Interface measurement for product quality

Refinery product quality often depends on how products separate. Accurate gathering of data about the interface levels within vessels is therefore crucial

Ingemar Serneby
Emerson Process Management

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Article Summary

Within the refining industry, ensuring product quality is essential. Consistent, high quality products, produced cost effectively, are paramount to profitability. Efficient separations are an integral part of many production processes and have a significant bearing on final product quality.

For example, water entrained in hydrocarbons can impede quality specifications and oil in water streams can place increasing demands on effluent treatment. Separation methods, which include gravity settling, centrifugal force or electrical charge, will invariably rely on an interface level measurement. Accurate and reliable instrumentation supports optimised control and enables efficient production and a high quality final product.

Advantages of guided wave radar
Guided wave radar (GWR) is increasingly being specified for level measurement applications, including those requiring the measurement of an interface. The technology is well established, field proven and suitable for applications involving liquids, slurries and some solids. Guided wave radar is a top down, direct measurement as it measures the distance to the product surface and interface. A key advantage of radar is that changes in pressure, temperature and most vapour space conditions have no impact on the accuracy of its level measurements. Moreover, no compensation is necessary for changes in the dielectric, conductivity or density of the fluid.

Changing density, caused by variations in process or ambient conditions, is one of the major issues when measuring level or interface using older technologies such as displacers influencing the reliability and accuracy of density-based technologies. GWR has no moving parts, which means a reduction in maintenance costs as well as improved accuracy of measurements. Advanced diagnostics ensure that operators are made aware of any degradation in performance. Easy system integration and the introduction of wireless enabled versions that remove the need for data or power cabling have made the decision to upgrade existing level technology to GWR much easier.

Principle of operation
GWR technology is based on the time domain reflectometry (TDR) principle. Low power nanosecond pulses are guided along a probe in the vessel. The probe is submerged in the process media and, when a pulse reaches the surface of the material it is measuring, some of the energy is reflected back to the transmitter. The time difference between the generated and reflected pulse is converted into a distance from which the total level or interface level is calculated.

The speed of travel of the pulse is impacted by the dielectric of the medium. As the pulse travels through a different medium, such as oil, the speed of travel changes. This change in travel time is predictable and allows compensation for the measurement to be accomplished.

The reflectivity of the product is a key parameter for measurement performance. Media with a high dielectric constant (DC) will provide a better reflection and a longer measuring range (see Figure 1).

For interface applications, the modern software built into GWR level transmitters allows the detection and tracking of the interface between two liquids, for example in a separator, giving users a reliable measurement to optimise control and maximise throughput. Radar works because when there are two immiscible layers of fluid and the surface of the fluid with the lower dielectric is the first seen by the radar, only a small part of the signal will be reflected back to the device with most of the radar signal travelling through the low dielectric material. For example, with a low dielectric material, such as oil with a dielectric of 2, less than 5% of the signal will be deflected back to the transmitter. The rest of the signal will travel through to the interface, with the lower fluid allowing the interface of the two fluids to be accurately detected (see Figure 2).

In an oil and water interface measurement, because water has a significantly higher dielectric constant, the interface of the two fluids can be easily detected. Since the speed of travel of the microwave signal changes as it moves through the upper fluid, the determination of the physical distance of the layer needs to include compensation for the change in travel time. If the dielectric of the upper fluid is known, this is easily calculated as:

Real distance = Electrical distance/√DK of media

For GWR with built-in dielectric calculators, the material’s dielectric can be calculated by the device if the actual physical distance of the upper layer is known.

GWR level transmitters are ideal for interface measurement installations in oilfield production tanks, free water knock-out vessels, water and skim tanks, accumulators, and storage and buffer tanks containing oil, condensate, water, or solvents. All of these applications are critical for safety and the quality of the finished products.

It must be noted that GWR is only suitable for interface measurement if the first fluid seen by the device has a lower dielectric constant than the next one and if there is a difference between the dielectric constant of the two fluids of at least 6. In typical applications the upper layer would have a low dielectric of less than 3, and the lower layer would have a high dielectric greater than 20. For example, the dielectrics of oil and gasoline range from 1.8 to 4. Water and water-based acids have high dielectrics of more than 50. It is important that the dielectric constant of the first product must be known and should be constant. If the DC value is not known, the latest GWR devices offer configuration software tools to determine it in the field. In cases where the lower dielectric material is denser than the higher dielectric one, the GWR can be mounted on the bottom of the vessel to measure the distance to the interface.

The maximum allowable upper product thickness/measuring range is primarily determined by the dielectric constants of the two liquids. For example, the maximum upper product thickness for the flexible single probe using Emerson’s Rosemount 5300 Guided Wave Radar is shown in Figure 3.

Fully submerged interface measurement
Most GWR devices can be used for measuring level or interface, with the interface only being measured in the submerged mode. A submerged interface application is one where the upper portion of the probe is in oil or a similar liquid and the interface between the upper fluid and lower fluid is the desired measurement. Often this measurement is made with the probe mounted in a bypass chamber.

A single lead probe should always be used as this provides the most distinct reference pulse. Ideally, there should be no air gap present at the top of the chamber, but this is not always possible. If there is an air pocket, this creates an offset in the measurement reading due to the difference in the speed of travel of the microwaves in the air space compared to the upper fluid. There are several ways to handle this. For example, if the air pocket is small and is within the upper blind zone of the device, it is possible to block any potential error in the reading of the surface (see Figure 4).

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