The sectional glass fused steel tank had perforated and the polyurethane sealant had broken down as a result of the aggressive environment presented by the food waste in the reception tank at the anaerobic digestion plant.
Another contractor had applied a tank lining which had not performed as the client expected and had significantly broken down after 6 months in operation.
This was a challenging tank lining application due to the following requirements given to us by the client;
- Aggressive chemical agents, with a variable pH and acidic elements
- Elevated temperature of up to 65 degrees centigrade
- Up to 14 tons of ring loading applied over panel joints – meaning a flexible solution is required
- Adhering to glass fused steel – a notoriously difficult surface to adhere tank linings to, as exhibited by the poor adhesion of the existing tank lining
The tank lining specification that could resist these conditions was Corrolastic UB, manufactured by Specialty Products Incorporated.
Removing the failed tank lining using high pressure jetting
Initially the existing failed tank lining was removed using our own Ultra High Pressure water jetting unit. The existing lining was approximately 1000 micrometers thick and slightly flexible – a difficult material to remove by abrasive blasting alone. The tearing action of the rotary head of our Ultra High pressure water jetting unit removed the failed tank lining with ease.
The tank had been cleaned by the client prior to handing over to ourselves. However what became apparent during the water jetting works was a large amount of fats and greases still present on the tank walls.
To ensure that these were removed prior to abrasive blasting and ensure excellent adhesion of the new tank lining an additional degreasing regime was introduced using an emulsifying degreaser and hot washer. This also served to remove soluble metal salts and chlorides prior to abrasive blasting.
The cleanliness of the surface was checked using a black light by one of our two Institute of Corrosion trained paint inspectors.
Abrasive blasting the tank
Once clean the tank internals were prepared by method of abrasive blasting. All exposed steel was prepared to SA2.5 as per ISO8501-1, with a minimum surface profile of 50 micrometers on steel and remaining glass enamel. The surface profile was checked using a surface profile needle gauge.
Following blasting and the clearing of spent media the tank was washed using clean water with the inclusion of an anti-corrosive agent. This served to ensure a spotlessly clean surface for the tank lining to adhere to whilst also preventing flash rusting of the blasted steel. The blast standard was also maintained using dehumidification equipment.
Soluble salt testing
As part of our standard tank lining quality assurance soluble salt tests were conducted to ensure that the levels were below the permitted levels from the tank lining manufacturer.
All bolt heads and seams were detailed using a polyurethane jointing compound to provide a simpler surface to spray apply the new polyurea tank lining to, and isolate these potential points of movement from the tank lining.
The stainless steel termination/overlap areas were primed using Corrolast DSP – our specifically designed epoxy primer for difficult substrates such as stainless steel.
Spray applying the polurea
Once satisfied with the preparation and priming works Corrolastic UB pure polyurea was spray applied to a thickness of 2.5mm as recommended by Specialty Products Incorporated. This included their AE4 adhesion promoter which allows the material to bond tenaciously to silica containing substrates such as glass enamel without the use of primers.
As part of our standard tank lining quality assurance the lining was checked for porosity using a DC holiday spark tester. Where pin holes were found these were marked, prior to touching in using the same material. This was simplified by the use of SPI’s Ultrabond molecule which allows application of polyurea after the conventional recoat window has been exceeded.