Two of the corrugated galvanized water storage tanks in the food processing factory had leaked since there installation, despite repeated attempts to fix the loose fitted bag type liners. This had included the complete replacement of one of the liners.
These types of tank liners are very susceptible to damage during routine maintenance. We suspected that a likely point of water egress was where the liners intersected plastic pipe that entered the tanks through the concrete base.
We proposed the removal of the existing linings and application of a food safe polyurea tank lining in the form of Versaflex FSS 45DC FDA approved pure polyurea.
Versaflex FSS 45DC is FDA approved for direct food contact, making it a quite unusual polyurea tank lining.
The reason we proposed a polyurea tank lining material is that the bolted and sectional nature of this type of tanks makes them susceptible to movement between the panels during emptying and filling, meaning that to ensure fluid tightness we felt this would best be served by a highly elastomeric polyurea tank lining .
Initially the existing tank lining was cut out and removed.
Some of the existing tank linings were repurposed to form part of the temporary containment that was required during the works due to the highly dust sensitive nature of the area that we were working within.
Galvanized steel must be properly profiled in order to ensure optimum adhesion of the new tank lining linings. Given the dust sensitivity of the area it would not have been possible to use conventional grit blasting.
Instead we proposed the use of low dust sponge blasting which captures 95% of the dust it creates. More information on this technology can be found here.
To ensure further prevent dust release during the tank lining preparation works temporary protection and extraction was introduced
This meant that the visibility during the sponge blasting preparation works was excellent. This combined with the controllability of sponge blasting in which both pressure and media feed can be adjusted allowed the delicate preparation of the galvanising to ensure that minimal zinc was removed as per best practice to ensure the maximum corrosion protection remained in place possible.
Once prepare all surfaces were vacuumed clean and the effectiveness of this checked and recorded as part of our standard tank relining quality assurance by means of dust tape tests.
Also as part of our tank lining quality assurance soluble salt tests were conducted and recorded. Failure to test for and remove it present soluble salts such as chlorides and sulphates can lead to premature failure of protective coatings and tank linings through osmotic blistering.
The surface profile created by sponge blasting was tested and recorded using a surface profile needle gauge in order to ensure that sufficient roughness had been achieved for good adhesion of the new tank lining.
During preparation works we identified that in some locations the tank bottoms did not actually site flush with the concrete slab which formed the tank base. We raised this with our client and proposed the filling of these gaps using a high strength epoxy slurry to ensure that the tanks were fully supported to minimise movement in the structures.
Vertical and horizontal panel joints were then isolated using a bond breaker tape – the purpose of this being to deliberately de-bond the tank lining from the substrate in the areas where the greatest potential for movement exists.
This is only possible with high tensile tank linings such as polyurea and serves to act as an elongation multiplier, allowing the elastomeric nature of the polyurea lining to be spread over a greater width.
The plastic pipes previously mentioned were abraded and treated using Corrolastic SG. This waterproofing grout is unusual in that it will adhere well the HDPE and LDPE pipes – something that is challenging for even the best epoxy coatings.
Prior to tank lining application each day and during the climatic conditions were tested and recorded to ensure compliance with the tank lining manufacturers recommendations.
A solvent free epoxy primer was applied throughout the tanks and dressed with a quartz aggregate to provide an excellent mechanical anchor for the subsequent polyurea lining to adhere to in addition to the chemical bond with the epoxy primer.
As per best tank lining practice the thickness was tested and recorded using a wet film thickness comb / gauge
The rapid curing properties of polyurea meant that it was not possible to take wet film thickness readings, however because the material gels in less than 1 minute dry film thickness readings could be taken during application instead to ensure that the required tank lining thickness was achieved.
As per our standard tank lining quality assurance the completed coating was checked for porosity using a DC holiday spark tester.
Once we had completed our works the client placed the tanks under hydrostatic test to ensure the water tightness of tanks before returning them to service.
