Water Tank Painting and Lining, Dorset

THE PROBLEM

As part of the refurbishment of the water tank / water tower a new protective coating system was required due to the flaking existing coatings.

The likely causes of this coating failure were two fold. Firstly galvanized steel can be a challenging surface to adhere to and typically requires sweep blasting to raise a surface profile or use of Mordant solution.

Secondly if an Alkyd based protective coating had been employed a process called saponification will occur through which the coating negatively interacts with the zinc to form a metal soap between the coating and the substrate.

THE SOLUTION

We proposed the removal of all existing coatings by method of high pressure water jetting (10,000psi) prior to the application of a protective coating system that is proven to adhere to galvanized steel substrates without the need for abrasive blasting or use of mordant solution.

The benefit of this is that there is no risk of loss of the protective zinc layer through abrasive blasting and the use of acidic mordant solution is avoided.

To facilitate the works at a time of year which would not be conducive to coating application a fully encapsulated scaffold was first erected.

This would later allow the introduction of heating and dehumidification equipment to ensure the correct climatic conditions for protective coating application.

Once access had been obtained all existing coatings were removed from the tank structure by high pressure water jetting, exposing the galvanized steel substrate.

This would later allow the introduction of heating and dehumidification equipment to ensure the correct climatic conditions for protective coating application.

Once access had been obtained all existing coatings were removed from the tank structure by high pressure water jetting, exposing the galvanized steel substrate.

Unfortunately during the preparation process several perforations in the tank structure were exposed.

We were requested to conduct a confined space entry survey of the tank and ascertain the condition of the tank internals.

To facilitate a survey it was first necessary for us to arrange a tank cleaning subcontractor to remove the approximately 200mm of sludge from the tank base and wash the tank internals. Fortunately the tank had only been supplying farm water and nothing for human consumption.

Once access had been obtained the cause of the perforations was instantly clear. Bacteria within the sludge had produced acids which had locally eroded the zinc and corroded the steel. This phenomenon of sulphate reducing bacteria is something that is often seen in oil storage tanks.

Following the submission of our report we arranged for an engineering subcontractor to attend site and install plates over the areas of steel where loss of thickness necessitated it.

Both internally and externally chloride tests were undertaken to ensure that chlorides beyond levels as prescribed by the coating manufacturers were not present.

If chlorides are left in place prior to protective coating /tank lining application, these will draw moisture through the coating film and lead to osmotic blistering.

The areas to be coated internally were then prepared to SA2.5 as per ISO8501-1, with a minimum surface profile of 75 microns as recommended and tested and recorded using surface profile needle gauges as part of our standard quality assurance.

All surfaces were then vacuumed clean and the effectiveness of this confirmed by the performing of dust tape assessments.

As standard climatic monitoring was instigated in order to ensure the correct climatic conditions for coating application.

A heavy stripe coat of Acothane DW was then applied to all angles, edges and corners. The purpose of this stripe coat is to apply additional coating thickness in areas where the tank lining film will pull thin through surface tension.

Once the stripe coat had cured a full coat was applied in a contrasting cream colour to a nominal thickness of 500 microns. Once this has cured another coat to the same thickness was applied in a contrasting blue colour.

Different colours for each coat is best tank lining practice as this aids applicators in ensuring full coverage for each layer of the tank lining system.

As per best tank lining practice and our standard quality assurance the cured tank lining was tested for porosity using a DC holiday spark tester, with any pin holes found being marked with chalk.

Dry film thickness readings were also taken and recorded, again with any areas below the required minimum thickness of 800 microns being marked.

All marked defects were then made good using the same tank lining material applied by brush.

Once the internal works had been completed we could then return again to complete the external protective coating works.

All nuts, bolts and seams received a stripe coat of Rustoleum Noxyde Plus applied by method of brush and roller.

This was followed by the application of two coats of Rustoleum Noxyde Plus to a nominal wet film thickness of 350 microns per coat.

Wet film thickness readings were taken throughout the application to ensure that the required protective coating thickness was achieved, followed by dry film thickness readings once cured.

As previously mentioned Rustoleum Noxyde Plus offers excellent adhesion to even the most challenging substrates such as glass and ceramic, without the need for extensive surface preparation.

This excellent protective coating material offers up to 15 years protection in the most demanding C5 corrosivity environment, meaning that in this much less aggressive environment a very high durability could be expected.

Water Tank Painting and Lining, Dorset

THE PROBLEM

THE SOLUTION

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