Dear Ricardo, I suggest that you use API 580 "Risk Based Inspection" as the basis for your work. Most companies nowadays use a commercially available RBI software, together with a multidisciplinary team of engineers that are knowledgeable on API 510, API 570, and API 571.
The RBI process is not very complicated, and API 580 will enable you to obtain your inspection frequencies for each individual heat exchanger. However, the team you assemble will need to set the ground rules and assumptions in order to get a successful RBI program. For example, your management, environmental, and safety team will have to define the risk matrix that you will be using in your RBI program. Similarly, your multidisciplinary team of engineers will need to set the assumptions for corrosion rates on those exchangers where you don't have existing data (you will need to "guess/estimate" values based on experience).
Hope this helps. Best regards,
Luis F. Garfias
Cell: +1 (832) 600 7811