Fabrication of Welded Tees for Cold Reheat Piping in Power Plants

Literature Overview

This paper by Wu Zhongqi (1990), published in Electric Power Construction, documents the field fabrication methodology for welded tees used in cold reheat piping of large-scale thermal power units. This represents one of the earliest documented Chinese engineering practices for on-site fabrication of large-diameter welded tees, providing valuable reference experience for subsequent installations of large thermal power generation units. The paper acknowledges that while the fabrication process requires further refinement, the initial innovation is worthy of reference and adoption.

Technical Context: Cold Reheat Piping Systems

Cold reheat piping connects the high-pressure turbine exhaust to the low-pressure turbine inlet through the reheater section. These piping systems operate at temperatures typically ranging from 300°C to 540°C (depending on the specific unit design) and pressures of 1.5 to 4.0 MPa. The piping materials commonly employed include P91 steel (ASTM A335 P91), 347 stainless steel (ASTM A213 T347), and in some designs, 9Cr-1Mo steel. The welded tees in this application must withstand both the sustained pressure load and the cyclic thermal stresses from frequent start-stop operations.

Typical Parameters for Cold Reheat Piping

Parameter	Typical Range
Operating Temperature	300–540 °C
Operating Pressure	1.5–4.0 MPa
Pipe Diameter	300–600 mm
Wall Thickness	14–32 mm
Material Grade	P91 / 347 SS / 9Cr-1Mo
Cyclic Loading	500–2000 cycles per year
Design Code	ASME B31.1 / GB/T 20801

Fabrication Process Description

The field fabrication methodology described involves several key stages. First, the header pipe is prepared by cutting and forming the intersection opening using plasma or oxy-fuel cutting, followed by mechanical finishing to achieve the required surface quality. The branch pipe is then prepared with the appropriate bevel geometry for welding. The fit-up stage requires precise alignment of the branch pipe to the header pipe opening, typically using temporary backing plates and alignment fixtures.

Welding Procedure Parameters

Parameter	Typical Value
Welding Process	SMAW (manual) + GTAW (root)
Preheat Temperature	150–250 °C (P91), 100–150 °C (347 SS)
Interpass Temperature	≤250 °C (P91), ≤150 °C (347 SS)
Heat Input	0.8–1.5 kJ/mm (P91)
Post-Weld Heat Treatment	720–760 °C × 2–4 h (P91)
Number of Weld Layers	4–8 (depending on wall thickness)
Electrode Type	E911-T1 (P91), E347 (347 SS)

Quality Control Measures

The quality control program for field-fabricated welded tees must address the unique challenges of on-site construction including limited access, variable environmental conditions, and the need for comprehensive non-destructive testing. The typical quality control sequence includes visual inspection of fit-up, radiographic testing (RT) of the root and intermediate weld layers, ultrasonic testing (UT) of the completed weld, and magnetic particle testing (MT) or penetrant testing (PT) of the weld surface.

Defect Analysis and Countermeasures

Defect Category	Typical Manifestation	Root Cause	Corrective Action
Root defects	Incomplete penetration, root concavity	Inadequate GTAW parameters	Optimize root welding procedure
Interpass defects	Lack of fusion between layers	Excessive interpass temperature	Enforce temperature control
Surface defects	Cracks at weld toe	Residual stress + HAZ hardness	PWHT + stress relief
Geometric defects	Out-of-roundness, misalignment	Poor fit-up technique	Use precision alignment tools
Material defects	Segregation in base metal	Casting quality	Material certification + UT

Engineering Practice Lessons

The 1990 paper represents a pioneering effort in field fabrication of large-diameter welded tees for power plant applications. The key lessons that have been carried forward include the importance of prequalification of welding procedures, the necessity of skilled welder certification, and the critical role of post-weld heat treatment in relieving residual stresses and improving the microstructure of the HAZ. For P91 steel components, the PWHT is particularly critical as it transforms the as-welded martensitic structure into tempered martensite, reducing hardness from potentially 400+ HV to the required ≤350 HV.

The field fabrication approach described, while innovative for its time, has since been supplemented by improved technologies including CNC pipe cutting machines, robotic welding systems, and advanced NDT techniques such as phased array ultrasonic testing (PAUT) and thermography. However, the fundamental principles of fit-up precision, welding procedure qualification, and comprehensive quality control remain unchanged.

Study Insights and Reflections

This paper holds historical significance as one of the first documented Chinese engineering practices for field fabrication of large welded tees in power plant applications. The author's candid acknowledgment that the process requires further improvement reflects the engineering maturity to recognize limitations while still contributing valuable initial experience. For contemporary engineers, the key insights are the importance of developing and documenting fabrication procedures even when they are not yet perfect, the critical role of welding procedure qualification (WPQ) in ensuring weld quality, and the necessity of comprehensive post-weld heat treatment for high-temperature alloy piping components. The transition from field fabrication to factory fabrication has improved quality consistency, but the fundamental welding metallurgy challenges remain the same, and the principles documented in this 1990 paper continue to inform modern practice.