Internal Tube Corrosion:
Internal corrosion is predominantly influenced
by the chemical composition of the process fluid, process and metal
temperatures, fluid velocity and tube metallurgy. Some critical species include
sulfur compounds and organic acids (naphthenic acid). The level of these
species in the fluid influences the type and rate of corrosion on the internal
surface of the heater tubes. Sulfur compounds in particular promote sulfidic
corrosion, which can manifest itself as localized and general wall thinning. Similarly,
corrosion from organic acid can appear localized in turbulent regions or
general thinning in areas. The corrosion rate can be aggravated by other fluid
components like chlorides, the type of sulfur compounds, and the presence of
hydrogen. Sulfur compounds, in the presence of hydrogen, usually increase the
corrosivity of the process.
Fluid and metal temperatures influence the
corrosion rate. The highest tube metal temperature predominantly occurs at the
fire side front face of the radiant tube where the heat flux is greatest. The
corrosion rate profile often follows the heat flux profile. Differences in the
corrosion rate along the length or around a cross section of a tube are often
the result of temperature differences between locations. An example of
temperature influence is the increase in corrosion rate with a rise in
temperature to 750°F (399°C) for processes with sulfur compounds. Above 750°F
(399°C) the corrosion rate decreases due to stable sulfide scale that inhibits
further corrosion. Figure 10 is a convection tube that failed from internal
corrosion attributed to high-temperature sulfidic attack.
High-velocity fluids, fluids containing
particulate, or tubes with two-phase flow can increase the corrosion rate by
stripping away protective scale and exposing fresh metal to continue the
corrosion process. Corrosion from organic acids and sulfur compounds are
significantly influenced by fluid velocity. Particular attention should be
given to high turbulent regions.
Tube failures resulting from corrosion are
generally due to local stress rupture in which the wall thickness becomes too
thin and is overstressed at the metal operating temperature. These failures can
appear as small leaks through pits or as “fish-mouth” ruptures if the thinning
is general or if it is longitudinal grooving.
External Tube Corrosion:
External corrosion of the tube depends on the heater
atmosphere and temperatures. Generally, the external surface of the tube will
corrode from oxidation. The heater atmosphere contains excess oxygen necessary
for combustion of the fuel at the burners. Oxidation rates for a metal increase
with increased temperature. Refer to API Std 530 for additional information on
oxidation. Oxidation may either be a localized condition or extend over the
entire length of the tube inside the heater. Excessive oxidation and scaling is
usually the result of operating the tubes above recommended levels. This could
be the result of over-firing the heater or internal fouling of the tubes, which
increase the tube wall temperature.
Combustion deposits may have the appearance of oxide scale, but they can
be distinguished by checking them with a magnet. Oxide scales are magnetic,
whereas combustion deposits are not.
Other types of corrosion attack are possible.
Heaters operating with insufficient oxygen or fuel-rich can cause corrosion
from the resulting reducing environment. Depending on the type and quality of
the fuel, corrosion could occur from sulfidation or carburization. Acid attack
can result from the combustion of heater fuels depending upon the sulfur
content of the fuel. When the gas or fuel oil has a high sulfur content, one of
the combustion products formed and deposited on the outside surfaces of the tubes
is a sulfate. This sulfate is harmless during operational periods, but when the
deposit is allowed to cool it becomes highly hygroscopic and absorbs moisture
from the air, hydrolyzing to produce sulfuric acid, which corrodes the
underlying metal.
When the fuel has a high vanadium content,
metal at temperatures above a critical point in the range from 1200°F (649°C)
to 1400°F (760°C) is subject to very rapid attack from low-melting vanadium
based compounds (vanadates) and sodium-vanadium compounds (sodium vanadates).
The vanadates and sodium vanadates deposit on the hot metal surface, melt and
act as a fluxing agent to remove the protective oxide scale on the tubes The
cycle repeats itself as oxide and deposit builds back up on the tube.
Convection sections where flue gas dew point
temperatures occur during operations suffer metal loss because of acid material
from the products of combustion. Metal loss on the exterior of convection tubes
may be difficult to evaluate because of inaccessibility.
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