Membrane Strip Fin Bars
What Are Membrane Strip Fin Bars
Membrane Strip Fin Bars are specialized flat steel bars used in the manufacture of water walls. In a boiler, the water wall is arranged on the inner surface of the furnace and is responsible for absorbing radiant heat from the flame to vaporize or superheat the water. A membrane water wall is formed by welding bare tubes and flat steel bars (i.e., fins) to create a gas-tight monolithic structure. The flat bars connect adjacent water wall tubes into a continuous wall surface, serving both sealing and heat conduction functions.
What Materials Are Commonly Used for Membrane Strip Fin Bars
The flat bars typically have a rectangular cross-section, and the material must match that of the water wall tubes. Under the American standard system, commonly used materials include carbon steels and carbon-manganese steels such as SA-36 and SA-516 Gr.60/70. For high-temperature service, chromium-molybdenum alloy steels such as SA-387 Gr.11/Gr.12 are employed.
Water wall flat bars are generally selected from pressure vessel steels. Although the flat bars themselves do not directly bear the internal pressure, they are welded to the pressure-containing water wall tubes to form the membrane wall. During boiler operation, the flat bars are exposed to high temperatures, thermal stresses, and corrosive flue gases; failure of the flat bars could lead to damage of adjacent tubes. Therefore, the material selection, manufacturing, and inspection of the flat bars are controlled in accordance with the pressure vessel steel system.
Functions of Fin Bars
Despite their small size, fin bars play multiple indispensable roles in the boiler system:
Creating a gas-tight structure
Before the advent of membrane water walls, conventional boilers often used bare tubes combined with refractory brick or insulating layers, which suffered from high air leakage and significant heat loss. By using fin bars to seamlessly connect the bare tubes, the membrane wall forms a fully welded, sealed furnace enclosure, greatly reducing furnace air leakage, improving combustion efficiency, and eliminating the risks of cracking and spalling associated with traditional furnace walls.
Enhancing the heating surface
The fin bars themselves also serve as part of the heating surface. Radiant heat from the flame acts not only on the water wall tubes but also on the fin surfaces; the heat is rapidly conducted through the fins to the tubes on either side and absorbed by the working fluid. This design significantly increases the heating area per unit projected area, resulting in a more compact boiler structure.
Increasing structural strength
The fin bars firmly connect the individual water wall tubes into an integral rigid beam structure, greatly enhancing the mechanical strength and impact resistance of the furnace wall. Under operating conditions such as boiler start-up, shutdown, and load changes, this effectively suppresses deformation and vibration of the tube panels.
Enabling a fully suspended furnace
Because the membrane wall has good self-supporting and sealing properties, the furnaces of modern large boilers are often of a fully suspended design, eliminating the need for heavy refractory brick outer walls. Only a thin layer of insulation is required, reducing boiler weight and civil engineering costs.
Is the Membrane Water Wall a Pressure Part
The water wall is a pressure part and is one of the most critical and important pressure parts in a boiler. The water wall tubes contain high-pressure steam-water mixtures and directly bear the pressure from the drum, qualifying them as pressure-containing heating surfaces. Relevant codes and standards explicitly classify the water wall as a pressure part.
Why Is the Membrane Water Wall a Pressure Part
The tubes contain boiler water or saturated steam at a pressure equal to the boilers rated working pressure. The service conditions involve a combination of internal pressure, high temperature, and thermal stresses. Rupture would directly result in tube failure, pressure release, and a safety incident.
Are the Flat Bars (Fins) Considered Pressure Parts
The flat bars/fins themselves are not considered pressure parts because they do not contain pressure. However, as they are welded to the water wall tubes to form the membrane wall, they are integral components of a pressure part. Their material, welding, and quality requirements are still controlled under the pressure part system.
Consequently, the water wall is a major pressure part of the boiler, and its flat bars (fins) are associated welded components of that pressure part, requiring strict quality control standards.
Quality Requirements for Membrane Strip Fin Bars
In actual membrane wall fabrication, the flat bars and tubes are typically joined using double-sided submerged arc automatic welding or high-speed resistance welding. Welding parameters (current, voltage, travel speed) must be precisely matched to the dimensions and materials of the flat bars and tubes to ensure that penetration depth, width, and weld bead formation meet the required standards.