Shell Corrosion Allowance for Aboveground Storage Tanks

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. A 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. Defining an appropriate corrosion allowance requires consideration of a number of factors: • Whether or not the product contains significant sediments and water; • Whether or not an internal lining will be applied in accordance with API 652; • The length of time to the first out-of-service inspection; • Whether or not the tank will be externally coated; • The temperature of the product stored; • The annual precipitation at the specified location; • The average chloride concentration in rainwater at the specified location. During 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 calculated using API 650 instead of API 653 could be designated as corrosion allowance. This paper will describe the corrosion allowance calculations in detail as well as address the issue of maximum allowable design stress.