Question about Inspection of storage tank

[javad64]

Hello my friends
We have several storage tanks at our company .
We use these vessels about 20 years old .
I want to know that is there any standard to explain how often do we must inspection of their nozzles and shells ? ( for example every year or once of three years , etc )
Thank you

[Nabilia]

API Standard 653 4th Ed. Apr. 2009 - Tank Inspection, Repair, Alteration, and Reconstruction.pdf

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