On double-acting cylinders (and some single-acting), the piston rod must be sealed to reduce gas leakage from inside the cylinders. This seal is called pressure packing. It is of the full-floating design so that the packing rings follow any lateral motion of the piston rod.
As with piston rings, PTFE is used extensively for packing rings. Figure 300-35 shows the forces on a packing ring. Figure 300-36 shows a typical arrangement of packing rings. The back-up ring limits deformation of the packing ring, and is usually not required below 500 psi. The back-up ring is sometimes made of bronze for better heat dissipation.
Packing cases with vent and buffer arrangements are shown in the Appendix of API 618. Figure 300-37 shows a packing case with passages for coolant. Guidelines for when cooled packing is applied are included in the text of API 618. Wiper packing is also shown on the diagrams in API 618. The purpose of this packing is to minimize leakage of crankcase oil.