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Water treatment tanks are highly susceptible to corrosion due to handling aggressive chemicals on a daily basis. In the presence of moisture at the vapour interface the biological conversion of hydrogen sulphide gas to sulphuric acid can take place.
Hydrogen sulphide gas can directly attack metals such as copper, iron and steel. Corrosion in turn leads to metal losses where through-wall defects can develop, as illustrated in the adjacent image.
Tank maintenance can present multiple challenges where shutting the process down to do the necessary repairs is too costly and working on live equipment using hot work is not possible due to the risk of sparks or damage to existing lining. These issues prompted the formulation of coatings and repair composites that are applied and cure at ambient temperatures and allow for the repair to be carried out on a live tank without the use of mechanical equipment.
Difficult problem
This September 2013, Belzona, polymeric repair composites and coatings manufacturer, was approached with a difficult problem. A sludge buffer tank at UK's Wessex Water sewage treatment site fabricated from glass fused to steel (GFS) was suffering from through-wall corrosion at the vapour interface level. The client had only been able to run the buffer tank at a reduced capacity of 65%.
Wessex Water is a regional water and sewage treatment business serving an area of the south west of the UK, covering 10,000 km2 including Dorset, Somerset, Bristol, most of Wiltshire and parts of Gloucestershire and Hampshire. The treatment site deals with various types of domestic and industrial effluent, which is treated at the sewage treatment works before being safely returned to the environment.
Wessex Water had been using anaerobic digestion to process sewage sludge at larger sewage treatment works since the early 1960s. The bacteria in the tanks break down the organic material in sludge, this process producing a mix of methane, carbon dioxide and hydrogen sulphide gas which has two mechanisms of corrosion to the tank.
Cold plate bonding
After carefully assessing repair alternatives for the sludge buffer tank, Belzona cold plate bonding technology was selected. This solution kept the site running with minimal disruption and was completed without hot work or the use of mechanical equipment, hence presenting no risk of sparks or damage to existing lining.
The repair was carried out by a team of two from Belzona’s application division. Steel plates to cover the leak areas were prefabricated. After general cleaning to remove some contaminants, surface preparation by hand abrasion was carried out to ensure good mechanical adhesion.
Belzona surface-tolerant epoxies were selected for this repair situation due to the fact that these materials are easily applied on-line to wet and oily surfaces where only minimal surface preparation is possible. Tensile share adhesion of Belzona 1831 (Super UW-Metal) when tested in accordance with ASTM D1002 to wet abraded steel is 1,600psi (11MPa) and to oily abraded steel – 1,300psi (9MPa).
Repair plates were pre-coated with Belzona 5831 (ST-Barrier) and allowed to solidify for 24 hours. Once cured, these plates and the repair area were wetted out using Belzona 1831 to ensure full contact and excellent adhesion. A layer of Belzona 1831 was then brush applied over the repair areas, plates lowered and pressure applied to allow the material to exude from the sides. Seeing material exude confirms full contact between the tank surface and the bonded plate and guarantees a long-term repair. Excess material was then cleared away and Belzona 5831 applied to the entire repair area to prevent future corrosion.
Chris Mills of Wessex Water commented: "We have been using Belzona cold plate bonding solution on GFS tanks and have saved a lot of time and money by doing so. We have also been able to repair tanks while they are still in service as the repair is a spark free repair."
No hot work
The application took less than two days to complete from start to finish; no hot work, no replacement, no lengthy shutdown required. Traditional repair method would have taken two weeks and would involve emptying the tank and riveting repair patches to walls internally and externally, whilst alternative storage for the sludge would need to be arranged.
Cold bonding technology was first introduced to tackle Health & Safety issues presented by welding, since application and cure take place at ambient temperatures. Additionally, businesses now require maintenance work to not disrupt their daily operations and for the work to be relatively easy to complete.
Belzona perfected this cold bonding technique by introducing surface-tolerant materials that allow for greater application flexibility in terms of surface preparation and adhesion to contaminated surfaces.
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