Vacuuming Out Power Losses
Renk’s etaX gear units utilize vacuum technology for power loss reductions
BY ROBERTA PRANDI
German gear manufacturer Renk recently celebrated its 100th TA73XT gear delivered for GE’s 6FA 100 MW gas turbine. The first Renk gear unit for a GE 6FA gas turbine was delivered in 1996 to the Sierra Pacific plant, while the first etaX gear to GE was delivered in 2002 to the Barranco power plant.
Toni Weiss, general manager of Renk’s Industrial Gear Division, said conventional high-speed gear units incur a power loss of 1 to 2 MW in the 10 MW-class turbines. “Power is lost during operation mainly in the form of heat,” said Weiss. “All components in the gear unit have a
temperature limit for the operation, and the cooling is achieved by oil.
Actually, in conventional gears the cooling requires much more oil than lubrication.” Oil inside a gearbox is moving and splashing, forming a high-density atmosphere, which causes windage losses. “Our etaX gears work with the principle of vacuum inside the gears, which allows them to significantly reduce the quantity of oil necessary for cooling. This, in turn, shrinks power losses, with a potential reduction of 50%,” said Weiss.
The etaX technology is a module composed of several elements that can be individually adopted according to the application requirements. The actual vacuum technology of the etaX gears, called etaX-VACUTECH, includes additional components for gas suction plus a vacuum seal. The etaX ECCO-bearings with ISOPRESS geometry contribute to advanced cooling of the bearings, said the company.
Among the additional elements to create vacuum inside the gears are a vacuum-suction pump mounted on the casing, valves for flooding prevention and a system for the detection of inner pressure. The third element comprising the etaX module is the etaX-CONTROL, designed for uninterrupted conversion to conventional operation in the event of failure with the vacuum
The etaX-CONTROL will automatically switch to normal operation in case of inoperability of the suction system, which will guarantee uninterrupted operation of the gas turbine, even if at the higher power loss rate given by conventional operation of the gears, said Renk. The danger detection and actuation of the safety devices are ensured by a redundant configuration of sensors and actuators. For installations that can sustain a shutdown of the system in case of inoperability or have a redundant VACUTEC system, the safety circuit is not necessary. In these cases, the oil system can be designed for a much lower quantity of oil.
The etaX technology holds the potential for retrofit applications. According to Renk, the revamping of existing Renk gears is both viable and economically attractive for the efficiency
gain, while at the same time maintaining the highest operational safety. Renk revamped gear units with etaX technology in a combined-cycle power plant in Finland.
Peter Boiger, head of sales and applications of Renk’s Industrial Gear Division, said the Vuosaari power plant in Helsingin Energia (HELEN) provides electricity and district heat to the city of Helsinki, Finland. The plant comprises two blocks, Vuosaari A and Vuosaari B — both operating two gas turbines.
The Vuosaari A block operates two 60 MW Siemens V64.3 gas turbines, which have both been working for 70 000 service hours so far. The two turbines were originally fitted with Renk conventional gear units with a rated power of 75 MW, an input speed of 5413 r/min and an output speed of 3000 r/min. An initial proposal was made to retrofit those gears with the etaX design to increase the efficiency of the plant, with a projected power loss reduction of 520 kW.
“You can imagine that with the flawless behavior of the two conventional gears, there was only an economic incentive to perform this revamping. Thus, a better efficiency rate within a very attractive timespan was the only consideration,” said Boiger. “On the other hand, a new technology could also risk the plant availability, which was also weighted by HELEN.”
The existing gear unit rotors were sent to Renk for reconstruction to replace the casing with a new one featuring the vacuum technology. The new gear units were also going to feature the redundant safety elements that would ensure uninterrupted operation of the gas turbine, in case of trouble with the vacuum function.
In the contract with HELEN, Renk guaranteed a reduction of power loss by 520 kW, and comprehensive measurements were taken to verify this parameter. “The power losses measured at 28, 40 and 57 MW proved to be below the value in the contract. Thus, HELEN
also decided for the reconstruction of the second gear unit,” said Boiger.
In the case of the two gas turbines of the Vuosaari A plant, the whole intervention resulted in fuel savings of 18 GWh per year, and the amortization period was calculated as less than two years.
Source: Diesel & Gas Turbine Worldwide