Dear Xinquan,
Since this is not a known problem, it would be great if you could submit you model to us for evaluation. A quick check of the computer code reveals that the inside radius is equal to the inside diameter/2.0 plus the corrosion allowance of the nozzle.
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Scott Mayeux
COADE Inc.
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Scott MayeuxCADWorx & Analysis Solutions
Intergraph Process, Power, & Marine