Chemical processors have an array of technologies to choose from for liquid level measurement. Through-air radar solutions are common, as are guided wave, pressure, and radiation-based level solutions, just to name a few. The options are plentiful and each measurement technology has a solid track record in the chemical processing industry. Given the volume of available instruments, users in chemical plants often stick with the level measurement they know instead of the right one for their process; they make no decision instead of the right decision. Call it the curse of good choices.
Yet the choice may be getting a little clearer for end-users in chemical plants. The recent introduction of liquid level radar sensors with 80 GHz transmission frequency offers certain advantages users in the chemical industry may find especially beneficial. This paper highlights three of them and gives a brief of overview of the limitations of 80 GHz radar sensors so users in chemical plants can make an informed decision for their liquid level applications.
In applications measuring harsh, caustic chemicals, many operators separate their level detectors from the process via a ball valve. This is a common practice and frankly, a smart move. External access allows the user to keep the process moving during maintenance work or when replacing a faulty sensor. You can also look at the wisdom of mounting a level instrument on a ball valve this way: If a problem arises with an instrument inside of a tank, that particular tank—or worse, an entire line—might have to be shut down, potentially leading to thousands of dollars in lost production. That’s a bad day at work. Many plants choose to play it safe and mount sensors on a ball valve.
Pairing a ball valve with a 26 GHz radar sensor has a history of difficulty because the interior surfaces of the valve reflect radar signals back to the sensor, creating an excess of signal noise at the top of the measurement. Similar noise is created when mounting a 26 GHz radar instrument on a tall nozzle, as reflections from the nozzle’s narrow walls reflect back to the sensor antenna.
80 GHz radar technology presents chemical-industry users new option for liquid level measurement, even when a sensor is mounted on a ball valve or a nozzle. These higher-frequency sensors emit a radar beam as narrow as 3°, so a smaller portion of the signals contact the valve or nozzle walls, minimizing noise and creating an easier experience for users who now have a clear picture of the level inside a tank. 80 GHz radar sensors have been installed successfully on a broad range of 3-in. and 4-in. ball valves and on nozzles as tall as 5 feet.
I’d guess that everyone who works in chemical processing has struggled to accurately measure a liquid with low dielectric constant (dK). These unreflective products demand a highly-sensitive measurement device to detect the weak signals they return to the antenna. In the past, if a liquid’s dielectric constant was too low, radar might not be sensitive enough to measure it. That’s changed due to advancements in dynamic range.
Measured in decibels (dB), dynamic range is an indicator of sensitivity. The larger the dynamic range of a radar sensor, the smaller the signals it can measure. VEGA’s 80 GHz radar for liquid level, the VEGAPULS 64, has a dynamic range of 120 dB, large enough to measure any liquid chemical regardless of dK value—dielectric constant isn’t even part of the conversation anymore, and 80 GHz radar is compatible with any liquid chemical.
Portal windows on process vessels are familiar sights in the chemical industry. Historically, users have installed these portholes to visually check the level, ensure mixes are reacting properly, and confirm everything looks as it should.
Vessels with windows permit users to measure liquid level in a unique way: by mounting a radar sensor above the glass. Radar microwaves penetrate the glass, reach the product inside, and reflect through the glass back to the sensor. This eliminates two major expenses because users are spared from retrofitting a tank to accommodate a sensor and can continue running a process during installation. Functionally, nothing changes as users can simply move the sensor for a moment to look through the glass and see what’s happening inside a vessel.
As a matter of disclosure, you should know that this isn’t new. Any radar sensor can measure liquid level through glass, but the ease and clarity of this measurement is new and is made possible by 80 GHz transmission frequency. Windows are often welded, bolted, or clamped directly onto a tank wall or roof with a circular flange, while others are mounted on a nozzle. As is the case with ball valves and nozzles, the sides of the flange reflect signals to the antenna and create noise. The interference forces operators to leave empty space inside a tank to make a clear distinction between the signal received from the vessel and the signal received from the product.
Further complicating the use of last-generation, 26 GHz sensors above such vessels is the fact that most windows are installed at a natural slope in the tank. This angle narrows the path to the liquid, increasing the degree of difficulty in setting up a sensor so the beam is perpendicular to the product. Perpendicularity is important because it’s in direct relationship to the strength of the signal the sensor receives. In order to minimize the small signals that bounce from the angled window back to the sensor, it’s recommended that users pair a 26 GHz radar sensor with a window installed at least a 30° angle. This forces users to choose between a strong signal from the product accompanied by reflections from the glass or a weak signal from the product and no reflections from the glass. Call that the curse of bad choices.
80 GHz radar sensors solve the problems signal noise and narrow paths create because of signal focusing. Installed above any window installed at a 5-10° angle, the narrow radar beam misses the sides of the flange and travels a tight path to the product without sacrificing signal strength.