Russell Hattingh, MD of BBE consulting, and specialist ventilation consultant, Hannes Potgieter, present their perspectives on ventilation on demand (VoD), what it is, why the mining industry should be interested and how to best apply it to optimise mine ventilation while minimising risks.
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Primary ventilation involves drawing surface air through the mine, usually using an exhaust fan on the surface that pulls clean and cool air through the mine’s ventilation shafts.
When planning a mine ventilation system, Hattingh says some key decisions need to be made in terms of the primary and secondary ventilation strategies. Primary ventilation involves drawing surface air through the mine, usually using an exhaust fan on the surface that pulls clean and cool air through the mine’s ventilation shafts and, once it has been distributed to active mining areas, bringing that ‘used’ air back up to the surface through the main exhaust-air shafts.
Once underground, the air has to be distributed to wherever miners are working. This is done through secondary underground ventilation systems that draw the fresh air off the main intake airways, usually via ducting, and distribute it to the working places – the production zones and development ends.
Every mine has a well-defined ventilation network that enables ventilation practitioners to know and control exactly where the air is coming from, where it is going, how it is being routed safely via return airways to the surface and finally discharged back into the atmosphere.
It is important, Hattingh cautions, to remember that the core purpose of ventilating a mine is to enable mining to progress safely. “Our job as ventilation specialists,” he says, “is not to provide office-type air conditioning, but to ensure that we support the dynamic process of mining and safeguard those working underground from harmful airborne contaminants and heat.”
Every ventilation strategy must be developed around the layout and mining method of the mine. This is particularly true of the secondary ventilation system, as these areas differ considerably from mine to mine, depending on the type of mine, mining method, mining equipment, nature of the ore body, as well as airborne pollutants and heat associated with the orebody and mining process. The key question here is: how do we effectively and efficiently ventilate production areas to enable mining to progress unhindered?
Which is where ventilation on demand comes in
Highlighting the motivation underpinning VoD, Potgieter points out that ventilation needs to remove and dilute heat and air-born pollutants, such as blast and diesel exhaust fumes, flammable gases and dust, so as to keep miners safe and to enable them to stay fully productive in that environment.
Ventilation and refrigeration are, in many cases, the largest consumers of electricity in underground mines, especially as mines become deeper. To draw in and exhaust say 500 m3/s of air for 24 hours, 7 days a week needs a primary ventilation fan, drawing anywhere between 600 kW and 3.5 MW, a huge amount of electrical power.
“The idea underpinning VoD,” Potgieter says, “is to pull in only the air that is needed by the operating production zones. By applying a VoD strategy, we strive to optimise the ventilation requirements at all the points of need; which, if done well, can lower a mine’s electricity use by a huge amount.”
While primary ventilation systems consisting of main intake and return airways remain fairly constant, the dynamic nature of production scheduling may call for ventilation supply in active production zones to be adjusted on a shift-by-shift basis. A VoD design involves mostly secondary networks, which tend to use smaller fans and ducting.
“This is the dream,” continues Hattingh, “to minimise the ventilation power requirements without affecting miner safety or production targets. VoD can turn ventilation on and off, like lights turn on and off when walking into an office.” However, while this is technically possible, VoD is nowhere near as easy to implement. While lights might not need to be on when nobody is in the room, underground ventilation can never be completely turned off without understanding the implications: re-entry, strata gas build-up, availability of sufficient air, etc.
“First and foremost miners have to have enough air to breathe, which is generally handled by bringing down a sufficient quantity of the primary air from surface. But the air must also always be at the right quality. As well as removing impurities, the key air quality issues of a productive atmosphere at the work face are temperature and humidity. Typically, we strive to maintain the air at each workface to a wet bulb temperature of 27 °C , with air pollutants below set Time Weighted Average (TWA) exposure rates.”
The wet bulb temperature indicates the potential cooling capacity of the atmosphere and, combined with air movement, how effectively the evaporation of sweat cools down the human body. At wet bulb temperatures in excess of ±32 °C, the risk of human cooling mechanisms failing becomes significant, causing heat exhaustion and worse.
Manual versus automatic VoD systems
Ventilation on demand is an existing practice in many mines where the risk of flammable gas presence is negligible. Shift overseers stop and start secondary fans, or choke ventilation ducting manually, to supply the required air volumes to work faces according to activity requirements.
Manual operation of VoD systems may not always be effective because it is time consuming and inaccurate, especially where multiple ventilation duct branch-offs are installed, which can result in over or under ventilating work faces. As a result, VoD is seen as part and parcel of mine-automation, not only to reduce electrical costs, but also to provide an optimum and reliable supply of ventilation to the right place at the right time to ensure safe and healthy work environments.
Key to implementing an effective VoD solution is measurement. By knowing what the airflows, temperatures and humidities at each critical point in a mine are, it becomes possible to calculate the exact air velocities required for optimal working conditions. Using a VoD Controller (PLC), the data set from the whole mine, including the areas being worked at any particular time, can be analysed and used to continuously manage and optimise ventilation delivery. A further advantage of automated VoD systems is that live results may be visible in a control room on-surface, allowing the immediate identification, and possibly correction, of substandard conditions. But there are clearly risks, particularly at the design stage. This necessitates that rules are followed when the mine is in operation to ensure there is enough air for the development and stoping. VoD systems would, by design, reduce the overall air requirement of the mine so it would not be possible to add additional production zones without impacting the air delivered to planned work zones. Good practices and procedures, and coordination with ventilation and mining personnel, will however enable a safe and efficient system.
A mine is not a static environment, continues Potgieter. Mining might start at one level, then move to the next level down. Mines are continually mixing the ore grades mined to optimise profitability, or close off mined out areas and open new production levels. Therefore, the ventilation system would need to be adapted or upscaled quickly with instrumentation and supply infrastructure to enable the PLC to support any new production zone. So, the total capacity of the primary system really needs to take as many of these possibilities into account if value is to be maximised across a mine’s lifetime.
An automated VoD system requires very careful design and there are no off-the shelf solutions, notes Hattingh. While standard control loops and routines can, and have been developed, implementation is not plug and play. A ventilation control system must be specifically designed and tailored to suit the intended application. This comes directly off the overall mining strategy and associated health and safety risks, which, along with the ventilation design require careful consideration.
“At BBE, we have our VUMA Ventilation modelling software that can quickly be used to model and simulate ventilation for a mine design or extension,” Hattingh says. “Also, we have a spin-off product called VUMA Live, which can access live data from key areas to provide VoD control and a SCADA-based overview showing how the ventilation system is performing. This delivers the transparency to enable mine managers to make informed decisions on how to safely operate and optimise the system, and to predict what might happen when they make changes.” “If used properly, VoD can be a risk minimiser,” Potgieter concludes. The consequences of decisions become more exposed, while unintended consequences can be quickly identified and rectified.
And while there are capital costs involved in installing VoD systems, the safety assurance and life-of-mine cost savings that can be achieved make it worthwhile.
