It is a requirement of NFPA 30. Using the 2000 edition, section 2.2.5.2.1 states Every aboveground storage tank shall have emergency relief venting in the form of construction or a device or devices that will relieve excessive internal pressure caused by an exposure fire.

If your silicone is a class IIIB combustible liquid, and the tank volume is 285 bbl. or greater, an emergency vent is not required so long as the tank is not located in the same dike or drainage path of tanks that contain class I or II liquids.

The tank volume is very small (160 gallons).  The tank has a breather on it, but not any type of relief valve.  I don't think silicone is too much of a flammable liquid.  Do I still need on?  There is a relief valve on the discharge side of the pump moving the fluid out of the tank.  The tank is also construcied of polyethylene.  Would that be the construction relief of pressure?

Thanks.

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.