Page 284 - Reciprocating Compressors Operation Maintenance

Overhaul and Repair of Reciprocating Compressors 260

Table 4-3 shows the clearance between piston and cylinder (liner) bore
on oil-free cylinders (conventional non lube) or lubricated cylinders with
rider ring (band).

TABLE 4-3
CLEARANCE BETWEEN PISTON AND CYLINDER

Cylinder Diameter Clearance
2-M"-5" .125"
5"~8" .156"
8"~12" .188"
12"-16" .219"

Rider Ring Diametrical Clearance

The clearance between the rider ring and cylinder (liner) bore on pis-
tons equipped with rider rings can be determined as follows:

Diametrical clearance = cylinder bore diameter x k + .005"
Where k = expansion factor based on piston material
Cast iron piston k = .0015"
Aluminum piston k = .0025"

Piston Ring Function

Two principal functions of piston sealing rings are:

• To prevent gas from blowing by the piston.
• To transfer heat from piston to the cylinder walls and hence to the
water jackets.

Piston ring groove width variations can have a significant effect on
these functions. To ensure proper operation, good contact is required
between the ring and the side of the groove. If the grooves become
tapered or edges "dragged" over as shown in Figure 4-22, poor sealing or
ring fracture may be expected. If shoulders or a tapered condition have