PORTLAND – If you think it’s hard getting around the Old Port after a snowstorm, wait until April when the snow is gone.

That’s when Unitil Corp. will begin replacing cast iron natural-gas lines, some of which have been underground for a century.

The company will tear up pavement and dig trenches on streets in the heart of the Old Port, including Middle, Exchange, Market, Fore and Temple streets.

The work, expected to begin April 1, is part of a $60 million project to upgrade more than 100 miles of underground gas lines throughout Portland and Westbrook.

Unitil plans to substantially complete the project by Memorial Day, and assured city officials it won’t do any work during the summer, said Mike Bobinsky, director of the city’s Public Services Department.

If crews don’t finish their work by Memorial Day, they will come back in the fall and pick up were they left off, Bobinsky said.

Several business owners and store managers in the Old Port said this week that they’re pleased the project won’t interfere with the summer tourist season. They also said the infrastructure improvements are necessary.

“If we want our city to be vital, it has to be done,” said Kathleen Tutone, owner of Tree House Toys on Exchange Street.

It will be important for Unitil to communicate with business owners so they can make plans to work around disruptions, said Doug Fuss, who owns Bull Feeney’s on Fore Street and serves on the board of directors of Portland’s Downtown District.

Some property owners welcome the project because they will be able to connect to gas lines without paying the city fees that normally are charged to open up streets for gas line extensions, said Jan Beitzer, executive director of Portland’s Downtown District.

And replacing the old cast iron pipes, which are too frail to support high pressure, with high-density plastic pipes will increase the system’s capacity and give some property owners access to natural gas for the first time, Beitzer said.

“It’s one of those things that has to be done,” she said, “not only because the pipes are old, but it’s an economic development improvement.”

Natural-gas prices have fallen in recent years because of increased domestic supplies. It now costs roughly half as much to heat a house with gas as with heating oil, said Alec O’Meara, spokesman for Unitil, which is based in Hampton, N.H.

This is the third year of the company’s 14-year-project. Last year, it replaced pipes in the Veranda Street and Washington Avenue area. In 2011, it replaced pipes on several streets downtown, including portions of Market, Fore and Middle Streets.

In the past two years, Unitil has replaced 10.5 miles of cast iron and steel pipe. It plans to replace 6.3 miles this year.

Residents who want the latest information on the project will be able to call (866) 821-4386 or go to a website, www.ngupgrade.com.

Tom Bell can be contacted at 791-6369 or at:

[email protected]