Low-Leakage PTFE Packing – PTFE cold forms easily and it does not have good memory; i.e., after PTFE becomes deformed, it does not return to its original shape. When PTFE is used as a packing ring material, a major concern is the extrusion of the packing rings and the resulting loss of sealing integrity. Refer to Figure 28 and note the anti-extrusion rings and anti-extrusion washers that prevent packing ring extrusion. Because only a few packing rings are included in the arrangement, there is extra space in the packing bore. Lantern rings are included to fill the empty space. The packing arrangement that is shown in Figure 28 is designed to meet the 500 ppm leakage requirement at temperatures up to 450 degrees F and at pressures up to 750 psig.