By Andrew Hooppell Principal Mining Consultant (Underground in Study) at Mining Plus
Originally published in September 2018.
The recent improved outlook for commodity prices, and the mining industry in general, has seen an increase in the number of underground mining projects being considered. In the Australian mining context these are almost exclusively accessed via declines from surface. In many cases the layout of the mine and/or geotechnical conditions of the site result in significant lengths of development being undertaken before a primary ventilation connection is established. Up until this point, auxiliary ventilation systems, typically axial fan and flexible duct, are be used to supply sufficient ventilating airflow to meet legislated requirements, guidelines and operational needs.
Peak decline development airflow requirements are generally driven by the bogging (mucking) and hauling fleet. In the not too distant past, a decline development haulage, fleet was a R2900 LHD and an AD45 truck, or equivalent, with a combined rated engine power of 743 kW, or less, on the assumption no other diesel-powered equipment was used concurrently. In the Western Australian (WA) context, applying the Diesel-engine Exhaust Emission (DEE) dilution requirements specified in the Mines Safety and Inspection Regulations, 1995 (MSIR) of 0.05 m³/s/kW of installed diesel power required a minimum airflow of 37 m³/s be delivered to face, or working area. Importantly the MSIR requirements are not mirrored in other mining jurisdictions in Australia and around the world, with a DEE dilution rate of 0.06 m³/s/kW of operating power being widely adopted, a 20% increase over the WA MSIR.
The standard ‘plan’ for ventilation of single-heading developing declines with this bogging fleet in Western Australia is simply to install a 2 x 110 kW (twin-stage), 1,400 mm diameter, axial fan in the primary air stream, or outside the portal, forcing air to the face through 1.4 m diameter flexible, or lay-flat, ducting. Correctly installed and maintained, this system provides adequate ventilation for a distance of 550 m, based on WA requirements. A plot of the specified system performance can be seen below.
The latest incarnation of larger capacity and higher engine-power mining equipment, typical for decline development activities (R2900G and AD60) can see total installed power exceed 900 kW, with airflow requirements increasing to 45 m³/s based on the WAMSIR. The 2 x 110 kW fan specified is capable of satisfying the specified airflow requirements for a distance of 200 m. This is a significant restriction, requiring additional capacity be installed or a primary ventilation circuit established at this point.
When considered against DEE dilution airflow recommendations and legislation from other mining jurisdictions, typically 0.06 m³/s/kW or more, the minimum airflow to be provided increases to a more than 54 m³/s. A volume in excess of the capacity of the 2 x 110 kW ‘workhorse’ fan.
This requires a significant change in recent ventilation planning practice. Options available to mine owners and operators to increase airflow to long, single-entry development headings include:
-Decrease size and/or engine power of diesel equipment.
-Install multiple ventilation systems in parallel.
-Increase size and/or power of single auxiliary systems.
-Establish primary ventilation connections at distances of no more than 200 linear meters.
-In many instances this will require parallel ventilation airways be developed, i.e. twin declines.
-Installation of combination forcing and exhausting systems, known as push-pull systems.
Primary Ventilation System
Two basic options exist for creation of a primary ventilation circuit to support a decline development project:
-Development of a concurrent, parallel airway to the Access Decline.
-Development of dedicated raises/shafts from surface connecting to the Access Decline at intervals not exceeding the performance capability of the auxiliary fan(s).
Early establishment of surface raises is the simplest, least capital-intensive method for creating a primary ventilation system, however is not suitable for all situations as the available surface footprint, geotechnical conditions or decline alignment can all work to prevent this. There is significant capital outlay required for raise development, particularly if raises/connections form no part of the longer-term circuit.
An alternative is to develop a parallel ventilation decline or drive, adjacent to the main decline, separated by a pillar. Cross-cuts are then developed between these to facilitate establishment of a simple ventilation circuit. Upon breakthrough of the first cross-cut between the two declines the ventilation decline portal has a temporary primary fan system installed. Typically, this would consist of appropriately specified axial fans, sealed in a wall constructed across the portal, creating a negative pressure and causing air to be drawn into the main access decline portal after which it would flow along to the first open cross cut and back along the ventilation decline to the fans which exhaust to atmosphere. A conceptual arrangement of this is shown below.
This option is capital intensive, requiring essentially 100% more development than a comparable single-heading decline, and is unlikely to provide an optimum economic solution from a ventilation perspective alone, unless it forms part of the long-term primary ventilation circuit for the mine or has some other functionality.
A combination forcing/exhausting ventilation system, otherwise known as a push-pull system provides a similar methodology to establishing a conventional primary ventilation circuit, however does not require the development of a parallel tunnel or airway. Instead the ‘parallel airway’ is created through the use of ducting, installed in the developing decline/heading.
Exhausting, or pulling, fans cause air to be drawn into the portal, flowing to the fan inlets. This is then forced back to atmosphere via ducts, with the exhaust point located adequate distance from the portal to prevent recirculation. Forcing fans that direct air to the working face are located on the portal side of the ‘pulling’ fans and force air to the working tunnel face. In this layout, the face fans must be located portal-side of the exhaust fans to prevent recirculation.
The same limitations apply to the face ventilation (push) fans as for a single fan forcing system with respect to development distance that can be ventilated. Upon reaching this, the exhausting fan system must be extended towards the working face. The parallel arrangement of the exhausting fans allows for each system, or bank, to be extended independently of the other, maintaining continual airflow in the decline, permitting non-ventilation intensive activities to carry on concurrently.
Duplicate Forcing Systems
Duplication of standard auxiliary ventilation systems, in parallel, is a relatively simple method of significantly increasing the development distance that can be ventilated. However, the simplicity of this must be traded-off against the additional work required to install and maintain the system combined with 100% increase in capital and operating expenses. Added to this is the likely increase in cross-sectional area of the decline/heading to accommodate an additional duct, while maintaining adequate clearance to equipment and other services.
For mining jurisdictions, requiring a DEE dilution airflow of 0.05 m³/s/kW, the maximum distance that can be ventilated using parallel systems each consisting of a 2 x 110 kW, 1,400 mm diameter fan combined with 1.4 m diameter duct is 1,500 m (23 m³/s per duct, delivered to the face), based on the prescribed haulage fleet. Where 0.06 m³/s/kW is required, the maximum distance that can be ventilated with the same system is 1,100 m (27 m³/s per duct, delivered to the face). In both cases the assumption is made that good quality ducting will be correctly installed and maintained.
Increased Single Auxiliary System Size
Increasing airflow through a single auxiliary ventilation system requires that higher duty fans are employed and/or duct diameter increases. In simple terms, upgrading fan performance while maintaining duct diameter at a standard 1.4 m diameter, as in previous examples, will give the least ‘bang for buck’ from a ventilation point of view. Increasing airflow through a given duct will increase pressure drop between the fan and outlet. This will increase leakage, and result in only marginal gains at the working face.
There are countless permutations and combinations that can be evaluated, with respect to fan and duct dimensions and fan motor power, with each of these having merit depending, on the application. One such example consists of a twin-stage 1,600 mm diameter fan, each stage fitted with 200 kW motors, combined with 1.6 m duct. This will support the identified decline mucking fleet to a distance of 450 m when considered in context of 0.06 m³/s/kW DEE dilution requirements, with a plot of system modelling shown below.
An important flow-on effect of increasing duct and fan diameter is that excavated dimensions of the developing decline, or heading, must be increased over that used for 1.4 m diameter systems, to maintain acceptable clearance between equipment and duct to prevent damage, with an associated increase in development cost.
At face value, the increased ventilation requirements for decline projects, or other long single heading development can be seen solely as an imposition in terms of capital cost, operating cost, and the increased work required for installation. However there are significant benefits that can be leveraged to improve project performance, including:
-Reduced blast re-entry times, equating to more time at the face.
-Increased dilution of non-diesel atmospheric contaminants where these exist.
-Increased air velocity at the workface providing increased cooling to personnel, particularly important in hot environments.
Increasing ventilating airflows for decline development projects, or other extended length development, when utilising diesel-powered equipment is a fact-of-life to which the mining industry is going to have to adapt. Ever increasing equipment engine power combined with a greater understanding of the health impacts of exhaust emissions from diesel engines and a willingness from regulators to enforce ventilation requirements will all contribute to this outcome. The methodology adopted will be dependent on what is the best-fit for the respective project and what provides the best outcome for the lowest cost. Failure to plan for this at the design stage could lead to costly rectification and/or decreased productivity.
What is almost certain is that doing nothing different is not going to be an option.