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-73
2.B.10.5
References
See References [125], [126], [127], [128], and [129] in Section 2.2.
2.B.10.6
Tables
Table 2.B.10.1—Acid Sour Water Corrosion—Basic Data Required for Analysis
Basic Data
Material of construction
pH
Water temperature, °C (°F)
Air or oxidants present (Yes or No)
Chlorides present (Yes or No)
Flow velocity, m/s (ft/s)
Comments
Determine the material of construction of the equipment/piping. Carbon
and low alloy steel (containing max. 6 % alloys) assumed as default.
Determine the lowest pH for the equipment/piping. The pH that is used
should be of the separated acid phase within this equipment or nearest
equipment downstream, e.g. the overhead accumulator boot water
downstream of the overhead condenser.
Determine the maximum temperature present in the equipment/piping.
This may be the maximum process temperature, but local heating
condition such as effect of the sun or heat tracing should be considered.
Presence of oxygen may increase the corrosion rates. Normal oxygen
concentration is ≤ 50 ppb and high is maximum > 50 ppb.
The present of chlorides in combination with a pH below 4.5 significantly
affect the corrosion rate.
Determine the maximum expected flow velocity.
Table 2.B.10.2—Acid Sour Water Corrosion Estimated Corrosion Rates for Carbon
and Low Alloy Steel (mpy)—CR pH
pH
Temperature (°F)
100 125 175 200
4.75 1 3 5 7
5.25 0.7 2.0 3 4
5.75 0.4 1.5 2 3
6.25 0.3 1 1.5 2
6.75 0.2 0.5 0.7 1
Table 2.B.10.2M—Acid Sour Water Corrosion Estimated Corrosion Rates for Carbon
and Low Alloy Steel (mm/y)—CR pH
pH
Temperature (°C)
38 52 79 93
4.75 0.03 0.08 0.13 0.18
5.25 0.02 0.05 0.08 0.1
5.75 0.01 0.04 0.05 0.08
6.25 0.01 0.03 0.04 0.05
6.75 0.01 0.01 0.02 0.03