msnoddon031,
In my opinion there are a couple of things to note here:
1) ASME Section VIII and the API 650/620 tank and pressure vessel Codes and standards do not cover HDPE...they are for the specific metals that are described within
2) Since your system developed a head of water that caused material failure of a tank component, it appears that your overflow was undersized for the application
3) When a steel, vertical axis, flat bottomed, "atmospheric tank" is designed to API-650, the purchaser decides upon the actual design pressure and vacuum. +10 IWC and -5 IWC are values commonly used (....again, Steve....in my opinion)
4) When an atmospheric, horizontal axis, metallic tank is desired, the UL-142 code is commonly used.....I do not know of any HDPE/plastic tank code that addresses the unique design requirements of horizontal axis tanks (can anyone out there help ???..??)
-MJC