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.
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