Maintain Level in Granular, Heavy, or Coarse Materials

Keeping track of level in a bin or silo full of solid materials has never been an easy task. The nature of the materials to be sensed conspires to subvert operation of the instrumentation used. Additionally, the medium itself often behaves in a manner that makes obtaining an accurate measurement close to impossible.

By Dick Johnson, Control Engineering May 1, 2002
  • Instrumentation and process sensing

  • Level sensing

  • Continuous level sensing

  • Point level sensing

Make little rocks out of big ones
Coal bunker application: Easy reliability retrofit

Keeping track of level in a bin or silo full of solid materials has never been an easy task. The nature of the materials to be sensed conspires to subvert operation of the instrumentation used. Additionally, the medium itself often behaves in a manner that makes obtaining an accurate measurement close to impossible.

Although non-liquid media are usually considered to be less aggressive or corrosive than their liquid counterparts, they can be heavy and have sharp edges, just perfect for damaging (or completely removing) any mechanical point-level device that protrudes into a vessel. Additionally, many granular materials produce copious amounts of dust and grit during a loading or unloading operation, which can interfere with the noncontact measurement technologies. Dust and grit can also damage mechanical devices over time if moving parts are not properly sealed against these contaminants. Coarse materials like crushed limestone can trap moisture that can cause rust and corrosion of metal parts in level instrument assemblies.

Solid materials, especially granular ones, rarely seek a level. The coarser the grain structures the more likely materials are to clump, bridge, leave voids, or pile up. And although this type of material flow behavior may not be a problem in many storage scenarios, it makes determination of accurate level difficult.

Get to the point

Point-level detection instrumentation performs alarm functions or feeds information to level control systems. Alarming point devices provide an indication that material has reached some point in the storage and handling process that the control system needs to ‘know’ about.

In a ‘working’ bulk solids system, level control devices often signal that a bin or vessel is either completely filled or completely empty. Reasons for tracking these points are usually to prevent overfilling and subsequent spillage or to avoid emptying of vessel contents in cases where interruptions in material feed or extended conveyor duty cycles need to be controlled.

Although quite of few technologies can do point detection, not all are adaptable to solids. Among the most common technologies in use are buoyancy (float-type), conductivity, capacitance, ultrasonic (gap-type), thermal dispersion, and vibration switches. Buoyancy and conductivity devices are not usable with solid materials. Ultrasonic gap sensors are not readily adaptable either. The gap sensor relies on the media to be sensed to fill an air gap between a transmitting and receiving piezoelectric crystal and allow generated ‘sound’ to travel to the target crystal. Once received, an electrical output signal is generated to sound an alarm or trip a switch.

Solid material in a gap would allow sound energy to reach the receiving crystal, but the device would not reset properly over time because solid materials eventually would build up in the gap rendering the switch useless. Large-sized solids (those near to or even larger than the instrument’s opening) would not fill the gap at all.

Depends on location

According to Wayne Shannon, product manager for Magnetrol International Inc. (Downers Grove, Ill.), several technologies are usable in bulk-solids applications. These include capacitance, thermal dispersion, and vibrating switch types.

Capacitance types measure the capacitance that can be stored between a probe and another conducting surface, usually the wall of a storage vessel. Capacitance devices are easy to calibrate (although they require changing the level during setup), feature adjustable setpoints, tolerate high temperatures and pressures, and available with self-diagnostic capabilities. The most serious drawback of using capacitance-based devices is that shifting dielectric constants (common in solid materials) can cause serious inaccuracy.

‘Although adaptable to bulk solids, they are not recommended,’ says Mr. Shannon. ‘Heavy, coarse materials are especially hard on probes and place high pull forces on them during an emptying process. Maintaining the air gap is also difficult with solid media.’

A similar technology called RF (radio frequency) admittance offers an accuracy advantage over capacitance sensing where probe coating is a problem. According to Bill Sholette, product manager at Ametek Drexelbrook (Horsham, Pa.), ‘Many people use the terms (RF admittance and capacitance) interchangeably, but there is a difference. In a capacitance measurement, only the capacitive property of the material is measured. Coating on the probe has a capacitive element, so it looks like ‘level’ to the transmitter. In admittance-based sensing both resistive and capacitive properties are measured. Using the transmitter’s electronics, conductive coatings on the probe can be ignored assuring accurate level determination.’

Thermal dispersion technology uses two RTDs that project into the media to be sensed, one that measures media temperature and one that is heated. When level rises or falls, the heated probe is cooled or reheated, a temperature differential is sensed, and an output produced. This technology withstands very high pressures and temperatures, requires only a small process connection, and can be mounted in any orientation. However, it has slow response time and can, due to invasive mounting, be easily damaged by heavy solids. Evaporative cooling caused by trapped pockets of moisture and condensation formed in solid material handling systems can lead to reading inaccuracies.

Vibration switches feature a probe-type sensor that resonates at a set frequency. When the sensor touches anything (solid or liquid) the vibration is dampened and its electronics provide an output signal. Vibration devices are low cost, very sensitive, and provide flexible mounting arrangements.

These devices are also invasive and, as such, are susceptible to damage from heavy materials and probe erosion unless protected. Mounting should be done so that solid materials do not build up around the process connection of the probe and permanently dampen its vibration. Because high heat damages electronic components over time, they are intended for use in low-temperature (under 320 °F) environments.

Measuring continuously

In processes where batching takes place or more accurate inventory values are required, continuous level values over the entire depth of the storage vessel are required. When solids, especially heavy ones, are the media to be measured, the number of available technologies are limited. Gone are the differential pressure, displacer, and magnetostrictive technologies that are adaptable to liquids only, because neither Bernoulli’s nor Archimedes’ principles carries over to the world of solid materials. RF capacitance-based technologies are adaptable to continuous measurement but the same problems that plague it for point measurement still apply here.

One mechanical system that does operate with solids uses a plumb bob that is lowered by cable to contact the solid’s surface and then immediately reverses. The device’s electronics provide a level measurement based on travel of the cable. Monitor Technologies LLC’s (Elburn, Ill.) SiloPatrol inventory monitoring system uses this technology. The sealed mechanical unit can lower a plumb bob to measure silos up to 150 ft high. It operates from -40 to 150 °F, has an accuracy of 0.5% of reading, and a repeatability of 0.1 ft. The Kingsford Charcoal plant in Springfield, Ore., uses these units on 60-ft high silos that store charcoal briquettes.

What’s left?

A number of technologies suit continuous-measurement applications. Included are ultrasonic (also known as air sonar), radar-based technologies (guided wave and through air types), nuclear, and laser.

Ultrasonic measurement uses a transducer to send an ultrasonic energy pulse through the headspace to the material’s surface where it reflects. The time it takes for the reflected beam to reach the transducer is measured (based on the speed of sound in air) and a level is calculated. Advantages of ultrasonic measurement include noncontact operation and ease of installation and setup. No emptying and refilling of the vessel is required to calibrate and commission these devices.

Disadvantages-such as limited pressure/temperature range, existence of surface turbulence and the need for low-frequency excitation (20 kHz or less) for use with low dielectric media-are usually not a problem in the bulk-solids arena. Measurement inaccuracy from dust floating in the path of the ‘ping’ can be a problem, however.

According to Russ Carlson, level product manager for SOR Inc. (Lenexa, Kan.), the combination of more powerful onboard electronics and stronger, lower-pitched audible signals can overcome many obstacles. ‘For instance, SOR’s U-Series ultrasonic devices overcome high amounts of dust and angle of repose issues through the use of high-power, low-frequency sound to penetrate extreme dust, and adaptive gain control to compensate. Using adaptive gain control, a unit automatically monitors the quality of the echo received and adjusts sensitivity to provide the most reliable detection available,’ Mr. Carlson adds.

Radar-based technologies work similarly to ultrasonics with the exception that the energy pulse is a microwave. The pulse can be sent down a probe into the material (guided wave) or through air and reflected from the surface.

Guided-wave radar sends a pulse down a probe that is reflected when it sees a dielectric change at the interface of air and the material to be measured. Transit time is measured and level calculated. Use of a probe permits very efficient energy transmission. Although guided-wave devices are unaffected by dust or coating of the probes, they are contact devices, so are subject to abuse from solid media.

Probes must be ‘matched’ to the media and mounted to avoid internal bracing and other bin features (exit gates, chutes, etc.). However, proper mounting ensures that the guided signal (essentially a non-diverging beam) totally ignores internal tank structures. K-Tek Corp. (Prairieville, La.) provides metallic probes for its MT2000 guided-wave radar transmitter that can be cut to size, from 2-100 ft, and can operate even when bent.

Through-air radar is a noncontact method that uses a reflected radio wave to determine level. The technology requires high power output for typically low dielectric constant solids and tall bin configurations so that the return echo is measurable. It is unaffected by vapor or vacuum conditions and signal strength can be matched to the media so that a usable signal is obtained from low dielectric solids. However, these units are sensitive to dusty atmospheres, antenna buildup, and inaccuracies due to stray reflections from internal vessel structures.

Chris Lamakul, product manager at Siemens Energy & Automation (Arlington, Tex.), relates that Sitrans LR 400 is an example of how advanced electronics and echo processing software can now suppress false signal returns. This feature ‘sees’ the location of obstructions in a vessel, ignores them, and returns to the correct level reading. Algorithms keep the unit locked on the proper return echo during dynamic process conditions.

Totally non-intrusive

Laser technology has also been adapted to continuous level measurement. Using either time-of-flight or phase-shift parameters to sense level, this simple concept is intrinsically safe, and non-intrusive-the beam can be ‘shot’ and returned through a window in an enclosed vessel. Alignment of the reflected beam is critical, however, and, like other noncontact methods, accuracy is effected by dust floating above the air/media interface.

Nuclear level sensing is also noninvasive, able to handle extreme pressures and temperatures, any material, and is highly accurate. In short, it works where nothing else will.

A radioactive source (gamma radiation) beams through the vessel to a target that runs along the outside of the opposite wall. The material in the vessel attenuates the radiation so that the target does not ‘see’ the source below the solid/air interface. Level is determined by the device’s electronics. Although continuous improvements in this technology have made it more accessible to the user community, it is still relatively high cost to purchase and install. It also brings with it real with improper handling or perceived health risks and regulatory red tape.

Even with the never-ending variety of solid materials that process engineers may have to deal with, level-sensing manufacturers have certainly leveraged a wide variety of technologies to provide control when needed. It almost seems that the rougher the material to be sensed, the better they like it.

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Make little rocks out of big ones

Luck Stone Co. (Richmond, Va.) is a producer of construction aggregates. In the process of making the aggregates, rocks of various sizes and shapes are fed into a large crusher bowl for processing and size reduction. From the initial crusher bowl, the rock is feed onto separating screens where smaller rocks are separated. Oversized rocks are fed into another crusher system or returned to the same crusher for further processing. The process repeats itself until all of the rocks meet appropriate size requirements.

According to Brian McMorris, product market manager for STI Automation Products Group (Logan, Ut.), ‘Accurate and reliable level monitoring and control is vital to feeding and maintaining a steady amount of aggregate product in each crusher. If too little rock is fed, the rock will not be crushed to the desired size and the crusher will not be used to capacity, resulting in lower productivity. If too much is fed, the crusher will overflow and excessive parts wear will result.’

In the past, Luck Stone used electromechanical level sensors but found that these sensors failed from constant contact with the stone, resulting in inaccurate monitoring and excessive downtime.

Once Luck Stone’s engineers settled on ultrasonic units, they found that not all of noncontact devices had the stability and accuracy to read level in rocks greater than 4 in. in size.

With some investigation, they found that STI’s US20 ultrasonic sensors accurately sense a wider range of rock sizes and provide the reliability and accuracy to maximize crusher throughput and overall productivity.

Coal bunker application: Easy reliability retrofit

Reliant Energy’s Shawville Power Plant (Shawville, Pa.) had a number of antiquated capacitance-type level switches in the coal storage system that fed the plant’s boilers. The switches, which turned the feed conveyor on and off, needed constant repair. Parts were getting hard to find. Thereplacement needed to be reliable, retrofitable with minimum modifications, and workable with the coal stored in the silo, which ranged from 1/32- to 1/4-in. dia.

According to Jeff Sell, group supervisor, electrical and instrumentation, ‘Ease of installation and maintenance were an absolute necessity. The 100-foot-high coal bunkers are located on the sixth floor of the power plant, making just getting to the site to work on them time consuming at best.’

Reliant Energy replaced the old units with side-mounting Magnetrol (Downers Grove, Ill.) Solitel standard vibrating rod level switches, which fit directly into the existing openings for high and low upper alarm points. A new low-level alarm switch was also added at this time. ‘The self-contained electronics of Solitel switches eliminated the extra wiring the older units required and used existing process openings, a real plus since all work had to be done from catwalks,’ Mr. Sell adds.


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