Page 95 - Compression Machinery for Oil and Gas

84 SECTION II Types of Equipment

FIG. 3.49 Swirl ratio in the shroud and the backside cavity.

Of particular importance for the topic of off-design operation is the fact that
the swirl coefficient changes when the impeller is operated away from its design
point (Fig. 3.50). Also, the magnitude of the swirl coefficient on the impeller
backside changes in the opposite direction from the swirl coefficient on the front
side of the impeller. This means that the thrust imbalance (for a given pressure
level and a given speed) changes not just due to the pressure difference between
the impeller eye and the corresponding backside, but also due to different swirl
factors in the cavities in the front and back of the impeller. This imbalance, in
particular, changes when the compressor moves from the design point to choke.
In general, the shroud side swirl is higher than the backside swirl, a result also
reported by Koenig et al. [18].
Because the thrust load has a direct impact of the thrust bearing temperature,
which can be measured conveniently, Figs. 3.51–3.53 establish the correlation
between nondimensional operating point (Fig. 3.51), thrust load at different
speeds (Fig. 3.52), the resulting bearing temperature of the loaded and unloaded
pads of the thrust bearing (Fig. 3.53), as well as the axial position of the rotor as
a result (Fig. 3.54). The inboard bearing shows a significant increase in temper-
ature (albeit not to a level that would cause concern) when the compressor
enters the choke region. The outboard bearing shows only a much lower
increase in temperature when the operating point moves toward surge. For this
particular application, with the particular selection of the balance piston size,