These transfer chutes are everywhere at mining sites, and one plugged chute can stop production, which incurs hundreds of thousands of dollars in downtime production costs. Screens, both wet and sizing, are also key to the smooth operation of a facility, and they are just as susceptible (if not more) to blockages based on the wet materials they handle. At an average selling price of $75 to $100 per ton, and production rates in the hundreds to thousands of tons per hour, it is very easy to calculate the cost per hour just in loss of production. Add in clean-up costs and the entire event becomes extremely expensive.
There are, and have been, many options for the detection of blockages in chutes. Some of them are invasive, others just intrusive, and some are even completely non-contacting. Each has had their place in the operation of a coal prep plant at some point in time. Today’s demands are for high reliability, a preference for a non-invasive technology, and an avoidance of nucleonic based detectors when possible. Plant personnel such as reliability engineers, operations managers, facilities engineers, maintenance and more are always looking for ways to increase throughput, reduce downtime and improve process efficiencies. Companies employing cutting edge technology are designing process instrumentation that offers many different types of techniques for providing reliable point level detection solutions for tough applications. To be successful in this instrumentation market, a company must offer solutions that are value added to customers and offer user friendly configurations with high reliability in mind. With today’s technology, upgrading of instrumentation at a plant location from older measurement techniques to newer designs will definitely lower maintenance costs, improve process efficiency and provide higher reliability devices, which ill provide many benefits. Since safety is a company’s number one goal, any blocked chute detector must be reliable, robust and accurate.
Consider the technologies that have been in use—vibrating devices (tuning fork type), capacitance (or even admittance as some prefer to call it), mechanical devices (such as the tilt switch, either mercury filled or its newer non-mercury version). In addition to the “contacting type devices” there are also the nuclear- or microwave-based detection systems, which have no entry into the chute. But now there is a new member of the detection family that falls somewhere in between. It needs an opening into the chute, but doesn’t protrude into the flow stream, and that is the acoustic switch.
Vibrating technology uses the principle of exciting a piezo-crystal to induce vibration onto a set of tines. When product comes into contact with the tines, the vibration frequency is dampened, and an alarm relay is triggered. A fine measurement in lighter, small particle applications, but somewhat out of place in the size fraction typically encountered in a prep plant. These systems are potentially subject to issues of false indication due to build-up of fines which need to be washed off.
Capacitance (or admittance) technolo-gy: uses the principle of applying a small radio frequency voltage to an element, and measuring the capacitance in picofarads of the element as an antenna installed into the chute. An electrical “bridge” is set to measure an imbalance, caused by contact with the product, and triggering the alarm. While the development of various “guard” elements has improved their ability to ignore coatings, they are still subject to being fooled by a coating. Varying probe styles have been developed, and some are actually flush with the chute wall, but they still are subject to coatings.
Mechanical or “tilt” switches use the principle of a “floating” element inside the chute. When the material rises to a preset level, the switch body is tilted by approximately 15°-25°, causing a conductive liquid (mercury in some cases) to produce an electrical connection across a pair of contacts, and activating the alarm. Simple and reliable in many installations, this device would seem less than optimum for use in the abusive and abrasive coal handling world.
Nucleonic (nuclear) technology uses a radiation source and a detector, mounted on opposite sides of the chute. During normal “free flowing” conditions, the rate of absorption of the emitted radiation is low. It rises significantly when a blockage occurs, and is used to trigger the alarm. Proper positioning and alignment of the components is required since the signals used are relatively small. It does have the advantage of no contact at all with the process material; however it is subject to licensing, regularly scheduled inspections, and mandates the employment of a nuclear safety officer for the site. These devices can have false trips caused by material building up on the wall of the chute, which necessitates either cleaning the chute, or making adjustments to the sensitivity setting, thereby leading to lack of an ability to see a real blockage
Microwave technology uses the high-frequency electromagnetic waves of radar, with an emitter and a detector mounted on opposite sides of a chute. Alignment of the emitter and detector is important, as they must face each other rather precisely to minimize signal losses. These devices are able to be tuned to accept free flowing material as the normal condition, and respond only to a level that attenuates the signal substantially, as occurs during a blockage. They are particularly well suited to dry materials, and also have the ability to have extensions to both the emitter and detector making them appropriate for elevated temperatures. They do not do well with materials that may leave a coating on the inside of the chute, as this tends to attenuate their signal. In cases like this, the user can try making adjustments to ignore the build-up, but as with the nucleonic device this can lead to a loss of ability to detect a real blockage.
But now there is a new member of the detection family that falls somewhere in between. It needs an opening into the chute, but doesn’t protrude into the flow stream, and that is the acoustic switch.
An acoustic switch uses a sender and receiver type of mounting, but with a difference. The essence of the measurement is that two transducers face each other from opposite sides of a chute, and using lower frequency high power sound waves; they send and receive an acoustic signal from each other when the material is freely flowing. Because they are using sound, which has a wider beam angle, there is minimal concern for proper alignment. A significant difference from the other send/receive type of devices is that this technique uses both transducers as senders and receivers. Each unit alternately sends, and then listens for a pulse from its “partner unit.” Additionally, because they are using acoustic energy, which is a physical pressure wave, the transducer faces are self-cleaning. With each pulse, the face of the transducer moves at high frequency, which forces material to be removed from the facing. In materials that have a tendency to adhere to the wall of the chute, this is a distinct benefit since it removes the need to clean the unit to ensure correct operation. With a standard titanium facing, the sensors are ideal for operation in highly abrasive and potentially impact prone installations. They can be used in virtually any ore type, from gold and copper, through coal and lignite.
Each end-user or designer will have to determine if the material will have the potential to change from a dry condition to a damp or even wet state which can lead to significant deposits or build-up on the internal walls. The advantages of a reliable, durable and self-cleaning system would seem to make it the most obvious choice.
Kulp is a sales manager with Hawk Measurement Systems. He can be reached at firstname.lastname@example.org.