The API 2000 committee issued a interpretation several years ago. The question that was asked was: "I have a 100 gallon tank. Is an emergency vent required?" The committee's answer: Yes.
You should review the flash point and boiling point data to make a determination as to the classification of the liquid (e.g., Class IA, IB, IC, II, IIIA or IIIB).
Without the liquid's classification I can't tell you if a PE tank would be acceptable using NFPA 30. NFPA 30 does allow combustible aboveground storage tanks for Class IIIB liquids so long as:
A) The tank is in an area not exposed to a spill or leak of Class I or II liquids, and
B) The tank is approved by the authority having jurisdiction. (NFPA 30, section 2.2.2 (b))
If this tank is in the US, you should ensure that you have the approval of the authority having jurisdiction (e.g., fire marshal).
One could consider that because the PE is a low melt point material that the tank emergency vent is by form of construction. This is assuming that this is a vertical tank. However, this is too important of a safety issue. Emergency vents are cheap.