When piping full of air is placed in operation, the air in the piping is to be displaced with fuel gas provided the piping does not exceed the length shown in NFGC/IFGC The air can be safely displaced with fuel gas provided that a moderately rapid and continuous flow of fuel gas is introduced at one end of the line and air is vented out at the ather end. The fuel gas flow shall be continued without intemption until the vented gas is free ed air,The point of discharge is not to be left unattended during purging, The vent is then to be closed. ithe piping exceeds the lengths whown in NFGC/IFGC, the air in the piping isto be displaced with an inert gas and the inert gas is then to be displaced with fuel gas.