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HYDROCARBON
ENGINEERING
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of these machines are mismatches of the head pressure
required for each section or stage and large swings in flowrates
(between design and off-design performance).
Refrigeration compressors differ from other process
compressors in that the head across the entire compressor
remains nearly constant at all flowrates. The discharge pressure
is set by the condensing temperature of the refrigerant
regardless of flow. The inlet pressure is directly related to the
required duty temperature of the evaporators. These
properties play an important role in dealing with centrifugal
compressors in refrigeration services in off-design cases.
As the duties change, the mass flowrate from the
evaporator(s) into the compressor changes, and thus the
operating point on the head vs volumetric flowrate
compressor curve also changes. The compressor designer
makes design decisions to best meet the customer’s
requirements. Early collaboration between compressor
designer, end user, and process designer allows for a better
understanding of the long-term operation of the equipment
and the design goals.
Challenges of refrigeration
compressors
The refrigeration process enhances its efficiency by
recuperation from the demethaniser reboilers and the ethylene
fractionator. These energy recovery processes can sometimes
reverse inter-stage flows in the refrigeration compressor,
causing a surge event in the refrigeration compressor. When
the re-cycle valve opens, the inlet gas temperature can get too
high because hot gas from the compressor discharge is sent
back to the compressor inlet. Compression of this hot gas
raises its temperature even more, causing the compressor to
shut down.
To prevent the discharge temperature from getting too
high and triggering a shutdown, a cool vapour stream must be
added. To address this challenge, hot compressed vapours
from refrigerant compressor discharge are used to vaporise
liquid refrigerant in a manner similar to how process stream is
used to vaporise refrigerant in the chillers. Ideally, if the
compressor anti-surge vapour flow and temperature control
valve on quench liquid are perfectly matched and there is no
time lag within the control system, the configuration will work
perfectly.
Unfortunately, if there is any mismatch, which in a
real-world situation always exists, either too much quench
liquid is added to the inlet stream (whereby there is high liquid
level in the suction drums) or not enough quench liquid is
added (whereby the discharge temperature keeps rising). In
either mismatch case, the refrigerant compressor will shut
down from high liquid level in the suction drum or from high
discharge temperature.
Gas impurities impose additional considerations for the
proper operation of refrigeration compressors. The purity of
the propylene refrigeration cooling has a significant impact on
the ability of the refrigeration system to work properly. If the
propylene refrigerant has a 2% impurity of ethylene, then the
compressor discharge pressure needs to be 13 psi (88 Kpa) or
higher. This would require a higher discharge pressure, higher
compressor speed, and hence higher horsepower. To handle a
2% – 4% variation of gas impurities, the compressor control
must be unaffected by a reasonable change in the molecular
weight of the gas.
Final control variable
The final control variable is the calculated surge process variable
that is a predetermined distance from surge line. This variable
should be independent of any factors related to gas properties
such as the gas’ molecular weight, the polytropic exponent.
Also, the compressor’s control should have a decoupling option
to remove possible unstable interactions between the stages of
a multistage compressor. Finally, the quench control function
should allow a variable set point based upon the dew point
curve of propane. This enhances the efficiency of propane
compressors by avoiding excess quenching and therefore lessens
the chances of compressor trips while allowing faster start-ups.
Control solutions
In addition to the specific control system recommendations
outlined, there are other fundamental control system
characteristics that enhance the operation and performance of
the plant’s ethylene compressor and help it to stay up and fully
operational. Some of these characteristics include:
Advanced control algorithms – including a ‘rate’ control
PID (proportional, integral, and derivative) that anticipates
impending surge events and takes evasive action to
prevent the occurrences.
Faster flow signal filtering, such as a 4-point filter which is
equivalent to a 20 millisecond sampling rate, to provide
more precise data for the control algorithm to use in its
calculations.
Control algorithms to ‘break’ surge cycles, such as a ‘surge
minimum position’ lock, so that frequent surges due to
upsets can be avoided.
Model-based simulation testing to verify system operation
and performance before installing on actual compressor.
Fast scan rate of signals with 40 millisecond scan rate,
coupled with fast response anti-surge valves to help avoid
surge events altogether.
Temperature control through quenching using dew-point
curve instead of simple temperature PID loop to mitigate
excess quenching.
Conclusion
It is no overstatement to say that how a compressor controller
monitors and manages the multiple process variables and
responds to ever changing process conditions contributes to
the successful operation of ethylene plant compressors. While
these compressors are the workhorses of the process, they are
also very challenging to control due to changing process
variations and loop times, coupled stage-to-stage interactions,
and long-term compressor stage fouling. Since any of these
process conditions can force the compressor towards its surge
line it is the challenge of the compressor controller to maintain
process flow, temperature, and pressures whilst protecting
each compressor stage from experiencing a minor or major
surge event. Designed to control and protect critical process
compressors, Woodward’s MicroNet compressor control uses
field proven algorithms to juggle all process control and
protection functions – ensuring stable operation in all the
process phases.