All turbomachinery is subject to degradation that, over
time, will affect the systems efficiency and operational
performance. Precise monitoring of turbomachinery performance
with continuous torque-monitoring systems can be used to
identify gradual efficiency loss, allowing for the development
of a more focused maintenance scope to return the
system to its optimum operation and efficiency.
Torque monitoring based on heat balance, energy balance and
other methods utilizes numerous parameters such as pressure,
temperature, flowrate, gas composition, etc., which require
precise instrumentation to measure with low
uncertainty.1 However, phase displacement technology can be used to accurately
measure torque directly at the coupling to within 1% of
full-scale torque, a combination of all electrical and
mechanical sources of error. This accuracy provides the lowest
amount of uncertainty when computing efficiency, compared to
A torque-monitoring system was recently installed on a
cracked-gas compressor (CGC) train at Qenos Olefins in
Australia to determine the causes of a power limitation. The
torque-meter coupling utilizes phase displacement technology for long-term reliability, eliminating the need
Torque meter installation
The meter consists of two rings with pickup teeth installed
on a torsionally soft spacer and intermeshed at a central
location. Two monopole sensors 180° apart are mounted on
the coupling guard. As the coupling rotates, the ferromagnetic
teeth create an AC voltage waveform in the sensor coil, which
is digitally processed using known calibration parameters. Due
to the intermeshed pickup teeth, the system is referred to as a
single-channel phase displacement system, producing two
independent torque measurements (Fig. 1). The
system will output torque, power, speed and temperature, which
can be easily integrated with any DCS system (Fig.
Fig. 1. The torque-meter coupling
two independent torque signals.
2. Typical output from the torque-meter
At the olefins plant, the operating cycle of the steam-driven,
CGC train is 78 years. During this cycle, the plant
reaches production limitations because this compressor train
encounters a power limit. To determine the cause of the power
limit as turbine fouling or compressor
foulingor a combination of bothwas not
confidently possible with the instrumentation installed.
One option was investigated to add more power by upgrading
the turbine power rating from 7.5 MW to 9 MW. This required a
capital investment of $2 million. The plant elected to defer
this investment and, instead, a torque meter was installed
during the major eight-year shutdown.
The installation involved replacing the existing coupling
spacer and flexible halves with the drop-in torque
meters integral flexible elements. The torque meter
assembly was dynamically balanced to API standards, so it was
not necessary for the user to return any coupling components
for the retrofit. The coupling guard was modified so that the
two variable-reluctance sensors could be installed, completing
the mechanical installation (Figs.
3. Completed mechanical installation
at Qenos Olefins.
4. Torque-meter coupling retrofit
at Qenos Olefins plant.
5. Existing coupling arrangement
and retrofitted torque-meter coupling
On restarting the plant and having completed a number of
compressor efficiency improvements, the torque meter clearly
showed that the 7.5-MW turbine did not require an uprate and
that the major power losses were coming from the CGC. The
torque meter also allowed online tuning of the seal gas system
of the compressor to establish the lowest power draw from the
recycles that the seal system introduces. An additional 200 KW
of power was reduced from the turbine load, with the manual
adjustments made on the seal gas system.
The torque meter is now being used to monitor turbine
steam-fouling issues and process-related compressor fouling so
that corrective online washing can be activated as soon as
The historical data collected from the torque meter will
also provide a baseline of mechanical loading through the
drivetrain of the CGC over time. This data will be used to
determine if increases in the maximum continuous operating
speed rating of the compressor and the turbine can be
accomplished at minimal costs. This would achieve increases in
the operating envelope of the compressor.
Furthermore, the value of the torque meter justified the
installation of a second system for the olefins plants
second steam-cracking plant turbine/compressor train in October
1 Kurz, R., K. Brun and D. Legrand, Field
performance testing of gas turbine-driven centrifugal
compressors, Proceedings of the 28th Turbomachinery
Symposium, Turbomachinery Laboratory, Texas A&M University,
College Station, Texas, pp. 216220, 1999.