By Vincent A. Carucci
The required internal inspection interval of an Aboveground Atmospheric Storage Tank (AST) must be determined based on corrosion rates measured during previous inspections or anticipated based on experience with similar tanks in similar service. Bottom plate corrosion normally controls the required internal inspection interval, and this corrosion may occur both from the top side and the underside.
Special consideration should be paid to tanks whose service has changed during their operating lives. Also, because underside corrosion of the tank bottom is generally the primary factor governing the required internal inspection interval, tanks in nominally the same service, even at the same location, could have significantly different underside corrosion and/or pitting rates. This is due to differences in soil conditions or contaminants or the presence of water under the tank. In this regard, there could be marked differences among different tanks located within the same geographic area or even among different locations on the same tank bottom.
The use of local experience gained from prior tank inspections is clearly the best tool for determining internal corrosion rates. These may then be used to help determine appropriate internal inspection intervals and the overall tank inspection plan for the site. Experience has shown that corrosion rates are extremely variable even for nominally the same service. Therefore in the absence of actual data, the topside corrosion rates indicated below may be used as a guide for scheduling internal tank inspections. They should be considered in combination with data from previous tank inspections, when available, including both topside and underside corrosion rate data. Underside corrosion is greatly influenced by tank foundation design, materials, and temperature. At elevated operating temperatures, severe corrosion can occur on the underside of the bottom plate around the perimeter of the tank.
The following lists typical tank storage services in descending order of internal corrosion severity based on information obtained from multiple sources.
Accelerated corrosion on the underside of bottom annular plates can occur with tanks that store heated liquids (e.g., asphalt or fuel oil). This underside corrosion can be especially severe when the tank pad is not well graded and sloped away from the shell on the outside to avoid rainwater accumulation at the shell and then running under it. When heated liquids are stored, a temperature gradient exists across the annular plate from inside to outside. This temperature gradient creates ideal conditions for accelerated underside corrosion of the annular plate when rainwater is present.
The corrosion is generally greatest just a short distance from the inside shell-to-annular plate weld, and has resulted in annular plate failures and loss of tank contents. Special care should be taken when inspecting the tank bottom near the shell in cases where there is a greater likelihood of such corrosion. This is especially true in situations where there is concern regarding rainwater accumulation at the tank, and/or if there are visible signs of corrosion of the bottom annular plate extension beyond the tank shell.