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A seal assembly forms a barrier between a compressor's interior and exterior regions. The seals assembly includes a primary seal stage and a secondary seal stage. The primary seal stage is formed of materials chosen to effectively block flow of gas through the seal assembly. The secondary seal stage

A seal assembly forms a barrier between a compressor's interior and exterior regions. The seals assembly includes a primary seal stage and a secondary seal stage. The primary seal stage is formed of materials chosen to effectively block flow of gas through the seal assembly. The secondary seal stage is formed of materials chosen to survive a failure of the primary seal stage.

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1. A seal assembly comprising: a primary seal stage including a primary ring arranged to be coupled to a housing and a mating ring arranged to be coupled to a rotating shaft, the mating ring formed of a brittle material; anda secondary seal stage coaxially positioned with respect to the primary seal

1. A seal assembly comprising: a primary seal stage including a primary ring arranged to be coupled to a housing and a mating ring arranged to be coupled to a rotating shaft, the mating ring formed of a brittle material; anda secondary seal stage coaxially positioned with respect to the primary seal stage and including a primary ring arranged to be coupled to the housing and a mating ring arranged to be coupled to the rotating shaft, the primary ring and the mating ring each formed of a ductile material:wherein, the brittle material fractures at a lower strain level than the strain level at which the ductile material fractures. 2. The seal assembly of claim 1, wherein the primary seal stage further includes a retainer arranged to be coupled to the housing, wherein the primary ring of the primary seal stage is disposed in an opening of the retainer of the primary seal stage, wherein the secondary seal stage further includes a retainer arranged to be coupled to the housing, and wherein the primary ring of the secondary seal stage is disposed in an opening of the retainer of the secondary seal stage. 3. The seal assembly of claim 1, further comprising a sleeve arranged to be coupled to the rotating shaft, wherein the mating ring of the primary seal stage is disposed in a first opening of the sleeve, and wherein the mating ring of the secondary seal stage is disposed in a second opening of the sleeve. 4. The seal assembly of claim 1, wherein the primary seal stage further includes a spring arranged to bias the opposed faces of the primary ring and the mating ring of the primary seal stage together, and wherein the secondary seal stage further includes a spring arrange to bias the opposed faces of the primary ring and the mating ring of the secondary seal stage together. 5. The seal assembly of claim 1, wherein the brittle material is silicon carbide. 6. The seal assembly of claim 1, wherein the brittle material is tungsten carbide. 7. The seal assembly of claim 1, wherein the ductile material is steel. 8. The seal assembly of claim 1, wherein the ductile material is able to be strained by at least five percent without fracturing. 9. The seal assembly of claim 1, wherein the primary ring and the mating ring of the secondary seal stage are strengthened by a surface treatment. 10. A compressor, comprising: a housing including a suction port and a discharge port;a shaft penetrating the housing;a centrifugal impellor fixed to the shaft and arranged for compressing a gas from the suction port to the discharge port;a seal assembly circumscribing the shaft near a location where the shaft penetrates the housing, the seal assembly comprising: a primary seal stage including a primary ring coupled to the housing and a mating ring coupled to the shaft, the mating ring formed of a brittle material; anda secondary seal stage coaxially positioned outside the primary seal stage and including a primary ring coupled to the housing and a mating ring coupled to the rotating shaft, the primary ring and the mating ring each formed of a ductile material;wherein, the brittle material fractures at a lower strain level than the strain level at which the ductile material fractures. 11. The compressor of claim 10, wherein the secondary seal stage is arranged for temporary operation. 12. The compressor of claim 10, wherein the ductile material is able to be strained by at least five percent without fracturing. 13. The compressor of claim 10, further comprising a second seal assembly circumscribing the shaft near a second location where the shaft penetrates the housing, the second seal assembly comprising: a primary seal stage including a primary ring coupled to the housing and a mating ring coupled to the shaft, the mating ring formed of a brittle material; and a secondary seal stage coaxially positioned with respect to the primary seal stage and including a primary ring coupled to the housing and a mating ring coupled to the rotating shaft, the mating ring formed of a ductile material;wherein, the brittle material fractures at a lower strain level than the strain level at which the ductile material fractures. 14. A seal assembly comprising: a primary seal stage including a primary ring arranged to be coupled to a housing and a mating ring arranged to be coupled to a rotating shaft; anda secondary seal stage coaxially positioned with respect to the primary seal stage and including a primary ring arranged to be coupled to the housing and a mating ring arranged to be coupled to the rotating shaft,wherein the primary ring and the mating ring of the primary seal stage are formed of materials chosen to effectively block flow of gas through the seal assembly, andwherein the primary ring and the mating ring of the secondary seal stage are formed of materials chosen to fracture at a higher strain level than the primary ring and the mating ring of the primary seal stage to survive a failure of the primary seal stage. 15. The seal assembly of claim 14, wherein the materials of the primary seal stage include a brittle material, and wherein the materials of the secondary seal stage include a ductile material, wherein the brittle material fractures at a strain less than five percent strain and the ductile material is strained at least five percent without fracturing. 16. The seal assembly of claim 14, wherein the mating ring of the primary seal stage is formed of a brittle material, and wherein the mating ring of the secondary seal stage is formed of a ductile material, wherein the ductile material is strained at least five percent without fracturing. 17. The seal assembly of claim 16, wherein the primary ring of the primary seal stage is formed of a brittle material, wherein the brittle material fractures at a strain less than five percent strain. 18. The seal assembly of claim 16, wherein the primary ring of the primary seal stage is formed of a carbon material. 19. The seal assembly of claim 16, wherein the primary ring of the secondary seal stage is formed of a carbon material. 20. The seal assembly of claim 16, wherein the primary ring of the secondary seal stage is formed of a ductile material, wherein the ductile material is strained at least five percent without fracturing.