Interstage Seals

Interstage and balance piston seals of the labyrinth type are universally used in centrifugal compressor service. Multistage compressors are equipped with impeller eye seal and interstage shaft seals to isolate the stages. Figure 5-47 shows various labyrinth configurations. Labyrinth seals consist of a tooth-like form with spaces in between. Leakage is a function of both the tooth or fin clearance and the spacing. As shown in the figures, the fins can be stationary or rotating. The basic labyrinth design is the straight seal, where the teeth are at the same height. Another, rarely used for the interstage seal but frequently used for the balance piston, is the staggered or stepped form. When rotating seals are used, they can be machined integral on a sleeve or into the rim of the balance piston. Another type of rotating seal is constructed of a strip material with one edge rolled. The rolled edge is then caulked into a groove in the rotating

Stepped Labyrinth Seal b

Stepped Labyrinth Seal

Interference Labyrinth Seal

Figure 5-47. Three types of labyrinth seals. (Courtesy of Elliott Company

Interference Labyrinth Seal

Figure 5-47. Three types of labyrinth seals. (Courtesy of Elliott Company element. The shape of the rolled edge gives the name J-Strip, which is sometimes used by the manufacturers who use the seal (see Figure 5-48). For use with the rotating seal, a soft backing surface is provided on the stationary surface opposite the seal. The backing can be lead, babbitt, or a stabilized fluorocarbon material. This arrangement allows the seal clearance to be set to a smaller value compared to the stationary finned seal with the objective of running the seal fins into the soft material to cut run-

Stationary Part

Stationary Part

Enlarged View Enlarged View

Stepped Type Labyrinth Straight-Thru Type Labyrinth

Figure 5-48. J-strip type labyrinth seal. (Courtesty ofA-C Compressor Corporation)

Enlarged View Enlarged View

Stepped Type Labyrinth Straight-Thru Type Labyrinth

Figure 5-48. J-strip type labyrinth seal. (Courtesty ofA-C Compressor Corporation)

ning grooves. While somewhat more expensive, the method does tend to keep leakage down and is particularly well-suited to fouling service; whereas, the stationary teeth would tend to fill and lose effectiveness.

When teeth are stationary, the material chosen for the labyrinth must be a relatively soft nongalling material, because the teeth tend to touch the shaft during upsets, startup, or shutdown. The clearance chosen must be large enough to avoid excessive rubbing yet close enough to control the leakage. If set too tight, the extra rubbing may cause the edges to roll and affect the performance. Overall, the stationary seal is simple and relatively easy to replace.

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