Analysis of welding methods for long-distance heating pipelines
With increasing environmental protection pressures, using long-distance or ultra-long-distance hot water pipelines to transport industrial waste heat from power plants, steel mills, chemical plants, and other industries to cities for centralized heating has become a popular method in the heating industry. These pipelines are mostly buried underground, and the quality of the welding directly impacts their lifespan and safety. Given the characteristics of these pipelines—long distance, large spans, high pressure, large diameters, thick walls, and outdoor construction—it’s essential to design and evaluate welding processes according to their unique requirements, geological conditions, and natural environment to enhance welding production efficiency, improve quality, and ensure longevity and safety.
SSAW welded steel pipe
Shielded Metal Arc Welding (SMAW)
Shielded Metal Arc Welding (SMAW) is widely used in various industrial fields due to its simple equipment structure, low cost, portability, flexible operation, and adaptability to weld in any spatial position. However, SMAW demands high operator skill, leading to high training costs. The welding quality is significantly influenced by the operator’s skills and environmental factors, resulting in low production efficiency and making it unsuitable for mechanized and streamlined welding operations of long-distance heating pipelines.
Electrode Coated Arc Welding
Low-hydrogen electrodes are widely used for repair and root welding of long-distance pipelines.
Iron powder low-hydrogen electrodes are used for small-diameter long-distance pipelines and special sections’ ring welds.
High-cellulose electrodes are employed for root and hot welding of ring welds in long-distance pipelines.
Cellulose electrodes are mainly used for welding pipeline connections.
ASTM A312 TP304 welded steel pipes
Manual Tungsten Inert Gas Welding (TIG)
Primarily used for root welding of ring welds in long-distance heating pipeline projects, though its low efficiency limits widespread use.
Gas Metal Arc Welding (GMAW or MIG)
Gas Metal Arc Welding (GMAW), or MIG welding, features high welding currents, deep penetration, fast filler metal deposition, and minimal deformation, enhancing production efficiency. It is extensively used for root welding of ring welds in long-distance heating pipelines, often combined with SMAW, CO2 gas shielded welding, and self-shielded flux-cored semi-automatic welding for filling and capping. However, MIG welding is sensitive to oil, rust, and other surface contaminants on the wire and base material, necessitating thorough pre-welding cleaning.
CO2 Gas Shielded Welding
CO2 Gas Shielded Welding has grown rapidly in low-carbon and low-alloy steel welding due to its low cost, minimal pre-welding preparation, reduced post-welding correction time, high production efficiency, and low energy consumption. Its low sensitivity to oil and rust, good arc visibility, and wide applicability make it suitable for all-position welding of long-distance heating pipelines. The combination of cellulose electrode root welding and CO2 gas shielded welding for filling and capping is popular in long-distance heating pipeline construction due to its low cost and high efficiency.
Mixed Gas Shielded Semi-Automatic Welding
Adding argon (Ar) to CO2 gas in mixed gas shielded semi-automatic welding refines droplet size and reduces spatter. Techniques like Surface Tension Transfer (STT) and Regulated Metal Deposition (RMD) have made mixed gas semi-automatic welding prevalent in pipeline welding. STT offers stable welding, thick weld metal, fast deposition, low hydrogen content, minimal spatter, attractive weld bead, low heat input, minimal deformation, and high qualification rates. RMD reduces joint defects during root welding, enhances welding quality, and optimizes spatter, heat-affected zones, and smooth transitions compared to traditional cellulose electrode root welding.
Self-Shielded Flux-Cored Arc Welding (FCAW-S)
High Efficiency: Compared to SMAW, FCAW-S has a faster deposition rate, increasing welding efficiency by up to 33% with qualification rates over 95%.
Better Performance: FCAW-S offers softer arcs, less spatter, and better welding performance than GMAW.
Wind Resistance: The flux core produces shielding gas and slag, providing combined protection with strong wind resistance (effective welding in wind speeds <8 m/s), making it ideal for outdoor and harsh environments.
Weld Appearance: FCAW-S yields flat, deep penetration, aesthetically pleasing welds with good low-temperature toughness.
Cost-Effective: While the wire cost is higher, increased welding speed and high first-pass yield reduce overall construction costs.
The combination of cellulose electrode root welding and FCAW-S filling and capping is particularly suitable for large-diameter long-distance heating pipelines, reflecting the adaptability and efficiency of these welding methods for critical infrastructure projects.