Different multiphase flowmeters can be effective in measuring liquids up to 95 to 97% gases. Some sensors such as Agar MPFM400 and Jiskoot can measure combined gas using an internal separator to send wet gas and residual gas along with oil and water using a multiphase separator. There is no standard in the design of multiphase measuring systems and there are different methods to define accuracy and performance.
There are three common ways to express the accuracy of a multiphase sensor:
- Percentage volume of phase volume
- Percentage of total multiphase flow
- Percentage of liquid and gas flow plus absolute uncertainty of water in the liquid phase
Percentage volume of phase volume
The first method is more popular with the relevant engineers and clearly presents the performance that has been stated. This method may not be very practical for excessive phase mixing.
Methods 2 and 3, despite relatively low mixing values of 5 to 10% for gas / liquid phase uncertainties and 2 to 3% for water, may show duct phase errors of 100% or more, depending on the absolute percentage of water.
The first requirement for the mixed phase of a liquid is the separation of hydrocarbons from water. If the liquid phase is oily, water is usually less than 40% of liquid hydrocarbons. Constant dielectric measurements at microwave frequencies can determine the water fraction. The dielectric constant of dry hydrocarbons is in the range of 2 to 4, depending on the composition, while this value is 82 for water and gives a sensitive measurement.
Another requirement is the tendency of the liquid flow from the gas flow in a system in which two substances tend to separate and flow at different speeds. Improving flow conditions in these systems is usually done using a mixer. Transverse correlation flowmeters use nuclear techniques to double-measure current density. In the vertical distance a little farther and the fluctuations in density relative to time are related to each other. In some other devices, the electrical characteristics are used in a similar way.
Because it is difficult to distinguish between different liquid and gas velocities, it is common to measure the velocity of one phase and to use flow modeling techniques to estimate the velocity of the other phase.
Most techniques are limited by the liquid / gas ratio. It is difficult to measure low gas flows with a liquid less than 5% because the density of the gas may be equal to the density of the liquid. At a pressure of 3000 PSI, the gas density may be 200 kg / m 3 and the liquefied gas density may be 600 kg / m 3 . Therefore, a change from 5% liquid content to 4% in the mixture is equivalent to a 20% change in liquid, but changes the density by less than 2%. To avoid this, a manufacturer uses a centrifugal separator to separate the gas from the three-phase mixture, measuring this value separately, and mixing them again after measuring the other phases. Removal of 80% of the gas in the multiphase sensor section is measured by a positive displacement sensor and differentiates venturi techniques and microwave water content of different phases.
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