THE PROBLEM
The anaerobic digester was exhibiting corrosion to the roof structure that had been identified as a concern. A tank lining solution was required to extend the life of the anaerobic digester roof until it could be replaced with a dome structure in 8 years time.
SOLUTION
Initially we washed the areas to be coated to remove soluble salts and contaminants. Soluble salts if left in place can cause premature tank lining failure through osmotic blistering as a result of the hygroscopic effect of salts on the steels surface.
The existing coating was then removed by method of abrasive blasting, preparing the steel to SA2.5 as per ISO8501-1.
As part of our standard quality assurance the blast profile was checked and recorded using a surface profile needle gauge to ensure that the correct surface roughness was achieved for the specified coating system.
Following preparation as per best practice a second soluble salt test was conducted and unfortunately the levels measured were above those permitted by the tank lining manufacturer. This required that the prepared steel was washed to remove these salts and the surface be re-blasted to the necessary preparation standard.
To prevent flash rusting occurring the humidity was controlled within the tank using a dehumidifier. All surfaces were vacuumed clean and a surface cleanliness test conducted and recorded to ensure that the steel was free from dust contamination prior to application of the new tank lining.
The tank lining material that was selected was Remmers Epoxy Universal. We have extensive experience using this material in Anaerobic Digester Tank Linings and this tank lining material has an expected service life in excess of 15 years until first maintenance – easily exceeding the 8 year life extension required.
Anaerobic digester tank linings are an aggressive environment for tank lining materials as they often have very high levels of Hydrogen Sulphide, which in turn creates biogenic sulphuric acid.
As per best coating practice a separate stripe coat was applied to and angles and edges – which as you can see there are plenty of in this tank lining application.
Remmers Epoxy Universal tank lining was then applied in three coats to a thickness in excess of 800 microns to all surfaces being relined.
Dry film thickness readings were taken after each coat and any areas below the required thickness treated to ensure that all areas of the tank lining were within specification.
As per our standard tank lining procedure the cured coating was tested for pin holing using a DC holiday spark tester. Any pin holes were marked and made good using the same tank lining material applied by brush.
In an aggressive environment such as an anaerobic digester, a single pin hole in a tank lining is a point of failure that will quickly be exposed and lead to premature tank lining failure.
