The API(1) 650 Oil storage tanks in the refinery have experienced accelerated corrosion from the soil side at the rate of 1-2mm/year primarily due to inefficiency of the impressed current cathodic protection system and under-deposit corrosion caused by the presence of corrosive bacteria in the soil (detailed failure analysis presented in NACE(2) Paper # C2017-9025). This has caused failure of several tank bottom plates within 8 years of commissioning resulting in loss of primary containment. With no other means of corrosion control from the soil side, the priority was to apply a short term stratagem by which the corrosion rate is substantially slowed and enable the refinery to distribute these tank turnarounds across as many years as possible. If this is not achieved, then a large number of tanks might be expected to perforate in any given year overwhelming refinery maintenance and disrupting operations.
The short term strategy adopted with best likelihood of success and minimal disruption to maintenance and operations was to drill under the tanks and inject vapor phase corrosion inhibitors. The primary objective is to increase the reliability by extending the floor life and meeting the agreed tank out of service schedule.
Considering the same probability of failure for all 48 nos. tank, a consequence based risk ranking was carried out to roll out the chemical injection plan in a phased manner. Based on the risk ranking, 21nos. tanks were identified for the first phase, which was implemented in 2016, followed by other tanks in subsequent years. The efficiency of the inhibitors was monitored by the use of a number Electric Resistance (ER) probes that have been installed at various locations underneath the tank bottom. A detailed cost benefit analysis was also conducted to realize the savings between the chemical injection and having to deal with multiple tank rebottoming. This article describes the online injection method, the inhibitor efficiency by measuring corrosion rates pre & post chemical injection along with the benefits achieved by implementing this online corrosion management strategy to extend the tank floor life.
Key Words: Above ground Storage Tanks, Soil Side Corrosion, VPCI, Electrical resistance probes,