Overcoming the challenges of blocked chute detection in the Quarrying and Mining Industries

Nuclear-based technology has long been thought the only blockage detection technology tough enough to withstand harsh mining and quarry environments, but its radioactive nature means it is far from ideal. Here, level measurement experts Hycontrol outline a safer, modern alternative.

BLOCKED CHUTE DETECTION
It is well recognized that transporting irregular-shaped bulk solid materials over long or short distances presents a number of operational challenges. Primary crusher crash boxes, conveyors, transfer lines, inlet/outlet chutes and inlet hoppers can all be prone to blocking, bridging or jamming, resulting in severe site problems. These include:

• Lost production
• Crippled and damaged plant
• Lost material
• Extended and expensive down times 

Therefore reliable, early detection of imminent blockages in chutes before they reach a critical level is of major importance.

NUCLEONICS
Traditional optical methods for detecting blockages, such as proximity through-beam sensors, cannot cope with the hostile environments encountered in these applications, where the extreme dust and high vibration culminate in repeated failures when sensitive optics are damaged or become coated in dust.

As a result, nucleonic devices have historically been considered the only reliable form of instrumentation capable of providing effective blocked chute detection in such demanding conditions. However, although nucleonics provides a technically competent technology, this measurement technique requires a radiation source from a radioactive isotope such as Caesium Cs137 or Cobalt 60 in order to detect the presence of material in the hopper. The radioactive source must be specially shielded so that radiation is only directed through the hopper wall towards the product. Levels of radiation detected on the opposite side of the hopper are then used to determine whether there is a blockage or not.

These devices are very expensive to install and, although considered relatively safe to operate, there is still concern about safety aspects and the long-term environmental impact. As one would expect, controls on the use of these radioactive devices are extremely strict and extensive training is required to operate them. They need initial licensing and also have to be periodically verified, either by the manufacturer or an authorised external authority.

MICROWAVE – THE SAFE, LOW-POWER ALTERNATIVE
Hycontrol’s new generation of microwave switches is proving to be an extremely reliable, robust and cost effective alternative to nucleonics, offering operators a 100% safe solution for blocked chute detection. This range of microwave level switches provides a simple non-contact, non-intrusive option suitable for many solids applications. A typical installation comprises of a transmitter and a receiver mounted facing one another on either side of the chute. During operation the transmitter emits a continuous, low-power microwave beam at 24 GHz to the receiver and an output relay is energised or de-energised when this beam is obstructed by the material being monitored. The switch trigger point is determined by the amount of microwave energy received and can therefore be adjusted to cater for different products and applications. As microwaves easily penetrate any surface contaminants it makes the switches ideal for applications where there can be high build-up of material on the inner surfaces of the chutes or vessel walls. This also means this switch technology is immune to problems with airborne contaminants such as powders or dust.

Microwave power levels are well below any required industry standards and therefore this device requires no special procedures for its operation and use. A wide range of flanges and connections make this range extremely easy to fit to new or existing installations, utilising original process connections if required.

A key advantage of this latest generation microwave technology is its ability to effectively see through low-dielectric materials such as refractory bricks, ceramics, plastics and polymers. This means that low-cost ‘sacrificial’ windows can easily be fitted into the sides of a vessel or container, keeping the process closed and causing no disruption to material flow. The probes can then be mounted at some distance from the application, well away from damaging vibration and abrasion. The switch is adjusted so that the beam passes unaffected through the windows and falling product, only being triggered if the material being detected builds up and permanently breaks the microwave beam. If the windows suffer any physical damage or wear they can be simply replaced, leaving the switch intact!

The potential health dangers, high costs and strict licensing requirements associated with the use of nuclear sources in industrial environments are extremely onerous. The straightforward cost comparisons alone give microwave technology a favourable five- to ten-fold advantage, but additional costs of ownership for the nucleonics system have to factor in the added mandatory site security, strict operator training, provisions for checks from the authorities, plus annual licensing fees. The nuclear source will degrade over time and its disposal must be carefully controlled, all of which adds further to the cost of ownership.

DETECTING BLOCKAGES IN CRUSHERS
Primary crusher crash boxes at quarries are a vital part of the crushing process and it is essential that adequate warning is given of any potential blockages before they result in costly plant down time. However given the ferocity of the environment where huge rocks are being crushed, peripheral equipment is constantly being subjected to extreme vibration, impact damage and abrasion.

To overcome these problems in one such recent application, Hycontrol mounted the microwave switch components up from the floor beneath the crusher head, 200mm back from the walls to prevent the units being shaken to pieces by the intense vibration. A 60mm window made in each side of the crash box allows the microwave beam to pass from one side of the box to the other. In the un-switched state the narrow beam from the transmitter is detected by the receiver. If the path of the beam is then permanently interrupted by a build-up of material in the crash box, the internal relay is triggered, initiating an alarm.

MAINTAINING CRITICAL MEASUREMENT SYSTEMS IN HARSH ENVIRONMENTS
Given the harsh environments that blocked chute switches and other level measuring equipment is expected to work in, regular structured and preventative maintenance is essential. Unfortunately all too often only cursory visual maintenance is carried out on critical equipment, exposing sites to potential safety hazards as well as premature and costly breakdowns.

All critical level components should be independently tested and calibrated on a regular basis. A well-designed level system should take this testing requirement into consideration so that maintenance work can be carried out effectively and safely. If the tell-tale signs of possible problems are uncovered during servicing the root cause should be investigated and rectified.
Choosing the correct level equipment and ensuring it is professionally installed in the correct location is vital for the long term reliability of quarrying and mining sites. It is now clear that microwave technology offers a reliable and cost-effective alternative to aging nucleonic systems.

In parallel a structured maintenance programme is essential to prevent unwanted and costly breakdowns, ensuring that new equipment continues to provide optimum performance.