Low Leakage Graphite Packing – When low leakage is required at higher temperatures and pressures, graphite packing rings are included in the packing arrangement. Refer to Figure 29 and note the following:
• The packing arrangement includes graphite rings and composite rings. The major role of the composite rings is to transmit the loading force to the graphite rings. The graphite rings perform the primary sealing function.
• To create a tight seal, PTFE packing washers are included between the packing rings. PTFE is typically limited to 400 to 450 degrees F to prevent extrusion. In this application, however, extrusion of the PTFE actually helps produce and maintain a tight seal. For this reason, the packing arrangement that is shown in Figure 29 is designed to meet the 500 ppm leakage requirement at a maximum temperature of 600 degrees F.
• Graphite packing is not as resilient as PTFE packing and any lateral movement of the stem will enlarge the packing bore, leading to leakage. To prevent packing damage and subsequent leakage, carbon guide bushings are located above and below the packing rings to provide stem guiding. The guide bushings are made of a non-metallic material to prevent scratching the stem.
