API RP 581 - 3rd Ed.2016 - Add.2-2020 - Risk-Based Inspection Methodology

2.B-60 API RECOMMENDED PRACTICE 581
2.B.7.6 Figures
Start
• NH 4 HS Concentration
• Velocity
Determine
corrosion rate
using Table
2.B.7.2.
• H 2 S Partial Pressure
H 2 S Partial
Pressure>50 psia
Calculate adjusted
corrosion rate per
Table 2.B.7.2
Calculate adjusted
corrosion rate per
Table 2.B.7.2
Adjusted corrosion
rate
Adjusted corrosion
rate
Figure 2.B.7.1—Alkaline Sour Water Corrosion—Determination of Corrosion Rate
2.B.8
Amine Corrosion
2.B.8.1 Description of Damage
Amine corrosion is a form of often-localized corrosion that occurs principally on carbon steel in some gas
treating processes. Carbon steel is also vulnerable to SCC in gas treating amines if it is not postweld heat
treated (see Section 7). Gas treating amines fall into two major categories—chemical solvents and physical
solvents. This supplement deals with corrosion in the most common chemical solvents, MEA, DEA, and
MDEA. These amines are used to remove acid gases, primarily H 2 S, from plant streams. MEA and DEA will
also remove CO 2 , but MDEA is selective to H 2 S and will remove little CO 2 if it is present. Generally,
corrosion in MDEA is less than in MEA and DEA when contaminants are well controlled.
Carbon steel corrosion in amine treating processes is a function of a number of interrelated factors, the
primary ones being the concentration of the amine solution, the acid gas content of the solution (“loading”),
and the temperature. The most commonly used amine concentrations are 20 wt% MEA, 30 wt% DEA, and
40 to 50 wt% MDEA. At greater concentrations, corrosion rates increase.
Acid gas loading is reported in terms of moles of acid gas per mole of active amine. “Rich” solution is amine
of higher acid gas loading, and “lean” solution has lower acid gas loading (typically < 0.1 mole/mole).
Corrosion in poorly regenerated amine with high lean loadings is not an uncommon problem, particularly
because lean solution temperatures are often greater than rich solution temperatures. Both H 2 S and CO 2
must be measured to determine the acid gas loading. In addition, only the amount of available or “active”
amine should be considered when calculating the loading. In H 2 S-only systems, rich amine loadings up to
0.70 mole/mole have been satisfactory. In H 2 S + CO 2 systems, rich loading is often limited to 0.35 to 0.45