence: The customer needed 1,920 tubes (3 exchangers * 640 tubes) delivered within 14 weeks. We committed to 11 weeks and delivered in 10. The Indian supplier quoted 18 weeks. In the customer's words: "Every week of delay was another $380K gamble on whether the old tubes would hold."
4. NACE MR0175/ISO 15156 compliance documentation: We provided detailed compliance statements for sour service at the specified H₂S partial pressure and pH range, including hardness test reports (max 28 HRC for 2507, max 35 HRC for Alloy 625 solution-annealed) and sulfide stress cracking test certificates per NACE TM0177 Method A.

Technical Solution

Material Selection & Rationale

Manufacturing & Quality Control

• Tube dimensions: 19.05 mm OD * 1.24 mm wall (reduced from 1.65 mm — the higher strength of 2507 allowed a 24% wall reduction while maintaining design pressure of 85 barg at 150°C per ASME Section VIII Div.1 calculations).
• Cold pilgering + bright annealing: All 2507 tubes were cold-pilgered to achieve ±0.05 mm OD tolerance and bright-annealed at 1,080°C with rapid water quenching to avoid sigma phase precipitation — a known risk with duplex grades if cooling rates fall below 50°C/min in the 900-600°C range.
• Eddy current testing (ECT): 100% of tubes underwent ECT per ASTM E426 with a 0.8 mm diameter flat-bottomed hole reference standard. Any tube exceeding 50% of the reference signal was rejected.
• Hydrostatic test: Each tube was tested at 1.5* design pressure (127.5 barg) with a 30-second hold. Zero failures across the entire 1,920-tube production batch.
• ASTM G48 Method A (ferric chloride pitting test): Sample tubes from each heat were tested at 50°C for 72 hours. Weight loss on 2507 samples averaged 1.2 g/m² — well below the 4.0 g/m² acceptance criterion. No pitting observed under 20* stereomicroscope examination.
• Ferrite content: Measured at 42-48% on 2507 tubes using Feritscope per ISO 8249, within the optimal 35-55% range for balanced corrosion resistance and mechanical properties.

Project Results

The three retubed exchangers were commissioned in August 2023. As of the most recent inspection in February 2026 (28 months of continuous operation), the plant reports:

• Zero tube leaks across all three exchangers. The previous 316L tubes had already required two partial retubing campaigns by the 28-month mark.
• Wall thickness measurements: Ultrasonic testing (UT) at the most recent turnaround showed average wall loss of 0.03 mm in the hot-end 2507 tubes and 0.01 mm in the Alloy 625 U-bends. Extrapolated service life now exceeds 12 years.
• Heat transfer performance maintained: Despite the thinner wall, the higher thermal conductivity of 2507 (15 W/m·K at 100°C vs. 316L's 14.6 W/m·K) compensated, maintaining the design UA value within 2% of the original specification.
• Reduced maintenance cost: The plant eliminated its quarterly emergency tube plugging procedure, saving approximately $120,000/year in labor and consumables alone.

"We haven't touched those exchangers in over two years. For a plant that used to schedule tube repairs like clockwork, that's transformative. The material upgrade paid for itself in the first three months of uninterrupted operation." — Operations Manager, Abu Dhabi Gas Plant

Cost-Benefit Summary

The 5-year TCO reduction of approximately 86% made the business case for the material upgrade straightforward. The incremental tube material cost was recovered in under 12 weeks of uninterrupted operation.

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