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Powder/Bulk Solids

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Measuring and Controlling Level

July 14, 2010

By Keith Riley, Endress+Hauser

Level measurement of solids sometimes presents technical problems that are difficult to solve with standard level sensors. In other cases, it’s technically simple to measure the level, but difficult to justify the expense of the appropriate sensors. Here are examples of both scenarios.

Measuring Level of Low Dielectric CVPC Powder

Low dielectric powders are difficult to measure during pneumatic filling operations because the process is very noisy and generates false echoes during filling. The powder also doesn’t provide a good echo for a radar level sensor. Other types of level sensors are not suitable for powder.

A Micropilot M FMR250 radar level sensor (red sensor) is attached to each of the moveable discharge chutes. When the chute is positioned over a silo, the sensor looks through a 6-in. hole in the top of the silo to measure level during filling.

Dust typically generated during the fill cycle effectively “blinds” other technologies such as ultrasonic or laser devices. The suspended dust cloud has sufficient density to reflect the mechanical ultrasonic pulse. Similarly, excess dust can also disrupt the beam from a laser device and interfere with its operation.

A specialty chemical producer in Kentucky makes a CPVC powder with a very low dielectric constant of 1.3 to 1.5. The powder is used as an additive in finished products such as engine oils, driveline lubricants, metal working fluids, over-the-counter pharmaceutical products, and performance coatings and inks. The company wanted to measure the level in a tall carbon steel cylindrical silo with a cone bottom and polyurethane lining. The silo is 78 ft tall with a diameter of 26 ft.

The goal was to provide continuous level indication during both the fill and empty cycles, and to measure as deep into the silo as possible. The fill stream is through a 6-in. nozzle providing a fill rate of approximately 20,000 lb/hr. The company tried without success to use a free-space radar unit, and other vendors declined to even attempt the application.

Endress+Hauser (E+H) installed a Levelflex M FMP40 guided wave radar sensor with a grounded cable probe that is connected to a threaded knock out in the vessel cone. Because of the low dielectric of the CPVC powder, the level measurement is typically based upon the End of Probe (EOP) signal evaluation.

With guided wave radar technology and low dielectric process materials there is always a portion of the electromagnetic signal launched by the electronics that is not reflected when the pulse contacts the process material. The portion of the signal that is not reflected travels on and is reflected at the end of the probe. This “end of probe” signal is used to create a redundant measuring principle so it will accurately determine the level, should there be no reflection coming from the product itself.

The Levelflex FMP40 is a top-mounted compact level transmitter that transmits microwave impulse radar signals with a frequency of 1 GHz. The unit has a redundant measurement signal system using standard signal reflection and EOP signal evaluation.

Even with the advanced algorithms used for EOP the level signal was at times a bit jumpy due to the fill noise. After making some necessary setting adjustments, E+H was able to correct this and provide smooth and accurate level indication.

Measuring Level in 60 Silos

The problem at a roofing manufacturing plant in California was not how to measure the level of roofing granules (radar level sensors made that part easy), but, instead, how to cost effectively measure the level in 60 silos. Installing 60 individual radar level sensors would have been too expensive, so E+H was asked to come up with a better solution.

Measuring level in multiple silos is often best accomplished by using a few movable level instruments as opposed to one instrument per silo.

The plant crushes, treats, and colors rocks for granules used on roofing shingles. It stores finished products in 60 silos, each of which are 45 ft tall. The silos are arranged in three rows of 20 connected silos. The solution was to use three Micropilot M FMR250 radar level sensors from E+H to measure level in all 60 silos.

The Micropilot M FMR250 is a two-wire, high-frequency, non-intrusive radar level device for solid applications up to 230 ft. The sensor works by launching a high-frequency electromagnetic wave from an antenna horn. Signal energy is reflected by the product and received by the antenna. The time it takes for the signal to travel to the level and reflect back to the antenna is measured. This time is then calculated into a 4-20 mA output signal proportional to level.

A FMR250 sensor was mounted on each of three moveable chutes, which discharge finished product into the silos. As a chute discharges, the FMR250 “looks down” into the silo to measure level. To position the sensor over each fill point, a laser controls the chute’s location so that the radar sensor is precisely aligned above a 6-in. hole, cut in the top of each silo.

A 4-in.-tall collar around the hole prevents material contamination. The FMR250’s horn “lands” about 1 in. above the collar and reads through the 6-in. hole to the product below.

The customer gets a reliable and repeatable level measurement, which helps inventory control and eliminates possible overfill to each silo during filling. Monitoring on moveable chutes provided the required level information, and eliminated the expense of putting a radar unit in every silo.

Keith Riley is a level product manager for Endress+Hauser (Greenwood, IN). Prior to that, he worked as a product manager and regional sales manager for L.J. Star and TycoValves (Penberthy). Overall, he has more than 15 years of experience in the process industry. Riley has a BA in marketing from Iowa State University.