Keeping on the Level

Measuring the Level of Solids Can Be Tricky. Ultrasonic and Laser Measurement May Be the Answer

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For Catherine Steukers, asset manager of the polystyrene and Styrolux plant at BASF, Antwerp, Belgium, measuring the level in hoppers containing various types of plastic granulate isn’t optional.

“We need the level measurement to know how many tons of product we have in storage,” she says. “Accurate level measurement allows us to fully utilize the hoppers to their maximums. Also, the laser can be pointed to the exit of the hopper to ensure that the cone is empty. Despite the ability to point the laser, there is always some inaccuracy in the amount of plastics remaining.”

Installation Guidelines

General guidelines for installing ultrasonic and radar level gauges include mounting them about one-third of the diameter from the hopper wall. Sonnier suggests that “The ultrasonic sensor initially should be aimed at the discharge point and positioned with an aiming device so as to increase signal strength. In general, the return signal will be strongest when the sensor signal is perpendicular to the angle of repose of the material.”
Eduardo (Eddie) Yi, electrical engineer for the Aggregate Division of Cemex Florida, in Miami, is quick to caution that not all vessels are symmetrical.

“We need to measure the level of aggregate in vessels that can have multiple draw points,” he says. “With three draw points, the cone bottom may exhibit ratholing and bridging at the same time. What the instrument sees could be very different than what a person sees. Sometimes a vessel with a 50% level measurement appears to be only 30% full when viewed by a person. Therefore, level technologies with small beam sizes may not work as well as those with larger beams.”

However Ivo Radanov, laser level product manager at level measurement systems maker K-Tek, points out that “Laser light energy is scattered from the material surface in all directions. This means that laser level measurements are independent of the angle of the material encountered by the laser beam because part of the scattered reflection will return to the sensor,” thus mitigating signal strength issues in many applications. (See “Lasers Come to Level Measurement,” Control, Feb., 2006.)

Mikos handles this phenomenon by mounting his ultrasonic transmitter to measure vertically approximately midway between the center of the tank and the tank wall.

De-Rating Ultrasonics

More powerful ultrasonic signals are necessary for solids applications as compared to liquids, so many suppliers de-rate their ultrasonic systems when they are used for solids. Most suppliers de-rate by a factor of two; that is, a 60-meter system for liquids would be de-rated to 30 meters for solids. However, Sonnier typically de-rates his ultrasonic equipment by a factor of three to account for material properties, dust and similar problems that degrade signal strength. He also cautions users to be sure that the transceiver can handle the next-sized sensor—just in case an unexpected problem surfaces that requires more power.

Ultrasonic level transmitter ranges may also need to be de-rated due to noise. Some of Yi’s applications “Include metal tanks that can be so noisy when filled with granular material that the acoustic noise drowns out the ultrasonic signal to the extent that the level transmitter fails to operate until the filling operation is stopped. Higher power transducers should be used in these applications.”

Application knowledge is important to help determine which technology is applicable. Between ultrasonic and radar level measurement, Sonnier tends to supply ultrasonic level gauges for applications that are under approximately 7.5 m (25 ft) and radar level gauges for applications over 15 m (50 ft). Technology selection for ranges between 7.5 m and 15 m typically is not related to distance, but rather other factors, such as the bin type, bin shape, material properties and the like. Further, ultrasonic level gauges usually exhibit a price advantage over radar level gauges, except that power requirements for longer ranges tend to increase the pricing for the ultrasonic solution, making the radar level gauges more attractive for long-range applications.

Radanov agrees. He points out, “Laser technology can be applied to measure level using a narrow beam at long distances (as far as 120 m) for any solid surface at any angle, which makes the application less application-dependent. Installation is generally simpler because virtually any existing nozzle size can be used, and the narrow beam allows installation near the wall of the bin. Laser level transmitters can also measure into the silo cone and chute to ensure that the bin is completely empty. This ability is important in many plastics and food-processing applications in order to avoid cross-contamination between campaigns. In addition, scanning laser transmitters are able to measure the non-uniform profile of the material and calculate the volume of material in the bin. This ability differentiates it from other solids level measurement technologies.”

Steukers found that the setup for radar level gauges can take three hours or more per hopper, depending on material properties and the number of false reflections received—especially with plastics that have relatively weak reflections. In contrast, “Laser level gauges are not dependent on material properties, so the configuration for one hopper can be transposed to identical hoppers and operation is more reliable—even for different types and colors of plastics. In other words, product properties are not a problem for us with this technology.”

Mikos advises users to remember practical matters when dealing with solids. In particular, some “applications utilize intermittent vibrators to keep the material from plugging. But do not get carried away with too much of a good thing because running the vibrators all the time will pack our material too hard and cause it to plug.

Also, be cognizant of the battery backup features associated with your equipment. Some of our ultrasonic transmitters are almost 10 years old, so the on-board memory backup battery is near the end of its service life. We now have to replace these batteries to avoid the potential of configuration loss in the event of a power failure. This can pose significant problems during both extended and intermittent power failures.” Overall, Mikos notes that this technology has improved to the extent that he notes fewer problems such as caused by false echoes.

Steukers also had some practical problems with level transmitters. Dust would stick on the laser and cause a loss of measurement. It appeared that the manner in which the dust tube was purged was incorrect,  she reported, “so an improved dust tube purge was installed that now appears to keep the laser clean and eliminate the loss of measurement.”

It should be clear that measuring the level of solids can be perplexing. Measurement errors can occur even when the level transmitter measures perfectly and without error. In many applications, solids level measurements are simply “good enough” for the application.

David W. Spitzer is a principal in Spitzer and Boyes, LLC. Contact him at 845.623.1830 or www.spitzerandboyes.com

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