3 replies to this topic

[ryn376]

According to API 2000, 7th Ed., 3.3.2.2.1b), volatile liquids have a vapor pressure greater than 0.73 psi. Is this psia or psig? 

[proinwv]

Correct me if I am wrong guys but I have only seen vapor pressure in psia.

[ryn376]

That is why I'm a little confused. If the units are psia, that would mean water above approximately 90^oF would be considered volatile according to API 2000. That means a very large number of chemicals would be considered volatile...

[MrShorty]

True, a standard of 0.74 psia means a large number of chemicals get classified as volatile. Consider the flip side. If it means 0.74 psig, then it is saying that something is not volatile until it is well above its normal boiling point. Under this variation, butane is a volatile liquid at room temperature, but pentane is not. Neither is acetone or methanol or benzene or other common solvents.

 

I am not familiar with the specific API standard that you mention, but, in my own lab work, I consider many liquids volatile even when they are well below their normal boiling point. Consider the intent of the standard. Does it seem to intend only to classify "liquified gases" and other chemicals that are above their boiling points as volatile, or is it intended to identify where a liquid will have a "significant" vapor pressure below its normal boiling point. My first impression is that it would be intended for the latter. A standard that waits until something is above its normal boiling point before calling it "volatile" does not seem very useful or meaningful.