The installation of pipes to expand Raynham’s sewer system is expected to begin April 1, Sewer Department Superintendent Chris Gallagher said. About 75 percent of the town has sewer access now, as part of a project begun in the late 1970s or early 1980s to help build the commercial tax base along Route 44, he said. The new phase, in the southern part of town, will increase sewer service to about 90 percent, he said. “I’m pretty excited to get it done. It’s been a long time coming.” He said the cost of the current phase is about $7 million, consisting of $5.6 million in construction contracts, plus engineering, inspections, and police time. Selectman Joseph Pacheco said the work helps fulfill a decades-old promise to taxpayers that if they funded early phases, the whole town would eventually get sewers.