A Reliable Solution for the Level Measurement of Powders and Bulk Solids

Measuring the level of powders and bulk solids in bins, silos and other types of vessels comes with significant challenges. These include the shape of the material surface, material characteristics, internal vessel structure, filling and discharging rates, vessel dimensions, bulk density variances, dust inside the vessel (especially during filling) and others. Some of these challenges deal with converting a distance or level measurement to volume or weight. Others challenge the reliability of the level measurement technology chosen. This article will review a specific technology that has proven successful dealing with many of these challenges and is especially suited for the challenges associated with the level measurement of powders.

A Technology Most Suited for Solids
Smart cable-based sensors, guided wave radar, through-air radar, ultrasonic, laser, capacitance, a rope, rocks and tape measurers have all been used at one point or another to measure the level of a powder or other bulk solid in a vessel. Matching the correct technology to a specific application in order to produce a reliable and reasonably accurate measurement can sometimes be elusive, and is as much art as science in some cases.
    Each technology mentioned offers advantages, disadvantages, different price points, varying cost of ownership. One technology reported to be increasingly gaining widespread acceptance by users in a wide variety of industries is guided wave radar. Venture Development Corp. (VDC)¹ recently reported in its white paper on Global Process Level Measurement and Inventory Tank Gauging Markets that guided wave radar is the fourth largest segment of continuous level measurement technology for solids applications. In addition, the VDC reports that guided wave radar (also known as time domain reflectometry, or TDR) is the fastest growing technology segment. The growth rate and increasing popularity in solids applications is for good reason. It works, as you will see, when real-world examples are examined in applications from liquids to heavy, dusty powders.

Guided Wave Radar—Why It Works
Since the early 1990s, the use of microwaves (radar) to measure the level of powders and bulk solids has continued to evolve technically and grow in popularity. The use of microwaves has moved into the mainstream of the level measurement arena. One reason for this is “because microwaves are practically immune to process conditions such as temperature, pressure and atmospheric composition,” as stated in a July 2004 article on radar devices.²
     Guided wave radar devices measure the time-of-flight of a transmitted radar signal toward the material surface and its reflection back to the instrument. The measured time-of-flight is directly related to the distance of open space between the level measuring instrument and the material surface. Regardless of dust during filling, vessel construction and most material characteristics, a guided wave radar device provides years of reliable and trouble-free operation, delivering accuracy of the measurement to within as little as 0.3 in. in an application measuring bulk solids.
    One challenge to microwave-based level measurement devices is measuring low-dielectric materials. The radar waves are emitted into the vessel atmosphere. As soon as they come in contact with a change in dielectric constant (the material surface), a certain amount of the radar energy is reflected. However, some is absorbed. The lower the dielectric of the material, the more radar energy is absorbed and the lower the amount of reflection. As one manufacturer reports in a Web site FAQ³ on radar technology, “a rule-of-thumb is that the value of the dielectric constant represents the percentage of energy that is reflected. Thus a DC [dielectric constant] of eight means that eight percent of the emitted energy is reflected back to the transmitter.”

    Guided wave radar devices will typically measure lower dielectric materials than through-air radar devices. This is a distinct advantage. The reason for this is easy to understand. Because guided wave radar units utilize a wave guide to focus the radar energy, more energy is delivered to the material surface. Energy from through-air technologies (through-air radar and ultrasonic) disperses from its emitter, and the energy is reduced significantly when it comes in contact with the material surface. Guided wave radar using TDR technology will reliably measure materials with dielectric constant as low as 1.4, and possibly even lower. This is especially true when the dielectric constant of the material remains stable.
    In addition to using direct measuring techniques, low dielectric materials can be measured using bottom reflection techniques. In a bottom reflection mode the radar pulses are guided down the cable probe (wave guide) and pass through the low dielectric material. They then reflect off the counterweight attached at the bottom of the cable probe. The time-of-flight reflecting from the bottom through material is then compared to the same through air. The reflection time through material will be longer and the difference is related to the level of the material.
    Guided wave radar with TDR has other advantages over through-air technologies, including the elimination of problems associated with reflectivity of through-air emitted energy “off a vessel’s walls and internal tank obstructions, such as piping, nozzles, ladders, etc.,” as noted in a March 2003 article.⁴ Table 1 illustrates some advantages of guided wave radar devices using TDR technology versus other common technologies. As a result of these advantages guided wave radar is generally the best choice for powders and other bulk solid materials. Let’s take a look at an example application.

Measuring the Level of Cement Powder
Cement powder is one of the key ingredients in making concrete. Concrete is an essential construction material and is used in a variety of ways. Concrete is manufactured as a batch process and each batch can have slightly different properties dealing with strength, aggregate amount, color and other properties that may be important and specific to the intended use.
     The manufacturing facility used to produce concrete is typically known as a concrete batch plant. There are numerous types of plants and tens of thousands of these plants in North America alone. In addition to being a key ingredient in concrete, cement powder is also an expensive ingredient. Keeping track of the amount of cement used and remaining in a storage silo has an impact on the productivity and resultant profitability of a concrete batch plant.
    Batavia Concrete is located in Montgomery, IL, and is one of many concrete batch plants operated by Prairie Materials in northern Illinois. Like most concrete plants, Batavia uses cement and fly ash as ingredients. While point level bin indicators told Batavia personnel when materials were at either a high or low level, they had no idea how much material they had left in the storage bins at any given time. They needed a better approach to make them more efficient.
    After reviewing a proposal from Monitor Technologies LLC, Batavia Concrete opted for the installation of two of Monitor’s Flexar guided wave radar continuous level measurement sensors. These were installed in December 2005.
Batavia Concrete uses a multiple compartment batch plant manufactured by Erie Strayer of Erie, PA. As a supplier for many years to Erie, Monitor Technologies was familiar with the batch plant and bin design. The units employed were manufactured and shipped from Monitor’s Elburn, IL, facility and installed using 2-in. ANSI flanges on existing roof structures.
    Both units are in bins approximately 34 ft high. Cement powder has a dielectric constant that can range from 1.5 to over 2.5. Fly ash also can have a relatively low dielectric constant in the area of 1.5. In addition, both the cement and fly ash vessels are filled pneumatically. This creates heavy dust during filling. Dust is present during discharge as well. While fly ash can be as light as 35-45 lbs/cu ft, cement powder is rather heavy and typically between 85-95 lbs/cu ft. Guided wave radar cable probes are designed to withstand up to almost 4 tons of pull during material discharge. Guided wave radar such as this design can handle applications such as cement powder at up to 100 ft in overall height.
    The results using guided wave radar technology in cement powder and fly ash have been exemplary. Fred Thompson of Batavia Concrete has been satisfied with this solution ever since it was installed. “Observations during filling cycles were very good right off,” he says. “The guided wave radar unit doesn’t skip a beat, even with the heavy dust kicked up during pneumatic filling.”

Measuring Whey in a Feed Product Application
Whey is typically a by-product from the cheese manufacturing process. Whey is the watery part that separates from the curds. Whey is typically rich in lactose, minerals and vitamins, and contains lactalbumin and traces of fat. Whey is also known as the watery part of milk produced as raw milk sours and coagulates.
    One use for whey is in the manufacturing of certain types of feed products for domestic and livestock animals. Whey can also be processed into a dry powder form, with a fairly low dielectric constant of about 1.7, for use in the food and beverage industry.
    Socorro Design is an engineering and consulting firm specializing in automation projects within a wide variety of industries. Tom Campbell, owner and President of Socorro, needed to replace some level measurement systems that were not working properly at Westway Feed Products in Texas. The existing sensors were a through-air radar design. However, they were not working reliably due to the effect of coating on the antennae. Socorro and Westway needed a solution.
    While whey powder has a low dielectric constant, the raw whey is really a watery fluid with about 40-60 percent solids as a result of pressing cheese curds during the cheese manufacturing process. The dielectric of the whey is higher than that of the dried whey powder. Guided wave radar was acceptable for the application, and any build-up or coating on the cable had no affect.
    Two of three tanks at Westway contained whey and one contained another liquid. The two whey tanks are 28 ft in diameter and 32 ft tall. The tank for the other liquid is 12 ft in diameter and 20 ft high. The whey is being pumped from a nearby cheese plant into either of the two whey tanks at the Westway facility. The two whey tanks can be circulated and the flow is metered and monitored using highly accurate Coriolis mass flow meters.
    The whey is processed on site at Westway and then either pumped into trucks for delivery to feed manufacturing facilities or pumped to a local ingredients manufacturing facility where it is dried to create a whey solid that resembles cottage cheese. This material is further processed by grinding the dried material into a powder, which is then stored in bins for bagging or bulk shipment.
    Whey is a valuable commodity and is added to a variety of animal feeds. The level sensing in each of the whey tanks and the other liquid tank needed to be reliable and accurate. Guided wave radar was selected due to its resistance to coating and the ability to handle relatively low dielectric constant materials.
    In this case all three vessels were outfitted with Monitor Technologies’ Flexar guided wave radar units. They performed right from the start. The level readings are connected into an Allen-Bradley PanelView system inside the facility. The entire process of loading and unloading the whey tanks will be automated and the level readings from the Flexar guided wave radar units are key.

Conclusion
Guided wave radar (TDR) technology has come a long way in a little over a decade as a viable, reliable and accurate level measurement technology. It is virtually universal in powders and bulk solids when properly applied by a reputable manufacturer that understands the challenges associated with powder and bulk solids applications. It offers technological and application advantages over the most common devices available and is of great value.

Joseph D. Lewis is Vice President—Marketing & Sales, Monitor Technologies, specialists in level measurement and inventory management. For more information call 800-601-6302 or visit www.monitortech.com.

1. Venture Development Corp. (VDC) is a technology market research and strategy company located in Natick, MA.
2. Chemie Technik, July, 2004, “Half-Full or Half-Empty—TDR Versus Through-Air Radar.”
3. Emerson Process located at www.emersonprocess.com.
4. Control Engineering, March, 2003, Technology Update at www.manufacturing.net/ctl.