Below are excerpts from the paper “Comparison of different variable speed compression train configurations with respect to rotordynamic stability and torsional integrity” by Reto Somaini, Baumann Urs and Yves Bidaut of MAN Diesel and Turbo at the 41st Turbomachinery Symposium in Houston, Texas.
“In the past, the driver of choice for Oil & Gas upstream and midstream applications was the gas turbine. Especially when speed variation was necessary to meet the process requirements for a wide operating range and a large turn down. Gas turbine driven compressor trains can either be directly driven (high speed gas turbine without gearbox) or can consist of a low speed gas turbine driving the compressor via a speed increasing gearbox. Environmental restrictions like local pollution regulations and noise requirements as well as the complexity of the gas turbine system (infrastructure and certainly also maintenance aspects) have led to a steadily increased demand for electric (VFD) driven compressor trains. Concurrently also the variable frequency drives (VFD’s) for electric motors have evolved greatly with respect to available powers and reachable frequencies.
Finally, in the last ten years a completely new type of compressor for these applications has entered the market, the so called sealed, oil-free, compact compression systems, levitated by active magnetic bearings (AMB) and driven by high speed motors.
Compressor drive configurations
In configuration ‘A’ the compressor is driven via a flexible coupling by the gas turbine after gearing. The gearbox increases the speed from the turbine shaft at 9048 to 11270 rpm for the compressor and thus has a gear ratio of ~1.25. When the gear ratio is less than two and the train has to run in a large speed spectrum, the low and high speed ranges merge in on single extremely large prohibitive region. It becomes almost impossible to shift the torsional frequencies out by tuning the coupling stiffness.
In configuration ‘B’, a VFD with 50 Hz line frequency feeds the 1800 rpm synchronous motor. The motor drives the compressor via gearbox and two flexible couplings. The torque ratings of flexible couplings for motor driven compressors are substantially higher because they have to cope with the occurring air gap torques. Therefore, for high power and torque applications a lateral drive end overhung critical speed is expected in or near the operating speed range causing increased coupling unbalance sensitivity (an on site trim balancing of the coupling might be necessary). The gearbox brings the speed from 1797 to 11277 rpm.
In configuration C, with respect to the compressor lateral critical speeds this configuration is identical to configuration ‘A’. The torsional analysis of this arrangement is usually uncritical due to the nonexisting torque excitations of the gas turbine drive and the fact that the first torsional eigenfrequency can be tuned below minimum speed (soft coupling) while the second clearly remains above the speed range. Since there is no speed increasing gearbox necessary in this configuration even the possible excitations from this source cannot occur. The operation speed range extends from 5638 to 11840 rpm.
Configuration ‘D’ is one representative for the new generation of high speed motor driven, magnetically levitated compact compression systems. Even though the schematics for the configurations ‘C’ and ‘D’ look very similar the real systems are completely different. In this configuration each train component – motor and compressor – comes with their own pair of radial bearings (as opposed to the following configuration ‘E’) and the axial bearing is moved from the overhang position to the new location between the radial bearings.
Since the compressor is driven via a flexible coupling the issue of the drive end coupling overhung mode is also present for this configuration. The key of this configuration is the ability to damp the first bending mode critically allowing the operation of this train in the full speed range of 30-105 percent which is 3375 to 11813 rpm. With regard to the torsional analysis all the issues of VFD driven trains do also apply here.
Configuration ‘E’ is a second representative of the new generation of high speed motor driven, oil-free, hermetically sealed compressors. Here the motor and the compressor shafts are connected to each other by a rigid coupling, thus resulting in one single shaft component. As the compressor is driven by frequency converter non-integer harmonics can occur also for this configuration.
Conclusions
The direct gas turbine driven compressor – configuration ‘C’ – is good-natured regarding the train torsional behavior. The lateral analysis classifies this compressor between solutions ‘A’ and ‘B’ because of the coupling unbalance sensitivity. The stability behavior (basic LogDec δb and Q0) is equal for the three configurations ‘A’, ‘B’ and ‘C’. If a gas turbine drive is mandatory, configuration ‘C’ is the best in terms of simplicity, rotordynamics and footprint. Thanks to the fact that no gearbox is needed, even the efficiency increases (losses for a 7 MW gearbox are in the region of 150 kW, which is 2 percent).
Configuration ‘B’ has higher coupling unbalance sensitivity, has many torsional resonances, in particular the lowest mode (20 Hz) which is excited to a considerable stress level. Hence a torsional measurement is recommended, maybe leading to a small barred speed ranges. This configuration is clearly the most difficult to handle.
Configuration ‘A’ has neither the coupling sensitivity drawback nor the torsional complications as the driver is a gas turbine and is therefore rated as third best solution. If the view can be extended to sealed, magnetically levitated systems, it is arguable that configuration ‘C’ is slightly better rated than ‘E’. In this case the decision may be driven by other considerations, like auxiliary systems, gas turbine rotordynamics (contain risks too), ecological aspects and service requirements (e.g. ramp up times), which are not addressed by this paper.
The main advantages of configuration ‘E’ are the better coupling sensitivity and the promising stability key figures. When comparing the two design with magnetic bearing technology ‘D’ and ‘E’ the last configuration has better damping qualities leading to less required damping effort to accomplish the amplification factor requirements and a better overall stability behavior. The advantages of configuration ‘E’ in the torsional analysis are negligible and thus the trains design considered coequal for this figure. On the other side, favorable arguments for configuration ‘D’ would be a smaller alignment effort at manufacturer workshop and less critically damped speeds in the speed range, both a direct profit from the flexible coupling.
Generally, due to its compactness, configuration ‘E’ probably requires more manufacturing attention than the other compressor train arrangements. Indeed the controller design for a four bearing rotor is more complex than for a two bearing system however several running units of this type prove that the required knowhow is present in the Oil & Gas industry. While the conventional trains are available from several manufacturers and in any power class, the magnetically levitated, direct driven concepts are limited to 20 MW and few suppliers of motor, VFD, compressors and magnetic bearings.”
