The rules in NFPA 30 are complex and it is not possible to provide an answer without further details pertaining to the installation including:
- Type of tank (floating roof, fixed roof, horizontal, etc)
- Contents (Class 1, Class 2, Class 3a, etc.)
- Tank size (diameter and volume)
- Tank operating pressure
I recommend you go to a site like Techstreet.com where you can purchase NFPA 30 for immediate download and then you'd be able to review the requirments and make an appropriate selection
Regards,