SHELL CORROSION ALLOWANCE FOR ABOVEGROUND STORAGE TANKS

摘要

Enbridge Pipelines Inc. utilizes aboveground crude oil storage tanks for operational flexibility and merchant storage purposes. Most of these tanks are built in accordance with the requirements of API 650. This standard requires that an appropriate corrosion allowance be included in the minimum shell thickness calculations.rnA variety of sources were researched in an effort to develop a process that ensures the selected corrosion allowance allows for the safe operation of a tank for its entire service life. Some of these sources include other API standards, historical API 653 tank inspection reports, published atmospheric corrosion rates, and corrosion allowance specifications of industrial counterparts.rnDefining an appropriate corrosion allowance requires consideration of a number of factors:rn1. Whether or not the product contains significant sediments and waterrn2. Whether or not an internal lining will be applied in accordance with API 652rnThe length of time to the first out-of-service inspectionrn3. Whether or not the tank will be externally coatedrn4. The temperature of the product storedrnThe annual precipitation at the specified locationrn5. The average chloride concentration in rainwater at the specified locationrnDuring the course of the corrosion allowance study, the issue of maximum allowable design stress was also considered. The allowable stress values specified in the standard for construction of new tanks (API 650) differs from the allowable stress values specified in the inspection standard for existing tanks (API 653). It has been suggested that the incremental difference between the minimum shell thicknesses calculatedrnusing API 650 instead of API 653 could be designated as corrosion allowance.rnThis paper will describe the corrosion allowance calculations in detail as well as address the issue of maximum allowable design stress.