The five Ammonia reduction tanks had previously had an epoxy tank lining applied to the profiled galvanized steel sheets that formed that soffit. This coating had failed and the concrete tank walls also required a protective tank lining in order to resist the high concentration of Ammonia and hydrogen sulphide that could be found in the gas space.
From experience in lining other Ammonia Reduction tanks we know that the only tank lining system that would withstand the aggressive chemicals and elevated temperatures would be a vinyl ester system. Also given the temperatures involved that the tank lining would need to be reinforced in case of cracking in the concrete walls.
Initially all surfaces were prepared by method of abrasive blasting to remove the failed epoxy tank lining and prepare the concrete. Concrete when poured forms a layer of surface laitance which must be removed to ensure adhesion of the new tank lining.
The surface profile on the steel surfaces was checked using a surface profile needle gauge as per our standard tank lining quality assurance procedure.
All surfaces were vacuumed clean and surface cleanliness tests conducted and recorded to ensure optimum adhesion of the new tank lining.
During and following preparation works we identified areas of moisture ingress from the adjoining digester tank. This transpired that the civils contractor had omitted the water proofing additive when pouring the tank walls. The problem this presented is that even if the tanks were forced dried to allow application of the tank lining, the moisture transmission would return and disrupt the adhesion of the tank lining. This would lead to blistering and eventual failure. This is especially pertinent as vinyl ester tank lining resins are particularly moisture sensitive.
Fortunately we had the perfect solution in the form of Aquron 2000. This novel colloid silicate forms a hydrogel within the concrete matrix, effectively stopping moisture transmission.
Initially the steel surfaces were primed using ATB Metal primer and the concrete surfaces primer using the equivalent concrete primer.
In the steel soffit the seams and voids made by pipe penetrations were filled using the same ATB vinyl ester tank lining resin filled with chopped strand powder.
ATB 300 was then applied throughout, incorporating two layers of chopped strand matting for reinforcement. This was then fully sealed by suturing with two coats of ATB 300 vinyl ester tank lining resin, with the second coat incorporating the Top coat additive for additional chemical resistance.
As part of our standard tank lining quality assurance the climatic conditions were monitored and logged throughout. Following completion the new tank linings were tested for porosity using a DC holiday spark tester – especially important given the demanding service where a single pin hole in the tank lining would lead to attack of the substrates.