Enhanced efficiency through rigorous monitoring
The fact that a Slack/Tight rope system is required by law (South Africa) to be installed on a winder means that, when the system is not functional, the winder is also unable to operate. This impacts negatively on the production of a mine in terms of lost production. It is therefore essential that Slack and Tight rope montoring systems are reliable and effective in performing Continous Slack and Tight rope monitoring in a vertical shaft.
By Russell Allsop
Slack and Tight rope conditions in a vertical shaft are more prevalent in certain areas of a vertical shaft and under certain conditions. The majority of these incidents are the result of a mechanical failure resulting in an obstruction in the shaft which will prevent the conveyance from descending (Slack rope condition) or from ascending (Tight rope condition).
Our experience in designing systems to detect these conditions over the past 20 years in this field has shown that the majority of incidents involve Rock winders.
This can mainly be attributed to the “Tipping” operation, which involves mechanical arrangements of some type, called “Camel Back”. “Camel Back”, which is used to open the door on the conveyance, is prone to causing Slack rope conditions as the empty conveyance gets stuck in the “Camel Back” on exiting the tip.
Similarly, pneumatically operated discharge devices can result in a Slack rope condition when the door of the conveyance is not completely closed and catches on the surrounding steelwork. Misaligned guides, loose buntings and other unsecured mechanical fittings within the shaft and on the stations are common causes of Slack and Tight rope conditions on Man and Rock winder conveyances.
A potentially disastrous situation occurs when a Slack rope condition on a winder results in the slack winding rope entering the compartment of another moving winder within the same shaft complex, the winding rope could be picked up by the moving conveyance resulting in extensive damage and possible loss of life. For this reason, it is always advisable to interlock the Safety Circuits of all the winders that share a shaft complex in order to suspend all winding operations in the event of any winder detecting a Slack/Tight rope condition.
Common oversights in rope monitoring
Slack and Tight rope monitoring systems are essentially Safety Monitoring Devices that are linked into the Safety Circuit of a winder i.e. in the event of a slack/tight condition being detected, the safety circuit’s status will become “Unhealthy” and the winder will be stopped. Hence, it is important to ensure the system is not detecting “False Trips”, for whatever reasons, as this results in the integrity of the system being compromised and the system being “Reset” without a proper investigation being carried out.
The ability of a system to allow the user to analyse the conditions at the time of the trip and to make an informed decision based on the Rope Load, Speed, Acceleration/De-acceleration, position of the conveyance in the shaft and other logged Events relating to the winder, for instance, Brakes On/Off, Safety Circuit Status, etc, is key. The objective is to strike a balance on the sensitivity of the system as a whole to detect and respond timeously to genuine trips and not generate “False” trips which reduce the confidence in the system as a whole.
The actions taken following a detected trip condition can compromise the situation. For instance, if a Slack rope is detected and the winder moves in the direction of the trip (Descends), the suspended conveyance is in danger of breaking free as the rope is slack. The system should only allow movement in the opposite direction to the condition. If the winder is moved “into” the condition, the Safety Circuit is set “Unhealthy” and the winder stopped.
The most practical approach
- Strain gauge technology
In order to perform continuous rope load monitoring, the actual measurement of the rope load needs to be repeatable and reliable. This measurement relies on the use of strain gauge technology in one form or another. There are two main ways in which this can be achieved.
Firstly, performing the measurement on the conveyance by inserting a custom designed load cell in the attachments of the conveyance, the signal is then amplified and transmitted via a radio link to the receiver on the surface using the winder rope as the transmitting medium (Aerial). Some systems utilise a microwave link which does not require the use of the winder rope. Although this approach generally produces a high level of accuracy, it is prone to high maintenance costs due to the harsh conditions encountered on a moving conveyance in a mine shaft and the need to provide power in the form of batteries.
The second method measures the rope load on the sheaf wheels using discrete load cells mounted under the bearings or by bonding strain gauge elements onto the structure supporting the sheaf wheels.
Both methods include the weight of the rope in the measurement. Winding ropes all have a kg/meter specification. The system calculates the weight and length of winding rope present in the shaft by counting the number of turns of the winder drum by means of a encoder mounted on the drum shaft. This is typically referred to as the “Predicted” measurement and is used to calibrate the “Measured” rope load, if at any pont in time the difference between the “Predicted” and “Measured” rope load is greater than the limits set for the Slack and Tight Alarm the Safety circuit of the winder is set to “Unhealthy” indicating a Slack ot Tight rope condition.
- Installation of discrete load cells
The installation of discrete load cells would also provide a high level of accuracy. However, this involves installation of the mechanical assemblies under the bearings, the mechanical assemblies would also require finite analysis testing and installation. The initial capital outlay for this option would also be higher than the strain element option. In the event of a load cell failure, the rope would have to be lifted off the sheaf in order to replace the damaged load cell.
- The convenience of strain elements
Compared to the other options above, Strain gauge technology has a number of advantages. Firstly, it results in reduced maintenance costs due to the measuring elements being located external to the shaft and relative ease of access to the sheaf wheels for maintenance. Secondly, installation of the strain element option would not require the mechanical assemblies to be installed under the sheaf wheels. Furthermore, the capital outlay, installation time and maintenance required would be minimal when compared to the other options above.
All in all, although not as accurate in some cases as the other options, the accuracy and repeatability is more than sufficient for Slack/Tight rope monitoring.
A typical installation of a contemporary system would include ancillary equipment in the form of encoders, filter amplifiers, strain gauge amplifiers etc., in conjuction with signals from the winder critical to the operation of the system e.g. Brakes, Clutches, Synchronisation switches etc. It is imperative that a failure of one of these devices or signals does not impact on the operation of the system by creating “False” trips or preventing the system detecting a Slack/Tight rope condition. For this reason, the hardware and software should include “Fail Safe” features to detect these conditions and change the status of the Safety Circuit to “Unhealthy” in such an event.
Systems that have the capabilities to remotely connect via the Internet to perform first level fault finding and data analysis can significantly reduce downtime and the cost of maintenance of a system. Another advantage of this type of ‘connectivity’ is the ability to perform software upgrades remotely.
The implementation of the SANS10294 specification relating to “Rope Load Monitoring” allows mines to enhance production by increasing the payload for a Rock winder without the need to install winder ropes with a higher breaking strength, systems that can integrate this option fulfill a growing requirement in the mining industry for increased productivity.
The general state of Slack and Tight Rope Monitoring
Slack and Tight rope systems have evolved from being a system required by law into a system which can not only prevent loss of life but in some cases decide the fate of a mine shaft. Due to increasing costs and illegal mining, the margins that some mines operate within are slim, if, for example, a Rock winder is no longer running in the shaft guides due to a Slack/Tight rope condition and the shaft infastructure is “Stipped”, the shaft, in many cases, would have no option but to close down.
Due to this situation, mines in neighbouring countries like Zambia, DRC and Ghana have opted to install Slack and Tight rope systems even though in some of these counties it is not required by law. In most instances, these systems include Bell and Event Recording facilities to provide the necessary information in the event of an incident or enquiry.
Furthermore, audits have become a requirement in order to ensure systems are tested on a regular basis and any issues that pose a risk to the correct operation of the system are identified and rectified.
Evidently, as mines around the world become deeper, the necessity for Slack and Tight rope detection is becoming more critical. Typically, a shallow shaft could rely on the trip wire (Tarzan Wire) installed under the rope as it exits the winder house. However, this method would not detect a Slack rope condition when the weight of the winding rope in the shaft prevents the rope on the surface reaching the trip wire, this method would also not detect Tight rope conditions. The installation of a instrument based system would become a requirement to protect the shaft.
In conclusion, in the coming years, many aspects of a typical Slack and Tight rope detection system will still be based on relatively simple technologies that have been around for some time, for example, strain gauge measurement. The advances will be driven primarily by the software applications. Hopefully, this will simplify time-consuming and costly slack and tight rope monitoring tasks, enhancing productivity and safety in mines.
Russell Allsop is a specialist in tight and slack rope monitoring from Control Solutions, a Johannesburg-based company involved in winder projects in mines throughout Southern Africa.