When gas piping is to be opened for servicing, addition, modification, the section to be worked on is to be turned off from the gus supply at the nearest convenient point and the line pressure vented to the outdoors or to ventilated areas of sufficient size to prevent accumulation of flammable mixtures. if the pipe section exceeds the lengths shown in NFGC/IFGC the remaining gas is to be , displaced with an inert gas.