Drinking Water Storage Tank Asset Protection Strategies: “Corrosioneering”

By Theodore Lund, NEWWA Distribution & Storage Committee

As printed in The Source, The Quarterly Newsletter of NEWWA, Summer 2022

Welded steel water storage tanks can provide service for more than 100-years. Tanks do not have a finite life span.

Full repainting costs for water tanks can range from $600,000 to $1.8M depending on tank size. Not only is there a substantial hard dollar cost for maintenance but there is a substantial energy cost for equipment. Even the paint product requires a large amount of energy to produce. Overall a tank painting project has a significant cost and carbon footprint.

It is critical to utilize all knowledge and Best Available Technology (BAT) at our disposal to preserve the steel tank asset at the lowest cost and environmental impact. This paper explores the BAT and engineered options for tank maintenance to achieve 100+ year life spans with a lower cost and environmental impact.

High solids epoxy and high-performance urethane coatings have been applied to tanks since the eighties. These coatings last for 20-35 years extending repainting schedules dramatically. Exterior coatings degrade due to ultra-violet light, pollution and mechanical damage which allows surfaces to corrode. While normal exterior corrosion would not be a problem, the poor appearance of the tank becomes unacceptable and reflects negatively on the quality of stored water.

By understanding how to assess the integrity of exterior coatings and using that data decisions can be made regarding the options to spot and/or overcoat a tank to preserve the original barrier coat.

As an example, a tank experienced premature failure of the exterior coating due to a weak inter-coat bond between the primer and the topcoat layers. An integrity assessment of the exterior coating determined that an overcoat approach could be successful. The tank was successfully overcoated in 2010 and is still performing well twelve years later.

The estimated cost to completely blast and repaint the tank exterior would have been $900,000 and it would have been off line for 8 weeks. The overcoating process that was selected cost $325,000 and took 6 weeks, saving an estimated at $575,000.

On the environmental side, full coating replacement would have consumed 3,500 to 4,500 gallons of fuel emitting 80,000-100,000 lbs of carbon dioxide. *(ref; US Energy Information Administration 2/9/22 data 22 lbs CO2 per gal of diesel burned).

The interior of a drinking water storage tank has two distinct corrosion environments. The above water surfaces are exposed to acidic mist due to chlorine fumes and temperatures up to 120 F. Underwater flat shell and floor plates are easier to coat and are not exposed to the same aggressive environment.

The complex above water surfaces can be protected using the best practices such as grinding sharp edges, application of zinc primers, applying multiple stripe coats to gaps, edges and seams, holiday testing the final coating and application of caulking to any small gaps.

It is not uncommon for the shell and floor coating to still be 95-98% intact after 20-years while the interior roof needs to be painted. Common practice has been to repaint the entire tank interior because the interior roof is in need of attention. By spot coating the interior corrosion cells and installing an Impressed Current Cathodic Protection (ICCP) system all the sound coating will remain for years.

Protection can be provided to the underwater surfaces with existing protective coatings and the application of ICCP. ICCP will halt all corrosion, prevent pitting and prevent under-film corrosion. Properly designed ICCP systems operate for 10-15 years with minor testing and maintenance. ICCP systems will postpone or eliminate the need to blast and paint the submerged surfaces.

By applying the right combination of corrosion solutions to the interior tank environments, maintenance costs are reduced. As with the exterior overcoating strategy the added benefit of applying BAT is a significant reduction in the amount of energy and carbon emissions.

The following financial comparison illustrates the cost savings that can be realized by using BAT and engineering practices as contrasted to premature full coating removal:

Example: 2.0MG tank 90-ft tall & 60-ft diameter
Traditional Approach: Complete Recoat . . . . . .$698,760
Sustainable Solution:
Install ICCP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . $ 40,000
Coating Above water areas only . . . . . . . . . . . . .$177,680
Total Cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .$217,680

The cost savings is $481,080 plus a huge reduction in fuel consumed which would reduce carbon dioxide emissions by 100,000-120,000 lbs.

By using the “sustainable” approach, a significant reduction in cost, energy consumption and emitted carbon dioxide is achieved. In this case there is no conflict between being better stewards of our water storage tank assets and protecting the environment. It is a true “Win-Win”!