The city is moving forward with a plan to replace a water storage tank on West First, a tank constructed in 1939 and on that is starting to show its age.
The plan is to replace it with two tanks, each about half the size of the current 3.8 million gallon tank. That plan will offer redundancy if there is work to be done on the tanks, but it also is predicated on not using the water transmission that bring drinking water 5.5 miles from the mission water plant into the city to pump directly to water towers.
That's because those transmission lines are aging as well, and staff does not want to put undue pressure on those lines.
“We do not want to have to bypass the First St. tank and pump straight to the towers, because this puts excessive pressure on these lines that could cause failures,” said city engineer Suzanne Loomis. “Those of us that are the managers of the public infrastructure always have concerns/worries of the 'what if' scenarios because we don’t want the public to ever have to suffer without those commodities and services we provide.”
The newest of those lines, also the largest, is a 24-inch line constructed in the 1970s. The other two are considerably older — a 14-inch line constructed in 1916 and a 12-inch line constructed between 1890 and 1910. All three are constructed of cast iron.
According to waterworld.com, a trade magazine for the water/wastewater industry, while many would believe that old age is the primary contributor to iron water main breaks, the problem is not age. It is corrosion. An older pipe can continue to operate as long as corrosion is controlled. The site claims that cast iron mains can last more than 100 years without major problems.
The American Water Works Association states that because pipe assets last a long time, water systems that were built in the latter part of the 19th century and throughout much of the 20th century have, for the most part, never experienced the need for pipe replacement on a large scale.
That, among other factors, is why Loomis said there is not a current replacement plan for the three transmission lines. Instead, she said, the city will replace portions of the lines if they fail.
“We do not have a replacement plan at this time. Just like we don’t have replacement plans for the majority of our more than 150 miles of waterline in place today,” Loomis said. “The items that might trigger a replacement are: multiple failures on a segment of line, increased usage requiring more capacity in a service area, growth due to new development, line corrosion, poor fire flows, etc. As we see problems like these, then we add a line replacement to our Capital Improvement Program. Many of our lines are very old, tied directly to the age of our town, but we haven’t had an inordinate amount of problems to date at all.”
If the city were to replace the transmission lines, it would likely replace them with either ductile iron or PVC pipe. Because there is no plan for replacement at this time, the city does not have a cost estimate for that work.
The project to build a new storage tank system is estimated at $3.1 millon.
However, the transmission lines have held up well to Father Time.
“Thus far, we have not had significant problems with these lines. We have had two leaks on the 14-inch line and one on the 12=inch in the past 20 years,” Loomis said “These were resolved with our crews. Fortunately, we have redundancy with the multiple lines, so we are not completely out of water when we have to work on one line.”