Page 52 - Compression Machinery for Oil and Gas

Centrifugal Compressors Chapter 3 43

FIG. 3.12 Block representation of a dry-gas seal.

FIG. 3.13 Force balance on a dry-gas seal.

The diameters of the faces are designed to achieve hydrostatic liftoff at
standstill with a certain pressure difference across the seal as shown in
Fig. 3.13. The axially displaceable seal element on the back of the stationary
face seals is the so-called hydraulic diameter, which is always larger than the
inner diameter of the sealing surface. This means that the effective pressure area
on the back of the stationary face is smaller than the sealing surface and the
prerequisite for a stationary liftoff is given.