Level measurement design: Guided wave radar (GWR) transmitters can detect a thinner top liquid layer in interface level measurement applications, making the separation process more efficient and preventing unwanted cross-contamination.
The ability to accurately measure an interface level, when a vessel contains two immiscible liquids such as oil and water, is vital in separation process applications, as it helps prevent costly cross-contamination when the liquids flow into independent channels. One of the most widely-applied solutions in interface level measurement applications, such as separators, is guided wave radar (GWR), which provides a number of advantages over other technologies.
Measuring the interface level in tanks
GWR provides accurate and reliable measurements in vessels of all sizes. No compensation is required when there are changes in the density, dielectric constant or conductivity of the fluid, while changes in pressure, temperature and most vapor space conditions have no impact on its measurement accuracy. In addition, GWR devices also are easy to install and need minimal maintenance because they have no moving parts.
GWR transmitters are based on microwave technology. The probe of the transmitter extends to the full depth of the vessel, and a low-energy pulse of microwaves is guided down the probe.
At the surface of the upper liquid level, a significant proportion of the microwave energy is reflected up the probe to the transmitter, which measures the time delay between the transmitted and received echo signal. An onboard microprocessor then calculates the distance to the liquid surface. As a proportion of the pulse continues down the probe, a second echo can then be detected from the interface between the two liquids. This enables the device to accurately measure the interface level.
Liquids with a dielectric constant below 1.5 can be difficult to measure because the echo signal is weak, but the high sensitivity of GWR transmitters enables them to meet this challenge, and also overcome liquid disturbances in the vessel.
Better oil and water separation
When performing a measurement from the top of the vessel, the upper liquid must have a lower dielectric constant than the liquid below. The upper liquid layer also must be of a certain minimum thickness to enable the transmitter to make a distinction between the echoes of the two liquids. This minimum detectable thickness is typically between 50 and 200 mm, depending on the transmitter model and probe style being used. The latest GWR transmitters incorporate advanced technology that enables the minimum detectable thickness to be reduced to 25 mm.
An interface algorithm allows the transmitter to detect signal peaks that are closer together without having to decrease its signal bandwidth, which would reduce its high sensitivity. Being able to detect a thinner top liquid layer helps prevent unwanted product ingress and optimizes the performance of the separation process, helping maximize operational efficiency and profitability.
Christoffer Widahl, manager solutions management, Emerson. Edited by Mark T. Hoske, content manager, Control Engineering, CFE Media, [email protected].
KEYWORDS: Automation design, process sensors
Interface liquid level measurement has challenges.
Separators are more accurate with reliable measurements.
Guided-wave radar measures a smaller upper layer.
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