Wednesday, July 30, 2014

Corrosion a constant battle for water tanks

Res 9 tank

LAMINAR CORROSION turns a rafter end in Reservoir 9 Tank into a metallic deck of cards. Photos courtesy EID

From page A1 | March 22, 2013 |

The El Dorado Irrigation District stores 71 million gallons of water in 36 storage tanks, with 27 of those being potable water for a total of 53 million gallons.

That’s a lot of metal that needs painting inside and out. Failure to regularly maintain the water tanks can result in rust so bad, such as the Reservoir 9 Tank where some of the rafter ends have delaminated. Or the Outingdale tank were the rafters are starting to come loose, with one already fallen part way into the tank.

In an informational presentation to the EID board March 11 engineer Bob Rice detailed the results of a photographic survey EID inspectors or divers had made of 23 of the district’s 36 tanks, Rice, who has specialized expertise in metal water tanks, ranked the 23, with three being excellent, eight good and two fair. There were six in poor conditions and four failing.

“The extent of this took us by surprise,” Rice said.

The failing ones added up to $1.85 million in capital expense estimated for this year. That includes replacement of the Outingdale tank at a cost of $250,000. Reservoir 9 Tank will need renovation and painting at a cost of $950,000. The Deer Creek Equalization Tanks will need renovation and painting at a cost of $650,000.

The 62,000-gallon Outingdale Tank was built in 1993 and the 1 million-gallon Deer Creek tank in 2007. The Outingdale Tank’s interior structure is perilous enough that Rice “cautioned staff about walking on top” of the tank. The 2 million-gallon Reservoir 9 Tank was built in 2001.

Two factors add to the corrosion. One factor, according to Rice, is water entering the tanks through a pipe extending too near the roof that spills water into the tank. This top loading creates a mist of chlorine that can more quickly corrode the tank’s roof. To remedy this water should enter the tank from a side inlet instead.

Another factor is painting flaws such as inadequate surface preparation, inadequate dehumidification, improper paint application technique and inadequate inspection.

“Inspection is the most important part of the process,” Rice said. “Since 2006 we have been hiring specialty paint inspectors.”

In fact, Rice said he has come to realize that certified paint inspectors must be on the job during the entire painting process.

“Now our paint specifications are 31 pages long,” Rice said.

“We have extensive qualifications on paint contractors,” Rice said.

The maintenance and inspection program includes inspecting the exterior of each tank annually and having divers clean and spot paint 15 tanks in the last four years. Wastewater and recycled water tanks are being included in the program.

The maintenance program’s goals include a five-year cycle for interior inspection of all tanks, dive seven tanks per year for cleaning and spot painting and establish a routine coating cycle for all tanks.

For the 2014-2019 period Rice is recommending renovating two tanks annually at a cost of $1.6 million, redoing the top coat of one tank annually for $250,000 and dive and clean seven tanks for $50,000. The annual total is estimated to be $1.9 million.

“It’s a 15-year cycle and we need to get ahead of this,” said EID General Manager Jim Abercrombie.

Again noting the need to issue capital improvement bonds, Abercrombie listed critical capital projects that included $10 million for water tanks, $17 million-$20 million for raising and strengthening Forebay Dam, and $20 million for three flume projects.

“Reinvesting capital in our assets is critical,” Abercrombie said.



Michael Raffety



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