Engineers have many choices when selecting tubing materials for the application at hand, whether it be a condenser or feedwater heater in a plant, or exhaust tube in an automobile. The wide variety of stainless steel choices available go far beyond 304 or 316! In fact, ASTM lists over 75 alloys, giving the engineer greater flexibility to choose the best option, while staying within specification guidelines, meeting budgetary goals and still providing the performance needed for the lifetime of the product. As you may well know, upset conditions are common in power generation and manufacturing, resulting in unexpected failure of tubing and piping materials. These may include differences in operation modes from design, changes in water chemistry due to leaks in other parts of the system, corrosion from unexpected sources, impact of improper lay-up practices, and the effect of corrosion product transport to other parts of the system. One phrase common to most ASTM tubing specifications is “It is the responsibility of the purchaser to specify all requirements that are necessary for material ordered under this specification.” It’s up to you!
Knowing the limitations of a material is crucial when making a selection for your application. Let’s talk about the factors that need to be considered when selecting a material.
Seamless vs Welded
The first choice that a user has in selecting the tube material is whether it should be made by a seamless or by a welded process. Traditionally, the seamless product has had a reputation of having higher quality. Seamless tubular manufacturing requires a process to force the hole into the billet. This is done by either a high temperature shearing operation, extrusion; or an internal tearing operation, rotary piercing. Both of these operations have the potential for creating small ID surface flaws. For projects that require heavy walls, seamless tubing is often preferred since it can stand up to high pressure environments.
Welded tube is produced from a strip that is roll formed and welded to produce a tube. Then the weld is often cold worked to produce a homogenous wall. Welded and drawn tube manufacturing has made many advancements in the last 65 years, creating technical and commercial advantages for the welded and drawn tubing over seamless products. Additionally, it is generally cheaper and has shorter lead times.
Corrosion and Erosion
Generalized corrosion occurs when a metal atom is oxidized by a fluid, leading to dissolution of surface metal. This loss reduces the wall thickness, making the tube more prone to mechanical failure. Rust is a commonly occurring byproduct of corrosion, resulting from iron corroding and forming iron oxide. General corrosion can be planned for and is typically not catastrophic. This is the only form of corrosion in which extra tube wall thickness adds extra life. In many instances, an alloy susceptible to this type of corrosion may be a cost-effective design option. Many designers and engineers commonly add a “corrosion allowance” to tube wall thickness.
However, other types of localized corrosion exist, including pitting, crevice corrosion, and MIC. Each type poses unique threats that must be evaluated during the design and material selection phase of your project. Electrochemical drivers such as voltage differences and cathode to anode ratio, can lead to increased rate of corrosion.
Erosion occurs when the protective layer on the surface of the tube dissolves, usually due to fluid velocity “scrubbing” the surface of the metal, or by water droplets when steam cannot properly be released. The occurrence of erosion should be considered when reviewing material options.
Temperature and Pressure
Pressure rating defines the minimum or maximum pressures the tube is designed to withstand, measured in pounds per square inch (psi). This rating is defined for tubes in fluid flow applications. Temperature range defines the full range of ambient temperatures that the tubing is designed to operate within. This is particularly important in applications where the transported fluid or outside environment could be subject to extreme temperatures. Other dynamics that can affect temperature include changes in operations, changes in cooling water source, and drop in flow rates.
Economic Considerations
Prices can vary considerably depending upon quantity purchased, availability, and OD-to-wall ratio. The supply and demand from foreign material has driven prices all over the map. Nickel, copper, and molybdenum prices have varied dramatically in the last few years, significantly impacting the price of steel tube. Therefore, one should be very careful when assembling long-term budgets for alloys that have higher alloy contents such as TP 304, TP 316, cupro-nickels, and the 6% molybdenum containing alloys. Alloys with low nickel such as admiralty brass, TP 439, and the superferritics are more stable and predictable.
Despite what some may think, there is not always a clear cut answer to which tube is always better. The application, including the potential for corrosion and erosion, environmental characteristics, and cost need to be considered. Then, think about the purchaser’s specifications and the manufacturer’s capabilities when making a decision. For certain sizes or cost limitations, there may only be a sole option. But when considering quality, all aspects of the application and the specific manufacturer’s capability must be determined.
