stud wobble test

[pgsturbine]

Project involves a 1 1/2" x 8tpi steam turbine stud and its fitment to the internal threads of the turbine casing horizontal joint flange. In defining the acceptability of the internal flange studs we would be looking to insert the stud into the flange a distance of 1 times the thread diameter. We would look to indicate the movement of the stud by dial indicator method. We anticipate that the acceptance criteria would be something less that .008" movement. The question would be at what distance from the flange would the indication of the stud movement be taken?

 

[MintJulep]

How did you arrive at your acceptance criteria without this distance?

Movement of a male thread in its female hole is largely determined by the thread geometry and fit class.

Rather than trying to inspect in an acceptable condition after everything is built and assembled, why not take the time to properly identify what thread you need, and guage the parts at their points of manufacture rather than at assembly in the field.

 

[metengr]

I believe that the Westinghouse large steam turbine division at one time issued a Technical Advisory on turbine governor and throttle valve stud wobble tests. These tests were recommended to evaluate the extent of thread deterioration in the stud holes of castings.

We had performed such tests on valve studs and typically measured the extent of wobble by hand or with a straight edge off of the gasket surface. If any wobble was detected, it was cause for possibly enlarging the stud hole (as permitted by analysis to assure adequate ligament spacing between stud holes) and using mushroom type valve studs.

For HP and IP turbine shell mounting studs, we typically do not measure wobble because it is difficult to remove the studs that are threaded into the lower shell.

 

[pgsturbine]

Thank You Metengr for your comments. In this instance the studs are remomved. This is a Westinghouse unit. Both GE and Westinghouse had Technical Advisory on the extent that they considered the internal stud threads acceptable, although the diction was vague. In this case the existing stud has been removed. The test is done in the effort to define the effect of any loss of material from the internal threads with the use of a new replacement stud. If determined that the stud hole is unacceptable there is the possibility of oversizing the hole and going to a bottle design stud, or oversizing the hole, manufacture and installing a bushing, and using the original stud design. I believe the acceptance criteria I mentioned was based on the measurement taken with the stud installed and the indication point being 1 1/2" (stud diameter) above the shell flange surface. Any thoughts?

 

[metengr]

On a 1.5" diameter turbine shell stud, inserted into the shell and hand tight, the deflection can be measured along any point off the flange surface. Frankly, I think if you can feel any wobble by hand, this would be an indicator that the threads need to be chased or an insert should be considered.

Your 0.008" deflection seems too restrictive for stud hole threads on large steam turbine shells.