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

2.B-106 API RECOMMENDED PRACTICE 581
M-506 gives some guidance on the calculation of the two-phase viscosity, density, and fluid velocity. These
factors depend on the volumetric ratio of the liquid flow to the flow, sometimes known as the liquid holdup,
the amount of water in the liquid fraction, the gas water and oil viscosities at the temperature and pressure.
These calculations are complex and go beyond the scope of an RBI study. If required, the analyst can refer
to the recommendations in NORSOK or use the software provided by NORSOK to perform the calculations
recommended in M-506 to approximate the two-phase flow characteristics. While M-506 provides some
guidelines for the calculation of bulk fluid properties, they do not explicitly account for the solubility of natural
gas in oil, which can also affect the all of these properties.
2.B.13.3.7
Adjustment for Inhibitors or Glycol
The addition of glycol reduces the CO 2 corrosion rate. M-506 accounts for this by simply reducing the
corrosion rate by applying a factor of the form:
( [ ] )
log ⎡
10 F ⎤
⎣ glycol ⎦
= 1. 6⋅ log 100 −G
−2
(2.B.36)
In Equation (2.B.36), G is the percent weight of glycol as a percent of water in the system. F glycol is limited to
a minimum value of 0.008. The base corrosion rate is multiplied by the factor F glycol to account for the
corrosion reduction due to glycol.
For other types of corrosion inhibitors added to the stream, M-506 requires that its effectiveness must be
independently determined and used directly as a reduction factor, F inhib . If both glycol and an inhibitor are
used, the minimum of F inhib and F glycol should be used as shown in Equation (2.B.23).
2.B.13.4
a
Nomenclature
is the fugacity coefficient
A is the pipe cross-sectional area, m 2 (ft 2 )
CR
CR B
D
eD
f
f CO2
f (T, pH)
F glycol
F inhib
G
m
P
is the corrosion rate
is the base corrosion rate
is the pipe diameter, m [ft]
is the relative roughness
is the fraction factor
is the CO 2 fugacity, bar
is the temperature-pH function
is the corrosion rate correction factor for glycol
is the corrosion rate correction factor for the inhibitor
is the percent weight of glycol as a percent of water in the system
is the calculated using the mass flux, kg/(m 2 -s) [lb/(ft 2 -s)]
is the pressure, psia