A distance piece provides the separation of the compressor cylinder from the compressor frame as shown in Figure 11-9. At the top of the figure is a standard distance piece.
The piston rod moves back and forth through packing that is contained within the distance piece. The packing keeps the compressed gas from leaking out of the cylinder through the piston rod opening. As the rod passes through the packing it is lubricated. As it goes back and forth, the rod is in contact with the frame lube oil and with the cylinder lube oil and gas. Thus, oil carry-over may occur on the rod from the cylinder to the crankcase. Impurities picked up by the oil from the gas being compressed could contaminate crankcase oil.
In a single-compartment distance piece, the frame end and the cylinder end contain packing. The space between the cylinder packing and the frame diaphragm and packing is sufficiently long to assure that no part of the rod enters both the cylinder and the frame. This minimizes contamination between the gas being compressed and the oil that is used to lubricate the crankcase. There are drains and vents off the distance piece and off the packing, so if there is a packing failure, the high-pressure gas has a place to vent and not build up pressure that could leak through the frame packing into the crankcase. An oil slinger as shown in Figure 11 -9 may be added to further reduce the amount of cylinder lube oil migrating down the rod into the crankcase.
A two-compartment distance piece may be used for toxic gases, but it is not very common. In this configuration, no part of the rod enters both the crankcase and the compartment adjacent to the compressor cylinder. That is, even if there were one failure, the crankcase oil cannot be contaminated
with the toxic gas.