Page 315 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

PETROLEUM REFINING 293

TABLE 4.44 Coating Systems and Initial Cost
Initial Cost
Coating System (“P” Is the Basic Cost of Carbon Steel Requiring Protection)
2-coat alkyd 0.2 P
3-coat vinyl 0.5 P
3-coat epoxy (with blasting) 0.9 P

4.27 PETROLEUM REFINING

High-temperature crude corrosion is a complex problem. There are at least three
mechanisms: (i) furnace tubes and transfer lines where corrosion is dependent on
velocity and vaporization and is accelerated by naphthenic acid; (ii) vacuum column
where corrosion occurs at the condensing temperature, is independent of velocity, and
increases with naphthenic acid concentration; (iii) side-cut piping where corrosion is
dependent on naphthenic acid content and inhibited somewhat by sulfur compounds.
Blending may be used to reduce the naphthenic acid content of the feed, thereby
reducing the corrosion to an acceptable level. Blending of heavy and light crudes can
change shear stress parameters and might also reduce corrosion. Blending is also used
to increase the sulfur content of the feed and inhibit, to some extent, naphthenic acid
corrosion.
Injection of corrosion inhibitors may provide protection for specific fractions that
are particularly severe with respect to corrosion. Monitoring needs to be effective
to check on the adequacy of the treatment. Process control changes may provide
sufficient corrosion control provided there is reduced charge and temperature. For
∘
long-term reliability, material selection is the best solution. Above 288 C with very
low naphthenic acid content, cladding with 5–12% chromium steels is suitable for
crudes of greater than 1% sulfur. When hydrogen sulfide is present, an alloy with
9% chromium is preferred. Carbon steel is found to be adequate in naphthenic acid
corrosion, and low-alloy steels containing up to 12% chromium do not provide bet-
ter performance over carbon steels. Type 316 stainless steel (>2.5% molybdenum)
or Type 317 stainless steel (>3.5% Mo) is found suitable for cladding vacuum and
atmospheric columns.
The selection of materials for refinery construction depends on the type of refinery,
the type of crude oil to be refined, and the expected service life for each vessel. As
with all materials selection, the life-cycle cost is of importance in addition to the
purchase price. Table 4.45 lists some of the common alloys and their material costs
relative to carbon steel. The costs listed are relative to carbon steel, which is assigned
a value of 1.0.
Carbon steel is the most commonly used structural material in refineries primar-
ily because of a combination of strength availability, low cost, and its resistance to
fire. The low alloy steels are meant for applications that require higher properties
than can be obtained with carbon steels. The workhorse refinery alloys for elevated
∘
temperatures greater than 260 C contain 0.5–0.9% chromium plus molybdenum.