N.Y.C. Admin. Code § FGC_AppG.5.2: G.5.2 Double wall piping.

Horizontal piping that traverses within a building from a protected room or shaft to a protected room or shaft shall be run in an outer pipe of the same pressure rating as the inner pipe. The outer pipe shall be welded and the annular space between the inner and outer pipe shall be equal to or greater than the inside diameter of the inner pipe. The annular space shall be monitored for natural gas with an approved gas detection alarm system and shall be vented to the exterior of the building. The outer pipe must open to the shaft and/or protected room or to the outside air.