Page 168 - Compression Machinery for Oil and Gas

156 SECTION II Types of Equipment

3.5
–
Ka y a y , a y –
3 – –
Ka y a y , a x
Normalized moment-angle stiffness 1.5 Ka x a y , a x Ka x a y , a y Ka x a x
2.5
2
–
–
–
–
–
1
0.5
–
0
Ka y a x
–0.5
–1
–0.2 –0.1 0 0.1 0.2
–
Normalized TC static misalignment angle a x , a y
–
FIG. 4.17 Influence of alignment on the moment angle stiffness that couples bull gear and pinion
dynamics. (From T. Cable, L. San Andres, K. Wygant, On the predicted effect of angular misalign-
ment on the performance of oil lubricated thrust collars in integrally geared compressors, ASME J.
Eng. Gas Turbines Power 139(4) (2016) 042503.)
Thrust collars also provide a mechanism that couples the bull gear and pin-
ion dynamics. The most obvious coupling is in the axial thrust within the fluid
film of the thrust collar. However, the angled thrust collar also provides a
moment coefficient that couples the lateral motion of the bull gear and pinion.
The moment dynamic coefficients are generated by the hydrodynamic oil film
in reaction to perturbations in the alignment conditions. Dynamic moment coef-
ficients are produced that couple the lateral motion of the bull gear and the pin-
ions. Fig. 4.17 shows how variations in the alignment may affect the moment
angle stiffness. Typically, the inertia of the bull gear is order-of-magnitude
greater than that of the pinion, meaning that bull gear motion is more apt to
be imparted on the pinion than the pinion onto the bull gear.

Casings
As opposed to inline compressors, IGCs require separate casings for each
impeller stage. Most prominently, the discharge of a radial compressor requires
a volume around the perimeter of the impeller to collect the discharge flow and
channel it to the discharge piping. This volume is commonly a scroll or volute,
which has improved flow characteristics compared to constant-area collectors.
Fig. 4.18 shows an image of an IGC with four pinions and with volute casings
for each stage. Note that the lower stages have cast volutes, while the last two
stages, which would be at significantly higher pressure, have bulkier fabricated
casings.