Formula Verification for length of plate required

[Faiz vhora]

Hello, Currently i am working on project and the industry calculate the length of plate required from this formula(dishend circum-3*shell thickness-2*3.14(dishend thickness-shell thickness) now i am unable to understand from where this formula comes actually we will have to use mean diameter method to calculate length of plate so can any one who knows this can you please tell me is it theoretical correct formula or wrong? Thank you.
Regards.

 

[Some Curious Guy]

Your query is incomplete. Plate length for which component ?

 

[DriveMeNuts]

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