Hydrodynamic torque converters in oil and gas applications

Below are excerpts from the paper titled, 'Hydrodynamic Torque Converters for Oil & Gas Compression and Pumping Applications: Basic Principles, Performance Characteristics and Applications' presented by Klaus Brun, Joseph Thorp, Christoph Meyenberg and Rainer Kurz at the 44th Turbomachinery and 31st Pump Symposia Houston, Texas in September 2015.

In the oil and gas industry, many centrifugal compressors are driven by electric motors. This represents a special challenge since many electric motors have low startup torque capabilities. The problem of startup torque occurs when rigidly coupling an electric motor with a compressor or pump. This problem can be overcome when a soft coupling such as a hydrodynamic transmission is utilized to couple the output shaft of the motor and the input shaft of the compressor.

(Vorecon variable speed plan­etary gear can control the speed of pumps, compressors, and fans in the 1 – 50 MW power range.)

Hydrodynamic transmissions such as hydraulic couplings and torque converters are fluid couplings that typically use oil as a process liquid to transmit power from an output shaft (driver) pump to an input shaft (driven) turbine.

In the oil and gas industry, torque converters are often used as integrated components in drive transmissions for electric motor driven compressors or pumps trains. They provide step-less speed variation along with progressive torque increase towards low speed. The unique power transmission features of the torque converter make it a viable option for equipment that requires start-up torque assistance and speed control. Modern torque converters up to 65,000 kW have been designed and are widely in operation.

Below case study presents an application of torque converters at SATORP Saudi Aramco Total Refining and Petrochemical Company for the off-gas compressor drivers. In 2013, at Al Jubail Industrial City in the Kingdom of Saudi Arabia, the new 400,000 bpd grass-root refinery was opened. The refinery produces ultra-low sulfur diesel, gasoline, LPG, and jet fuels as well as Paraxylene, Benzene, and Propylene from heavy crude oil. The process requires coker gas, H2 rich gas, recycle gas, and off-gas compressors. All those compressors required to be variable speed controlled to maintain efficiency and provide flexibility in the complex refining process.

The off-gas compressor takes the heavy gas from the mild hydrocracker MHC and the distillate hydrocracker DHC as well as Hydrogen-rich gas for the continuous catalyst regeneration CCR. This causes significant changes in pressure requirements and gas properties like mole weight, compressibility, and isentropic exponent. For the drive system, this means speed variation in range down to 70% and constant or even rising torque requirements towards lower compressor speed.

Steam turbine drivers were not considered because of the expenses for large condensers and demineralized water/steam supply. Therefore, electric drivers came into focus. For speed control of compressors either variable frequency drives (VFDs) changing the frequency of the electrical power supply or variable speed drives (VSDs) changing the gear output speed are available. Especially the spread of the operation points of the off-gas compressors demonstrates the capabilities of the hydrodynamic VSD.

The hydrodynamic torque converter with its turbine-like characteristic meets these requirements without oversizing the equipment. As the motor runs all the time at rated speed, the upper power limit given by the electric motor is almost constant and only de-rated by efficiency decrease at part load.

A VFD-driven motor would show decreasing output power at the same ratio as the motor speed decreases because the design is limited by the torque capability of the shaft and the rotor windings. Due to the climatic conditions between dust storms from the desert and corrosive saline atmosphere, the use of VFD would also have required large air conditioned buildings. While the footprint of a VFD system is determined by all the components like isolation transformer, harmonic filter, VFD, step-up gear, separate lube oil system, and cooler, the hydrodynamic variable speed planetary gear integrates all these functions in a single compact unit.

The same considerations were applied to the components regarding reliability. As each component has a failure potential, the use of an almost wear-less hydrodynamic torque converter mitigates the risk of unplanned outage considerably. In the oil and gas industry, torque converters are often used as integrated components in drive transmissions for electric motor-driven compressors or pumps trains. They provide step-less speed variation along with progressive torque increase towards low speed.

The unique power transmission features of the torque converter make it a viable option for equipment that requires start-up torque assistance and speed control. Modern torque converters up to 65,000 kW have been designed and are widely in operation.

(Klaus Brun, Ph.D., is Program Director, Southwest Research Institute, San Antonio, Texas, USA. His experience includes positions in engineering, project management, and management at Solar Turbines, General Electric and Alstom.)

(Joseph Thorp is Engineering Consultant, Aramco Services, Houston, Texas, USA. He has served as an advisory committee member of Texas A&M's International Pump User's symposium and the Middle East Maintenance Conference. He is a registered Professional Engineer in the State of Texas.)

(Christoph Meyenberg is Head of Technical Sales, Voith Turbo, Crailsheim, Germany. Since 2006 he is employed at Voith Turbo as Head of Technical Sales Americas for variable speed drives.)

(Rainer Kurz, Ph.D., is Manager, Solar Turbines, Inc., San Diego, California, USA. He has authored numerous publications about turbomachinery related topics, is an ASME fellow, and a member of the Turbomachinery Symposium Advisory Committee.)