Rotary machine shaft seal

In a rotary machine that includes a housing, a shaft rotatably extending from said housing, an opening in said housing surrounding said shaft and coaxially spaced therefrom, a fluid passage extending from said opening to externally of said housing, and a shaft seal within said opening for preventing leakage of fluid along said shaft, the improvement wherein said shaft seal comprises:

• an annular shell of rigid construction separate from said housing and removably received within said housing opening surrounding said shaft,
• first and second annular sealing means mounted within said shell spaced from each other along said shaft and extending radially inwardly from said shell for self-biased rotary sealing engagement with said shaft independently of each other, and
• pressure relief valve means comprising at least one opening extending radially through said shell between said annular sealing means and an elastic band resiliently surrounding said shell externally of said at least one opening such that pressure of fluid between said sealing means urges said band away from said shell to permit flow of such fluid through said at least one opening into said housing opening and thence through said passage.