Page 79 - Compression Machinery for Oil and Gas

Centrifugal Compressors Chapter 3 69

of springs (representing stiffness) and dampers (representing damping). Seal
rotordynamic characteristics are impacted by a number of parameters, including
seal type, geometry, location, and gas conditions. Labyrinth hub seals generally
have a small pressure differential and can generally be neglected from a rotor-
dynamic analysis. Impeller shroud or eye seals are generally labyrinth seals, and
impact rotordynamic stability. Balance piston or division-wall seals generally
have the largest rotordynamic impact because of the large pressure differential
and long sealing length. For centrifugal compressors, the balance piston or divi-
sion wall seal can be a labyrinth or a honeycomb/hole-pattern seal.
The destabilizing effect of annular seals is largely a function of the swirl or
bulk circumferential velocity of the flow entering the seal. Centrifugal impellers
by design impose high circumferential swirl at their exit. The flow field in this
region communicates with the secondary passages feeding swirl to the various
annular seals throughout the machine. Fluid swirl in the direction of rotation is
the primary destabilizing mechanism in annular seals. This circumferential flow
entering the seal can be reduced with de-swirl methods, including swirl brakes
or shunt injection [9]. Swirl brakes (or antiswirl vanes) include geometry, typ-
ically consisting of radial vanes oriented in the axial direction to impede cir-
cumferential flow entering the seals. Swirl brakes have little effect on
aerodynamic efficiency and can be designed integral to the seal material; how-
ever, they can increase leakage by reducing the swirl in secondary flow cavities.
Swirl brakes can be applied to interstage and balance piston seals, as well as
shroud cavities. Shunt injection, in contrast, bleeds off higher pressure gas
and feeds a reverse swirl annulus near the entrance to the seal, hindering the
swirling flow. Shunt injection is limited to the balance piston seal and increases
the secondary leakage, thus lowering compressor efficiency. Successfully des-
wirling the inlet flow turns annular seals into stabilizing components, increasing
the system log dec values. Labyrinth seal rotordynamic coefficients can be cal-
culated via bulk flow methods (such as assuming a single control volume
between labyrinth teeth) or with CFDs methods. It is noted that the bulk flow
seal calculations are the most common for centrifugal compressors compared to
CFD analyses.
Damper seals such as honeycomb or hole-pattern seals can be used as the
balance piston or division-wall seals to provide significant stiffness and damp-
ing to improve rotor stability [10]. These seals provide substantial damping
while generally sacrificing leakage compared to traditional labyrinth seals.
Damping increases with increasing seal length as well as increasing pressure
differential. The rotordynamic coefficients of honeycomb or hole-pattern seals
are frequency dependent and should be properly accounted for in the stability
analysis. In addition to notable damping, the direct stiffness of damper seals can
become large enough to influence critical speeds. In addition, the stabilizing
effect of honeycomb or hole-pattern seals is significantly influenced by the seal
clearance taper (inlet seal clearance relative to exit clearance). While more
damping can be achieved with a slightly diverging taper compared to a