Two workers were hurt in an explosion at a hydraulic fracturing tank site in south Texas early on Wednesday, a sheriff’s dispatcher said. U.S. workplace safety regulators cited the company that owns the site, Vann Energy Services LLC, three months ago for what inspectors called serious health and safety violations there. A secretary for Vann Energy said the men were cleaning a tank at the maintenance facility that holds hydraulic fracturing or fracking tanks in Nixon, Texas when the blast happened. She declined to identify herself and said no further information, such as what the tanks contained, was available.