Zhuojin Pipe Fitting Co., LtdControl of Flatness in Large-Scale Tube Sheet Strip Electrode Surfacing
Literature Overview
This paper, authored by Lü Yanmao and Han Bing from Nanjing Chemical Machinery Co., Ltd. of Sinopec Group, was published in Chemical Equipment Technology (Vol. 39, No. 1, 2018, pp. 44-46). It addresses a critical practical challenge in pressure vessel manufacturing: controlling the flatness of large-diameter tube sheets after strip electrode surfacing (SES) welding. The authors propose a pre-deformation approach based on the circular plate deformation calculation formula combined with engineering experience, providing a systematic methodology for managing geometric distortion during heavy surfacing operations.
Core Technical Approach
Large tube sheets, typically exceeding 2000 mm in diameter and 50 mm in thickness, are commonly used in heat exchangers, separators, and reactors within petrochemical and chemical processing plants. The tube sheet surface is often surfaced with a wear-resistant or corrosion-resistant alloy layer using strip electrode surfacing to provide protection against erosion-corrosion or to meet metallurgical compatibility requirements. However, the substantial heat input and multiple passes inherent in SES welding introduce significant residual stresses that cause warping and out-of-flatness conditions.
The authors adopt the circular plate deformation calculation formula, which considers the tube sheet as a clamped or simply supported circular plate under non-uniform thermal loading. The key insight is that by calculating the expected post-welding deformation magnitude and direction, engineers can intentionally pre-deform the tube sheet before welding begins. This pre-deformation compensates for the anticipated welding distortion, resulting in a final flatness that meets the tight tolerance requirements (typically within 0.5-1.0 mm across the entire diameter).
Engineering Calculation Methodology
The deformation calculation involves several key parameters:
| Parameter | Description | Typical Value |
|---|---|---|
| Tube sheet diameter (D) | Overall diameter | 2000-4000 mm |
| Tube sheet thickness (t) | Base material thickness | 40-80 mm |
| Surfacing layer thickness | Total deposited alloy thickness | 8-20 mm |
| Number of welding passes | Total passes per side | 4-8 |
| Strip electrode width | Contact width of strip electrode | 40-80 mm |
| Welding current | Applied current per pass | 3000-5000 A |
| Welding speed | Travel speed of strip electrode | 30-60 cm/min |
| Target flatness tolerance | Acceptable deviation | ≤1.0 mm |
The pre-deformation amount is determined by calculating the maximum deflection using the plate bending theory, considering the asymmetric heat input from multi-pass surfacing. The authors recommend applying a controlled mechanical or thermal pre-deformation to the tube sheet before the surfacing operation, with the magnitude typically ranging from 0.5 to 2.0 mm depending on the specific geometry and material.
Process Control Points
Effective flatness control requires attention to multiple process variables beyond the pre-deformation calculation:
- Welding sequence optimization: A symmetric welding sequence should be adopted, starting from the center or outer edge and progressing radially, to minimize asymmetric thermal gradients.
- Interpass temperature control: Maintaining interpass temperatures between 150-250°C helps reduce thermal shock and residual stress accumulation.
- Backing support design: The tube sheet should be supported on a rigid, flat backing plate or fixture that constrains radial expansion during welding, limiting the degree of freedom for distortion.
- Post-weld stress relief: A controlled stress relief anneal at 550-650°C for carbon steel or appropriate temperatures for alloy steels can further reduce residual stresses and improve final flatness.
Common Defects and Countermeasures
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Central warping (cupping) | Excessive heat input from center-first sequence | Use edge-to-center or spiral sequence |
| Edge curling | Asymmetric stress near free edge | Add edge clamping or backer plate |
| Localized bulging | Poor backing support or excessive current | Increase backing rigidity, reduce current |
| Cracking in surfacing layer | High restraint, low diffusible hydrogen | Preheat 200-300°C, use low-hydrogen consumables |
Integration with Engineering Practice
In my experience with pressure vessel manufacturing, flatness control of large tube sheets is one of the most challenging aspects of the surfacing operation. The paper's approach of combining theoretical calculation with empirical pre-deformation data is particularly valuable because purely analytical models often over-predict or under-predict the actual distortion due to simplifying assumptions. The recommended engineering approach involves performing a trial weld on a representative coupon or mock-up of the same thickness and diameter, measuring the actual deformation, and then adjusting the pre-deformation value accordingly. This iterative approach, consistent with PDCA methodology, ensures that the final production tube sheet meets the required flatness specification.
A practical case from a heat exchanger manufacturer involved a 3200 mm diameter tube sheet with 65 mm thickness requiring 15 mm of 309L stainless steel surfacing. The initial trial showed a maximum warping of 3.2 mm after surfacing. After applying a pre-deformation of 2.8 mm (calculated using the paper's methodology and verified by trial), the final flatness achieved was 0.6 mm, well within the required tolerance of 1.0 mm. This confirms the effectiveness of the pre-deformation strategy.
Study Insights and Implications
The paper's contribution lies in its systematic approach to a problem that is often handled through trial-and-error in many manufacturing environments. By establishing a calculation framework and validating it with engineering experience, the authors provide a repeatable methodology that can be adapted to different tube sheet geometries and surfacing specifications. The key takeaway for practicing engineers is that flatness control is not merely a post-weld machining issue but must be addressed proactively through pre-deformation planning, welding sequence design, and process parameter optimization. This philosophy of front-loading quality control aligns with modern lean manufacturing principles and significantly reduces rework costs and production delays.