Why did it take the city roughly two and a half hours to shut off water last Thursday as it gushed onto Hennepin Avenue from a massive rupture in a key distribution pipe running through downtown?
City Engineer Steve Kotke has heard the question, and he and his staff say there’s a good answer.
Simply turning off the 36-inch water main at the nearest valve could have turned the leak into a bigger disaster. The water pipe could have started hammering like a home faucet, posing the risk of breaking a pipe.
“You can’t just slam a valve shut,” said Marie Asgian, the city’s water distribution supervisor. That’s because the main water valves on the pipe would need to work against massive volume and pressure from the water flow.
“When you have mains of this size, there’s a procedure,” Kotke said. That involves closing off water valves farther from the site so that the water flow and volume in the affected area is gradually reduced, a process utility officials called pinching off. The break was ultimately isolated in a three-block stretch of main on N. 2nd Street.
To reduce the water pressure enough that big gates controlling the flow to the broken main could be closed, water crews shut 37 other smaller valves that reduced the pressure. Those shutoffs ranged as far north as Plymouth Avenue and as far south as 5th Avenue S. Those were closed with a manually operated key in the shape of a T, according to foreman Mark Ebert.
But to close the big gates once pressure was reduced, the city uses machine power, keeping track of the 260 to 300 rotations of the valve stem required to fully close the valve. They are guided by torque readings that help show how much pressure the machine is fighting against.
That pressure-reducing process meant it took from the 2:30 p.m. rupture time to 5 p.m. to isolate the damaged 36-inch main enough for water pressure to rise in the area. The break was completely isolated by 6 p.m.
“Every available person worked on this problem,” Ebert said.
Meanwhile, even as it was responding to the massive break in the 1891 vintage cast-iron pipe for days on end, two other unrelated breaks happened in smaller water mains miles away in the city on Friday and Monday. The city averages about 40 main breaks a year, Ebert, said, which he said is better than similar cities. Often they involve unstable soils that shift, dislodging pipes.
Most of the city’s water mains were laid before 1920, Ebert said. The Minneapolis water department dates to 1867, when the city needed a source of water to put out fires. By 1872, it added providing drinking water as a public service.