Reid Vapor Pressure (RVP) is a common measure that’s typically used for assessing the volatility of hydrocarbon liquids. To determine the RVP for any hydrocarbon liquid, the standard procedure detailed in ASTM-D323, "Standard Test Method for Vapor Pressure of Petroleum Products (Reid Method)," should be followed. This procedure requires heating a chilled sample to a temperature of 100 degrees Fahrenheit at atmospheric pressure using specific test equipment. Following the standard RVP test procedure allows for the comparison of volatility across a range of hydrocarbon liquids.
Among the practical considerations related to Reid Vapor Pressure are:
- Controlling liabilities associated with the physical loss of product;
- Monitoring the potential for emissions;
- Identifying requirements for safe transportation; and
- Maximizing profit from downstream processing.
Using methodology outlined in API MPMS (Manual of Petroleum Measurement Standards) Chapter 19.4, "Evaporative Loss Reference Information and Speciation Methodology," and results from the measured RVP of a hydrocarbon liquid, the True Vapor Pressure (TVP) can be calculated for a range of temperatures. TVP is useful because it indicates what pressure is required to prevent flashing into the gaseous phase as a function of temperature. TVP is also the highest vapor pressure possible for a specific hydrocarbon liquid at a given temperature. Hydrocarbon liquids which are under pressures that are greater than their calculated TVP at any given temperature will remain in the liquid phase. Knowing the TVP is essential for designing processes and equipment for various hydrocarbon products with different properties.
Equilibrium Vapor Pressure (EVP) is a more generalized term that describes any vapor pressure for which a liquid is in equilibrium within a closed system. RVP and TVP can be considered as EVP for their specific conditions. However, when dealing with transportation of hydrocarbon liquids, there are several methods that have varying conditions for temperature and vessel size that do not match the conditions specified for RVP and TVP. As a result, it’s important to know if the vapor pressure determination conditions properly apply to its intended use.
For more guidance on the differences and uses among the various vapor pressure designations, be sure to consult with your organization’s measurement experts, whether in-house or third-party.
"The only way to handle pressure is to apply it."
~ Darrelle Revis, American football player