Filter Membrane Tank Lining, Bedfordshire

THE PROBLEM

The steel filter membrane tank was exhibiting corrosion and breakdown of the existing epoxy tank lining which was affecting water quality which had been identified during routine sampling.

THE SOLUTION

We proposed the application of Acothane DW – a solvent free polyurethane which is tested and approved to the highest regulatory standards for UK potable water.

The high build nature of this material offers outstanding corrosion protection and the flexible nature of the material offers good impact resistance – important given the loading and unloading of filter membranes in this application.

Initially all surfaces were thoroughly cleaned by high pressure washing to ensure that all contaminants such as soluble salts were removed prior to preparation.

This is a critical part of the tank lining process as if left in place soluble salts can lead to tank lining failure through osmotic blistering. Failure to remove these before preparation by abrasive blasting can lead to contaminants being forced into the surface profile of the prepared steel.

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The effectiveness of the cleaning operations and the absence of soluble salts was confirmed by the conducting of soluble salt tests using the Brestle method.

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All existing coatings were then removed and the steel prepared to SA2.5 as per ISO8501-1 in line with the manufacturers recommendations and instructions for use.

The blast profile was tested and recorded using a surface profile needle gauge, ensuring that an average roughness in excess of 75 microns was achieved.

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Following blasting all spent blast abrasive was removed and all surfaces vacuumed clean. To confirm the effectiveness of the cleaning operations dust tape tests were conducted. Ensuring surfaces to be coated are free from dust contamination ensures proper adhesion of the tank lining.

Prior to and during coating application the climatic conditions were tested and recorded to ensure conformance with the tank lining manufacturers recommendations. Heating and dehumidification had been introduced prior to preparation works to maintain the blast standard.

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To ensure the best finish possible the decision was made to apply Acothane DW in three coats to avoid sags and runs.

A full coat in a cream colour was first applied in order to ensure the highest possible surface preparation standard was maintained.

A stripe coat in a contrasting blue colour was then applied to all angles, edges and welds. Stripe coating ensures full tank lining thickness is achieved on edges where tank linings can pull thin through surface tension.

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Throughout the tank lining application process wet film thickness readings were taken in order to ensure the required tank lining thickness was achieved.

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Once each coat had cured dry film thickness readings were taken to further validate that the correct tank lining thickness had been achieved.

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Finally following the application of the final coat a DC holiday spark test was undertaken to check for holidays. By applying a high voltage across the tank linings surface any breaks in the lining can be identified and marked for rectification.

Post application data loggers were installed in order to monitor and record the curing of the completed tank lining to ensure compliance with the instructions for use before returning the tank to service.