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

2.B-104 API RECOMMENDED PRACTICE 581
In Equation (2.B.24), x is the ratio of the mass of water to the mass of dry gas, or %w ( 100 %w)
− , where %w
is the percent water by weight in the stream. The parameter P is the pressure and P sat (T) is the saturation
pressure for water at temperature, T, which can be obtained from steam tables. The 0.622 is the ratio of the
molecular weight of water (≈18) to the average molecular weight of air (≈29). Using Equation (2.B.24) and
standard steam tables, an approximate equation for the dew point temperature T d (°F) can be derived; see
Equation (2.B.25):
log 2 0866 0 2088 log ⎡ %w⎤
⎢ 0 2242 log
100
⎥
⎣ ⎦
10T d = . + . ⋅ 10 + . ⋅ 10 [ P]
(2.B.25)
In Equation (2.B.25), P is the pressure in psia, and %w is the percent water by weight in the stream
(lbm/100lbm), between 0 and 30.
2.B.13.3.3
Base Corrosion Rate
The base corrosion rate in mm/y is calculated from Equation (2.B.26).To obtain mpy, multiply the result in
mm/y by 39.4.
0.
62 ⎛ S ⎞
CR base = f ( T , pH ) ⋅ fCO
⋅
2 ⎜
⎝ ⎟
19⎠
0146 . + 0. 0324 f CO 2
(2.B.26)
In Equation (2.B.26), f (T, pH) is the temperature-pH function that is tabulated in Table 2.B.13.2. This
tabulated function was calculated by combining the M-506 temperature and temperature dependent function
pH function into a single quantity. It should be noted that the f (T, pH) function jumps sharply between 80 °C
and 90 °C (176 °F and 194 °F.).
The CO 2 fugacity, fCO 2
in bar, pH, and the shear stress from the flow S in Pa needed to calculate the basic
corrosion rate are discussed in the following paragraphs.
2.B.13.3.4
Determining the pH
For RBI purposes, the pH term in temperature-pH function tabulated in Table 2.B.13.2 may be calculated
using one of the following approximations. For condensation:
[ ] ⎡
2
pH = 2. 8686 + 0. 7931⋅log10 T −0. 57⋅log10
p ⎤
⎣ CO ⎦
(2.B.27)
for Fe ++ saturated water:
[ ] ⎡
2
pH = 2. 5907 + 0. 8668⋅log10 T −0. 49⋅log10
p ⎤
⎣ CO ⎦
(2.B.28)
and for water with salinity slightly greater than seawater (salinity = 46 g/l):
[ ] ⎡
2
pH = 2. 7137 + 0. 8002⋅log10 T −0. 57⋅log10
p ⎤
⎣ CO ⎦
(2.B.29)
In Equations (2.B.27), (2.B.28), and (2.B.29), T is the temperature in °F, and p CO 2
is the CO 2 partial
pressure in psi. These approximations were developed from Monte Carlo simulations. These simulation
studies used the equilibrium equations recommended and by M-506 to calculate the concentration of
hydrogen cation and then the pH for a wide range of temperatures, pressures, and CO 2 mole fractions.