# Design Temperature and Pressure for Large Propane Tank



## bmac9927 (Mar 26, 2019)

I'm spec'ing a large propane tank (18,000 gal), and it seems like the industry standard in the US is to use a design temperature and pressure of 125 F and 250 psig. The vapor pressure of pure propane is ~247 psig at 125F, which would explain these values, but commercial propane can have lighter hydrocarbons and a higher vapor pressure. In fact, NFPA 58 lists a vapor pressure for commercial propane of ~300 psig at 125F (interpolating, Annex B). Meanwhile the ASME Code says the design pressure should be based on the design temperature (max).

Does anyone know the reason for the apparent discrepancy?

As I see it, there are a few possibilities:


The design temp of 125F is overly conservative and rarely seen in a white-painted tank.

Commercial propane is rarely used in ASME vessels, and either high purity propane or a mixture of propane and butane is used instead.

These tanks just burp off excess pressure through their relief valves on hot days.



I'm inclined to spec a design pressure of 300 psig, but there should be a good explanation for why the standard practice is to use 250 psig.... I can't be the first one to run into this. Anyone have experience here?


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