By Robertson Gibb, P.Eng
Water pressure and distribution in most communities is maintained via steel water storage tanks, which operate relatively maintenance and trouble free for extended multi-year periods. However, the time comes when gradual corrosion, OH&S regulation updates, water quality fluctuations or even ‘loss-of-curb’ appeal, can result in this mostly ‘passive’ infrastructure requiring upgrades. This can cost up to one-third of the initial construction cost. Therefore, a rehabilitation program should be designed to ensure asset preservation, process water quality improvements, and operator safety.
With tanks constructed 30 or more years ago, the entire exterior structure required recoating. These include multi-leg, spheroid, and single-pedestal fluted steel tanks. Both the water containing tank shell and supporting structure (i.e., pedestal or column used to raise the storage tank for optimal distribution pressures) required a coating system to prevent rapid and excess rusting.
In the early 1980s, composite welded steel tanks began to appear. While the elevated steel tank still required refurbishment every 15-20 years, the concrete pedestal was essentially maintenance free. The relatively low construction costs and reduced maintenance helped lead to an almost exclusive adoption of this tank type. Recently, a relatively new tower type, comprised of a concrete pedestal, supporting a bolted glass-lined steel tank, has emerged.This option is similar to the standard composite elevated welded steel tank except that the glass-lined bolted steel structure does not require recoating.
Regardless of tank type, a routine inspection schedule should be implemented. Initial inspections should take place on the first and second year anniversary of a new coating application (or after new tank construction). These can be completed by a remotely operated vehicle inspection camera, by partially draining the tank, or by a completely dry confined space entry.
After the warranty period, the AWWA recommends a comprehensive inspection every three to five years. Once the coating starts to break down, the rate of deterioration of the coating system and underlying steel substrate will be accelerated. Also, the amount of metal loss and pitting within the tank increases. It is most important to monitor the condition of the tank in the latter half of it’s service life to avoid more extensive repairs.