The Seal in Space
Georgia Tech-developed intelligent seal holds promise
Georgia Tech-developed intelligent seal holds promise
Mechanical engineering professor Richard Salant has developed an electronically controlled mechanical seal that promises a boost for the U.S. space program. By reducing onboard system weight, the new seal could increase the payload capacity of liquid-fueled rockets, space shuttles and other space vehicles.
To demonstrate the feasibility of using the new seal for
aerospace applications, Salant and graduate student Paul Wolff focused
on replacing conventional buffer gas seals. These seals contain the
buffer gas, helium, that separates the hot gases on the turbine side of
the turbopump from the oxygen on the pump side.
"Liquid-fueled space vehicles use a turbopump to pump liquid oxygen from the storage tank to the combustion chamber," says Salant. "The pump is driven by a turbine wheel instead of an electric motor. Some of the hot gases are bled off from the combustion chamber and sent back through the turbine. The gases spin the turbine wheel which turns the shaft and spins the impeller which pumps the liquid oxygen."
To prevent the hot exhaust gases from touching the liquid oxygen and setting off a dangerous explosion, an inert buffer gas is injected near the middle of the pump assembly. The pressurized buffer gas surrounds the shaft and fills the pump cavity, effectively isolating the turbine wheel.
Currently, floating ring seals are used in the buffer gas assembly so that the mating surfaces can never touch. Thus the probability of excessive wear or seal failure is extremely low. However, because of the relatively large clearances required, floating ring seals exhibit a high leakage rate, thus requiring more buffer gas.
An alternative is the traditional mechanical face seal which pushes one of the faces toward the other by means of springs.
The mechanical face seal has much less leakage than a floating ring seal, but also has a major disadvantage: mechanical contact can occur if operating conditions vary significantly. "Sustained contact produces mechanical and thermal damage to the faces, leading to excessive wear and reduced reliability," says Salant. "Therefore, in aerospace applications where high reliability and low wear are important requirements, floating ring seals are generally preferred."
Salant's electronically controlled seal combines the high reliability and low wear rate of floating ring seals with the low leakage rate of mechanical face seals. The seal operates by continuously monitoring and adjusting the spacing between the two faces for optimum performance.
Salant replaced one of the faces with a deformable face assembly that consists of a thin carbon face bonded to a piezoelectric crystal. When a voltage is applied, the crystal and carbon face interact, changing the pressures and distance between the two faces. "By adjusting the voltage, you can control the separation between the two faces," explains Salant.
"If operating conditions change such that face contact is imminent, the control system automatically increases the face separation," says Salant. "Similarly, if the control system senses that the leakage rate is excessive, the separation is reduced. The control system can respond quickly to rapid changes in the operating conditions."
During the five-year study -- funded by NASA's Lewis Research Center -- Salant and Wolff designed, built and tested the seal assembly. Based on their results, the researchers believe the controllable mechanical seal could be a viable low-leakage alternative to the floating ring seal for certain aerospace applications. "You can save weight by reducing the amount of helium required, and at the same time maintain safety and high reliability," says Salant.
Because this was a laboratory demonstration program, additional work is required before the seal can be installed in a space vehicle.
In addition to potential use in the space program, there are a number of industrial applications for the seal, as well.
"Intelligent seals could be used on feedwater pumps in factories, on coolant pumps in nuclear power plants, or as pipeline seals in unmanned pumping stations," says Salant. "The seals could be used wherever low maintenance, low leakage, and high reliability are required."
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