Page 232 - Reciprocating Compressors Operation Maintenance

Operation and Maintenance of Reciprocating Compressors 217

5, Alignment. For both piston and packing rings, it is very important
that true alignment be maintained. This ensures that, in the recipro-
cating motion of the piston and rod assembly, the piston and/or rod
does not rise and fall. Such action would result in high loadings at
the sealing surfaces.
6. Surface finishes. These finishes must be maintained to prevent
abnormal and rapid wear of the piston rings, rider bands, and pack-
ing rings. It has been shown that Teflon performs optimally on
cylinder bore surface finishes of 8 to 16 RMS and 8 to 10 RMS sur-
face finish of piston rods.

Teflon wears and fills the uneven metallic structure resulting in Tefion-
against-Teflon contact. It is not unusual that an initial set of rings wears
rapidly because of this phenomenon. Surfaces that are too smooth do not
allow Teflon to deposit and may result in higher ring wear.
Any cylinder that has been rebored and honed should be given an
additional final honing using Teflon blocks substituted for the abrasive
stones. This impregnates the cylinder with Teflon for initial break-in.
A daily log of compressor pressures and temperatures should be
maintained. Inspection of such critical areas as cylinder bores, piston
and rider rings, and piston rod surfaces is recommended on a regularly
scheduled basis.
When replacing piston rings and rider bands, it is important that the
side clearances and end gaps are checked. Remember that the coefficient
of expansion of Teflon, which determines these clearances, is seven
times that of cast iron.

LABYRINTH-PISTON COMPRESSORS*

Low maintenance costs and reduced downtime make labyrinth-piston
compressors attractive for virtually any gas in chemical and petroleum-
refining processes—even severe fouling services. They are especially
well-suited for oil-free gas compression, where the common alternative—
a compressor with dry-running piston rings—often has maintenance costs
that are higher than those for an equal-size lubricated-piston machine.

*Bloch, H. P., "Consider a Low-Maintenance Compressor," Chemical Engineer-
ing, July 18, 1988.