Abstract: At 12:00 a.m. on April 30, 2014, flow-back employees opened the 4H well head that was connected to a sand separator which was under pressure. The sand separator is connected to a sand box collector on one end and a manifold on the other end. Employee #1 was seen closing the valve between the sand separator and the sand box collector. The pathway from the sand separator and the manifold were closed, including all 5 valves. Employee #2 delivered some product and walked towards employee #1 to get a haul delivery ticket signed. Both employees were in the pressurized zone area of wellhead #4H when the explosion occurred. The pressure increased in the sand separator which was not equipped with a pressure relief valve. According to witnesses, Employee #1 and Employee #1 sustained severe head trauma from shrapnel released by the exploded metal pressure vessel and were killed.