Dear Ketan,
The consideration of local shell thickness in WRC 297 has been addressed, it is available a hot-fix at this time. In future we will package it as a build update (the next one after March 2010) and it will be available on our website.
If you need the hot-fix just send us an email or as a workaround insert a small shell section with thicker plate and put the nozzle on it.
In my opinion, the stress concentration factor is useful for computing peak stresses which are used in the fatigue evaluation. For an elastic analysis you may leave it as 1. But, some users also think that the presence of opening causes an increase in the local primary stresses. So, they may specify a smaller stress concentration factor. If you use WRC-107 and checked the box "Use kn and kb" it will compute stress concentration factor too.
It is up to you what you use.
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Best Regards,
Mandeep Singh
CADWorx & Analysis Solutions
Hexagon PPM