Please note in sizes 1/2" through 12" pipe is known by its nominal inside diameter, which differs from its actual inside diameter. To distinguish pipe sizes from actual measured diameters as used for tube, the terms iron pipe size (IPS) or nominal pipe size (NPS) are usually used. For all pipe sizes, the outside diameter (OD) remains constant; the variations in wall thickness affect the inside diameter (ID).
PTFE is a material subject to plastic flow. When cycled through heat and then cooled down in process lines, PTFE gaskets will not return to the original configuration and it will continue to compress as the cycling continues. This plastic flow will allow leakage if clamp torques are not maintained.
Pipe size and Tube OD size materials are not interchangeable.