A man was sent to the hospital Sunday morning with first-degree burns to his face after a propane tank exploded at a glass-bottling plant in Milford, according to fire officials. The 55-year-old man, whose identity was not released, was filling up a forklift’s tank just outside the Saint-Gobain Containers, Inc. plant when the propane tank exploded, said Milford Fire Lieutenant Frank Ferrante. The man, who is a plant employee, was in stable condition at Milford Regional Medical Center. He was the only one near the tank when it exploded, officials said. Ferrante said firefighters took nearly an hour to control the blaze because the propane tank holds about a thousand gallons of fuel. Ferrante said that tank explosions were rare at the plant. The cause of the explosion is under investigation, Ferrante said.