Bins and silos management

By |  June 29, 2017

With so many level technologies on the market today, choosing one is much more difficult and can be confusing.

Process measurement and controls are essential components for any industrial plant attempting to conform and abide by the strict safety and environmental regulations set forth by state agencies. Not only is it important to know what is contained within any silo or vessel, but it is vital to know whether a silo or flow area has material blocked.

Whether that material is too high or low in the containment is also critical, as it can cause safety hazards to plant personnel and force cleanup costs and agency fines.

Reliable, continuous level measurement and redundant point level detection are important parts of any processing plant, particularly at a time when improving energy efficiency and reducing operating and maintenance costs are important considerations. Plant safety and meeting stricter environmental regulations become challenges.

Whether an industrial site is a mine, power generation facility or a cement plant, these sites all require technologies that will withstand the tough environmental conditions and the harsh nature of the solids applications. These include heavy dust in the airspace, steep angles of repose, high temperatures, changing process conditions, corrosive media, abrasive solids materials and more.

In addition, so many different sizes and shapes of containment mean many installations have to deal with obstructions like mechanical bracing for structural support.

Plant personnel, like reliability engineers, operations leaders, facilities engineers, maintenance personnel and others, are regularly looking for ways to increase throughput and improve efficiencies. Now, companies are designing process instrumentation that offers different types of technologies to provide reliable level and point level detection solutions for tough applications.

Tech exploration

Reliable, continuous level measurement and redundant point level detection are important parts of any processing plant. Photos courtesy of Hawk Measurement Systems

With safety being a priority, any basic level measurement must be reliable, robust and accurate, and there must be robust systems to guard against spillages from overfilling vessels.

Unfortunately, even with today’s advancements in process instrumentation, there isn’t one technology that provides undaunted measurement results in every application. The mechanical installation constraints, the conditions within the containment and the capabilities of the level device will all affect the choice of measuring device.

Within the level instrumentation spectrum, there are several different technologies, but the major technology contenders are ultrasonic, TDR (guided wave radar) and non- contact microwave radar.

When dealing with solids materials in an industrial environment, the conditions for measurement are extremely difficult. The conditions require a technology that can endure heavy dust, undulated material surfaces and, sometimes, hot conditions with buildup problems on equipment installed in the application.

If the height of the material containment for level measurement is more than 30 to 40 ft., then it is more appropriate and practical to choose a non-contact level measurement technology like ultrasonic or microwave radar.

TDR, or guided wave radar, can provide continuous level measurements up to 80 ft., but the tensile forces and loading on the cable become extreme in solids materials. Thus, breakage and shearing are possibilities. Also, as material shifts from one point to another in solids, the cable follows that line of movement.

With level measurement in solids beyond 30 to 40 ft., a non-contact technology can be a wise choice. Choosing between non-contact ultrasonic and microwave radar for solids materials can be challenging, but there are some simple rules to keep in mind when considering the choice for an application.

Remember, solids come in many different sizes and shapes, and regardless of the particle size, the material will be very dusty in the airspace. The method of fill and removal from the containment will also increase the dust in the airspace, causing further deterioration of the measurement technology’s signal.

Using a dense phase pneumatic conveying system during fill – essentially blowing the material into the silo from the top – the airspace conditions are extremely clouded and difficult for some level technologies to perform reliably. In these conditions, the transmitted signal must be strong and have the ability to penetrate dust without being attenuated.

Keys to microwave radar

With microwave radar, the frequency of the device used and the antenna design are important in how well it will perform in dusty conditions. Non-contact microwave radar designs typically operate in the frequency band from 5.8 to 26 GHz, and some go higher than that with use of either pulse or FMCW (frequency modulated continuous wave) technique. The technique of pulse wave radar seems to be used most often these days, along with a frequency band of 24-plus GHz.

The correct size and type of antenna are key when choosing this technology for solids level measurements. The antenna type should be a horn style and the size should be as large as possible, but most manufacturers offer 2 to 6 in. in diameter, with some offering 10-in. parabolic dish-type versions.

Applying a 2- or 3-in. horn antenna is not appropriate for solids applications, as there is not enough of a collection source at the receive area for the microwave signal. Choosing a horn diameter of 4 in. or larger is best for penetrating the dust in the airspace, as well as allowing for a better collector on the returning signals.

The technology works well on measurement ranges up to 150 ft., but after that the readings become somewhat unreliable and usually buildup of dust becomes a deterrent to the propagation of the microwave energy.

The application of a Teflon-fabricated dust cover is applied onto the end of the horn antenna to prevent the dust from entering and buildup inside the horn. However, the dust then builds on the dust cover and, over time, will impede the signal regardless of its dielectric value and moisture content. There are no self-cleaning properties for a microwave design, and the use of these antenna purges does not work properly.

The purging option requires an air supply for the removal of the solids material, and most industrial facilities have instrument air. But, there is moisture in that air. This moisture causes more problems than anyone wants to deal with, as it will induce more chance of solids to build up.

In dealing with long, dusty airspace measurement on solids, the larger parabolic horn antenna is recommended. But this horn size requires an opening of 10-plus in. in diameter. Buildup is also a realistic problem with this large antenna because it is a large surface area and, again, has no self-cleaning properties.

Ultrasonic technology

As for ultrasonic technology and its use in level applications, this applies to operating frequencies in the 40- to 50-KHz band, as well as sizes of 2 to 9 in. in diameter.

For liquid-level applications, the use of 30- to 40-KHz frequencies is suitable because the airspace conditions do not contain dust particulate. Therefore, propagation of the acoustic wave is only affected by the vapor space.

For solids-level applications with heavy dust in the airspace, a low frequency of high power is essential.

There are also other things to consider for the proper propagation of the acoustic wave signal in dusty conditions. The dust particles in the airspace will most likely attenuate or absorb the acoustic wave if not properly sized to the application. The distance of the measurement, the airspace conditions and the mounting availability are all factors to be considered when applying the right transducer.

In the case of ultrasonic technology and solids-level applications, size does matter, meaning the lower frequency transducers will make the long-distance shots and penetrate the dust particulate with minimal attenuation. These 5- or 10-KHz frequency acoustic wave transducers are audible in sound and have a lot of power applied to them with a variant gain scheme.

The key to the performance on these difficult applications is not just the lower frequency design, but rather a trio of things. It requires a low frequency and high power with a variable gain applied to the system to adjust for changing signal conditions.

With long-range measurements beyond 50 ft. and very dusty airspace conditions, the selection of the transducer frequency is important and should be at a minimum of 15 KHz or lower. Remember though, it is not only the frequency for succeeding in these applications, but the power applied, the transducer design and the dynamic gain circuit.

Don’t forget buildup

With the right transducer selection, the next thing to consider is the buildup potential of the solids materials in the application.

Remember, there are no self-cleaning properties associated with the microwave radar. So buildup can be a factor in impeding the energy from sensor to material surface.

The acoustic wave technology uses high energy applied to a crystal set, which causes mechanical vibration on the transducer surface and results in enough movement to keep solids particles of dust off the transducer face.

This self-cleaning technique allows for proper propagation of the low-frequency signal, even under the dustiest of airspace conditions, as no buildup will adhere to the transducer face. Also, the reliable, continuous performance of the acoustic wave system is dependent upon the adjustability of the gain circuit.

As the acoustic signal decreases in amplitude, the dynamic gain circuit automatically increases gain to the signal so there is an increase in the amplitude and the level can be maintained. This ability to vary the gain dynamically throughout the measurement proves to be a strong point when having the lower frequency and high power system. It takes every bit of technology savvy to accomplish a reliable level measurement on solids applications.

Level measurement on liquids applications is considered to be much easier in terms of a reliable acoustic signal. Still, the choice of the right technology for difficult solids applications does not have to be a brainteaser.


Jack Evans is president of Hawk Measurement America and director of global sales and marketing for Hawk Measurement Systems. www.hawkmeasure.com

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