The Tank Inspection That Shut Down a Town's Water
By Marisol Vega, Municipal water operations manager. Reviewed by Dana Whitcombe, technical reviewer.
Potable Water Tank Inspection & Cleaning | Wastewater & Sewer Diving October 14, 2022. Millbrook, New Hampshire. I was standing in tank 14, a 2-million-gallon bolted steel reservoir that serves the north side of town. My headlamp cut through the residual chlorine haze. The water had been drained to a thin puddle at our boots. That's when I saw the crack: a hairline running vertical from the third joint, about four feet long, with rust seeping through. My immediate thought was not 'we need a welder.' It was 'we need to shut the whole town down.' Every five years, our department inspects each of our 40 finished-water storage tanks. It's not a quick glance. We drain the tank, isolate it from the distribution system, and send in a two-person team with cameras, flashlights, and torque wrenches. We're looking for coating blisters, pitting, failed gaskets, and structural cracks. It's tedious work. Most inspections turn up nothing or small flaws that get scheduled for patching. Tank 14 was next on the list, and I expected a boring afternoon.
The Crack That Changed Everything
The crack was at the seam between panels three and four, about 20 feet above the floor. It was not a dry crack. Water was weeping through, carrying iron and oxygen. We measured it with a wire gauge: 1/16th of an inch wide. That's thin, but on a tank with pressure from 20 feet of water, it was a failure in progress. A crack in bolted steel can propagate quickly once it starts. We didn't know how long it had been there or how deep it went. We did know we couldn't argue with the likelihood of a catastrophic rupture.
Calling for a Shutdown
I dropped the tank out of service, which meant half the town lost water pressure almost immediately. We have four other tanks, but they serve different zones. We rerouted what we could, but the north side had to rely on a single 24-inch main. I called the town manager at 4:15 pm. His first question was, 'How long?' I said, 'At least a week.' He was silent for a moment, then asked, 'What do we tell people?' We told them: restore pressure overnight, then a boil water order for at least 48 hours. No one wanted it, but the alternative was risking a tank collapse that would have sent a million gallons of water through a residential neighborhood.
What I Got Wrong
The first mistake was thinking we could patch it with an epoxy plate. The manufacture's spec said welded repairs are the only acceptable fix for a crack through the full thickness. We lost a day waiting for a mobile welder who had to be certified for steel tanks. The second mistake was not checking the tank's coating history. If we had, we might have caught that the interior lining was past its service life, and corrosion was already active.
The Repair and the Hidden Cost
The actual repair took three days. We burnt an access hatch, ground out the crack, welded a new steel plate over the joint, and then applied a NSF-approved coating. Before we refilled, we pressure tested at 150 percent of normal work pressure. Then we chlorinated to 50 ppm, flushed, and tested for bacteria. The tank was back online in nine days. The hard costs—welder, coating, testing—blew through the year's repair budget in one shot. That doesn't count the boil water order, the restaurant closures, or the school's emergency case of bottled water. The real cost is trust. People remember that you told them they couldn't use their tap.
What Changed After
We now inspect all interior coatings every two years, not five. We installed acoustic emission sensors on the two oldest tanks. And we keep a local welder on retainer. But the bigger change is how we talk to the town. We now tell them ahead of time that storage tanks get old like roads and bridges. It's not a matter of if we'll find something, it's when. No one claps for that forecast, but it's honest.
Frequently Asked Questions
Why did you have to shut off the whole town? Couldn't you just bypass the tank? We had a single tank serving that zone. There was no parallel tank to switch to. The distribution system allowed some cross-feed from other zones, but not enough to keep water moving at full pressure. When we drained tank 14, the pressure in the north side dropped below 20 psi, which is the threshold for a boil water order. So we shut it down entirely to avoid drawing sediment and air into the mains. How did you decide it was safe to lift the boil water order? We followed state guidelines: two consecutive days of negative coliform samples from six locations in the affected zone. We also flushed all dead-end lines and tested chlorine residuals. The order was lifted when we had two mornings of clean samples after the tank was back in service. Did this happen because of deferred maintenance? Sort of. The budget for tank maintenance had been cut over the years, but we never missed an inspection. The deeper issue is that tank coatings are rated for 15 to 20 years. Ours were 23 years old. We were on borrowed time. It happened because we didn't budget for the coating replacement when we should have.