Union workers have called a strike at Husky Energy's (HUSKF, HSE.T) refinery in Lima, Ohio, leaving management to run the 155,000 barrels-a-day plant Friday.

The strike is the first called as local unions continue to negotiate with individual refineries over the details of a general contract agreed to by the national United Steel Workers and a consortium of refiners at the end of January. The union has been pushing for stronger safety measures in an industry that has had 18 reported deaths since 2009.

Spot prices for Midwest N-grade regular conventional gasoline traded 1.75c up on the day, at a discount of 8c to June Nymex RBOB futures, on worries that the strike could cut into production at the refinery. Traders said some last-minute buying before the weekend also boosted the differentials. The strike news had no impact on trading in Chicago.

The refinery produces about 2 billion gallons of refined fuel a year, including about a quarter of the gasoline consumed in the state of Ohio, according to the company's website.

The strike comes after months of negotiations to renew the three-year contract covering union workers that expired on April 14. About 235 members of the union walked off the job in Lima after concerns that refinery management was negotiating in bad faith, said national USW spokeswoman Lynne Hancock.

The union filed 23 unfair labor practice charges with the National Labor Relations Board against the company, Hancock added.

"They struck over the company's bad-faith bargaining," Hancock said. The local union "still believes there is room to negotiate."

The refinery is operating normally with no expected slowdown in production, Husky spokesman Mel Duvall said. Husky presented several offers to the union in the hours leading up to the strike, Duvall added.

"We put forward a good offer that fairly rewards workers for the work they do," Duvall said.

Besides Husky, workers at Tesoro (TSO) refineries are also still negotiating contracts. Workers at Tesoro's chemical plant in Wilmington, Calif., rejected management's offers Thursday, Hancock said.

--Rose Marton contributed to this article.

Copyright (c) 2012 Dow Jones & Company, Inc.