Non-contacting radar level transmitters are widely implemented throughout the manufacturing and process industries, especially in measurement applications where high accuracy, reliability and minimal maintenance are important requirements. These versatile devices are suitable for measuring the level of liquids, sludges, slurries and solids. However, one of the most challenging applications for this technology has traditionally been measuring level in tanks with internal obstructions, such as agitators, heating coils, pipes, ladders or baffles.
Vessels containing these types of structures can create difficulty for non-contacting radar devices because they can interfere with their signal processing. Radar level sensors work by emitting microwave signals that bounce off the material surface and are reflected back to the transmitter. These reflected signals are known as echoes, and they enable the material level to be calculated. Obstructions within a tank can, however, also reflect microwave signals, creating what are known as false echoes. When this happens, it can become challenging for the radar device to identify which echo is from the material surface and which are false echoes coming from obstructions. As some liquids have a low dielectric constant – meaning that they poorly reflect radar signals – even slight interference from obstructions can have a major impact on the signal in these cases, which further complicates level measurement.
The consequences of false echoes
When a radar device interprets a false echo as a surface echo, it results in an inaccurate level reading which could have serious consequences, including an overfill and subsequent spill. In tanks containing volatile or flammable substances, a spill would not only create an obvious safety hazard and endanger personnel but could also cause environmental harm and require a costly cleanup. False echoes can also prematurely stop the filling process, leading to tanks being underfilled. This can disrupt production schedules, result in product shortages or cause inefficiencies in downstream processes. In addition, false echoes can compromise product quality and reduce process efficiency, for example in batch processes that rely on accurate volume measurements and may yield inconsistent results if level readings are inaccurate.
Transmitter positioning
Although tanks with obstructions can present a challenge for non-contacting radar level transmitters there are measures that can be taken to mitigate the difficulties involved. The first thing to consider should be the position of tank openings. If there is an existing nozzle located with an entirely unobstructed view to the product surface, then it is obviously best practice to position the radar device there. However, it is rarely the case that a nozzle will be ideally situated, which means that other solutions are required.
Deflector plates
In tanks with internal obstructions, some technology vendors will recommend using deflector plates. These plates are typically installed at angles, close to the obstructions, so that they redirect radar waves that would otherwise reflect off these objects back to the transmitter. By angling the plate, unwanted echoes are directed towards tank walls or other areas where they will dissipate instead of reflecting straight back to the radar sensor. Eliminating these echoes enables the transmitter to more easily distinguish the true material level allowing for a more stable and consistent reading.
However, installing deflector plates can be challenging, especially in tanks with limited access or high complexity. Plates positioned incorrectly can cause unwanted reflections or even block part of the radar beam, resulting in signal loss or creating blind spots in the measurement. In addition, if the tank contains sticky, viscous or dusty materials, buildup on the plates can alter the angle of reflection or create false echoes.
False echo suppression
Traditional radar transmitters are equipped with user-initiated routines that can help mitigate these challenges. The user must tell the device which echo is the surface echo. The device will then create a threshold that blocks the false echoes from consideration as valid echoes.
False signal suppression can be effective, but it has limitations. To ensure all false echoes are managed, the tank should be emptied so the device can see them. Having to empty the tank is an inconvenience that can needlessly prolong the commissioning process. Also, we know that the character of echoes from obstructions can change over time. This can happen, for example, with changes in the vapour space of the tank, changes in temperature and also from coating from the process. Because the device creates the threshold based on current echo characteristics, when those echoes change, the threshold setting the device originally created may no longer be sufficient to exclude a false echo from consideration. This could reintroduce an inaccurate level measurement with the associated risks described above and would require having to empty the tank to re-run false signal suppression.
Advanced signal processing technique
Rosemount non-contacting radar level transmitters from Emerson feature an advanced solution that enables accurate and reliable level measurements in tanks with obstructions, without the complexity of having to install deflector plates or run false echo suppression. Instead, these devices feature a patented intelligent signal processing technique known as Smart Echo Supervision, which dynamically evaluates the behaviour of all viable echoes in real time. It then ranks all the echoes according to which behave the most like a material surface echo over time. The echo that most behaves like a reflection from the material surface is then tracked as the genuine surface echo and all other echoes are automatically suppressed. The technology continually adapts to changes in the echo profile, which can happen as the surface level changes or as tank conditions vary.
These features enable Smart Echo Supervision to reduce and often eliminate the need for any type of intervention against false echoes. On occasions when false echoes do require intervention, it can be performed quickly and easily through a graphical user interface, simply by right clicking an echo to suppress it.
Smart Echo Supervision has been specifically designed to overcome the unique challenges posed by obstructions and complex tank configurations. By automatically excluding false echoes, adapting to changing conditions and providing stable performance over time, this comprehensive approach ensures faster device commissioning and reliable level readings that operators can trust, even in the most complex tanks.
For more information contact Brenna Roseth, Emerson Automation Solutions,
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