Unitil Corp. is partnering with Portwalk Place, a mixed-use development in Portsmouth, NH, to upgrade the natural gas infrastructure in the downtown area at no cost to taxpayers. Unitil is replacing older steel pipes in the surrounding area with high-density plastic pipe. The project will modernize a portion of Portsmouth’s gas infrastructure, making it more reliable, efficient and better able to meet the growing demand for natural gas, Unitil said. The company has laid more than 2,500 feet of plastic pipe in and around the downtown area, allowing the natural gas system to operate at an elevated, more consistent pressure similar to other areas of the city. Unitil is implementing a global positioning system for all new pipe laid in the ground that will provide the company access to a database of infrastructure information. Unitil is a public utility holding company with operations in Maine, New Hampshire and Massachusetts.