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-105
Equations (2.B.27), (2.B.28), and (2.B.29) apply to water, seawater, and Fe ++ saturated water and do not
account for the buffering action of bicarbonate ions or for the increase acidity caused by salinity levels higher
than 45 g/l. In cases where these factors are significant, the analyst should consider using an alternative
analysis such as direct application of the provisions in M-506 for salinity and dissolved bicarbonate.
2.B.13.3.5
Determining the CO 2 Fugacity
Fugacity has units of pressure. It is used in place of the pressure—or partial pressure—in calculations
concerning the equilibrium of real gas mixtures. When the fugacity is used in place of the pressure, real
gases can be modeled using the equations for ideal gases. M-506 uses the following equation for modeling
the fugacity, f .
CO 2
14
log ⎡
.
10 f ⎤ log ⎡
CO2 10 p ⎤
CO min ⎡250,p ⎤
⎛
⎞
⎣ ⎦
=
⎣
0 0031
2⎦ +
⎣ CO ⋅ . −
2⎦ ⎜
⎝
⎟
T + 273⎠
[ ]
log ⎡
10 f ⎤ log ⎡
CO2 10 p ⎤
⎣ ⎦
=
⎣ CO + log
2⎦
10 a
(2.B.30)
(2.B.31)
In Equations (2.B.30) and (2.B.31), p CO 2
is the CO 2 partial pressure in bar and T is the temperature in °C.
Note that the fugacity coefficient, a, is the ratio between CO 2 fugacity and the partial pressure, or:
f = p ⋅ a
(2.B.32)
CO2 CO2
2.B.13.3.6
Determining the Flow Velocity
M-506 uses the fluid flow shear stress to model the effect of flow velocity on the base corrosion rate. M-506
recommends the Equation (2.B.33) to calculate the stress, S, in units of Pa. In the calculation for the
corrosion rate, the shear stress need not exceed 150 Pa.
S
2
f ⋅ρm
⋅um
= (2.B.33)
2
In Equation (2.B.33), f is the friction factor, u m is the mixture flow velocity in m/s, and ρ m is the mixture mass
density in kg/m 3 . The friction coefficient may be approximated for turbulent flow (Re > 2300) by using
Equation (2.B.34):
⎡ ⎛
6
⎛ e ⎞ 10 ⎞
f = 0.
001375 ⎢1+ 20000 +
⎢ ⎜ ⎜ ⎟ ⎟
⎝ ⎝ D⎠
Re
⎢
⎠
⎣
0.
33
⎤
⎥
⎥
⎥⎦
(2.B.34)
The term eD is the relative roughness and Re is the Reynolds Number of the mixture given by Equation
(2.B.35):
Dρmum
mD
Re = = (2.B.35)
µ Aµ
m
m
In Equation (2.B.35), D is the diameter in meters, and μ m is viscosity of the mixture in Pa·s (0.001cP = Pa·s).
This equation also shows that the Reynolds number can be calculated using the mass flux, m , and the
cross-sectional area, A.