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

2.B-108 API RECOMMENDED PRACTICE 581
2.B.13.5
Tables
Table 2.B.13.1—CO 2 Corrosion—Basic Data Required for Analysis
Basic Data
Temperature—Required
Pressure—Required
Comments
The corrosion phenomenon is highly temperature dependent. The
maximum temperature of the process is required. Temperatures
above 140 °C (284 °F) are not considered.
Total pressure of the system. The total pressure of the gas is a big
contributor in the corrosion rate up to about 250 psig.
CO 2 concentration (mole %)—Required Determine the CO 2 partial pressure (p CO2 ) = (mol fraction of CO 2 ×
total pressure), a maximum 4 MPa (580 psi) partial CO 2 pressure is
considered.
p CO2 —Required, if CO 2 concentration is
not given
Material of construction—Required
pH—Required
Stream properties: bulk density, ρ m ,
viscosity, μ m , gas to liquid ratios—
Required
For systems with liquids:
Water cut—Optional
For gas systems: Relative humidity, RH
or the dew point temperature, T d —
Optional
Glycol/water mix—Optional
Inhibition efficiency—Optional
CO 2 partial pressure, which is converted to CO 2 fugacity to account
for non-ideal behavior.
Determine the material of construction of equipment/piping. Stainless
steels and copper alloys are assumed to be resistant to CO 2
corrosion.
If known explicitly, the pH of the stream should be used; otherwise
Equations (2.B.27), (2.B.28), and (2.B.29) can be used to estimate the
pH based on the CO 2 partial pressure, whether the water in the stream
is Fe ++ saturated or water with salinity slightly larger than seawater.
Guidance with respect to typical values properties expected in natural
gas–oil mixtures (i.e. reservoir fluids) is provided. Estimation of
densities can be made on the basis of the oil density (°API), gas–oil
ratio (GOR), pressure, P, and temperature, T. For other streams, a
process engineer should assess these parameters.
Determine the percentage of water in the system. The default for this
field is 30 %.
Determine the dew point temperature, T d , based on the water content.
Equation (2.B.25) is provided for guidance, but should not be
assumed to be accurate within ±10 °F. If not provided, the gas stream
temperature is assumed to below the dew point.
Water content of glycol/water mix in %weight (%water in the total
glycol/water mix). The default value would assume no glycol added in
the system.
Requires %efficiency of the inhibitor. No inhibitor injected as a default
value.