CLADDING TECHNOLOGY SHANXI CO., LTDMeasurement of Rolling Thinning and Elongation and Cladding Shrinkage of Hot-Wall Hydrogenation Reactor Shell
Literature Overview
This paper, published in Petrochemical Equipment in 1990 by Fei Yutao and Yu Yifeng from Jinzhou Heavy Machinery Factory, addresses a fundamental engineering challenge in the fabrication of hot-wall hydrogenation reactors. The study presents methods for measuring and predicting the rolling thinning and elongation of reactor shell plates during the roll forming process, as well as the dimensional changes caused by cladding layer shrinkage. Hot-wall hydrogenation reactors are critical equipment in petroleum refining, where the reactor wall itself serves as the heat transfer surface for the exothermic hydrogenation reaction. The shell plates must be precisely dimensioned to ensure proper fit-up, cladding quality, and final vessel geometry.
Core Technical Analysis
The fabrication of hot-wall hydrogenation reactor shells involves several critical forming operations that cause dimensional changes:
- Plate rolling (cold or hot) of the base carbon steel or low-alloy steel shell.
- Cladding of the inner surface with a corrosion-resistant alloy layer using ESW, SAW, or GTAW overlay welding.
- Machining of the cladding surface to achieve the required final dimensions and surface finish.
- Final heat treatment and dimensional verification.
Rolling Thinning and Elongation
During the plate rolling process, the shell plate is progressively bent to form a cylindrical shape. This bending causes thinning at the outer surface (tension zone) and thickening at the inner surface (compression zone). The degree of thinning depends on several factors:
| Parameter | Symbol | Typical Value | Effect |
|---|---|---|---|
| Initial plate thickness | t0 | 40-80 mm | Thicker plates thin less in percentage |
| Final rolling radius | R | 1,500-3,000 mm | Smaller radius causes more thinning |
| Number of rolls | n | 3 or 4 | 4-roll mills provide better control |
| Rolling temperature | T | 20-800 degrees C | Hot rolling reduces thinning |
| Plate material | - | 16Mn, 20G, 15CrMo | Higher strength materials thin more |
The thinning can be calculated using the following empirical relationship:
t = t0 (1 - k t0 / R)
Where t is the final thickness, k is a material-dependent constant (typically 0.15 to 0.25 for carbon steel), and R is the final rolling radius. For a typical hot-wall reactor shell with t0 = 60 mm and R = 2,000 mm, the expected thinning is approximately 0.9 to 1.5 mm, or 1.5 to 2.5 percent of the original thickness.
The elongation in the circumferential direction is correspondingly greater, and must be accounted for in the flat pattern development of the shell plate. The elongation can be calculated from the volume constancy principle:
L = L0 * (t0 / t)
Where L is the final length, L0 is the initial length, t0 is the initial thickness, and t is the final thickness.
Cladding Shrinkage
The cladding layer introduces additional dimensional changes due to thermal contraction during cooling. The cladding shrinkage has both radial and circumferential components:
| Shrinkage Type | Direction | Typical Magnitude | Measurement Method |
|---|---|---|---|
| Radial shrinkage | Toward axis | 0.5-1.5 mm per 100 mm diameter | Dial indicator |
| Circumferential shrinkage | Along circumference | 0.2-0.8 mm per 100 mm length | Laser tracker |
| Longitudinal shrinkage | Along axis | 0.1-0.5 mm per 100 mm length | Coordinate measurement |
The cladding shrinkage is influenced by the cladding process, alloy composition, cladding thickness, and heat treatment conditions. ESW cladding typically produces less shrinkage than GTAW cladding due to the lower cooling rate and reduced thermal gradient. The shrinkage can be minimized through:
- Using a cladding alloy with thermal expansion coefficient close to that of the base metal.
- Employing a multi-pass cladding technique with controlled interpass temperatures.
- Applying post-weld stress relief heat treatment.
- Designing the cladding layout to minimize residual stress accumulation.
Engineering Practice Implications
The accurate prediction and control of rolling thinning and cladding shrinkage are essential for ensuring the dimensional accuracy of hot-wall hydrogenation reactors. The final vessel must meet tight dimensional tolerances to ensure:
- Proper fit-up of shell courses during assembly.
- Adequate cladding thickness throughout the vessel interior.
- Correct alignment of nozzle attachments and other attachments.
- Acceptable out-of-roundness and ovality after fabrication.
Fabrication Sequence Optimization
| Step | Operation | Dimensional Control | Tolerance |
|---|---|---|---|
| 1 | Flat pattern development | Account for rolling thinning | +/- 2 mm |
| 2 | Plate cutting | Laser or plasma cutting | +/- 1 mm |
| 3 | Plate rolling | In-process thickness monitoring | +/- 0.5 mm |
| 4 | Shell assembly | Out-of-roundness check | +/- 3 mm |
| 5 | Cladding welding | Thickness control | +/- 0.5 mm |
| 6 | Machining | Final dimension control | +/- 0.3 mm |
| 7 | Heat treatment | Dimensional stability | +/- 0.5 mm |
| 8 | Final inspection | Dimensional verification | As per drawing |
The study recommends that the fabrication sequence be carefully planned to minimize cumulative dimensional errors. For example, cladding should be performed before final machining, and heat treatment should be performed after all welding is complete to minimize distortion.
Key Questions and Reflections
The study raises important questions about the interaction between rolling thinning and cladding shrinkage. These two effects are not independent; the rolling thinning changes the base metal geometry, which affects the cladding thermal field and, consequently, the cladding shrinkage. A comprehensive dimensional analysis must account for both effects simultaneously.
The paper also highlights the importance of empirical data in fabrication planning. While theoretical calculations provide a starting point, the actual dimensional changes depend on many factors that are difficult to predict analytically. The study recommends that fabrication engineers maintain detailed records of dimensional changes for each fabrication job, building a database of empirical corrections that can be applied to future projects.
Study Insights
This research from 1990 remains highly relevant to modern hot-wall hydrogenation reactor fabrication. The fundamental physics of rolling thinning and cladding shrinkage have not changed, although measurement and control technologies have improved significantly. The study's emphasis on empirical measurement and process optimization is a sound engineering approach that continues to be applicable today.
The broader implication is that dimensional accuracy in pressure vessel fabrication requires a systematic approach that considers all forming and welding operations in sequence. Each operation introduces dimensional changes that must be predicted, controlled, and verified. The cumulative effect of these changes must be within the final dimensional tolerances specified by the applicable code and design requirements.