By Mark Kulp
In the current “high volume equals higher profits” mining world, is there any excuse for settling for less than the best technology for the detection of blockage in chutes? Installing point detection devices in transfer chutes for blockage detection is necessary as it is an inexpensive way of pre-empting a chute blockage. These transfer chutes are everywhere at mining sites, and one plugged chute can stop production. 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. Add in clean-up costs for any production stoppage and the entire event becomes extremely expensive.
There are many options for the detection of blockages in chutes. Some of them are invasive, others just intrusive, and some are even completely non-contacting. Today’s demands are for high reliability, a preference for a non-invasive technology, and an avoidance of nuclear-based detectors when possible. Plant personnel 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 add value for customers and offer user-friendly configurations with high reliability. 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 will provide many benefits. With safety as the goal, any blocked-chute detector must be reliable, robust and accurate.
Acoustic Switches Arrive
Consider the technologies that have been in use—vibrating devices (tuning fork type), capacitance (or 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 intrusion into the chute. But now there is a new member of the detection family that falls somewhere in between. Acoustic switches require an opening into the chute, but don’t protrude into the flow stream.
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. It is fine for measurement in lighter, small particle applications, but somewhat out of place in the size fraction typically encountered in a coal prep or mineral processing plant. These systems are potentially subject to issues of false indication due to build-up of fines that need to be washed off.
Capacitance (or admittance) technology uses the principle of applying a small radio frequency voltage to an element, and measuring the capacitance in picofarads of the element by an antenna installed into the chute. An electrical bridge is set to measure an imbalance, caused by contact with the product, and triggers 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. Various probe styles have been developed, and some are actually flush with the chute wall, but they still are influenced by 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 abrasive and harsh environments.
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 weak. 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.
Acoustic Advantages
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, clearing material from the face. In materials that have a tendency to adhere to the wall of the chute, this is a distinct benefit since it eliminates 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.
Mark Kulp is a sales manager with Hawk Measurement Systems. He can be reached at mark.kulp@hawkmeasure.com.
