Carried out under a work-study contract on the Gonfreville-l’Orcher petrochemical platform, within the maintenance department responsible for the steam-cracking furnaces.
Context
A TLX (tubesheet exchanger) cools the gases leaving the cracking furnace using pressurised water at 120 bar: the gases enter at 470 °C, pass through roughly 290 cooled tubes, and leave at 350 °C. Every furnace on the unit has one.
In the early 2010s several of these exchangers suffered tube perforations caused by corrosion, and a progressive replacement plan was started.
![Figure 1 - Shell-and-tube exchanger: the tube bundle is held at each end by a tube sheet, and the shell-side flow is turned across it by the baffles. Diagram from [1], annotations translated.](/assets/tlx-heat-exchanger-en.png)
Figure 1 - Shell-and-tube exchanger: the tube bundle is held at each end by a tube sheet, and the shell-side flow is turned across it by the baffles. Diagram from [1], annotations translated.
Corrosion that persists
Ultrasonic measurements on an already-replaced exchanger revealed local corrosion that was still significant, with a thickness loss rate estimated at 0.1 mm/year. The tubes, installed at 4.2 mm, were already below the manufacturers’ calculated thresholds in places.
Working from those latest inspection visits, I wrote the specification for the next tender and adjusted its technical requirements — in particular a corrosion allowance on the tubes of the next TLX to be replaced.
A new view on the design
Once the tender was out, several rounds of technical clarification with the suppliers exposed an anomaly: the proposed tube thicknesses varied widely from one supplier to another, without any corresponding difference in design.
To understand why, I re-ran the external-pressure tube sizing calculations. The tubes are sized to ASME VIII-1. ASME also publishes Code Cases — alternative calculation rules for particular situations — which here give a substantially lower minimum required thickness than the classical method.
| Supplier | ASME VIII-1 UG-28 | Code Case 2286 | Outside diameter |
|---|---|---|---|
| Supplier A | 3.47 mm | ~1.94 mm | 33.4 mm |
| Supplier B | 3.70 mm | ~2.15 mm | 33.7 mm |
Reproducing those calculations, I identified that some suppliers had simply used the gap to answer the tender: keeping the installed 4.2 mm and calculating through the Code Case, the corrosion allowance requirement already appears to be met — without changing anything in the design.
The design question then became whether the tubes — which also stiffen the tubesheets against deformation under service pressure — would remain structurally sound at such a thickness. A preliminary study was run to check that point.
What the study shows
Does the Code Case hold up mechanically?
A calculation supplied by one manufacturer, assuming generalised thickness loss over the full tube length, showed the plate stress staying under the allowable limit defined by ASME Section VIII, Division 2 (Part 5) — the division used for the finite element verification, as distinct from Division 1 (UG-28) used for the tube thickness calculation.
- The outlet plate is the most heavily loaded, particularly at the tube connections
- Assuming a linear evolution of the stresses (a simplification), the available margin would comfortably exceed the required corrosion allowance — an indicative result, computed on few points
- The thin-plate exchangers already installed could stay in service for at least two more cycles
The final choice
This design was not the only one on the table: other suppliers had submitted genuinely reinforced, more expensive versions. To decide between the offers I set the selection criteria, then weighted the decisional weight of each technical and commercial criterion with the department. The standard design was retained over the reinforced version: the latter offered more technical guarantees, but the margin study on the Code Case showed that the standard design already met the corrosion allowance requirement — with no extra cost and no additional validation lead time.
What it changed for the department
- The next replacement secured, with a corrosion allowance requirement that is genuinely justified technically
- A better grasp of the limits of the suppliers’ sizing methodology — the structural role of the tubes, the real reach of the Code Cases
- A reusable cost/benefit analysis base for future design trade-offs on this type of equipment
References
[1] TotalEnergies, « Échangeur thermique : fonctionnement, types et efficacité ». Consulté le: 9 septembre 2026. [En ligne]. Disponible sur: https://www.totalenergies.fr/particuliers/parlons-energie/dossiers-energie/chauffage-et-climatisation/echangeur-thermique-fonctionnement-types-et-efficacite