The figure presents a simplified view of a shaft seal that provides a small flow of neutral buffer gas to prevent contact between different gases contained in two adjacent regions along the shaft. In the original application, the shaft couples a hydrogen-burning turbine with a high-pressure oxygen pump, and helium is used as the buffer gas to prevent hydrogen/oxygen contact along the shaft. Other potential applications could involve the use of helium or other buffer gases or liquids to prevent mixing of fluids or to prevent escape of process fluids that are poisonous, odorous, or harmful to equipment or to the environment.
The seal (see figure) is a combination of a primary and a secondary seal. The main structural component of the seal is a solid carbon ring with a T-shaped meridional cross section. The buffer gas flows from a plenum, through 36 equally circumferentially spaced narrow radial holes in the ring, to the clearance [a gap 2 to 3 mils (0.05 to 0.08 mm) thick] between the ring and the shaft. Upon reaching the shaft, the flow splits into two smaller flows in opposite directions along the shaft. The combination of the clearance and the split flow of buffer gas constitutes the primary seal.
The secondary seal is a pressure-assisted, fluid-film seal between T ring and a housing that bounds the plenum. This housing is shaped approximately as a hollow ring of rectangular meridional cross section. The fluid-film seal between the housing and the T ring is effected by two tapered land seals. The pressure of the buffer gas in the plenum pushes these seals toward the housing, forming a low-friction clearance with the T ring. The tapered land seals are also lightly spring-loaded to hold them in place before gas pressure is applied. The secondary seal is a low-friction seal that tolerates sliding between the T ring and the housing. Thus, the T ring is free to track the runout (if any) of the shaft.
This work was done by Wilbur Shapiro and Henry F. Jones, Jr., of Wilbur Shapiro& Associates, Inc., for Lewis Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Mechanics category, or circle no. 176 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).
Inquiries concerning rights for the commercial use of this invention should be addressed to
NASA Lewis Research Center
Commercial Technology Office
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21000 Brookpark Road
Refer to LEW-16339.