Welcome to the aboveground storage tank forum, a bulletin board dedicated to tanks, fuel tanks, plastic tanks, fuel storage tanks, and above ground storage tanks (AST) API650, API653 & UL142 aboveground storage tanks (AST) and chemical plastic tanks. Remember the forum is made up of volunteers from experts to beginners. Unfortunately, we cannot allow free commercialism that promotes products, services, or sites.
DavidR on Tank Datasheet Appendix LBy: DavidR@Fast.net 
Date: Friday, May 20, 2011
Time: 6:48:47 AM
Sir - In my view, if your tank has vents then it will be at atmospheric pressure inside and the vapor pressure can be ignored. If you are designing an Appendix F tank then you should add the vapor pressure to the liquid pressure. If you are pressurizing the Appendic F tank by some other means - nitrogen blanket for instance - then you only need to add your vapor pressure to the applied pressure to the extent that your vapor pressure exceeds the blanket pressure. My opinion only - independent verification is suggested!
Click on picture for full size view or to view PDF!