Best Technology for Reliable Plugged Chute Detection
Myth or Reality: The Facts about Radar, and the Right Choice for Level in Solids Applications
With so many level technologies on the market today, the choice of technology is much more difficult and can be confusing. Process measurement and controls are an essential component 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 enormous safety hazards to plant personnel as well as clean-up costs and agency fines. Additionally, installing point detection devices in transfer chutes for blockage detection is also important as it is an inexpensive way of preempting a nasty chute blockage. These transfer chutes are all over the place throughout a mining site, and one plugged chute can stop production, which incurs hundreds of thousands of dollars in downtime production costs. So with that stated, reliable continuous level measurement and redundant point level detection are an important part of any process 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 a challenge in this tough competitive marketplace.
Many level applications pose special problems for process level equipment and technologies. Whether the industrial site is a mine, power generation facility, or cement plant, these sites all require technologies that will withstand the tough environmental conditions as well as 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 that many installations have to deal with obstructions like mechanical bracing for structural support.
Plant personnel like reliability engineers, operations managers, facilities engineers, maintenance, and more are always looking for ways to increase throughput, reduce downtime, and improve process efficiencies. With technology on the constant cutting edge, companies are designing process instrumentation that offers many different types of techniques for providing reliable level and point level detection solutions for tough applications. In order to be successful in this instrumentation market, a company must be offering solutions that are value added to customers, and offer user friendly configuration with high accuracy and reliability in mind. With technology like it is today, upgrading of level instrumentation at a plant location from older measurement techniques to newer designs will definitely lower maintenance costs, improve process efficiency and provide higher accuracy devices, which will provide many benefits. With safety being most industrial company number one goal, any basic level measurement must be reliable, robust and accurate and there must also be robust systems to guard against spillages from overfilling vessels.
Unfortunately, even with today's advancement in process instrumentation, there is not one technology that will provide undaunted measurement results in every application. Although, it is the technology of microwave radar that has been promoted over the last several years as the panacea for all liquid or solid level materials. Is this really the case? What has happened in this instrumentation market to the idea of providing the right engineered solution for the customer's application? Let's really look at the technologies out there for liquids and solids level measurement like through air radar, guided wave radar, ultrasonic, and what's being referred to by Hawk as acoustic wave. In applications, there are mechanical installation constraints, the conditions within the containment, and the capabilities of the level device will all affect the choice of measuring device. In the level instrumentation spectrum, there are many different technologies, but the major technology contenders are ultrasonic or acoustic wave, TDR (guided wave radar), and non-contact microwave radar. It is interesting to note too that the technology of ultrasonic or sometimes promoted as acoustic wave technology has flat lined or hit a road block in growth. The technology of microwave radar has been growing at the "speed of light" and being regarded, or at least touted as the end all beat all technology for measuring level in liquids and solids. Well, choosing the proper technology from one of these three can be a challenge, but if you're looking for high reliability, low maintenance, and repeatable performance, then look below for some guidelines on each technology.
So, when one looks at level applications, the split is either liquids or solids. With liquids, many technologies can be applied depending upon the conditions in the application (temperature, pressure, air space conditions above the liquid surface, mounting, mechanical obstructions, and more. Liquids though are not nearly as difficult to solve with level technologies as the solids materials, which can range from fine powders to chunked aggregate materials, to the worst conditions of wet, moist fine powdery material that adheres to almost anything. When it comes to the technologies of through air radar, guided wave radar, or ultrasonic or acoustic , the choice of the technology is relatively straight forward with a few exceptions. If the liquid material is water based, with virtually conditions of a non-vaporous atmosphere, and temperatures / pressures in the ambient / atmospheric range, then ultrasonic or acoustic is suitable With microwave radar applied, the liquids are probably going to be of a chemical or hydrocarbon formulation, probably have some excessive temperat. ures or pressures, and have heavy vapor conditions in the airspace. Guided wave radar can be applied as well in the aforementioned conditions, with the exception maybe of the range being too lengthy for a rod or flexible cable antenna or if there is an agitator in the vessel.
But, make no mistake about the fact that when dealing with solids materials in an industrial environment like a metal or coal mine, or fly ash in a load out silo at a power generation facility, the conditions for measurement are usually much more difficult. It requires a technology that can endure the atmosphere conditions like heavy dust, undulated material surfaces, wet or moist conditions from process sprayers, and sometimes hot conditions with build-up problems on any equipment installed in the application. If the height of the material containment for level measurement is more than 30 to 40 feet, then it is more appropriate and practical to choose a non-contact level measurement technology like ultrasonic, acoustic, or microwave radar. TDR or guided wave radar can provide continuous level measurements up to 80 feet; However, in solids materials, the tensile forces and loading on the cable become extreme, and thus will potentially cause breakage and shearing. It is just not practical to outfit any solids measurement application with something of a contacting design like guided wave radar when there is any sort of build-up potential, or lengths beyond 30 feet (10 meters). Also, as material shifts from one point to another in the solids, the cable follows that line of movement. Cost also becomes a factor too for guided wave radar in long measurements as cable lengths increase, so does pricing. With level measurement in solids beyond 30 to 40 feet, it is a wiser choice to go with a non-contact technology.
So let's get down to the facts about non-contact technologies, both new and older in the market place today. The technology known as ultrasonic has been around for many years, and it is as the name implies, sub sound technology in the kilohertz frequency band. The designers of ultrasonic technology have made valiant attempts to solve the difficult solids applications with frequencies down to as low as 8 to 12 kHz and various transducer designs in size and shape, but the overall measurement success has been inconsistent at best. Then along comes non-contact microwave technology with the claims that it is the new "sexy" technology to measure the long range, dusty solids measurements. Great claims for something that performs well in dry materials, but induce moisture into the solids materials along with heavy dust, water sprayers for dust abatement, and that's a formula for disaster. This new technology is not the panacea for all level applications as many companies tout, and it definitely does not have carte blanche performance in the industries like coal, metal mining, minerals, and other solids industries. With the less than desirable results on solids using "ultrasonic" and the through air radar not capitalizing in the mining industries, what technology is out there to solve these applications? Well the overlooked technology, which is a variation on a technology theme of ultrasonic, but designed in a way to offer significant application benefits, is acoustic wave technology. The magic behind this technology is the fact that it utilizes audible frequencies (5 to 30 KHz) in a transducer design that is harnessed as a balanced resonant mass. The combination of low frequency, high applied power, and variable adaptive gain control makes this acoustic wave technology a real solids solution that can't be beat and is really underestimated. On the transducer, the low frequency with high applied pulsing power to the face creates a pressure wave that literally offers consistent and proven self-cleaning properties. Effectively, there are no materials that will adhere to this transducer face regardless of their moisture or sticky properties.
So in mining applications, where there are wet screens from sprayers, or ROM bins with dust abatement controls causing heavy build-up on anything in the area, the acoustic wave technology can reliably provide level measurement under those conditions. Microwave radar CAN Not function under these moist solids conditions as it would be disastrous with material build-up adhering to the emitter on the inside of the horn antenna. Or worse yet, adherence of moist, powdered ore fines on the face of a "dust" cover that is designed to keep material from entering the horn antenna, but does not prevent adherence on the dust cover face. Many suppliers of non-contact radar designs today will recommend the use of antenna purging with either water or air within the plant site. This purging option sounds great in design, but in reality, the air purge causes more problems than it's worth because most instrument air supplies have moisture, and this moist air will increase the chances of dust build-up on the emitter within the horn. Additionally, the instrument air is not inexpensive to supply on a regular basis.
The key to measuring solids materials in conditions where moist, wet, powders, ores, aggregate exist, then there needs to be a technology used where there are self-cleaning properties available. With acoustic wave technology, the power to the transducer with low frequency is one key design criteria, however, it takes a lot more than just that, and that's where an Australian company has led the solids measurement charge within the level industry. The long wavelength of the low frequency designs also makes them appropriate for the tough stuff. Guaranteed for high performance without fail in the worst conditions known to man, the acoustic wave technology will absolutely amaze the doubting customer, until they see in action, and "how it take a beating, yet keeps on repeating" in the measurement.
So again, choosing between non-contact acoustic wave and microwave radar for solids materials can be challenging, but there are some simple rules to keep in mind when considering the choice for the application. Remember that solids materials 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 which can cause further deterioration of the measurement technology's signal. During fill using a dense phase pneumatic conveying system, which essentially blows the material into the silo from the top, the airspace conditions are extremely clouded, and difficult for most level technologies to perform reliably. During these conditions, the transmitted signal must be strong in power, have the right wavelength, and have the ability to penetrate the dust in the airspace without being attenuated.
For these dusty airspace conditions, let's evaluate and compare the two technologies of non-contact design and see which one is the most applicable under the toughest conditions. With microwave radar, the frequency of the device used and the antenna design is very important in how well it will perform in these dusty conditions. Non-contact microwave radar designs typically operate in the frequency band from 5.8 to 26 GHz, and some even go higher than that, with use of either pulse or FMCW technique. The technique of pulse wave radar seems to be most often used these days, and a frequency band of 24+ GHz. The correct size and type of antenna is essential 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 inch diameter, with some offering 10 inch parabolic dish type versions. Applying a 2 or 3 inch size 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. So choosing a horn diameter of 4 inch 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 125 to 150 feet, but after that, the readings become somewhat unreliable, and usually build-up of dust becomes a major 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 build-up 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. Remember what was stated earlier in this article, and that is when suppliers recommend the use of purging options like air or water. Well, this is not a practical solution to removing adherence of solids particles. Suffice it to say that there are no self-cleaning properties for a microwave design and the use of these antenna purges do not work properly and they are not practical for most industrial applications. 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+ inches in diameter. Build-up though also is a realistic problem with this large antenna as it is a large surface area and again has no self-cleaning properties.
When we speak about ultrasonic technology (also acoustic wave) for use in level applications, we are talking about operating frequencies in the 40 to 5 KHz band, and sizes of 2 to 9 inches in diameter. For liquid level applications, the use of 30 to 40 KHz frequencies are suitable as the airspace conditions are not containing dust particulate, so propagation of the acoustic wave is only then affected by the vapor space. Keep in mind too, that acoustic wave technology is different than ultrasonic technology in that the application of lower frequency designs with high pulse power will create this pressure wave effect that literally atomizes any type of condensation adhering to the bottom of the transduce face. Any other ultrasonic design on the market today does not offer these cleaning values. When you are speaking about solids level applications with heavy dust in the airspace, then a low frequency of high power is absolutely 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 assuredly 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, which means that 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 the application of the lower frequencies.
Oversizing the transducer based on frequency and knowing the conditions in the measurement will prove to be successful. The lower frequency with power will deal with the harsh conditions of dust, build-up, and moisture in the airspace, and much more. With long range measurements beyond 50 feet and very dusty airspace conditions, the selection of the transducer frequency is important and should be at minimum, 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. With the right transducer selection, the next thing to consider is the build-up potential of the solids materials in the application. As we discussed in the previous paragraph with microwave radar, there are no self-cleaning properties associated with that technology, so build-up 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, thus resulting in a movement enough to keep solids particles of dust off of 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 build-up 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 that 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 also. It takes every bit of technology savvy to accomplish a reliable level measurement on solids applications.
Level measurement on liquids applications are considered to be much easier with regards to a reliable acoustic signal as compared to solids measurement on things like coal, lime, mined ores, cement, and gypsum. The choice of the right technology for these difficult solids applications does not have to be a brain teaser. Most companies are astute at assisting in the applicability of their designs, but it is important for you as the user to understand the limitations of the technologies. Below is a summary chart for the technologies discussed in this article along with others and the various conditions under which there could be exposure. It serves as a guide for the selection of technology for your application conditions.
Now for every continuous level application in your facility, you should be considering the application of a reliable point level technology. The practice of using an alternate technology point level device with a continuous level measurement should be adopted with every company. And no, it's not because the suppliers want to make or sell more product, but because it only makes logical sense. Think about it, if you have a malfunction or an application upset with your continuous device, and there is no point level shut-off for high level, then you will have a spill and that spill requires clean-up, which results in unnecessary costs , and potential fines by governmental agencies like the EPA. Additionally, these spills could also result in a safety violation with harm caused to employees or the process. In addition to the high level back-up, there should be precaution taken and applicability of a point level switch for a low level shut-off as well as point detection in a chute with solids material. Using point level technologies for back-up protection provide a high degree of cost prevention to replacing damaged pump systems, screw conveyors, valves, and other process control devices. With the cost of point level switches being anywhere from $ 200 to $ 2000 depending the severity of the application, these are relatively low cost and provide a low cost of ownership as they serve to prevent problems.
With the importance of having a point level back-up to your continuous level technology, it is wise to choose an alternate technology from what your continuous device is in the application. So for instance, if you have an acoustic wave system for measuring coal in your load out silos, then you could apply a point level technology of vibration, capacitance, rotating paddles, or microwave. With this point level in mind, there are many different technologies to choose from. The most common used for solids applications would be capacitance, vibratory forks, rotating paddles, acoustic wave, and microwave designs. With solids materials, the abrasive and heavy loading of the material can be a factor in causing more problematic issues with a point level device, especially on low level or high flowing materials, so choosing the right one is important. Other factors like build-up on the probe elements or impact from falling material can also affect the performance and reliability of the product.
The technologies of microwave and acoustic wave lend themselves to the more difficult solids applications, although the applications of both are also seeing the easy applications. These two technologies are more often seen though on the difficult applications where an indication of material absence / presence is critical in the customer's process, and therefore reliable detection is mandatory. The microwave detection technology is such that the faces of the transmit and receive sensors are across from one another over a certain short or long distance, but looking though a plastic window like Teflon. There is no contact with the material in the silo and no protrusion thus no wear and tear and reliable performance provided the material is dry. If the material has some moisture or it can be dry, then the applicability of the acoustic wave technology can be done. The beauty of this technology is the fact that it is also not protruding into the vessel and uses a very wear resistant titanium face for long lasting durability in abrasive applications. The costs for the microwave or acoustic wave design are more than conventional point level technologies like capacitance or rotating paddle wheels, but the replacement of these devices does not occur once installed in the applications. It's set up with minimal configuration, and then literally walks away with no problems after that point.
So in summary what I wanted to share with every reader is the idea that there are many technologies for measuring continuous and point level within the solids industry, but making the right choice for long term reliability, low maintenance, and high performance is where the rubber meets the road. If safety, improving process efficiency, or saving costs are your concern, then take to heart this information, and contact your local level expert or me if you'd like some guidance. And finally, let me say that the success and performance reliability of any technology is not chosen based upon its popularity, but on its capabilities to deal with adversities. Don't sell short the technologies that have been around for many years.
Jerry Boisvert ( [email protected] , 978-5308588)
Hawk Measurement Systems
7 River Street
Middleton, Massachusetts 01949