A fire broke out at BP's largest US refinery on Wednesday night, but the company said the heavy crude facility was still in production.A compressor in one of the refinery's units exploded, according to local reports, shaking local homes and knocking pictures off walls.The FTSE 100 oil major said operations at the Whiting refinery, just east of Chicago, "were minimally impacted as a result of the incident and the refinery continues to produce products for customers".BP said there had been "an operational incident" on a process unit at the refinery, but the plant's in-house fire department had snuffed out the blaze within two hours.The Whiting refinery, which was refurbished in November, processes heavy crude oil from Canada's tar sands fields in Alberta, which is discounted to international oil prices.The Indiana refinery processes 400,000-plus barrels per day and could process as much as 300,000 barrels of oil per day of heavy crude, chief executive Bob Dudley recently said, delivering up to $1bn of operating cash flow per year.Shares in BP were down 0.5% to 483.1p at 10:40 on Thursday. OH