Tubes Materials – Internal Corrosion
Corrosion Allowance. Besides choosing a corrosion-resistant tube material, you need for it to be thick enough to compensate for the degree of corrosion you expect. Usual practice is to design tubes to last at least 10 years. Typical corrosion allowances vary from essentially nothing in non-corrosive services (boilers, steam-methane reformers), to ¼ inch or more (furnaces handling sour hydrocarbons). Figure 700-1 gives corrosion allowances for tube materials in various services.
Although tubes are typically designed for a corrosion life of 10 years and a creep life of 100,000 hours (11.4 years), it is unusual when tubes fail in this short a time. In the majority of furnaces, tubes last 20 to 30 years because of the cumulative effect of all the built-in safety factors. When tubes fail in 10 years or less, it is usually the result of either a change to a more corrosive feed stock, or a deliberate decision to sacrifice tubes by overheating them to attain greater plant productivity.
Experience in Similar Service. We generally rely on this experience when selecting tube materials for fired heaters. However, be very careful in deciding what is “similar”. Sour hydrocarbons are especially difficult to deal with, since corrosion depends on both temperature and H2S concentration. In a crude oil heater, H2S concentration can progressively change because H2S is often produced by thermal decomposition of non-corrosive sulfur compounds in the crude.
Internal Tube Temperature. It is the tube’s inside metal wall temperature that determines corrosion rate, not stock temperature or skinpoint temperature. Corrosion rates may therefore be extremely sensitive to furnace heat flux and inside fluid film heat transfer coefficients. Internal fouling may either increase corrosion by raising metal wall temperature, or decrease it by shielding the metal from the corrosives.
Unless selection of the appropriate tube material is relatively clear-cut, consult company corrosion experts on materials selection.