Voith Redesigned Variable Speed Vorecon : After almost 25 years in oil and gas and power generation applications worldwide, Voith Turbo redesigned its Vorecon variable speed planetary gear by roberta prandi
Vorecon, the variable speed planetary gear by Voith Turbo, has been a longstanding player in the oil and gas and power generation fields worldwide. The company accounts for 300 units in operation since the product came to market in 1986, almost twothirds of which are in oil and gas applications with the remaining mostly in power plants.
At its very beginning, Vorecon had been designed for boiler feed pump applications, but soon moved into the realm of gas and pipeline compressors, due to its output range, from 1000 to 50 000 kW and up to 20 000 r/min. After almost 25 years of service, Vorecon has recently undergone a restyling, which has focused mainly on reduced complexity and a more service-friendly design.
The principle at the basis of Vorecon is a power-split concept: the system is a combination of a hydrodynamic component and a planetary gear, with superimposed couplings that control the speed variations. Most of the power is mechanically transmitted via the main shaft, and only smaller portion is deviated via the hydrodynamic torque converter and superimposed into the planetary gear to increase or decrease output speed.
Christoph Meyenberg, sales manager Americas and Africa at Voith Turbo in Crailsheim, Germany, said most of the power is transmitted mechanically, allowing the efficiency of the system to surpass 95%. Regarding power loss, a variable speed gear like Vorecon competes with the variable frequency drives (VFD) available on the market, said Voith. “Taking the whole system into analysis, we can determine that Vorecon is more efficient than a VFD-based drive system,” said Meyenberg. “The VFD system must account for several losses along the way, so to speak. Transformer, VFD, filter, all cause power losses to add to the driver and gear losses. “In the case of Vorecon, the only losses are from the driver and the Vorecon itself. So, taking the whole system into account, we can safely say that power losses are lower than in a VFD-based system and the efficiency is higher,” said Meyenberg. “The same applies to the size of the whole system — it is quite obvious that a system using a variable speed gear takes up quite less space than a VFD, which is composed of several more components. We estimate that Vorecon needs about one-third of the space of an average VFD system. Building costs and installation are consequently very competitive too.”
In this respect, the new Vorecon has introduced two innovations. First, the Vorecon has moved from a “tunnel” design to a horizontally split housing. “The upper part of the casing can be lifted in two pieces from the bottom, which requires much less lifting height, less crane capacity and allows faster access to the rotor set of the unit,” said Meyenberg.
The second innovation also relates to ease of maintenance. The concentric design of the gear has been replaced by two symmetrical parallel shafts. This allows direct access to the revolving planetary gear and eliminates the heavy-weight rotating sleeve, said the company. “The new solution is even more compact and offers easier access. All rotating parts can be accessed by opening up the upper part of the housing, and the removal of the housing ‘cap’ needs a smaller crane capacity and height,” added Meyenberg.
In addition, the new design grouped all outside connections in the bottom half of the housing all oil connections on the left side and all instrument connections on the right side, viewed from the motor. The new Vorecon is available in two models: type RWE, the most compact, for a speed-control range from 60 to 100% and type RWC, which includes two hydrodynamic couplings and allows for unloaded motor start for a speed control range from 60 to 100%. For the full speed range from 10 to 100% and a fully disengaged motor start a typical requirement for pumps and fans the type RW of the earlier design is still available, which features an integral lock-up clutch to the hydrodynamic variable speed coupling. The first application of the new Vorecon has been with a large U.S. pipeline company for compressors in four stations in Florida. One unit has already been delivered in the configuration with integrated oil circuit in the base frame, and four more will follow.
The compressors are driven by synchronous motors with a rated power of about 16 000 kW and a rotation speed of 1800 r/min. Of the five compressors to be driven, two have 12 000 r/min maximum continuous speed, while three have a speed of 8000 r/min. All of the compressors can be controlled down to approximately 60% speed.
Meyenberg also hinted that Voith Turbo is working on an additional version of the Vorecon variable speed planetary gear, featuring a dual torque converter, reducing losses by half in the lower part-load range. The project is still in the development phase.