THE PROBLEM
The glass fused steel anaerobic digestion tank was exhibiting corrosion and heavy breakdown to the lower two rows of panels. We suspect this was caused by damage from grit contained within the feedstock impacting the glass fused steel. Glass fused steel when damaged can have a micro porous layer beneath.
More information on this can be found here
SOLUTION
The food waste that enters the anaerobic digester is pasteurised, meaning that it can enter the anaerobic digester at up to 70 degrees centigrade. Conventional epoxy tank linings and polyurea tank lining soften from 60 degrees centigrade. The requirements for this tank lining were also further complicated by the potential low pH.
Because of the sectional nature of the tank also a flexible tank relining solution was definitely required as these types of bolted glass fused steel tanks have potential for lateral movement and deflection with the panels themselves.
Because of these requirements we enquired with our proven polyurea Specialty Products Incorporated. They recommended the application of Raven Lining AquataFlex 506 an innovative Tri Hybrid material.
Initially all of the existing sealant was removed. This is essential when relining bolted glass fused steel tanks as with some tank manufacturers the glass enamel coating does not extend over the panel edges or through the bolt holes, meaning these areas are prone to corrosion. We often see inexperienced tank lining contractors overcoating old panel sealant, which can be hiding corrosion beneath.
Following the removal of the old sealant the tank was thoroughly cleaned by method of steam cleaning to remove fats, greases and chloride contamination. The effectiveness of this with regard to soluble salts was confirmed by testing prior to abrasive blasting.
Removal of soluble salts prior to abrasive blasting is essential as the blasting process can force them further into the steel profile. If soluble salts are left in place when applying tank linings they can cause osmotic blistering and premature lining failure.
The steel was then prepared to SA2.5 as per ISO8501-1 with a minimum surface profile of 100 microns. This was tested and recorded using a surface profile needle gauge. When applying tank linings which operate at elevated temperatures a high standard of surface preparation is required as the elevated temperatures impart a stress upon the adhesion of the lining, which will quickly be exposed if it is insufficient.
All surfaces we vacuumed clean and a dust tape test conducted to verify the effectiveness of the cleaning operations to ensure excellent adhesion of the new tank lining.
To maintain the blast standard dehumidification was introduced and heating to create the correct climatic conditions for coating application. These were tested and recorded as part of our standard quality assurance.
All surfaces to be coated were primed using a surface tolerant high temperature epoxy primer applied by method of brush and roller. To promote adhesion of the polyurea tank lining kiln dried quartz aggregate was blown into the wet primer to create a perfect sand paper like finish for optimum adhesion of the polyurea lining.
A termination chase was cut into the concrete slab as per best polyurea application methodology in order to provide a strong finish point for tank lining.
Raven Lining Aquataflex 506 was then spray applied to a thickness of 2mm by method of plural component spray unit. This product is unique in that it combines polyurea, polyurethane and epoxy novalac technology to give increased chemical and temperature resistance compared to standard polyurea linings.
As per our standard quality assurance the new polyurea lining was tested for porosity using a DC holiday spark tester. In aggressive immersion environments such as this as single pin hole is a point of tank lining failure. This has to be completed within the short overcoating window to enable repairs to be made within the time required.
Finally the termination points of the tank lining were sealed using a flexible jointing compound applied using sealant guns.
