Heaters used in hydrodesulfurization, hydroforming, hydrocracking, and similar processes often have austenitic stainless steel tubes and usually process reactor feed or recycled gas containing hydrogen sulfide and sulfur compounds. The austenitic stainless steel tubes in these services can be susceptible to polythionic acid stress corrosion cracking. Polythionic acids form from sulfide scales exposed to oxygen and water in the stainless steel that are sensitized which can occur in most stainless-steel tube materials after exposures to temperatures in excess of 700°F to 1500°F (371°C to 815°C) during manufacturing, fabrication or in service. Relatively short exposure times are necessary to sensitize stainless steels at the high end of the temperature range while prolong exposure is necessary to sensitize stainless steels at the lower end of the temperature range. Generally, the risk of cracking increases during downtime when water and air are present. Cracking can be rapid as the acid corrodes along the grain boundaries of the stainless steel.
Cracking can initiate from either the inside or outside of the tube. Cracking from the process side is more common because the process often contains sulfur compounds resulting in sulfide scales. However, cracking can occur from the tube OD if the firebox operates fuel rich and there is sufficient sulfur in the fuel.
Preventive measures include using materials less susceptible to sensitization, preventing acid from forming, and neutralizing the acids. Specific details are as follows.
a. Stabilized grades of stainless steel (e.g., Type 321 or Type 347) are more resistant to sensitization but even these materials can become sensitized after a longer exposure to slightly higher temperatures. A thermal stabilization heat treatment of a stabilized grade of stainless has been shown to significantly improve resistance to sensitization and thereby minimize the potential for cracking.
b. Preventing oxygen and moisture exposure will not allow the polythionic acid to form. This can be accomplished by purging with an inert gas, like nitrogen, and keeping the tubes pressurized with it. When blinding is required, a positive flow of inert gas should be maintained while the flanges are open and a blind is being installed. If desired, a small amount of ammonia can be added to the inert gas as a neutralizing agent. Maintaining a positive flow of inert gas excludes air and moisture.
c. A wash with a soda ash solution can effectively neutralize acids and maintain a basic pH. Soda-ash wash tubes crossovers, headers, or other parts of the heater which must be opened. The usual solution is a 2 wt. % soda ash (Na2CO3) with a suitable wetting agent. The solution should be circulated so that all gas pockets are moved and all surfaces are wetted. Sodium nitrate at 0.5 wt. % may also be added to the solution to inhibit chloride cracking. The solution may then be drained and reused in piping or another heater. The 2-percent solution contains enough soda ash to leave a film, but a weaker solution may not. The film is alkaline and can neutralize any reaction of iron sulfide, air, and water. It is important to remember that the film, the residue from the soda-ash solutions, must not be washed off during downtime. Most units are put back on stream with the film remaining. If the film must be removed, flushing during start-up followed by inert gas may be acceptable.
d. Preventing moisture exposure by maintaining tube temperatures above the dew point will also prevent acid from forming. This is typically applied to external tube surfaces which are not neutralized. Depending on the dew point temperature, this may be accomplished either by keeping pilots burning during down times or keeping a burner at minimum fire when access is not needed and safety procedures allow. Tube temperatures should be monitored to (Part 2) ensure they are above the target dew point temperature. These preventive measures are described in detail in NACE RP 0170.