Application of ASME A335 P9 Studded Tube in Coal-Fired Power Plant Boilers

In coal-fired power stations, particularly in advanced ultra-supercritical units pursuing extreme efficiency, the use of ASME A335 P9 studded fin tubes represents a precise engineering choice to tackle extreme operating conditions and break through efficiency barriers. Modern utility boilers continuously push for higher steam parameters to achieve greater thermal efficiency, resulting in metal wall temperatures in the high-temperature superheater and reheater regions consistently within the critical range of 580°C to 650°C, while simultaneously withstanding immense internal steam pressure. This environment presents seemingly contradictory demands on the material: it must possess sufficient high-temperature strength to resist creep deformation, endure oxidation corrosion and fly ash erosion from sulfur-bearing flue gas, and have the thermal fatigue resistance to cope with frequent load cycling and start-ups/shutdowns. Facing this comprehensive challenge, ASME A335 P9 material provides a fundamental solution. As a standardized 9 chromium-1 molybdenum ferritic alloy steel, its approximately 9% chromium content grants it superior oxidation resistance compared to common low-alloy steels, while the addition of molybdenum significantly enhances its creep resistance at high temperatures. This makes its load-bearing capacity outstanding around the key temperature of 600°C, forming the core skeleton that supports the safe operation of high steam parameters.

However, a robust pressure-bearing tube alone is insufficient for efficiently capturing the vast thermal energy in the flue gas, as the low convective heat transfer coefficient on the boiler's gas side is the main bottleneck constraining overall efficiency. This is precisely where the studded fin structure plays its decisive role. By welding numerous sturdy, stud-like fins onto the outer wall of the P9 steel tube, this design expands the heat transfer area on the flue gas side by several to over ten times, forcefully breaking the heat transfer barrier and efficiently transferring high-grade thermal energy to the steam inside the tube. More importantly, this studded structure demonstrates unique advantages in fly-ash-laden flue gas: its wide, unobstructed flow paths are less prone to ash fouling, and the sturdy weld points effectively withstand long-term erosion by dust. Combined with the inherent good wear resistance of the P9 material, this ensures the long-term reliability of the heat exchange element in harsh environments.

The ASME A335 standard governs seamless ferritic alloy-steel pipes for high-temperature service, and the P9 grade (9Cr-1Mo) represents a medium-to-high grade of chromium-molybdenum alloy steel.
P9 contains approximately 9% Chromium (Cr) and 1% Molybdenum (Mo). Chromium significantly enhances high-temperature oxidation and corrosion resistance; molybdenum substantially strengthens high-temperature strength and creep resistance.
The standard ensures not only chemical composition but also, through the specification of heat treatment processes (normalizing and tempering) and mechanical properties (minimum yield and tensile strength at elevated temperatures), guarantees the material's long-term microstructural stability and load-bearing capacity at service temperatures.

The steam parameters of modern ultra-supercritical units have been increased to 600-620°C and 25-30 MPa and above, posing "three-high" challenges for boiler heating surface materials: high-temperature strength, resistance to flue gas corrosion, and thermal fatigue resistance. The physical properties of P9 provide targeted solutions:

In key boiler sections like the high-temperature superheater and reheater, the high-temperature strength of the P9 tubing is the foundation, and the studded fin structure is the "amplifier" that transforms this foundation into efficient heat transfer capability.

• Core Contradiction: The low convective heat transfer coefficient on the boiler's flue gas side is the bottleneck limiting overall heat exchange efficiency.
• The Studded Fin Solution:
  1. By welding dense studs, it expands the flue gas side heat transfer area by 8-15 times, powerfully enhancing the recovery efficiency of high-grade thermal energy and directly increasing boiler thermal efficiency.
  2. The wide channels between studs are not easily blocked by fine ash and can guide the flue gas to create turbulence, enhancing mass transfer. Its robust welded structure is also superior to vulnerable wound or embedded fins, making it more suitable for high-velocity, high-dust environments.
  3. The welded studs effectively add radial support to the tube, improving the tube bank's resistance to vibration and sway within the flue gas.

In the 580-650°C temperature range, which is crucial for power plant efficiency, its high-temperature strength far exceeds that of cheaper low-alloy steels like P22, while its cost is significantly lower than that of advanced austenitic stainless steels like TP347H. It is the most cost-effective and reliable mature material choice for achieving 600°C grade ultra-supercritical technology.
Its excellent thermal fatigue performance enables boilers equipped with such heating surfaces to better adapt to frequent start-ups/shutdowns and deep load cycling, which is vital for power grids with an increasing share of renewable energy.