The rig crew was in the process of running casing in the hole. A mud line with a low-pressure valve was being used to fill the casing. At one point while filling the casing with mud, the floorman closed the fill line valve when the casing was full. When the valve was closed, pressure built up and the pressure surpassed the working pressure of the valve. The floorman was struck with debris and required medical attention resulting in a lost time incident.