Page 53 - Compression Machinery for Oil and Gas

44 SECTION II Types of Equipment

With appropriate evenness and parallelism of the facing surfaces, only the
pressure pad in the sealing gap is able to keep the closing force on the back of
balance. This gives the seal the ability to regulate the height of the sealing gap
itself. Once any additional force tries to disturb the balance the gap height will
change, and the pressure pad in the seal gap will create immediately an appro-
priate counter force. The height of the sealing gap is only a few micrometers.
The spring force is only dimensioned so that it can safely overcome the
frictional force of the dynamic sealing element in the unpressurized state,
and therefore, plays a minor role. The closing force is mainly represented by
a rectangular pressure pad.
Starting from the outer diameter so-called gas grooves (5) are incorporated
in one face. These are usually incorporated in the rotational face and reach into
the middle of the sealing surface and are only a few micrometers deep. The main
function of these grooves is to achieve an additional pressure pad and thus liftoff
during dynamic operation. The rotation creates a shear flow in the grooves
which in turn induces pressure generation (Fig. 3.14).
The available groove geometries can be divided into unidirectional and
bidirectional ones. Above all, due to the hardly avoidable possibility of reverse

Gas in
Direction of rotation Gas inlet

Compression

Direction of rotation
FIG. 3.14 Grooves in a DGS (top) and resulting pressure dam action (bottom).