The major factors to consider when selecting tube materials are cost, strength, internal corrosion, and external oxidation. Temperature limits imposed by strength and external oxidation tend to be about the same for any given tube material, but the temperature limits imposed by internal corrosion are often much lower. For this reason, internal corrosion requires careful consideration. Typically we base our choice of tube material and of corrosion allowance on experience in similar services.
Also see the Corrosion Prevention Manual for further discussion and applications of tube materials for specific process applications. Selection of tube materials is an important decision and consulting a specialist is recommended.
Tubes made of carbon steel, the chrome-moly steels, and the austenitic stainless steels are all strong enough for services that subject the metal to temperatures below 1000°F. In this case, materials are generally chosen on the basis of internal corrosion resistance requirements.
For very hot services like hydrogen and ammonia plant reforming furnaces, high temperature strength generally controls materials selection. In these 1500°F and hotter services, only alloys with extremely high hot strength can be used. Typical materials include HK-40, HP-45, HP-50, and Incoloy 800H.
A second factor in selection of tube materials is external oxidation, caused equally by flue gases and air. The amount of oxidation increases with the skinpoint temperature in a furnace. Ordinary carbon steels resist oxidation well to 1000°F. If your skinpoint temperature will be higher than that, you will want to choose tube materials that contain some chromium. The more chromium in your material, the better it will resist oxidation. See the Corrosion Prevention Manual for a detailed discussion of high-temperature oxidation and for oxidation rate curves. The Company’s fired heater specification, HTR-MS-1350, gives oxidation limits for specific tube materials.