Page 157 - Compression Machinery for Oil and Gas

Integrally Geared Compressors Chapter 4 145

levels. Regardless of the type of motor, a variable frequency drive (VFD) can be
used to achieve compressor control via speed variation. Another important
driver family is turbines, especially steam turbines. The relatively high speed
of turbines must be adapted to the bull gear speeds of an IGC either by way
of an intermediate speed decreasing gearbox or by a dedicated gear pinion in
the IGC itself. The advantage of an integrated driver pinion is the reduction
of train elements and the compactness of the arrangement. This arrangement
is in use for IGCs with up to eight compressor stages. In addition, a compressor
stage can be mounted on the other end of the driver pinion.
The driver selection has considerable impact on the gear design since each
driver type exerts specific loads onto the gearbox, and this is reflected in the
recommendation for service factors for the different driver types. An induction
motor has the lowest requirements on the gearbox. The oscillating torque ampli-
tudes at start-up are relatively low, and the only other loads that have to be con-
sidered are electrical faults within the motor. Since these occur very
infrequently, they can be taken into account with a low number of load cycles,
which means they may not be critical for the design. A synchronous motor, by
contrast, excites the torsional natural frequency of the compressor train during
each run-up, which may lead to transient torque amplitudes in the train elements
several times higher than the rated torque. All train elements and the gear teeth
have to be designed to withstand these torques for the desired number of starts,
meaning that start-up is usually the critical design case for an IGC with a
synchronous motor driver. This effect can be mitigated by using a soft-starting
device. The use of a VFD, either for start-up or for normal operation, leads to
high frequency ripple superimposed on the steady-state torque. Torsional
natural frequencies of any shaft in the compressor system must either not coin-
cide with any of the excitation frequencies or be proven to be sufficiently
damped. Finally, a steam or gas turbine can produce steady-state torque which
is greater than the rated torque for a considerable period of time. This, too, has to
be considered in the gearbox design.

Aerodynamics
A typical stage in an IGC for oil and gas applications consists of an axial flow
inlet, IGVs (usually only the first stage), a radial flow impeller, a radial diffuser,
and an overhung volute. A wide range of fluids are seen in oil and gas applica-
tions. When sizing a compressor stage, both mechanical and aerodynamic lim-
itations must be met. The maximum impeller tip speed, U 2 , is typically used as a
preliminary mechanical guideline to ensure that stresses in the impeller do not
exceed the material limitation, and this also depends on the type of impeller. As
a rule of thumb, covered impellers are applied up to approximately 360m/s, and
open impellers are used up to 500m/s. While higher tip speeds are possible, they
are not common in oil and gas applications since the reliability and service life
,
decreases with an increasing tip speed. The machine Mach number, M U 2