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-111
2.B.14 Atmospheric Storage Tank Bottom Corrosion
2.B.14.1
Description of Damage
Corrosion occurs from the product side (internal corrosion) and soil side (external corrosion) of AST bottoms
constructed of carbon steel. Product-side corrosion can result in general or localized thinning. Factors
affecting product-side corrosion is the stored product corrosivity characteristics, operating temperature,
steam coil practices, and the presence of water in the AST. Soil-side corrosion results in localized thinning.
Factors affecting soil-side corrosion are soil type, pad type, water draining, CP, AST bottom design, and
operating temperature of the process stored.
2.B.14.2
2.B.14.2.1
Basic Data
Soil-Side Corrosion Rate Equation
The data listed in Table 2.B.14.1 are required to determine the estimated corrosion rate for soil-side service.
If precise data have not been measured, a knowledgeable corrosion specialist should be consulted.
2.B.14.2.2
Product-Side Corrosion Rate Equation
The data listed in Table 2.B.14.2 are required to determine the estimated corrosion rate for product-side
service. If precise data have not been measured, a knowledgeable process specialist should be consulted.
2.B.14.3
2.B.14.3.1
Determination of Corrosion Rate
Soil-Side Corrosion Rate Equation
The steps required to determine the corrosion rate are shown in Figure 2.B.14.1. The corrosion rate may be
determined using the basic data in Table 2.B.14.1 in conjunction with Equation (2.B.37).
CRS = CRSB ⋅FSR ⋅FPA ⋅FTD ⋅FCP ⋅FTB ⋅ FST
(2.B.37)
The base soil-side base corrosion rate, CR SB , should be determined based on actual inspection data. If these
data are not available, then the base soil-side corrosion rate may be assumed to be 0.13 mm/y (5 mpy). This
base corrosion rate is the expected or observed corrosion rate for a typical AST under average conditions
(see Table 2.B.14.3), neither highly susceptible to corrosion nor especially resistant to corrosion.
The adjustment factors in Equation (2.B.37) are determined as described below.
a) Adjustment Factor for Soil Conditions, F SR —The corrosion rate adjustment factor is given in
Table 2.B.14.4. The resistivity of the native soil beneath the AST pad can affect the corrosion rate of the
AST bottom. The resistivity of the AST pad material may be higher than the existing surrounding soil.
However, corrosive soil beneath the high resistivity AST pad material may contaminate the AST pad fill
by capillary action (see API 651, 1997, Section 5.3.1). Therefore, resistivity of the surrounding native
soil may be used to determine the likelihood of corrosion on the AST bottom. A common method of
measuring soil resistivity is described in ASTM G57. If the soil resistivity is not known, then assume
Moderately Corrosive soil (adjustment factor equals 1). Note that an adjustment factor of 1 is used for
ASTs with RPBs, since RPBs effectively prevent the contamination of the AST pad material by the
native soil.
b) Adjustment Factor for AST Pad, F PA —The corrosion rate adjustment factor is given in Table 2.B.14.5.
The type of pad or foundation that the AST rests upon will influence the corrosion rate. The adjustment
factors are assigned in a similar manner to those for the native soil beneath the AST pad.
c) Adjustment Factor for Drainage, F TD —The corrosion rate adjustment factor is given in Table 2.B.14.6.
Rainwater collecting around the base of the AST can greatly increase corrosion. The adjustment is