Page 286 - Reciprocating Compressors Operation Maintenance

Overhaul and Repair of Reciprocating Compressors 2,7 1

Clearance Straight Edge
I

Rider

FIGURE 4-24. Piston ring periphery should be below that of rider band.

Piston Ring Sealing

For a piston ring to seal properly, it must have contact with the cylinder
wall and also the side of the groove. Piston rings are made with a free diam-
eter larger than the cylinder so that, when installed, they exert outward ini-
tial unit pressure against the cylinder wall, as shown in Figure 4-25,

Operating
Free » End Clearance
Gap I I

B-
FIGURE 4-25. Free gap must exceed operating gap of piston rings.

The initial pressure varies from about zero in plastic or segmental rings
to possibly several hundred psi in some situations (with metallic rings).
Unit pressure will vary around the ring circumference, depending on ring
circularity. Positive circularity exists when A is greater than B, and nega-
tive when A is less than B. Normally, rings have positive circularity.
During operation the primary sealing force, however, is a result of gas
pressure in the ring bore and against the side. This serves to hold the ring
against the cylinder and against the flat groove face as shown in Figure 4-26.