THE PROBLEM
The process drum had layers of old epoxy linings which in places were up to 20mm thick. Because of the heavy profiling of the substrate and aggressive environment these epoxy coatings were no longer protecting the steel substrate, which was allowing the aggressive chemicals to attacked the steel. This had resulted in perforations and leaks.
Within each drum is a double helix divided into bays, presenting an extremely challenging working environment.
SOLUTION
We proposed the removal of all of the existing epoxy tank lining by method of ultra high pressure water jetting. The 40,000 psi spinning jet effectively removed the thick layers of epoxy lining which would have taken significant time to wear away by abrasive blasting alone.
The additional benefit of an initial preparation by method of ultra high pressure water jetting is that soluble salts and chemical residues are also removed. If left in place, particularly in an aggressive immersion environment this can lead to premature tank lining failure through osmotic blistering, by which the soluble salts left on the steel surface draw moisture through the coating film.
The removal of these soluble salts to a point approved by the tank lining manufacturer was verified by method of a soluble salt test as part of our standard quality assurance.
Preparing the steel surface
Following removal of the existing epoxy tank linings the steel surface was prepared to SA2.5 as per ISO8501-1 by method of abrasive blasting. This revealed the true extent of some of the loss of steel thickness and the client employed engineers to fill the deepest penetrations with weld and even cut out and plate some sections. A minimum surface profile of 75 microns was required, tested and recorded using a surface profile needle gauge.
This delay in coating application necessitated the repreparation of the steel by abrasive blasting. This blast standard was then maintained using dehumidification equipment and the sealing of the process drum.
All surfaces were then primed using Chemco International RC300P. This vinyl ester primer is designed to adhere to stainless steel well, which was of benefit given the overlap past the weld joining the stainless steel dividing walls to the outer drum shell in carbon steel. This primer was applied by brush only to best work it into the heavy pitting and profile of the steel substrate in order to achieve optimal tank lining adhesion and corrosion protection.
Following priming pitting in the shell was filled using Chemco RB300 vinyl ester putty, applied using filling knives. This had to be undertaken in one shift to ensure that the optimal recoating window of the primer was respected.
Applying the tank lining
Chemco RB332 glass flake vinyl ester tank lining was then applied in two coats of contrasting colour as per best tank lining practice. We selected the medium grade of glass flake reinforced vinyl ester to achieve a thicker film build within the available application window. Each coat was applied to a target thickness of 1mm by method of airless spray.
Between coats further filling works were undertaken as an iterative process to ensure all voids and pits were fully filled.
Following application of the second coat of glass flake reinforced vinyl ester tank lining an extremely thorough DC holiday spark test was conducted to ensure the effectiveness of the tank lining application. As seen by the degradation of the steel in aggressive environments such as this a single pin hole is a point of failure in a tank lining.
Dry film thickness readings were taken and any areas below the 2mm thickness marked.
Any defects identified were repaired using the same materials applied by filling blade and brush.
Whilst this might not be the most attractive tank lining application it is proven to perform as previously demonstrated in other areas which are still performing well after application over 8 years ago at the time of writing.
