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

RISK-BASED INSPECTION METHODOLOGY, PART 2, ANNEX 2.B—DETERMINATION OF CORROSION RATES 2.B-59
2.B.7.4 References
See References [110], [111], [112], [113], and [114] in Section 2.2.
2.B.7.5 Tables
Table 2.B.7.1—Alkaline Sour Water Corrosion—Basic Data Required for Analysis
Basic Data
NH 4 HS concentration (wt%)
Stream velocity, m/s (ft/s)
H 2 S partial pressure, psia (kPa)
Comments
Determine the NH 4 HS concentration of the condensed water. It is suggested to
determine this value with ionic process models. However, approximate values may be
calculated from analyses of H 2 S and NH 3 as follows.
If wt% H 2 S < 2 × (wt% NH 3 ), wt% NH 4 HS =1.5 × (wt% H 2 S)
If wt% H 2 S > 2 × (wt% NH 3 ), wt% NH 4 HS =3.0 × (wt% NH 3 )
The vapor phase velocity should be used in a two-phase system. The liquid phase
velocity should be used in a liquid full system.
Determine the partial pressure of H 2 S by multiplying the mole% of H 2 S in the gas
phase by the total system pressure.
Table 2.B.7.2—Alkaline Sour Water Corrosion—Baseline Corrosion Rates for Carbon Steel (mpy)
NH 4 HS
(wt%)
Velocity (ft/s)
10 15 20 25 30
2 3 4 5 8 11
5 6 9 12 15 18
10 20 27 35 43 50
15 45 70 100 150 200
NOTE 1
⎡⎧⎪⎛Baseline
CR ⎞
⎫⎪
⎤
For pH 2 S < 50 psia: Adjusted CR = max ⎢⎨⎜
⋅(pH2S − 50) + Baseline⋅CR ⎬,
0⎥
⎝ 25
⎟
.
⎢⎣⎪⎩
⎠
⎪⎭
⎥⎦
NOTE 2
⎡⎧⎪⎛Baseline
CR ⎞
⎫⎪
⎤
For pH 2 S ≥ 50 psia: Adjusted CR = max ⎢⎨⎜
⋅(pH2S − 50) + Baseline CR ⎬,
0⎥
⎝ 40
⎟
.
⎢⎣⎪⎩
⎠
⎪⎭
⎥⎦
Table 2.B.7.2M—Alkaline Sour Water Corrosion—Baseline Corrosion Rates for Carbon Steel (mm/y)
NH 4
HS
(wt%)
Velocity (m/s)
3.05 4.57 6.10 7.62 9.14
2 0.08 0.10 0.13 0.20 0.28
5 0.15 0.23 0.30 0.38 0.46
10 0.51 0.69 0.89 1.09 1.27
15 1.14 1.78 2.54 3.81 5.08
⎡⎧⎪⎛Baseline
CR ⎞
⎫⎪
⎤
NOTE 1 For pH 2 S < 345 kPa: Adjusted CR = max ⎢⎨⎜
⋅(pH2S − 345) + Baseline CR ⎬,
0⎥
⎝ 173
⎟
.
⎢⎣⎪⎩
⎠
⎪⎭
⎥⎦
⎡⎧⎪⎛Baseline
CR ⎞
⎫⎪
⎤
NOTE 2 For pH 2 S ≥ 345 kPa: Adjusted CR = max ⎢⎨⎜
⋅(pH2S − 345) + Baseline CR ⎬,
0⎥
⎝ 276
⎟
.
⎢⎣⎪⎩
⎠
⎪⎭
⎥⎦