Mass Flow Measurement in Petrolium Industry
The petroleum industry most commonly measures fluids in volume units. When volume is used as the measurement
unit, a specific set of conditions of temperature and pressure are designated to be the standard or base conditions for the custody transfer of the
fluid. Thus the measurement of volume requires that the gross volume metered be adjusted to an equivalent net volume at these base
conditions. This correction requires knowledge of the relative density or specific gravity of the fluid in addition to the temperature and pressure at which the gross volume was
Tables and equations are available for Certain fluids that will provide the user with the correction to net volume when the temperature, pressure, and relative density ere known. This empirical data predicts the change in density of the fluid with temperature and pressure, Crude oil, refined products, and pure compounds are predictable fluids that can be measured in volume units by Ibis method with very little uncertainty.
The physical behavior of mixtures of hydrocarbons is not very predictable. Different compositions can cause the density to vary differently with temperature and pressure, Another characteristic of mixed streams is known as volume shrinkage. The mixed stream has more volume due to its molecular structure than if all the components were measured by volume separately. However, a pound of one component added to a pound of another component always produces two pounds of the mixture. Custody transfer contracts for mixed streams are still written in volume units, however, the fluid is metered on a mass basis and then converted to volume by a ratio of weight per unit volume for each individual component in the stream.
There are some fluids with rapidly changing density at the temperature and pressure at which they are being measured. This region of rapidly changing density is known as the critical region. Such fluids as ethylene and carbon dioxide are often measured at conditions in this region, making it difficult to determine the net volume of these fluids. Contracts for custody transfer of these fluids are often written in mass units.
Mass measurement is utilized in the two Instances above to solve the problems related to determining the net volume. Mass measurement is also a desirable alternative to volume determination for inventory balance, as in underground storage, plant balance, and loading facilities. It can be argued that mass units are more logical units for custody transfer than volume units, since mass measurement does not require the measurement of temperature, pressure, and the use of empirical data to convert to a quantity that can be utilized for custody transfer. Mass units are a fundamental unit of measurement.
However, the metering technology for continuous mass measurement has only been available in the last thirty or so years, way after the petroleum industry had adopted standards for volume measurement. It has never been practical to weigh large quantities of fluid especially in a dynamic mode, though the weighing of tank trucks is often utilized for measurement of smaller quantities. Mass measurement also requires the use of electronics that again were not available in the earlier years.
The petroleum industry is continually searching for better and morn accurate means of measurement. Being "better" Is not only related to accuracy, but should be evaluated on a cost or ownership basis. Often better measurement technology can offer a higher degree of safety, reliability and/or benefits misted to efficiency, thus contributing to the overall profit of the Industry.
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