Des Moines-based MidAmerican Energy Company is paying 928-million dollars to purchase the Northern Natural Gas Company. MidAmerican spokesman Kevin Waetke says Northern is a 16-thousand mile pipeline that pumps gas from Texas into the Midwest. Waetke says the purchase increases MidAmerican’s ability to move natural gas.MidAmerican is buying the pipeline, which he equates to an interstate highway, and other companies will pay to use the pipeline to move gas. Waetke says the purchase expands the transport unit of the company.This purchase builds on the western pipeline the company purchased in March. Waetke says the purchase won’t impact natural gas prices in Iowa.He says it’s simply an example of MidAmerican diversifying its holdings. Waetke says the sale could become final in August — once the company gets regulatory approval.