The separation of two immiscible liquids contained in the same tank or vessel is a fundamental aspect of many production processes. Optimising the performance of these applications can have a significant effect on final product quality. To achieve this, plant managers need to implement devices that can accurately and reliably measure the interface level between the two liquids.
Examples of immiscible liquids requiring separation include oil over water, oil over acid, low dielectric organic solvents over water, and low dielectric organic solvents over acid. Low dielectric organic solvents include toluene, benzene, cyclohexane, hexane, turpentine and xylene. In most applications the liquids will separate naturally due to their differing densities, with the lower density liquid settling on top of the higher density liquid. For example, when oil and water occupy the same vessel, the oil floats on top of the water, and the interface would be the upper level of the water and the lower level of the oil.
In some interface level measurement applications, it is necessary to know the thickness of the upper liquid layer. This includes instances where only the upper liquid needs to be poured off and an accurate indication of when to stop pouring is required. It is also essential when controlling the flow of both liquids out of the vessel and into independent channels, as this process must be achieved with as little cross-contamination of products as possible. For example, in the oil production process, it can prove extremely costly if oil is channelled to the water tank, or if water is sent further along the process.
The most straightforward way to measure an interface level is through a sight glass on the side of a vessel, but this basic method has some obvious disadvantages. By requiring operator inspection, it is both labour-intensive and time-consuming. Also, sight glasses require regular maintenance, and in applications that are prone to condensation, an operator may be unable to see the interface well enough to make an accurate measurement. Alternative methods of measurement include floats and displacers, capacitance transmitters, differential pressure transmitters, and magnetostrictive sensors. However, these technologies can also have limitations in terms of their accuracy and reliability under certain process conditions, and can have complex maintenance and calibration requirements.
Guided wave radar (GWR) is a well-established and field-proven technology in interface level measurement applications such as oilfield production tanks; free water knock-out vessels; water and skim tanks; accumulators; desalters; scrubbers; and storage and buffer tanks containing oil, condensate, water, or solvents.
GWR transmitters deliver a top-down, direct measurement of the distance to the product surface and the interface, and provide many advantages compared to other technologies. Changes in pressure, temperature and most vapour space conditions have no impact on the measurement accuracy of GWR devices. Also, no compensation is required when there are changes in the dielectric constant, conductivity or density of the liquid. GWR provides accurate and reliable measurements in vessels with tight geometry, in chambers, and in tanks of all sizes. GWR transmitters have no moving parts and therefore require minimal maintenance, and their advanced diagnostics ensure that operators are quickly alerted to any degradation in performance. Easy system integration and the introduction of wireless-enabled devices that remove the need for data or power cabling are further significant advantages of GWR transmitters.
How an Interface Level Is Measured
GWR technology is based on the time domain reflectometry principle. A low-energy pulse of microwaves, travelling at the speed of light, is guided along a probe that is submerged in the process media. When a pulse reaches the surface of the material it is measuring, a significant proportion of the microwave energy is reflected up the probe to the transmitter. The time difference between the generated and reflected pulse is converted into a distance from which the level is calculated. As a proportion of the pulse will continue down the probe through low dielectric fluids, a second echo can be detected from an interface between two liquids at a point below the initial liquid level.
Influence of the Dielectric Constant
The speed of travel of the pulse — and therefore the accuracy of the interface level measurement — is dependent on the dielectric constant of the two products. Top-down GWR transmitters are suitable for use in interface level measurement applications only if the first liquid detected by the device has a lower dielectric constant than the second one, and if there is a difference between the two dielectric constants of at least six.
In typical applications, the upper liquid would have a low dielectric constant of less than three, while the lower liquid would have a high dielectric constant greater than 20. For example, the dielectric constants of oil and gasoline range from 1.8 to four, while water and water-based acids have high dielectric constants of more than 20. Therefore, in an oil and water interface level measurement application, because water has a significantly higher dielectric constant than oil, the interface of the two fluids can be easily detected by a top-down GWR transmitter.
If the upper product has a higher dielectric constant than the liquid below, this prevents a top-down interface measurement using GWR. However, in this circumstance the mounting position of the transmitter can be inverted, so that the device is mounted on the bottom of the vessel to measure the distance to the interface.
This article is protected by copyright. You want to use it for your own purpose? Contact us via: support.vogel.de/ (ID: 46139169 / Control & Automation)