CLADDING TECHNOLOGY SHANXI CO., LTDCladding of Tube Plates for Phenol Distillation Kettle Heaters
Literature Overview
This 2000 paper by Zheng Xingang from Panzhihua Steel Jinjiang Machinery Factory's Riveting and Welding Workshop addresses the specific engineering challenge of cladding tube plates for phenol distillation kettle heaters. The publication appeared in "Welding Technology" and represents practical field experience from a major Chinese steel complex. Phenol distillation equipment operates under conditions that demand high corrosion resistance while maintaining mechanical integrity under thermal cycling.
Service Environment Analysis
Phenol distillation processes present a unique combination of corrosive and thermal challenges:
| Parameter | Operating Condition | Implication for Cladding |
|---|---|---|
| Temperature | 150-250°C | Moderate thermal stress |
| Pressure | 0.5-2.0 MPa | Pressure vessel requirements |
| Medium | Molten phenol, acid condensates | Severe chemical attack |
| Cyclic loading | Daily start/stop | Fatigue considerations |
| Tube pitch | 25-35 mm | Dense tube layout |
| Tube diameter | 25-38 mm | Large number of tubes |
The tube plate serves as the critical interface between the tube bundle (typically stainless steel or alloy tubes) and the shell (carbon or low-alloy steel). Corrosion of the tube plate leads to tube leakage, which is the primary failure mode in heat exchanger service.
Cladding Method Selection
For tube plate applications, several cladding methods are available, each with distinct advantages:
| Method | Deposition Rate | Dilution Control | Equipment Required | Suitability |
|---|---|---|---|---|
| SAW (submerged arc) | High (10-15 kg/h) | Moderate | Welding machine, flux | Large tube plates |
| GTAW (TIG) | Low (1-3 kg/h) | Excellent | TIG machine | Small areas, repairs |
| PTA (plasma transfer) | Medium (5-8 kg/h) | Good | PTA system | High-quality overlays |
| SAW + GTAW combo | Medium | Good | Both systems | Complex geometries |
| Explosive cladding | N/A (bonding) | None (mechanical) | Explosives | Full-surface coverage |
Recommended Process for Phenol Distillation Tube Plates
Based on the study's findings, the following multi-step process is recommended:
- Surface preparation: Grind the tube plate surface to expose bright metal, chamfer tube holes to facilitate tube insertion.
- First pass (GTAW): Deposit a thin stainless steel layer using E309L or E316L wire to establish metallurgical bonding.
- Main overlay (SAW): Apply 2-3 passes of E309L or E316L using submerged arc welding with low-hydrogen flux.
- Final pass (GTAW): Apply a thin finishing pass of E316L to ensure surface quality and complete corrosion protection.
- Post-weld machining: Machine the overlay surface to achieve required flatness and tube hole dimensions.
Process Parameters
| Parameter | GTAW Pass | SAW Pass | Final GTAW |
|---|---|---|---|
| Current (A) | 130-160 | 450-550 | 120-150 |
| Voltage (V) | 18-22 | 28-32 | 18-22 |
| Travel speed (mm/min) | 250-350 | 300-400 | 250-350 |
| Wire/feed (mm) | 1.6 | 2.4 | 1.2 |
| Shielding gas | Ar | HJ431 flux | Ar |
| Preheat (°C) | 150-200 | — | — |
Key Technical Challenges
Challenge 1: Tube Hole Integrity
The tube holes in the cladded tube plate must maintain dimensional accuracy and hole wall quality after cladding and machining. Distortion of tube holes due to welding-induced deformation can lead to:
- Poor tube-to-tube-plate joint fit
- Increased tube expansion difficulty
- Potential leakage paths
Countermeasures: Use back-up rings during cladding to protect tube holes, apply symmetric welding sequences, and verify hole dimensions after machining.
Challenge 2: Residual Stress Management
The tube plate is subject to complex residual stress patterns from cladding, tube insertion, and tube joint welding. These stresses combine with operating thermal stresses and can accelerate fatigue failure.
Countermeasures:
- Post-weld stress relief at 550-600°C for 2 hours per 25 mm of plate thickness
- Symmetric welding sequences to minimize distortion
- Consider stress-relief annealing of the substrate before cladding
Challenge 3: Overlay Thickness Control
Insufficient overlay thickness leads to premature corrosion failure, while excessive thickness increases cost and may introduce residual stress concerns.
| Overlay Material | Minimum Thickness | Recommended Thickness | Maximum Thickness |
|---|---|---|---|
| 304/304L | 2.0 mm | 3.0-4.0 mm | 6.0 mm |
| 316/316L | 2.0 mm | 3.0-5.0 mm | 8.0 mm |
| Alloy 20 | 3.0 mm | 4.0-6.0 mm | 10.0 mm |
Inspection Requirements
For pressure vessel heat exchanger tube plates, the following inspections are required per applicable standards:
| Inspection | Method | Standard | Acceptance |
|---|---|---|---|
| Surface defects | PT (dye penetrant) | JB/T 4730.5 | Level II |
| Bonding quality | UT (shear wave) | JB/T 4730.3 | No lack of fusion |
| Thickness verification | UT thickness gauge | GB/T 19624 | Within tolerance |
| Hardness | Portable hardness tester | GB/T 231 | Within material range |
| Hydrostatic test | Water pressure | GB/T 150 | 1.25 × design pressure |
Engineering Practice Reflections
This paper provides valuable practical insights from a major industrial setting. Several observations are particularly relevant:
- The multi-process approach (GTAW + SAW + GTAW) represents a practical compromise between quality and productivity that remains widely used in Chinese fabrication shops.
- The emphasis on tube hole protection during cladding is a frequently overlooked but critical aspect of tube plate fabrication.
- The paper highlights the importance of post-weld machining in achieving the dimensional tolerances required for tube-to-tube-plate joint fabrication.
- The choice of overlay material should be driven by the specific corrosive environment rather than a default selection of 304 stainless steel, which may be insufficient for phenol service containing acidic condensates.
The experience documented in this paper underscores the principle that successful tube plate cladding requires integration of welding engineering, machining, and inspection disciplines, coordinated through a comprehensive fabrication plan.