When transferring liquefied petroleum gases, flammable liquids, combustible liquids, or products, all hatch covers shall be closed. Ullage plugs shall only be removed from such tanks as are being loaded or discharged. All such ullage plugs removed shall be replaced with approved flame screen. Such ullage screen covers shall be in place while transferring when not being used to take samples, check tanks, or when topping off. Unless the vessel is gas-free, tank tops and ullage plugs of tank vessels shall be closed while any towboat or other vessel is alongside and shall not be opened until any such towboat or vessel has left the tank and there is no longer any danger of sparks or other sources of ignition therefrom.