Tube Supports Material
Maximum tube support temperatures for various materials are summarized in Table 8 of API-560. Intermediate tube supports exposed to radiant heat and full firebox temperature are usually made of HH or HK alloy, the cast versions of 25-12 and 25-20 chrome-nickel, respectively. Some wrought alloys of comparable properties which have been used to fabricate replacement tube supports include Incoloy 800H and RA-330.
HH and HK will eventually embrittle in service and will then be sensitive to mechanical breakage. They may also corrode in furnaces burning residual fuels, especially those which are high in vanadium. Where vanadium corrosion is severe, greatly improved life has been obtained through the use of a 50-50 chrome-nickel alloy. Company experience suggests that HH and HK suffer appreciable attack when fuel oil vanadium contents exceed 20-30 ppm. The 50-50 Cr-Ni alloy has performed well at vanadium contents in the 100-200 ppm range.
Materials for end-wall tube supports in convection sections should be chosen on the basis of the oxidation limits in Figure 400-2. Normally, carbon steel will be used for metal temperatures below 1000°F and Type 304 for temperatures in the 1000 to 1500°F range. Where the cost differential between these two materials is small, it may be desirable to use Type 304, even below the 1000°F level. End supports are expensive to replace. Type 304 provides insurance against errors in predicting operating temperature or corrosion due to acid condensation from a high-sulfur flue gas.