Hydrocarbon Processing - May 2021 - 35

Special Focus

Maintenance and Reliability
M. RAMALINGAM and B. CHANDRAGUPTHAN,
Wood Plc., Chennai, India

Restaging/rerating of centrifugal compressors:
Fundamentals, practices and challenges
The primary requirement for rerating or revamping an existing centrifugal compressor is to match the new operating
requirements, such as increased or decreased flow, increased
or decreased head, or a combination of both. Rerating or revamping is necessary where the existing compressor or compressor train is incapable of meeting the new operating requirement efficiently.
The rerating/restaging of operational existing compressors
is an attractive cost-effective solution to address the latest development in process, changes in gas composition, debottlenecking of the existing plants and production maximization.
Successful rerating/restaging plays a significant role in capital
cost optimization processes.
Brief descriptions of fundamental concepts, practices, challenges and process checklists involved in the process of rerating/restaging of centrifugal compressors are discussed here.
Thermodynamic analysis. Centrifugal compressors are

sensitive to operating conditions, molecular weight, gas properties, etc. However, the new operating environment may call
for different operating scenarios, different molecular weights
and different gas properties, as described in the subsequent
paragraphs. During the restaging study, the adequacy of an
existing compressor system will be analyzed and the requirements for modification or opportunities for improvement will
be identified.
In centrifugal compressors, a decrease in inlet pressure will
shift the operating envelope toward a lower flowrate, and the
surge margin will decrease in proportion with the decrease
in suction pressure. The compressibility factor (Z) also increases as the suction pressure/inlet pressure decreases, as the
reduced pressure decreases while the reduced temperature remains constant. A decrease in inlet pressure decreases the gas
density, which may result in higher power consumption.
Lower suction pressure results in a lower Reynolds number, influencing the boundary layer and friction loss. At a
lower Reynolds number, friction loss will be higher, resulting
in lower performance. As a result, a small fall in the inlet flowrate due to higher pressure at the inlet piping/nozzle often
requires reduced rotational speed. On the other hand, higher
suction pressure will result in higher discharge pressure at a
given speed. As a result of a change in composition, the compressor operating envelope will change and consequently calls
for revised operating procedures.

Temperature. An increase in the inlet/suction temperature

will result in an increased discharge temperature; however, the
influence of suction temperature is important when approaching the stonewall point (≥ mach flow). When the suction temperature is high at choke flow, the pressure ratio is reduced.
The molar heat capacity has non-linear propositional relation
with temperature, and an increase in temperature will result in
a lower specific heat ratio. Additionally, higher inlet temperature will result in higher compressibility and lower gas density.
Lower suction pressure results in a lower Reynolds number
because of lower density and influences on the induced boundary layer and frictional loss in the flowing path, leading to lower
overall efficiency. When suction pressures increase, discharge
pressure will increase and must ensure the sufficient flow/
adjust speed to avoid surge. TABLE 1 summarizes the impact of
lower suction pressure and composition change.
The high specific heat ratio will result in higher discharge
temperature and slightly lower discharge pressure at the same
pressure ratio. On the other hand, lower specific ratio gases
consume more specific power for a given pressure ratio.
Polytropic head will increase along with specific heat ratio.
The head is increasing proportionally with the volume polytropic exponent, which in turn increases as the specific heat ratio goes up. However, the influence of the pressure ratio on the
head value is less significant compared with volume polytropic
exponent effect. The impact of gas heat ratio on the compressor head becomes more pronounced at high rotational speed.
Operating range decreases as the specific heats ratio of the
handled gas reduces.
Polytropic head is influenced by gas constant (R), which decreases as molecular weight increases. Compressor efficiency is
also affected by gas constant. The high molecular weight value
leads to a reduction in the gas constant, yielding a lower volume
polytropic exponent, which is inversely related to the stage efficiency. The required polytropic head of low molecular weight
tends to increase as the flowrate reduces. This indicates higher
frictional losses associated with low gas density and viscosity.
The effect of gas molar mass on the compressor head
causes the required number of mechanical stages to vary. The
low molecular weight gas raises the required head to achieve
the desired discharge pressure, leading to greater pressure coefficient on the impeller blades.
During rerating, the probability of formation of deposits/
hydrates on the impeller and diffuser surface of the compressor
Hydrocarbon Processing | MAY 2021

Hydrocarbon Processing - May 2021

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