NASHUA – Pieces of city history don’t usually require a portable electromagnet to lift, but when dealing with Nashua’s unique triangular manhole covers, the device comes in handy.

“For 20 years, we’ve been lifting these by hand. This is a lot better,” said Bill Baker, part of a six-man team working recently on Greenlay Street, just off Broad Street.

The team was removing triangular covers and replacing each with a round one. The covers weigh at least 150 pounds apiece, so everybody appreciated the magnet, which can be turned on with a switch and clamped onto a cover, allowing it to be shifted by a front-end loader.

The triangular covers were first built by Nashua Foundry starting in the 1920s or 1930s at the suggestion of an engineer named Walter Ellis, according to company president Peter Lyons.

Ellis thought the triangular shape would rattle less than traditional circles. As an extra incentive, the triangles were aligned to point in the direction of flow for the underlying sewer line, a valuable piece of information for sewer workers hunting down problems.

Thousands were installed over the decades throughout the city. As far as The Telegraph has been able to determine, Nashua is unique in the world in its triangular manhole covers. The closest alternative we’ve found is a San Francisco development called Mission Bay that has triangular water valve covers in the street to differentiate potable from non-potable water.

Unfortunately, the triangular covers are small by modern standards: 25½ inches across at the widest point, well under the 31½-inch diameter of the round covers, made by Neenah Foundry of Wisconsin, that replace them. Because of the geometry of the shapes, the circle has twice the area of the triangle.

Notably, the triangles aren’t big enough to handle workers wearing breathing apparatus, which is needed when descending up to 25 feet underground into a sewer line, where noxious gas can build up without warning.

“It can knock you out instantly; it can kill you,” said Paul Wallace, foreman for the team.

Except for some very shallow lines, city workers don’t go down sewer holes without using a gas monitor, which can detect buildup, and without wearing a harness that allows them to be hauled up quickly if they pass out.

So for many years, the city has been replacing the old covers with new ones, usually as part of road repaving or maintenance work, or taking out an old cover that had gotten jammed shut after being hit by one too many snowplows.

On Greenlay Street, each replacement job took a little over an hour, from jack-hammering asphalt from around the triangular counter to sealing in the round cover with concrete, depending on how much brickwork needed to be replaced. The covers and their iron “collars” rest atop a vertical brick-lined shaft, at least 32 inches wide, that leads down to the horizontal sewer pipe.

Some of these tunnels are a century or more old, and lack a ladder; if workers are lucky, some bricks will have been left jutting out to act as steps.

Still, the process of replacing the manhole covers is a relatively quick one because the city Department of Public Works has a lot of experience.

“We’ve been doing this ever since I came here,” said Wallace, who has worked for the city Department of Public Works for 26 years. “I couldn’t begin to guess how many are left to do.”

Here’s one guess: At least 2,000.

“The city has about 7,500 sewer manholes, and I’d say 30 to 40 percent probably still need to be replaced,” said Roy Sorenson, city superintendent of streets.

Nashua replaces 100 or 200 each year as part of regular street maintenance or repaving, so we’ll be driving over them for time to come. Eventually, though, the triangular covers will be relegated to novelty status, kept as collectors items (the Nashua Historical Society, for example, has sold them as a fundraiser), and Nashua will lose a little of its uniqueness.

Don’t expect much regret from the folks who have to work with them daily, however.

“We know that they’ve outlived the nostalgia when we have to do confined-space entry in them,” Sorenseon said.

Back on Greenlay Street, Jerry LaFleur, a driver for the department, agreed. He also pointed to another issue with really old sewer entry points that he has encountered in his work driving the huge, 6-wheel snowplows: Age has often left the covers rising well above the asphalt.

“When you hit one of these in the winter, your plow goes up 2 or 3 feet,” he said. “You definitely feel it.”

David Brooks can be reached at 594-5831 or dbrooks@nashuatelegraph.com.