Fastener assembly for securing a turbomachine casing and method for securing the casing

1. A casing comprising:

a first casing section and a second casing section configured to be joined to the first casing section to form the casing;

a first hole extending through a portion of the first casing section;

a second hole extending through a portion of the second casing section, wherein the first and second holes are configured to be in alignment while the first and second casing sections are joined;

a bushing seated in the second hole, wherein the bushing has a threaded outer surface confined to a first end region of the bushing and a threaded inner surface confined to a second end region, which is opposite to the first end region, wherein the threaded outer surface of the busing engages a threaded interior surface of the hole in the second casing section;

a fastener having a shaft configured to extend through the first hole, extend into the second hole, and seat in and engage the threaded inner surface of the bushing,

wherein a thermal expansion coefficient of the bushing is greater than a thermal expansion coefficient of the shaft of the fastener, and the thermal expansion coefficient of the fastener is greater than the thermal expansion coefficients for each of the first and second casing sections.

2. The casing of claim 1 wherein the threaded outer surface of the bushing engages a threaded inner surface of the second hole and the threaded inner surface of the bushing engages a threaded outer surface of the shaft of the fastener.

3. The casing of claim 1 wherein the fastener includes a nut having a threaded inner surface engaging an end of the shaft and the nut has a lower surface abutting a surface of the first casing section.

4. The casing of claim 1 wherein the casing is an inner casing which is encased in an outer casing.

5. The casing of claim 1 wherein the casing is an inner casing of a steam turbine.

6. The casing of claim 1 wherein the fastener is a bolt and nut assembly and the shaft is a threaded shaft of the bolt.

7. The casing of claim 1 wherein the bushing has a length sized such that thermal expansion of the bushing extends the shaft into the second hole a distance commensurate with a lengthwise thermal expansion of the shaft.

8. The casing of claim 1 wherein the first casing section is an upper casing section and the second casing section is a lower casing section.

9. A casing for a turbomachine comprising:

a first casing section having a first mating surface;

a second casing section having a second mating surface configured to abut the first mating surface of the first casing section;

a first bolt hole extending through a portion of the first casing section and having an opening at the first mating surface;

a second bolt hole extending through a portion of the second casing section and having an opening at the second mating surface, wherein the first and second bolt holes are configured to be in axial alignment while the first mating surface abuts the second mating surface;

a bushing seated in the second bolt hole, wherein the bushing has a threaded outer surface confined to a first end region of the bushing and a threaded inner surface confined to a second end region, which is opposite to the first end region, wherein the threaded outer surface of the bushing engages a threaded inner surface of the second bolt hole in the second casing section; and

a bolt configured to extend through the first both hole, extend into the second bolt hole, and seat in and engage the threaded inner surface of the bushing,

wherein a thermal expansion coefficient of the bushing is greater than a thermal expansion coefficient of the bolt, and the thermal expansion coefficient of the bolt is greater than the thermal expansion coefficients for each of the first and second casing sections.

10. The casing of claim 9 wherein the threaded outer surface of the bushing engages a threaded inner surface of the second bolt hole and the threaded inner surface of the bushing engages a threaded outer surface of the bolt.

11. The casing of claim 9 further comprising a nut configured to engage an end of the bolt.

12. The casing of claim 9 wherein the casing is an inner casing which is encased in an outer casing.

13. The casing of claim 9 wherein the casing is an inner casing of a steam turbine.

14. A method to fasten casing sections for a turbomachine comprising:

inserting a bushing in a first hole in first casing section, wherein the bushing has a threaded outer surface confined to a first end region of the bushing and a threaded interior surface confined to a second end region, wherein the second end region is opposite to the first end region;

engaging the threaded outer surface confined to the first end region of the bushing with an inner threaded surface near an open end of the first hole;

inserting a bolt in the bushing and the first hole;

engaging a threaded outer surface near a first end of the bolt with the threaded inner surface confined to the second end of the bushing;

joining the first casing section to a second casing section such that the first hole is aligned with a second hole in the second casing section;

extending the bolt through the second hole, and

engaging a nut to threads near a second end of the bolt,

wherein a thermal expansion coefficient of the bushing is greater than a thermal expansion coefficient of the bolt, and the thermal expansion coefficient of the bolt is greater than a thermal expansion coefficients for each of the first and second casing sections.

15. The method of claim 14 wherein the joining of the first casing section to the second casing section occurs before or after the engagement of the threaded outer surface of the bolt with the threaded inner surface of the bushing.

16. The method of claim 14 further comprising housing the casing sections in an outer casing.

17. The method of claim 14 wherein the engagement of the nut includes tightening the nut against a flange of the second casing section.

18. The method of claim 14 wherein the turbomachine is a steam turbine and the first and second casing sections form an inner casing which is housed in an outer casing.