Page 91 - Reciprocating Compressors Operation Maintenance

78 Reciprocating Compressors: Operation and Maintenance

A piston used in a non-lube cylinder is usually made .125" to .250"
smaller in diameter than the cylinder bore, depending on the size.
The piston shown in Figure 2-35 has a center rider band to support the
piston, while the one shown in Figure 2-34 has the rider band on the
upper half of the piston. Location and number of rider bands are dictated
by the design, diameter, length, and weight of the piston.
Rider bands and piston rings are made from PTFE (Teflon) with vari-
ous fillers such as glass, carbon, bronze, or high performance polymers.
Carbon filled Teflon is customary in low-pressure air service. It has good
heat conductivity, is compatible with many cylinder materials, and is nor-
mally used with hydrocarbon gases as well as dry gases such as nitrogen
and helium. Bronze may be used for pressures above 2000 psi, and
graphite is used for very high temperature applications.
Rider bands are designed to support the weight of the piston and piston
rod. Minimum rider band widths are given in Table 2-1.

TABLE 2-1
MINIMUM WIDTH OF TEFLON RIDER BANDS

Piston Teflon band
diameter, in. width, in.
To 7 1
7 to 11 VA
12 to 15 IK
16 to 21 2

Piston ring widths and gaps are based on the coefficient of expansion
of the materials and a 250°F temperature differential is assumed.
Example: A cast iron piston ring width for a 10"-diameter cylinder is
%"; a W ring width is recommended for glass filled Teflon. Side clear-
ances in the piston ring groove and the end gap are adjusted for the
greater expansion of the ring material.
With the rider band installed, the diametrical clearance between the
rider band and the cylinder bore should be .00125" per inch of diameter
of a piston made of cast iron, and .002" per inch of diameter for an alu-
minum piston.
Rider band life can be extended by rotating a piston 120° to 180° at
each overhaul.