Fabrication of Welded Tees for Cold Reheat Piping Systems

Literature Overview

This paper by Wu Zhongqi, published in Electric Power Construction (1990, Vol. 11, No. 3, pp. 39-42), documents the field fabrication of welded tees for cold reheat piping in large-scale thermal power generation units. The work was conducted by Anhui Electric Power Construction Company No. 2 and represents a pioneering effort in on-site fabrication of critical pipe components for power plant construction. The author explicitly acknowledges that while the process requires further improvement and the work needs deeper investigation, the initial innovation is worth referencing and learning from.

Fabrication Process Analysis

The cold reheat piping system in a thermal power plant carries steam from the high-pressure turbine to the reheater and then back to the intermediate-pressure turbine. This piping is subject to significant thermal cycling during unit startup and shutdown, as well as sustained thermal stresses during normal operation. The welded tee components in this system must withstand these demanding service conditions while maintaining structural integrity over the design life of the unit.

The fabrication process described in this paper involves the following key steps:

1. Pipe Selection and Inspection: Selection of appropriate grade pipe material with verification of mechanical properties and dimensional tolerances.
2. Marking and Cutting: Layout and cutting of the tee geometry from the run pipe and branch pipe, including preparation of the branch opening in the run pipe.
3. Groove Preparation: Machining or grinding of weld grooves at the branch-run junction to achieve proper fit-up and root gap.
4. Welding: Execution of the butt weld joining the branch to the run, likely using SMAW or GTAW processes depending on the pipe diameter and wall thickness.
5. Post-Weld Treatment: Application of post-weld heat treatment to relieve residual stresses and ensure proper metallurgical properties in the weld and HAZ.
6. Non-Destructive Testing: Inspection of the completed weld using radiographic testing (RT) and/or ultrasonic testing (UT) to verify weld quality.

Quality Control Considerations

The field fabrication of welded tees presents unique challenges compared to factory fabrication. Environmental conditions, limited workspace, and the need to coordinate with ongoing construction activities all introduce additional variables that must be controlled to ensure weld quality. The author's acknowledgment that the process requires further improvement is a realistic assessment that reflects engineering maturity.

Engineering Practice and Lessons Learned

Based on my experience with field-fabricated pipe components, several lessons emerge from this early work:

Material Considerations: Cold reheat piping in thermal power plants typically uses materials such as P91 (ASTM A335 P91 / GB 5310 12Cr1MoVG) or similar chromium-molybdenum steels that require careful control of welding parameters and post-weld heat treatment. The HAZ in these materials is susceptible to temper embrittlement and creep damage if not properly controlled.

Welding Procedure Qualification: The welding procedure used for field fabrication must be qualified in accordance with applicable codes (ASME B31.1, B31.3, or GB/T 20801). The procedure qualification must account for the specific geometry of the tee, the welding positions (which may include fixed vertical or overhead positions), and the environmental conditions.

Distortion Control: Field-fabricated tees are prone to distortion during welding, particularly when the branch-to-run diameter ratio is large or the wall thickness is relatively thin. Distortion control measures such as proper welding sequence, back-bar usage, and拘束 (constraint) fixtures are essential.

Residual Stress Management: The residual stress field in a field-fabricated tee is complex and depends on the welding sequence, thermal input, and constraint conditions. Post-weld heat treatment is critical for reducing residual stresses to acceptable levels, particularly for materials subject to creep or fatigue.

Documentation and Traceability: Field fabrication requires meticulous documentation of all welding parameters, heat treatment cycles, and inspection results. This traceability is essential for regulatory compliance and for future maintenance and repair activities.

Study Insights and Implications

This 1990 paper represents an early and valuable contribution to the field of field-fabricated pipe components in thermal power plants. The author's candid acknowledgment of the need for further improvement is commendable and reflects a mature engineering mindset. The work demonstrates that field fabrication of welded tees is technically feasible but requires careful attention to welding procedures, quality control, and documentation. For contemporary engineers, this paper serves as a historical reference that highlights the evolution of field fabrication practices and the ongoing importance of process improvement in ensuring the reliability of critical power plant components.

The experience documented in this paper is particularly relevant to the current situation in China, where rapid expansion of thermal power capacity has created a significant demand for field-fabricated pipe components. The lessons learned from early field fabrication efforts continue to inform current practices in welding procedure development, quality control, and personnel training. The principle that field fabrication must achieve quality equivalent to factory fabrication remains a fundamental requirement, and the experimental and practical work documented in this paper contributes to that ongoing effort.