The distance piece (refer to Figure 300-1) separates the cylinder from the crosshead guide (doghouse). It provides a housing for both wiper packing and pressure packing. The distance piece prevents entry of compressed gas into the crankcase, and provides access for maintenance of the packing and piston rod.
Four types of distance pieces (with corresponding diagrams) and their applications are covered in API 618. Vent and purge connections are shown on the API 618 diagrams. Further information on venting and purging is provided in Section 360, “Application and System Considerations.”