BY TALAL AL-RASHIDI & HAMAD K. AL-RUZIHI
Two
multistage 16,000 HP electrical motors are used to drive two onshore gas
compressors feeding a gas plant commissioned in 1984 (Figure 1).
Figure 1: Onshore compressor skid.
These compressors needed to produce
gas at a higher capacity. Unfortunately, the existing units were unable to meet
the new demand. One option considered was purchasing and installing an
additional compressor. Engineers also evaluated the possibility of utilizing
the available capacity of adjacent offshore
compressors. Although onshore and offshore gas compressors are
located on the same platform and discharge to a common header, they were
designed to process different
gas molecular weights due to separate feeds for onshore and offshore
units (Figure 2).
Figure 2: Compressor flow schematics.
The challenge was to ensure
that offshore
compressors could handle not only the new capacity but the new gas conditions. As
the plant receives gas from two associated crude oil fields that produce different
API oil grades, changing gas molecular weight is prevalent due to the number of online
producing wells. Understanding the effect
of changes in gas molecular weight can enhance performance prediction and
capacity control of centrifugal compressors.
The original 1980s
design assumed a molecular weight of 32 g/mole. But recent simulations and
forecasts predicted a 12-18% reduction. This simulation was validated through
test samples that showed a molecular weight of 27 g/mol. In addition, inlet
temperature and pressure have significant impact on centrifugal compressor
performance. The required system polytropic head and differential
pressure are indirectly proportional to molecular weight. Generally, centrifugal
compressors operating at higher molecular weight than design can deliver higher
actual volumetric flow rate at the new gas condition. But at a lower molecular
weight, more head needs to be developed. Therefore, the inlet flow rate would
need to be decreased, moving to the left on the Head-Flow curve, to satisfy
higher head requirements. In fact, varying centrifugal compressor operating conditions
result in different dynamic behavior
which in adverse cases become impossible to accommodate.
To assess existing compressors for the new conditions, a site performance test was conducted. It revealed that the onshore compressors could handle an additional 5% capacity. When added to offshore compressor capacity, this enabled the facility to meet the new requirements by expanding an existing jump-over line that connects the feed stream header.
MORE HEAD REQUIRED
During the conducted site
performance test, the inlet conditions differed
from the originally designed inlet parameters in terms of high pressure and low
temperature. This change had a positive impact on compressor capacity and
developed head. This is mainly attributed to the fact that onshore compressors
are operated within different
compression stages which allow for better control of any compression stage
inlet conditions in case of any compression stage poor performance. It is
evident that differential
pressure is not maintained due to increased gas rates. The more pipeline
compressors operating, the lower the required differential
pressure and the lower the system head pressure required from onshore
compressors. The compressor was operated at five different
flow rates but stopped before the choke region as the targeted flow was achieved and
to prevent any negative impact on the downstream compressor performance (Figure
3).
Figure 3: Performance test plot.
To estimate the impact of molecular weight changes in the available compressor
flow, a correlation formula was devised between flow element design conditions
and the measured gas conditions. Compensation or correction of flow measurements
are common pitfalls that can be major sources of errors in site tests (Figure
4).
Figure 4: Plot of onshore compressor capacity showing the corrected flow of the onshore compressor based on the corrected flow measurement correction and molecular weight changes
CONCLUSION
The current onshore compressors at an average molecular weight of 27 g/mole can
exceed the required new flow demand. Therefore, instead of an additional
compressor, the plant fully utilized existing compression capacity at adjacent
units. The new rated point per each compressor has conservatively 5% more
capacity available than the design point. Also, the motor has enough horsepower
to run at the new capacity and molecular weight.
Turbomachinery International
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