Boilers typically contain tubes made of alloy steel, a material that stands up well in the machine’s high-pressure, high-temperature environment.

Alloy pipes are durable to high-heat and other harsh conditions, allowing the correct volume of material to pass through the pipe at the correct pressure. This aids the boiler in proper function and efficiency. The right alloy pipes can also mean spending less money on replacement parts. Over time, this translates into cost-savings’and perhaps less maintenance spending’for the facility owner.

Alloy pipes can withstand temperatures up to 1,000 ‘C. As such, they are a critical part of proper boiler operation and overall facility maintenance.

Besides being strong and able to hold up well, let’s take a closer look at the factors that go into alloy pipe production and selection.

Creep Rupture Strength

One factor in choosing the right alloy boiler tube is the nature of the heat-resistant materials. Alloy pipes possess exceptional creep rupture strength, which enables them to hold up well in high-temperature, high-pressure conditions found in boilers. That creep rupture factor indicates the strength of the pipe.

Stainless steel alloy pipes with the highest creep rupture strengths are known as supercritical grades, and those are the types used in many commercial boilers. The strength of DMV 304 HCu, for example, is a result of the fine copper-rich precipitates in the austenite matrix. The high creep rupture rate in high-nickel steels such as DMV 800 H, DMV 800 HT and DMV AC 66 also have high precipitates in the matrix.

High-temperature Corrosion and Oxidation

Due to the nature of a boiler, alloy pipes are also susceptible to high-temperature corrosion, which is another factor in choosing the right alloy pipe.

In order to stand up to harsh conditions, the material’s sensitivity to oxidation on the steam-side of the boiler should be considered. As the temperature in the boiler goes up, oxide scale is generally formed more quickly and more thickly. The higher material loss lowers the thickness of the pipe’s wall and boosts the stress on the pipe, which causes creep rupture.

The oxide scale leads to an insulation of the tube material, which causes the temperature of the metal to rise. Those increased heats can cause high temperature corrosion and creep rates on the flue gas side.

During service, the buildup of scale also can cause blockages where alloy steel tubes bend. Scale can erode valve seats and turbine blades, which can lower the flow of steam, causing overheating and ultimately causing the boiler to fail.

Chromium has a strong effect on the corrosive behavior of steels, and the sensitivity of the material to steam-side oxidation and fire-side corrosion is an important aspect to consider for boiler operators. For example, when sulphides are deposited on the tube’s surface from high sulphur coal on the fire-side, it speeds up corrosion. When hot enough, the sulphur trioxide can eat through the protective oxide scale, forming iron and chromium-based sulphates that increase the rate of corrosion.

Boiler operators not only have to consider factors from the heat, but the condition of the water can also affect the boiler and its alloy pipes, and contribute to corrosion.

Contact American Boiler Company today for expert advice on selecting and maintaining the right boiler for your facility.

To learn more about alloy pipe materials, high-temperature corrosion and oxidation, click here. Salzgitter Mannesmann Stainless Tubes has put together a brochure that explains how these materials work.