CLADDING TECHNOLOGY SHANXI CO., LTDCladding Process Technology for Heat Exchanger Tube Sheets
Literature Overview
The paper by Shi Liang, published in 2009 by Tianjin Guanjie Petrochemical Engineering Co., Ltd., addresses the cladding process technology for heat exchanger tube sheets, a critical component in petrochemical and refining industries. Tube sheets serve as the interface between the shell-side and tube-side media in heat exchangers, and they are frequently subjected to corrosive environments that necessitate protective cladding layers. The study focuses on practical process development and optimization for producing reliable clad tube sheets that meet the stringent requirements of pressure vessel codes.
Technical Background and Challenges
Heat exchanger tube sheets typically require cladding with stainless steel or nickel-based alloys to resist corrosion from process fluids. The challenges in tube sheet cladding are more pronounced than for flat plate cladding due to the complex geometry involving numerous tube holes, the need for precise hole alignment between the base and clad layers, and the requirement for uniform cladding thickness across the entire surface. The cladding must also maintain sufficient mechanical strength to withstand pressure loading while providing adequate corrosion resistance.
Cladding Process Selection
The author evaluated multiple cladding approaches for tube sheets and recommended a combination of processes depending on the specific requirements:
| Cladding Method | Applicable Scenario | Advantages | Limitations |
|---|---|---|---|
| Strip cladding (roll-bonded) | Large-diameter tube sheets | Uniform thickness, high productivity | Limited to flat or low-curvature surfaces |
| Submerged arc welding (SAW) overlay | Medium to large tube sheets | High deposition rate, good penetration | Requires multi-pass deposition |
| Gas metal arc welding (GMAW) overlay | Small to medium tube sheets | Flexibility, good weld quality | Lower deposition rate than SAW |
| Plasma transferred arc (PTA) cladding | High-alloy overlay requirements | Low dilution, precise composition control | Lower deposition rate, higher equipment cost |
| Explosive cladding | Specialized applications | Excellent bond strength, no dilution | Limited to specific geometries, high cost |
Process Parameters and Quality Control
The study emphasized the importance of preheating the tube sheet to prevent cracking in the base metal, particularly for higher-strength steels. The recommended preheat temperature was 150-250 °C for carbon steel base materials and 200-350 °C for low-alloy steels. The interpass temperature should be maintained below 300 °C to avoid excessive grain growth in the overlay layer.
Quality control measures included:
- Visual inspection of all weld seams for surface defects
- Magnetic particle testing (MT) for surface and near-surface cracks
- Ultrasonic testing (UT) for lack of fusion and internal porosity
- Penetrant testing (PT) for tube hole edges and difficult-to-access areas
- Dimensional verification of cladding thickness using ultrasonic thickness gauging
Tube Hole Machining and Post-Cladding Operations
After cladding, the tube holes must be machined to ensure proper alignment and fit with the heat exchanger tubes. The author stressed the importance of maintaining a minimum cladding thickness of 1.5 mm after machining to ensure adequate corrosion protection. The machining process should be performed with appropriate tooling and cutting parameters to avoid damaging the cladding layer or introducing residual stresses that could lead to stress corrosion cracking.
Standards Compliance and Code Requirements
The cladding process and inspection procedures must comply with applicable pressure vessel codes and standards. For Chinese fabrication, GB/T 150 and NB/T 47002 govern the design and fabrication requirements. The welding procedure qualification must follow NB/T 47014, and non-destructive testing must conform to JB/T 4730. For international projects, ASME VIII Division 1 or Division 2 and ASME IX are the governing codes. The study implicitly references these standards in its discussion of quality requirements, which is consistent with industry practice.
Study Insights and Engineering Recommendations
This paper provides practical guidance for engineers involved in heat exchanger tube sheet fabrication. The key insight is that the selection of cladding process must be driven by a careful evaluation of the tube sheet geometry, material requirements, and applicable code standards. For large-diameter tube sheets with moderate corrosion requirements, SAW overlay with stainless steel wire offers an economical and reliable solution. For high-performance applications requiring low dilution and precise alloy composition, PTA cladding is the preferred approach. The study reinforces the importance of comprehensive quality control, including both destructive and non-destructive testing, to ensure the long-term reliability of clad tube sheets in demanding petrochemical service conditions.