Heater Tube Supports

1. Purpose

This procedure provides information about metal corrosion in heaters and boilers, and recommends preventative measures.

2. General

Vanadium, sometimes in combination with Sodium, Sulfur, Nickel, or Potassium, appears to be the main cause of corrosive effects of ash components of heavy fuel oils on metal surfaces in heaters and boilers.

The most comprehensive treatise on the subject (though nearly all of the conclusions are qualitative) is a 1964 Literature Survey done by the BP Research Center for the British Admiralty Fuels and Lubricants Advisory Committee. Inspector’s reports on problems with a Crude Unit heater at Toa Oil and a Calciner heater at Nikki-Universal provide additional information.

Inspector G. C. Weldon's report of June 8, 1970, indicates failure in six months of 25-12 support castings in the Toa Oil Co. Crude Heater, where Vanadium content of the fuel oil was given as 173 ppm and sulfur content 2.4 wt. %. Sodium was reported as 10 ppm. R. V. Roberts' report of failure of 25-20 alloy tubes at Nikki-Universal gave fuel oil Vanadium content as 42.3 ppm (no data given on other contaminants), and flue gas temperatures in range 1200-1600ºF. In this case, because of particular tube layout, deposits and corrosion could take place on tubes directly.

Pertinent points taken from these sources are as follows:

2.1 Causes of Corrosion

a. Vanadium oxides, particularly V2O5, may cause corrosion from ash deposits when in a molten or partly molten state. Melting point of V2O5 is 675-690ºC (1250-1275ºF).

b. Complex compounds of Vanadium with Sodium and Sulfur (oxides or sulfates) may have eutectics with melting points in the range 1000-1200ºF. Attack by these compounds is more rapid and more severe than V2O5 alone.

c. Sulfur oxides are not themselves appreciably corrosive at high temperatures under oxidizing conditions. However, V2O5 is a catalyst for the oxidation of SO2 to SO3 at relatively low temperatures (825-1025ºF) and, as pointed out above, sulfates, particularly if sodium is present, cause accelerated rates of corrosion.

2.2 Preventative Measures

a. Removal of sulfur and sodium from the fuel oil can minimize the corrosion problem. Desalting, if properly done and properly monitored, is effective in removal of sodium (and potassium) salts. Desulfurization down to levels required to prevent sulfur complications is not practical.

b. Surface coating of metals with resistant materials such as ceramics or cements may be ineffective because of bonding difficulties, differential expansion rates and porosity of the surface.

c. Additives in the fuel oil, particularly magnesium oxide together with kaolin or some other siliceous material, can be quite effective in reducing or eliminating corrosion.

d. High degree of corrosion resistance is exhibited by 60/40 chromium/ nickel and 50/50 chromium/nickel alloys. With the degrees of uncertainty involved in other corrosion control methods, use of these alloys is probably the most practical general solution when it is known that Vanadium is present in the fuel oil.

3. Inflection Point Engineering Vanadium Limit

Despite the information from the BP survey report, no quantitative data is available to indicate the Vanadium limit above which protective steps must be taken. Inflection Point Engineering has adopted the following policy.

Whenever fuel oil containing a total of at least 50 ppm of Vanadium and Sodium is fired, use 50/50 Chrome/Nickel alloy casting (ASTM A560, Gr 50 Cr-50 Ni-Cb) for tube supports, guides, retainers, etc., for temperatures above 1100ºF.

Suitable wording is included in Inflection Point Engineering Standard Specification 2-12.