Page 142 - Compression Machinery for Oil and Gas

Centrifugal Compressors Chapter 3 129

FIG. 3.86 Gas compressor restage principles.

optimum levels, the compressor needs to be speedup by removing stages to
improve turbine efficiency. This type of restaging reduces engine fuel
consumption.

Compressor Choking Compressor choking is a phenomena happening often
due to trend of more and more demand on natural gas. The compressor keeps
running at the choke region to deliver more gas. The compressor efficiency is
low. Although the compressor may not be physically choked, the available
power can limit the capacity throughput due to the low efficiency. When this
happens for long time, larger flow stages should be used to replace smaller flow
stages. Fig. 3.86 shows a typical restage due to choke using larger flow stages to
change the performance map to better match the conditions. The flow capacity
is dramatically improved.

Gas Recycling Opposite the choke, when the compressor does not get enough
flow, the antisurge valve opens to avoid surge. Some amount of discharge flow
is cooled to feed back to the compressor. The energy of recycling gas is wasted.
Replacing higher flow stages with smaller stages is an effective way to accom-
modate the lower volumetric flows. After restage, the wasted power can be used
to increase pressure ratio for more capacity or more oil production.
Besides economic reasons, running in recycle mode causes high discharge
temperatures if insufficient cooling is supplied. DGSs, balance piston babbitt,
and antisurge valves can be damaged during long period of recycling.

Restage Criteria The economic study of payback period requires interaction
between the user and the OEMs. Four parameters stood out as good indicators
for restaging: inlet flow coefficient (Φ), isentropic head coefficient (Ψ), inlet
pressure (P 1 ), and the required power. The detailed criteria for each parameter
are shown in Table 3.3. Generally, for power, suction pressure, and head