NASHUA – Perhaps the most unusual product ever made by a Nashua company is still going strong, despite efforts to replace it with a more modern version – not to mention the fact that you and your friends run it over frequently.

The city’s triangular manhole covers are nearly unique in the world, which helps explain why the Nashua Historical Society has auctioned some off to raise money. Auctioning circular manhole covers probably wouldn’t draw many bids.

The triangular covers were first built by the Nashua Foundry in the 1920s or 1930s at the suggestion of an engineer named Walter Ellis, who thought the triangular shape would rattle less than traditional circles. The triangles are aligned to point in the direction of flow for the underlying sewer line, a valuable piece of information for sewer workers hunting down problems.

Many thousands were installed throughout Nashua over the decades, covering most of the city’s 7,500 sewer connections. They never caught on anywhere else, however, and the Nashua Foundry wasn’t able to turn its design into a good business.

Unfortunately, the triangular covers are small by modern standards: just 25½ inches across at the widest point, well under the 31½-inch diameter of the round covers that replace them.

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

For many years, the city has been replacing the old covers with circular ones, usually as part of road repaving or maintenance work, sometimes when an old cover had become jammed shut after being hit by one too many snowplows. Nobody makes triangular covers of any size anymore, so circular replacements are inevitable.

A replacement takes a little over an hour, if all goes well. It begins with jackhammering the asphalt away so the triangular cover can be removed; there’s often brickwork underneath that needs to be replaced, depending on the age of the shaft leading to the sewer tunnel. Then the new cover and its iron collar are put down and asphalt replaced.

Roy Sorenson, city superintendent of streets, estimated that in 2011, at least 30 percent of the city’s sewer manholes were still covered by triangles. Since Nashua replaces 100 or 200 each year, you’ll be driving over them for some time to come.

David Brooks can be reached at 594-6531 or dbrooks@nashuatelegraph.com.