Zhuojin Pipe Fitting Co., LtdApplication of TIG-SMAW Combined Welding in Boiler Chemical Injection Piping Systems
Literature Overview
This paper by Zhao Qiaoliang and Jin Qiaofang, published in Hot Working Technology (2010, Vol. 39, No. 7, pp. 164-165), documents the practical application of a TIG-SMAW combined welding process for the chemical injection piping system of a 75 t/h circulating fluidized bed (CFB) boiler. The specific focus is on the welding quality at the connection between globe valves and the chemical injection piping, where previous welding attempts had produced quality issues. The authors report that after implementing the TIG-SMAW combined welding process, the piping system has operated without any weld-related problems for five years, demonstrating the effectiveness of the approach.
The classification number TG444 corresponds to welding methods, and the keywords include TIG-SMAW combined welding, boiler chemical injection piping, and globe valves. This is a practical engineering paper that bridges the gap between welding technology and field application in power generation equipment.
Technical Background and Process Analysis
The chemical injection system in a CFB boiler is critical for controlling sulfur dioxide emissions and managing ash behavior. The piping carries chemicals such as sodium hydroxide, lime slurry, or other desulfurization agents, which are often corrosive to carbon steel. The globe valves used in these systems typically have threaded or flanged connections with the piping, and the weld joints at these connections must withstand both mechanical stress and chemical attack.
The TIG-SMAW combined welding process is a two-step approach:
| Step | Process | Purpose | Typical Parameters |
|---|---|---|---|
| First pass (root weld) | TIG (GTAW) | Achieve clean, penetration-quality root weld with full gas shielding | Current: 80-120 A; Travel speed: 2-4 mm/s; Shielding gas: Ar or Ar+CO2 |
| Subsequent passes | SMAW | Build up weld metal to full thickness efficiently | Current: 100-160 A; Electrode: E7018 or equivalent; Layers: 2-4 passes |
The TIG root pass is essential for ensuring complete penetration and a clean, oxide-free weld root. The SMAW fill and cap passes provide efficient deposition of weld metal at a higher deposition rate than TIG alone, which is important for production efficiency in field welding conditions.
Key Technical Points
The quality issues encountered before adopting the combined welding process likely stemmed from one or more of the following:
- Incomplete root penetration when using SMAW alone for the root pass, leading to lack of fusion defects at the backside of the joint.
- Excessive oxidation of the weld root when TIG welding was not used, resulting in porosity and reduced weld strength.
- Poor weld geometry at the valve-to-pipe transition, where the joint configuration may involve a groove joint with unequal thicknesses.
The TIG root pass solves the penetration and cleanliness problems by providing a controlled, gas-shielded weld pool with precise arc control. The SMAW fill passes then efficiently fill the remaining groove volume. This combination leverages the strengths of both processes: TIG for quality-critical root welds and SMAW for high-deposition-rate fill work.
For the specific application of globe valve connections in chemical injection piping, the following additional considerations are important:
- The valve body material must be compatible with the piping material to avoid galvanic corrosion.
- The weld joint geometry must accommodate the angular misalignment that can occur during field assembly.
- Post-weld treatment may be required to restore corrosion resistance if the chemical being injected is aggressive.
Engineering Practice and Quality Control
The five-year successful operation record reported by the authors provides strong evidence of the process's reliability. However, from a quality control perspective, several measures should be implemented to ensure consistent weld quality:
- Pre-weld inspection of the joint fit-up, including gap width, root opening, and alignment.
- Welder qualification testing on similar joint configurations to verify operator skill.
- In-process monitoring of welding parameters, particularly for the TIG root pass.
- Post-weld non-destructive testing, such as visual inspection (VT) and potentially radiographic testing (RT) for critical joints.
- Hydrostatic pressure testing of the completed piping system to verify leak tightness.
The paper's value lies in its demonstration that a well-established welding process combination can solve practical field problems when applied correctly. The TIG-SMAW combined welding approach is widely used in pressure vessel fabrication and pipeline construction, and its application to boiler auxiliary piping systems is a natural extension of existing practice.
Study Insights and Conclusions
This paper serves as a practical case study for engineers working on power plant auxiliary systems. The key takeaway is that welding quality problems in field applications often have straightforward solutions when the correct process is selected and applied with attention to detail. The TIG-SMAW combined welding method is not a novel technique, but its successful application to this specific service condition demonstrates the importance of process selection based on the actual welding conditions and service requirements. For similar applications involving chemical injection piping or other corrosive service piping, the combined welding approach should be considered as the default method for ensuring long-term weld integrity. The five-year failure-free record is a testament to the reliability of this approach when properly executed.