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Inconel 718 bolt fracture Ball mill feed head shell.

Inconel 718 bolt fracture Ball mill feed head shell.

Inconel 718 bolt fracture Ball mill feed head shell.

(OP)
Dear All,

Kindly seeking all your assisting in finding the failure for Ball mill feed end shell Inconel 718 bolt breakage/fracture in same location.
The area of concern for this joint is located between bolts 38 and 55 and incorporates an area of approximately 16 bolts this is the area which is affected the most.
To date 11 bolts have broken since August 2019, of the bolts which have broken, only two were of the inner circumferential ring of bolts.
The actual bolts used before on the ball mill FEED Head bolts -1½’’ B7 which had fractured and was upgraded to Inconel 718 – size: 1½’’UNC round treaded to increase the overall load applied however, they also fractured recently on the 7/4/2020.
The original Feed head bolts B7 which had failed/fracture it was discovered by a third party group that the Ultrasonic bolt load verification of the critical circumferential joints and re-tension of any fasteners found to be below tolerance. This was correct.
For the Inconel 718 recent breakage it is believed to be because there is a significant difference in the flexing of the shell and head resulting in the Inconel 718 fasteners brunt and falling which resulted in the Inconel 718 bolt fracture on the fixed nut end.

Also note: The Interior End Trunnion shell area is identified with internal erosion with minor cracks

Possible causes that may have resulted in the Inconel 718 bolts failing;
1. No mill certificated for Inconel 718 bolts provided.
2. Significant difference in the flexing of the shell and head resulting in the fasteners bearing the brunt and falling.
3. Incorrect bolt torque sequence.

Corrective action/recommendation
3. Send samples of fracture bolts for root cause failure analysis
4. When a replacement of bolts occurs should we look at replacing the complete flange set?
5. Correct torque sequence should be done.
6. Upgrade Inconel 718 to AISI 4140 Alloy Steel for the affected area only.
7. Calculating Fatigue Life of Threads & Thread Stress Concentration Factor so we know the next change out date.

Apologise I cannot upload pictures.

RE: Inconel 718 bolt fracture Ball mill feed head shell.

I would suggest that you review: the equipment drawings, bolts ( type/ grade /material), survey foundations for settlement and check alignment.

I would recalculate the required bolt torque.

I ran into something about 10 years ago. One area of the facility was breaking (20) 2" Studs that mounted one of the Trunnion Journals of several Ball mills.

A decision had been made, before I hired in, that changed the Studs to a different grade of material and hardness. The difference was enough between the feed or discharge end, that We were pulling threads out of tapped holes instead of actually stretching the studs.

The other things that needed to be addressed were minimum shell and Trunnion thicknesses.

And lastly, we had foundation issues....

RE: Inconel 718 bolt fracture Ball mill feed head shell.

(OP)
Thanks JS for your response dual noted.

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