API RP 581 - 3rd Ed.2016 - Add.2-2020 - Risk-Based Inspection Methodology
Share Embed Flag
Download ePaper

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

luis.alberto.mayorga.plaza
from luis.alberto.mayorga.plaza More from this publisher
17.12.2020 Views

2.B-112 API RECOMMENDED PRACTICE 581made so that storm water collecting around a AST will cause the base corrosion rate to increase by afactor of 2. If the drainage is so poor that more than one-third of the circumference of the bottom edge ofthe AST is underwater for extended periods of time, then the base corrosion rate is increased by afactor of 3. Good drainage is considered normal, so the multiplier is set to 1 if water does not normallycollect around the base of the AST.d) Adjust for CP, F CP —The corrosion rate adjustment factor is given in Table 2.B.14.7. CP is one of theprimary methods used to avoid corrosion of AST bottoms from the soil side. However, the system mustbe installed and maintained properly. The factor is established so that the most credit is given for aproperly functioning CP system in accordance with API 651, but no penalty is assessed for lack of CP.This assumes that the base corrosion rate is for systems without CP. Note that unless a High Levelinspection can verify that the CP system is effective (verified by inspection in compliance with NACERP0169 or equivalent), no credit is obtained for the CP system.e) Adjust for AST Bottom Type, F TB —The corrosion rate adjustment factor is given in Table 2.B.14.8. ASTswith properly installed RPBs tend to have bottom corrosion rates comparable to those with a singlebottom.f) Adjustment for Operating Temperature, F ST —The corrosion rate adjustment factor is given inTable 2.B.14.9. The operating temperature of the AST may influence external corrosion.2.B.14.3.2Product-Side Corrosion Rate EquationThe steps required to determine the corrosion rate are shown in Figure 2.B.14.1. The corrosion rate may bedetermined using the basic data in Table 2.B.14.2 in conjunction with Equation (2.B.38).CRP = CRPB ⋅FPC ⋅FPT ⋅FSC ⋅ FWD(2.B.38)The product-side base corrosion rate, CR PB , should be determined based on actual inspection data. If thesedata are not available, then the base product-side corrosion rate may be assumed to be 0.05 mm/y (2 mpy).The base corrosion rate is founded on the conditions stated in Table 2.B.14.10.The adjustment factors in Equation (2.B.38) are determined as described below.a) Adjustment for Product Condition, F PC —The corrosion rate adjustment factor is given in Table2.B.14.11. Wet conditions should be used if significant bottom sediments and water are present.b) Adjustment for Operating temperature, F PT —The corrosion rate adjustment factor is given inTable 2.B.14.12.c) Adjustment for Steam Coil, F SC —The corrosion rate adjustment factor is given in Table 2.B.14.13. If asteam coil heater is present, the internal corrosion rate is adjusted upwards slightly due to extra heat,and the possibility of steam leaks from the internal coild) Adjustment for Water Draw-off, F WD —The corrosion rate adjustment factor is given in Table 2.B.14.14.Water draws, when consistently used, can greatly reduce the damaging effects of water at the bottom ofthe AST. To receive the full benefit, water must be drawn weekly or after every receipt.

RISK-BASED INSPECTION METHODOLOGY, PART 2, ANNEX 2.B—DETERMINATION OF CORROSION RATES 2.B-1132.B.14.3.3Combined Atmospheric Storage Tank Floor Corrosion RateThe internal and external corrosion rates are estimated by multiplying the base corrosion rate by therespective adjustment factors. This will produce two separate corrosion rates that are combined as describedbelow. It is assumed that the soil-side corrosion will be localized in nature while the product-side corrosionwill be either generalized or localized. Note that in order to avoid understating the risk, it is recommendedthat the combined corrosion rate should not be set lower than 2 mils per year.a) Option 1—If the internal corrosion is generalized in nature, the corrosion areas will likely overlap suchthat the bottom thickness is simultaneously reduced by both internal and external influences. In thiscase, the internal and external rates are additive.b) Option 2—For pitting and localized corrosion, the chances are low that internal and external rates cancombine to produce an additive effect on wall loss. In this case, the user chooses the greater of the twocorrosion rates as the governing rate for the proceeding step.2.B.14.4CR PCR PBCR SCR SBF CPF PAF PCF PTF SCF SRF STF TBF TDF WDNomenclatureis the product-side corrosion rateis the product-side base corrosion rateis the soil-side corrosion rateis the soil-side base corrosion rateis the soil-side corrosion rate correction factor for CPis the soil-side corrosion rate correction factor for AST pad typeis the product-side corrosion rate correction factor for product conditionis the product-side corrosion rate correction factor for temperatureis the soil-side corrosion rate correction factor for temperatureis the soil-side corrosion rate correction factor for soil conditionsis the product-side corrosion rate correction factor for temperatureis the soil-side corrosion rate correction factor for AST bottom typeis the soil-side corrosion rate correction factor for drainageis the product-side corrosion rate correction factor for water draw-off

RISK-BASED INSPECTION METHODOLOGY, PART 2, ANNEX 2.B—DETERMINATION OF CORROSION RATES 2.B-113

2.B.14.3.3

Combined Atmospheric Storage Tank Floor Corrosion Rate

The internal and external corrosion rates are estimated by multiplying the base corrosion rate by the

respective adjustment factors. This will produce two separate corrosion rates that are combined as described

below. It is assumed that the soil-side corrosion will be localized in nature while the product-side corrosion

will be either generalized or localized. Note that in order to avoid understating the risk, it is recommended

that the combined corrosion rate should not be set lower than 2 mils per year.

a) Option 1—If the internal corrosion is generalized in nature, the corrosion areas will likely overlap such

that the bottom thickness is simultaneously reduced by both internal and external influences. In this

case, the internal and external rates are additive.

b) Option 2—For pitting and localized corrosion, the chances are low that internal and external rates can

combine to produce an additive effect on wall loss. In this case, the user chooses the greater of the two

corrosion rates as the governing rate for the proceeding step.

2.B.14.4

CR P

CR PB

CR S

CR SB

F CP

F PA

F PC

F PT

F SC

F SR

F ST

F TB

F TD

F WD

Nomenclature

is the product-side corrosion rate

is the product-side base corrosion rate

is the soil-side corrosion rate

is the soil-side base corrosion rate

is the soil-side corrosion rate correction factor for CP

is the soil-side corrosion rate correction factor for AST pad type

is the product-side corrosion rate correction factor for product condition

is the product-side corrosion rate correction factor for temperature

is the soil-side corrosion rate correction factor for temperature

is the soil-side corrosion rate correction factor for soil conditions

is the product-side corrosion rate correction factor for temperature

is the soil-side corrosion rate correction factor for AST bottom type

is the soil-side corrosion rate correction factor for drainage

is the product-side corrosion rate correction factor for water draw-off

logo

products

Resources

Company

Terms of service
Privacy policy
Cookie policy
Cookie settings
Imprint
Change language
Made with love in Switzerland
© 2024 Yumpu.com all rights reserved

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!