Page 68 - Compression Machinery for Oil and Gas

Centrifugal Compressors Chapter 3 59

FIG. 3.28 Typical pipeline operating points plotted on a typical centrifugal compressor
performance map.

opening a recycle valve. For this reason, virtually all modern compressor instal-
lations use a recycle line with a control valve that allows the flow through the
compressor to increase if it comes near the stability limit. The control system
constantly monitors the operating point of the compressor in relation to its surge
line, and automatically opens or closes the recycle valve if necessary. For most
applications, the operating mode with open, or partially open recycle valve is
only used for start-up and shutdown, or for brief periods during upset operating
conditions.
Of importance for compressor control is the fact that the operating map is
continuous, that is, there are no areas within the defined boundaries that are
off-limits for operation. The only exception is found in some electric motor-
driven trains, where the variable frequency drives (VFDs) may induce unac-
ceptably high torsional vibrations at certain speeds [4].

Variable Speed
Compressor drivers that can operate at variable speed (two-shaft gas turbines,
steam turbines, turbo expanders, and electric motors with VFDs or variable
speed gearboxes) allow the compressor to operate over a range of different
speeds. The faster the compressor runs, the more head and flow it generates,
and the more power it consumes. The efficiency characteristics of the compres-
sor are retained for different speeds, so this is a very efficient way of adjusting
the compressor to a wide range of different operating conditions. Figs. 3.28
and 3.29 show the resulting map.