As the operating temperatures of industrial systems and advanced
gas turbines continue to rise, designers are facing increasingly
more difficult challenges in implementing high-temperature structural
materials and seals to meet system performance goals. To maximize
efficiency, they are reducing seal purge flows to their practical
minimum and are requiring low-leakage seals to be made of temperature-resistant
superalloy and ceramic materials. Seals are being designed to
both seal and serve as compliant mounts, allowing for relative
thermal growths between high-temperature, but brittle, primary
structures and the surrounding support structures (see figure).
Under a cooperative agreement with a major supplier of industrial
gas products, the NASA Lewis Research Center has demonstrated
the feasibility of all-ceramic and hybrid rope seals for a tube
seal application. In this application, a seal is designed to serve
as a seal and a compliant mount, allowing relative thermal growth
between a high-temperature, low-expansion rate primary tube structure
and a higher expansion rate structural support, thereby preventing
excessive thermal strains and stresses. The all-ceramic seal consists
of a tightly packed ceramic core (alumina-silica) overbraided
with a ceramic sheath for low-leakage, low-scrubbing environments.
The hybrid seal, which consists of a tightly packed ceramic (alumina-silica)
core overbraided with a superalloy (cobalt base) wire sheath,
was tested as an abrasion-resistant alternative.
High-temperature flow and durability were measured for both the
all-ceramic and hybrid seals. Flow tests were performed in a unique
NASA Lewis high-temperature test rig capable of 1500 °F. Compression
tests in displacement control mode were used in conjunction with
pressure-sensitive film to determine seal contact pressures and
establish required groove-depths to set seal preload in the flow
tests. (Braided seals exhibit hysteresis (i.e., nonrecoverable
displacement) during loading and have a memory of previous loading
conditions.) The seal flows met the acceptable flow goal for the
design preloads. Furthermore, sheath damage was minimal over the
thermal cycles.
On the basis of these observations, which were made during Phase
I, the braided seal was deemed feasible for the industrial tube
seal application. As a result, NASA has entered into a Phase II
Space Act Agreement with our industrial customer to transfer the
seal technology to the actual industrial application. Lewis personnel
will be helping the industrial customer insert these seals into
their proprietary system. The industrial partner has expressed
interest in licensing the rope seal technology from NASA after
the seals have been qualified in the prototype system.
Steinetz, B.M., et al.: High Temperature Braided Rope Seals for
Static Sealing Applications. AIAA Paper 96-2910 (Also NASA TM-107233
Revised), 1996.
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