THE PROBLEM
The sealing membrane in the top of the steel hot water tank had burst, allowing oxygen to enter the vessel. This had lead to the corrosion of the tank structure.
SOLUTION
As requested we proposed a suitable tank lining system that would be able to offer the necessary protection and withstand the 80 degree centigrade operating temperature. When undertaking high temperature tank linings the works must be conducted to the highest standard as the elevated temperatures will cause additional stresses in the tank lining which will quickly expose poor preparation and application.
From experience relining hot water tanks we know that they can have high soluble salt levels as a result of evaporation concentrating the relatively low levels of soluble salts found in tap water. This was confirmed when we tested the surface. Soluble salts if left in place can cause premature coating failure as they draw moisture through the coating film, which can lead to osmotic blistering and premature tank lining failure.
To remove the soluble salts we high pressure washed the tank internals with clean water. The effectiveness of this was verified by further soluble salt tests.
Preparing The Tank By Sponge Blasting
To prepare the tank ready for the new tank lining we propose preparation by sponge blasting – a low dust form of sponge blasting. The reason that this was proposed was to minimise dust released into the plant room which would damage plant and equipment. In conjunction with minimal extraction this form of grit blasting was completely dust free, something the client was especially pleased about.
Unlike conventional grit blasting, sponge blasting employs micro encapsulation technology. The compressing and subsequent rebounding of the sponge as it hits the surface and exposed the abrasive embedded, captures the dust it creates.
The grit blasting prepared the surface to SA2.5 as per ISO8501-1 and we tested the surface profile to ensure that a minimum surface profile of 75 microns was achieved.
All surfaces were vacuumed clean and a surface cleanliness conducted. It is important to verify that there is not a layer of fine dust present on the surface as this will be detrimental to the adhesion of the new tank lining.
Once cleaned the tank was primed using Chemco RE500P – a high temperature and surface tolerant primer. Although we had maintained the blast standard using dehumidification, having a surface tolerant primer gives peace of mind. As per good tank lining practice an additional stripe coat was applied to the tank welds to apply additional thickness where paint pulls thin through gravity and surface tension.
Following priming Chemco RW500 epoxy novalac tank lining was applied in two coats, again with an additional stripe coat. This tank lining material contains self-leafing glass flake which laminate within the coating film, creating a more tortuous route for oxygen and moisture to reach the steel substrate, offering improved corrosion resistance.
We were aiming for a minimum tank lining thickness of 600 micrometers which we comfortably achieved, as verified using our dry film thickness gauges.
As part of our standard tank lining quality assurance the tank lining was checked for porosity using a DC holiday spark tester. Where found pin holes were marked prior to touching in using the same material.
We warranted the works for 10 years from completion.
