Pressure-balanced carbon seal

1. A seal segment comprising an arcuate body having:

a first end;

a second end circumferentially opposite the first end;

a first face;

a second face axially opposite the first face;

an inner diameter (ID) face; and

an outer diameter (OD) face,

wherein:

the seal segment is shaped to interfit with a plurality of identical seal segments first end to second end to form a seal surrounding a central longitudinal axis;

the first face has:

a circumferential channel closer to the ID face than the OD face; and

a plurality of channels extending from the circumferential channel to the OD face;

the ID face has:

a circumferential channel closer to the first face than the second face; and

a plurality of channels extending from the circumferential channel to the second face; and

the ID face circumferential channel has an open end at the first end.

2. The seal segment of claim 1 wherein:

the body is a majority by weight carbon.

3. The seal segment of claim 1 wherein:

the first end has a circumferential protrusion at the first face and ID face;

the second end has a circumferential rebate at the first face and ID face; and

the interfitting first end to second end is of the protrusion of each segment in the rebate of an adjacent segment.

4. The seal segment of claim 1 wherein:

the first face circumferential channel has a closed end proximate the first end.

5. The seal segment of claim 1 wherein:

the first face circumferential channel has a depth H 1 ; and

the ID face circumferential channel has a depth H 2 120% to 500% of the first face circumferential channel depth H 1 .

6. The seal segment of claim 1 wherein:

the first face circumferential channel has a width W 1 ; and

the ID face circumferential channel has a width W 2 50% to 150% of the first face circumferential channel width W 1 .

7. The seal segment of claim 1 wherein:

the first face has a dam radially inward of the first face circumferential channel and having a radial span S 1 ; and

the ID face has a dam axially between the ID face circumferential channel and the first face and having an axial span S 2 of 20% to 60% of S 1 .

8. The seal segment of claim 7 wherein:

the first face circumferential channel has a depth H 1 ;

the ID face circumferential channel has a depth H 2 120% to 500% of the first face circumferential channel depth H 1 ; and

the ID face dam span S 2 is greater than the first face circumferential channel depth H 1 .

9. A turbomachine comprising:

a bearing in a bearing compartment;

a buffer chamber; and

a seal system isolating the bearing compartment from the buffer chamber and comprising:

a seal runner;

a segmented seal encircling the seal runner comprising a plurality of seal segments according to claim 1 interfitting first end to second end;

a garter spring holding the seal segments together;

a seal housing carrying the seal and having a first wall and a second wall; and

at least one spring biasing the seal segment first faces against the housing first wall,

wherein:

for each seal segment, a buffer flowpath from the buffer chamber to the bearing compartment extends from the second face, through the ID face plurality of channels to the ID face circumferential channel and out the ID face circumferential channel open end.

10. The turbomachine of claim 9 wherein:

the garter spring holds the seal in contact with a seal runner.

11. The turbomachine of claim 10 wherein:

the plurality of seal segments is three to twelve segments.

12. The turbomachine of claim 9 wherein:

the seal is a non-archbound seal.

13. The turbomachine of claim 9 wherein:

the at least one spring is a plurality of coil springs.

14. The turbomachine of claim 9 wherein:

for each seal segment:

the first end has a circumferential protrusion at the first face and ID face; and

the second end has a circumferential rebate at the first face and ID face;

the interfitting first end to second end is of the protrusion of each segment in the rebate of an adjacent segment;

the open end is at the first end; and

the interfitting leaves a gap between an end of each protrusion and a base of each rebate.

15. A method for using the turbomachine of claim 9 , the method comprising:

driving rotation of a shaft carrying the runner; and

passing an airflow along the buffer flowpath from the buffer chamber to the bearing compartment.

16. The method of claim 15 further comprising:

directing another airflow for each seal segment from the OD face, through the first face plurality of channels to the first face circumferential channel to pass as a leakage flow over a first face dam radially inward of the first face circumferential channel.

17. The method of claim 16 wherein:

the airflow has a first flow rate; and

the another airflow has a second flow rate less than half of the first flow rate.

18. The method of claim 15 wherein:

the rotation is in an angular direction from the ID face circumferential channel out the ID face circumferential channel open end for the segments.

19. The method of claim 18 wherein:

the rotation is effective to drive debris from the ID face circumferential channel.

20. A method for using a turbomachine, the turbomachine comprising:

a bearing in a bearing compartment;

a buffer chamber; and

a seal system isolating the bearing compartment from the buffer chamber and comprising:

a seal runner;

a segmented seal encircling the seal runner comprising a plurality of seal segments each seal segment comprising an arcuate body having:

a first end;

a second end circumferentially opposite the first end;

a first face;

a second face axially opposite the first face;

an inner diameter (ID) face; and

an outer diameter (OD) face, wherein:

the seal segment is shaped to interfit with a plurality of identical seal segments first end to second end to form a seal surrounding a central longitudinal axis;

the first face has:

a circumferential channel closer to the ID face than the OD face; and

a plurality of channels extending from the circumferential channel to the OD face;

the ID face has:

a circumferential channel closer to the first face than the second face; and

a plurality of channels extending from the circumferential channel to the second face; and

the ID face circumferential channel has an open end; and

the plurality of segments interfit first end to second end;

a garter spring holding the seal segments together;

a seal housing carrying the seal and having a first wall and a second wall; and

at least one spring biasing the seal segment first faces against the housing first wall,

wherein:

for each seal segment, a buffer flowpath from the buffer chamber to the bearing compartment extends from the second face, through the ID face plurality of channels to the ID face circumferential channel and out the ID face circumferential channel open end,

the method comprising:

driving rotation of a shaft carrying the runner; and

passing an airflow along the buffer flowpath from the buffer chamber to the bearing compartment,

wherein:

the rotation is in an angular direction from the ID face circumferential channel out the ID face circumferential channel open end for the segments; and

the rotation is effective to drive debris from the ID face circumferential channel.

21. A method for using a turbomachine, the turbomachine comprising:

a bearing in a bearing compartment;

a buffer chamber; and

a seal system isolating the bearing compartment from the buffer chamber and comprising:

a seal runner;

a segmented seal encircling the seal runner comprising a plurality of seal segments each seal segment comprising an arcuate body having:

a first end;

a second end circumferentially opposite the first end;

a first face;

a second face axially opposite the first face;

an inner diameter (ID) face; and

an outer diameter (OD) face, wherein:

the seal segment is shaped to interfit with a plurality of identical seal segments first end to second end to form a seal surrounding a central longitudinal axis;

the first face has:

a circumferential channel closer to the ID face than the OD face; and

a plurality of channels extending from the circumferential channel to the OD face;

the ID face has:

a circumferential channel closer to the first face than the second face; and

a plurality of channels extending from the circumferential channel to the second face; and

the ID face circumferential channel has an open end; and

the plurality of segments interfit first end to second end;

a garter spring holding the seal segments together;

a seal housing carrying the seal and having a first wall and a second wall; and

at least one spring biasing the seal segment first faces against the housing first wall,

wherein:

for each seal segment, a buffer flowpath from the buffer chamber to the bearing compartment extends from the second face, through the ID face plurality of channels to the ID face circumferential channel and out the ID face circumferential channel open end,

the method comprising:

driving rotation of a shaft carrying the runner; and

passing an airflow along the buffer flowpath from the buffer chamber to the bearing compartment; and

directing another airflow for each seal segment from the OD face, through the first face plurality of channels to the first face circumferential channel to pass as a leakage flow over a first face dam radially inward of the first face circumferential channel,

wherein:

the airflow has a first flow rate; and

the another airflow has a second flow rate less than half of the first flow rate.