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Dear Friends,
I am involved in the investigation & remedy for cracks which have appeared in welded connections of a circular sludge tank after 20 years in service. The tank, 46.0 meters in diameter, is installed with a raker mechanism inside, which is used to separate the soilds from a mixture of mining ore (density 2.8 to 3.0 gm/cc ) +water + sodium hydroxide ( 1.25 gm/cc). The raker rotates at a speed of 0.1RPM. The bottom of the tank is supported on 16 radial beams each of which is supported on 3 columns ( total 48 cols in I section with webs oriented along radial direction). In the outermost ring of columns, 'x' bracings are provided in every alternate bay in vertical plane. The innermost ring of columns are connected , at top & below tank bottom, by a ring of horizontal tie beams ( ' I ' sections) which are welded to the web of radial beams. The cracks have appeared in some of the weld joints ( 5 places in one tank & 7 in another such tank) in the entire web of tie beams after 20 years in service. Rewelding was done by the client but the cracks have appeared again. [ I hope I have stated the problem with clarity ] My guess is that this is due to fatigue. A Raker blade must creating +ve pressure in its front & -ve pressure in its rear as it rotates. Every sweep must be creating stress variation in the 16 bottom sectors. My problem is how to calculate this pressure variation required for the analysis.
  I will be thankful to anyone who can reply or comment.

Dileep L. Kulkarni

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