Could someone pls. advise me how to calculate the design pressure of tank. i have palm oil to storage in 16.5 m diameter and 12.1 height tank. Do i need to install PVRV, or any kind of venting equipment or just provide openvent with bird protection screen. i found many sources that tell me like +- 0.02 barg, 0.007 to -0.002 or something like that. API650 said 15 psig. and so on. Are there any exactly procedure to calculate design pressure?

You need to be very careful as you do not appear to understand the mechanics of what you are doing. I don't believe that you will be able to find an answer to your question in a forum. You are asking the questions that you should already have the answer to.

My first suggestion would be to hire a competent engineer familiar with the issues.

There are several industrial standards for designing tanks and determining the working pressures/vacuums. API is certainly one, ASME also but probably not for your application. I don't believe that you need to store at a high pressure. The design pressure is dependent upon many factors in the detailed design of the vessel, material, construction, product needs, etc. Don't forget local codes.

You may need to provide inert gas blanketing to protect the product. That is your call.

Good luck.