Measuring True Vapor Pressure in oil & gas process applications requires the right equipment. Not all Vapor Pressure Analysers work in the same way.
Measuring True Vapor Pressure – How it differs from Reid Vapor Pressure
True Vapor Pressure (TVP) differs from Reid Vapor Pressure (RVP) in distinct ways, and deciding which to measure will often depend on the process taking place at any one time.
In essence, the difference is as follows. TVP is the pressure you will see at the reference temperature when the ratio of gas to liquid is zero. In other words, it is the vapor pressure of the liquid before any components are volatilized. The TVP is often referred to as the bubble point pressure as it is the pressure at which a bubble first forms in a liquid.
RVP, on the other hand, is the pressure exerted by the liquid under test at a reference temperature when the ratio of gas to liquid is 4:1. It is intended to give a feel for the pressure build up you will see when filling a tank with the liquid under test.
To have meaning TVP and RVP are quoted at a reference temperature. By convention, this is usually 100°F. For pure liquids, TVP and RVP would be the same. TVP is the more fundamental measure of the liquid properties, and it is always higher than the RVP.
Measuring True Vapor Pressure & Reid Vapor Pressure – where it matters
In mixtures, the situation is more complicated. The mixture’s components may differ in terms of their volatility, and as such, the more volatile components are preferentially dispersed into the vapor phase and depleted in the liquid phase. The larger the gas to liquid ratio becomes, the lower the measured pressure will be, as put simply the liquid runs out of volatile molecules as they are all in the vapor phase.
DCS software usually has numerical models to convert TVP to RVP and to generate TVP or RVP at various temperatures from measurements at the reference temperature.
Depending on the process at hand, measuring True Vapor Pressure or Reid Vapor Pressure will be preferred.
How TVP can be measured
Depending on the type of analyser employed, either TVP will be measured and RVP calculated, or RVP is measured and TVP is calculated. Many of the leading Vapor Pressure Analysers (Bartec, Icon etc.) use a piston system to create the conditions required to measure the RVP. However, the ATAC 1354, is based on the design by Hallikainen, and utilises the bubble formed when the sample liquid is jetted through a venturi/nozzle to measure the TVP. Unlike the piston-type analysers, this method is continuous, meaning that the data produced is not cyclical, as it would be on the other systems.
Other advantages include the fact that the venturi set up has no moving parts that require maintenance through wear and tear. No atmospheric drain is required. Most importantly, changes can be seen in real time.
Best to Measure TVP
The ATAC/Hallikainen design has proven to be the preferred Vapor Pressure Analyser when it comes to measuring True Vapor Pressure for many in the oil & gas process industries. And for those needing the RVP, it can be set up to provide a read-out of that too. It has been on the market for many years, and it is undergoing continuous development to improve its accuracy and reliability, and to lower its carbon footprint.
For more information on the ATAC 1354, click the button below to be taken to our PPA Product page. Alternatively, email us at sales@atacgroup.com or phone our team of experts on +44 (0)1380 818411. We always look forward to discussing your needs and guiding you to the best products on the market.
Photos courtesy of Ben Wicks & Clyde Thomas
