Digital Feature: Enhanced radar level measurement performance solves key challenges in downstream interface applications

J. Söderström, Emerson, Gothenburg, Sweden

Improvements in the latest guided wave radar (GWR) level transmitters have solved some key challenges in interface measurement, potentially saving energy and petrochemical companies millions of dollars per year. The efficiency with which oil and water are separated and flow out of a separator into independent channels with as little product cross-contamination as possible, has a significant effect on the final product quality and profitability. Optimizing the separation process requires instrumentation that can accurately and reliably measure the interface level between the liquids, which is the point at which the upper level of the water meets the lower level of the oil.

GWR. GWR is a well-established and field-proven technology in interface-level measurement applications (FIG. 1). GWR transmitters deliver a top-down, direct measurement of the distance to the surface of the oil and the interface between oil and water. Changes in pressure, temperature and most vapor space conditions have no impact on the measurement accuracy of GWR devices. Moreover, changes in dielectric constant, conductivity or density do not need any compensation. GWR devices are easy to install and provide accurate and reliable measurements in vessels of all sizes. They have no moving parts and therefore require minimal maintenance, and their advanced diagnostics ensure that operators are quickly alerted to any degradation in performance.

FIG. 1. A typical GWR level and interface measurement application.

Traditional technologies in level and interface applications, such as displacers, have several disadvantages that GWR circumvents. Changes in fluid density can affect the measurement accuracy of displacers, while their moving parts require frequent cleaning and replacement, creating higher maintenance costs. Displacers are also affected by mechanical vibration, creating the potential for false readings and safety risks. Consequently, many refiners have implemented GWR transmitters to replace displacers.

Even though the advantages of GWR transmitters have led to the technology being widely applied across the energy and petrochemical industries, there are still certain aspects of their performance in interface applications that were causing major concern for refiners and need improvement.

How do GWR transmitters measure interface level? GWR transmitters emit low-energy microwave pulses that are guided along a probe submerged in the process media. When a pulse reaches the surface of the oil, a significant amount of the microwave energy is reflected 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 certain proportion of the pulse continues to travel through the oil, a second reflection (or ‘echo’) occurs at the interface between the oil and water. The interface level is calculated from the time difference between the first reflection and the echo.

Required thickness of the upper layer. When using a GWR transmitter for interface level measurement, the oil layer must be of a certain thickness, so that the device can distinguish between the signal echoes from the oil and water. This required thickness has ranged between 50 mm and 200 mm, depending on the transmitter model and probe style being used.

Although GWR has become the preferred technology to provide interface level measurement in oil production and refining, its inability to detect an oil layer less than 50 mm thick can be costly. For example, at a typical multiple-well pad facility, unaccounted-for or unauthorized hauling of produced oil in the water tank could cost the producer more than $1 MM in lost revenue, while excess water in the oil tank could cause unexpected capacity loss or a spill. Similar examples can be found throughout the energy and petrochemical value chain, including in downstream applications.

Working with a global energy and petrochemical company to develop an improved solution. Determined to improve the performance of GWR transmitters in interface measurement applications, the author’s company worked in collaboration with engineers from a global energy and petrochemical company on product development initiatives. Research and development (R&D) engineers from the author’s company spent almost 2 yr working with members of the company’s automation, control and optimization team to test and develop the enhanced device. The collaboration resulted in several innovative technological developments in the author’s company’s level transmitter (FIG. 2) that increased its measurement robustness in interface applications, potentially saving refiners millions of dollars per year.

FIG. 2. The author’s company’s proprietary level transmittera provides functionality that enables detection of a thinner top liquid layer.

The engineers developed a solution to reduce the detectable thickness of the oil layer in the form of a unique peak-in-peak software algorithm. This algorithm enables a transmitter to detect signal peaks that are closer together without having to decrease its signal bandwidth, which would reduce its high sensitivity and its ability to overcome liquid disturbances. Using this new algorithm, transmitters can now detect a top liquid layer that is just 25 mm thick (FIG. 3). The lowering of the previous minimum detectable thickness optimizes separation process performance, minimizes product cross-contamination and increases operational efficiency and profitability.

FIG. 3. The ability to detect an upper liquid layer that is only 25 mm thick improves insight into the separation process.

Measuring to the top of a tank or vessel. Another important aim of the collaboration was to create a means of enabling GWR transmitters to perform accurate level and interface measurements right to the top of a tank or vessel. This has traditionally proved challenging, even when using a large diameter coaxial probe. These probes provide the strongest return signal, have no upper dead zone and their measurement accuracy is unaffected by external disturbances such as protruding welds and side taps. However, they have been unable to provide accurate measurements right to the top of the tank due to high amplitude noise.

High amplitude noise is created as microwaves pass through the process seal between the transmitter and the probe. The engineers were able to eliminate the noise by improving the design of the process seal. The improved design enables accurate and repeatable measurement right to the top of the tank and maximizes tank capacity, thereby increasing throughput and profitability.

Configuration. A further challenge that the engineers were determined to solve was to simplify the configuration requirements of GWR transmitters to prevent costly measurement inaccuracies from occurring during critical process stages, such as start-up and shutdown.

GWR transmitters are often installed within still-pipes or chambers in interface applications, as this eliminates issues with disturbing obstacles, agitation and turbulence, which can affect the accuracy and reliability of the measurement. There are some interface measurement applications where during normal operation the chamber is completely filled with the process material and others where it is partially filled. A GWR transmitter will be configured specifically for one of those two applications. However, a device that is configured for a normally completely filled chamber will give inaccurate measurements on any occasions when the level drops so that the chamber becomes partially filled. This can happen during critical times such as startup, shutdown and certain process upsets, and can therefore potentially have safety implications. Likewise, a transmitter that is configured for a normally partially filled chamber will give inaccurate measurements when it is completely filled.

The engineers solved this challenge through an enhanced signal processing algorithm that enables transmitters to have a single configuration for both completely filled and partially filled chambers, thereby reducing complexity, ensuring measurement accuracy and increasing safety. Further helping to make transmitters easier to configure, operate and maintain, both the device descriptor and device type manager configuration methods have been updated to the latest device dashboard. In addition, the configuration software can switch between different alarm limit preferences, thereby further increasing safety.

Double bounces. A further challenge when using GWR technology for interface level measurement has been the so-called ‘double bounce.’ This is when a radar signal bounces back and forth between the liquid surface and the tank roof or another object in the tank before the transmitter detects it. These signals often have low amplitude and are ignored by the transmitter. However, the amplitude may sometimes be strong enough for the transmitter to interpret them as a reflection from the liquid surface, leading to a false reading and inaccurate measurement.

The author’s company’s engineers overcame this challenge by creating a factory-configured threshold setting that determines the minimum signal amplitude limits of key parameters such as surface echo, interface echo, reference peak and end-of-probe peak. In interface applications, an echo below the threshold is assumed to be oil, and an echo above the threshold is assumed to be water. This increases measurement reliability by preventing double bounces from being mistaken as an interface level measurement and enables more predictable behavior. This creates a true plug-and-play device that simplifies installation and operation.

Displacer replacement at large refinery. The energy and petrochemical company with which the author’s company collaborated performed a site inventory check at one of its large refineries and found that a significant number of its instruments had some form of incorrect setting, including many that were installed within safety loops. Having a device that is very easy to use and has only a single configuration that is suitable for any application helps to enhance safety considerably as it minimizes the risk of errors that occur during every implementation step, from ordering, through commissioning to operation.

As a result of the improved performance achieved by these developments, the company is making GWR its technology of choice for interface applications. It has embarked upon a program to replace thousands of its displacers with GWR transmitters and will select GWR devices for its new interface measurement applications around the world. With its minimized maintenance requirements, more robust measurement and ability to reduce cross-contamination by measuring a thin oil layer, GWR technology can provide refiners with increased peace of mind and save them millions of dollars per year.


a Emerson’s Rosemount 5300 level transmitter

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