Easily accessible, high grade ore is becoming scarce. To meet glo- bal mineral demand, mining com- panies must look to challenging locations for economically viable deposits. Naturally, this includes mountainous regions at high elevations. While some opera- tions are already producing at altitudes exceeding 5,000 meters (16,405 ft), high elevations can have adverse effects on people and machinery starting as low as 1,500 m (4,922 ft).
is similar to water running through a garden hose. Running unrestricted, a large volume of water comes out, but at a low pressure. If you put your thumb over the end of the hose, the pressure will increase but the volume will be reduced. In our example, if the nozzles are sized properly, the pressure at the bit might be 60 PSI. At this lower pres- sure, the volume would be higher than at 110 PSI, but still much lower than the intake volume. As the air moves through the bit and up the hole, the pressure will eventually drop to atmos- pheric pressure again, at a high volume. At 15,000 ft (4,572 m), the same compressor is still taking in 1,900 cubic feet of air per minute. However, the number of molecules of air is reduced by approximately 42%, calculated by comparing the air pressure at sea level
(14.7 PSI or 101 kPa) to the pressure at elevation (8.6 PSI or 59 kPa). To compensate for the lower density, cor- rection factors are used to calculate the effective, not actual, intake CFM required to produce the same perfor- mance as at sea level. In this case, the reduced air pressure effectively cuts the compressor to an equivalent intake capacity of 1106 CFM (assuming the same temperature as sea level). Taking into account the lower temperature and corresponding increase in density, the intake capacity would move up to 1,221 CFM at 7°F (-14°C).
To determine how much air is re- quired, a calculation of uphole velocity is required. Uphole velocity is calculat- ed from the intake air volume and the annular area, which is the gap between the wall of the hole and the drill rod.
A minimum uphole velocity is often stated as 5,000 ft/min but in reality is affected by the density, size and shape of the chips, and factors such as the pressure of cuttings or water in the hole and the condition of the borehole wall. Using a Pit Viper 271 as an example, the table below shows a comparison at various elevations.
At sea level, 1900 CFM for a 10-⅝” bit with 7-⅝” rod works well, even as the rod wears down. At high alti- tude, the large reduction in air den- sity requires either a larger compressor (2600 CFM in the example) or a larger (10-⅝”) diameter drill rod. The pre- ferred option is a larger compressor, as using larger rods cuts the clearance between the wall of the hole and the rod to such a small gap that larger cut- tings must be reground to a smaller size before exiting the hole, thus accelerat- ing bit wear. Going to a smaller diam- eter bit will improve uphole velocity as well, though this is not always feasible given the blasting requirements.
Power drain
Diesel engines face considerable dif- ficulty at high altitude as they rely on the oxygen in the air for proper com- bustion. Engines have varying alti- tude limits and power de-rate curves. Manufacturers are able to maintain full power to the altitude rating by changing the engine timing, turbocharger con- figuration and compression ratios. This altitude rating is the point where horse- power begins to decrease. As a rule of thumb a diesel engine will de-rate
Comparison of compressor performance at high altitude vs sea level and subsequent effect on uphole velocity in rotary drilling using a Pit Viper 271 drill rig.
Uphole Velocities
compressor
at Sea level/15° c (59° F) at 15 000 ft/ –14° c (7° F)
Rating: 1 900 cfm Rating: 2 600 cfm Rating: 1 900 cfm Rating 2 600 cfm
hole Ø (in)
Drill Rod
Ø (in) Effective Vol. 1 900 cfm Effective Vol. 2 600 cfm Effective Vol. 1 221 cfm Effective Vol. 1 670 cfm
10 ⁵⁄₈ 7 5⁄8 6 434 ft/min 8 804 ft/min 4 087 ft/min 5 592 ft/min
10 ⁵⁄₈ 8 ⁵⁄₈ 9 149 ft/min 12 520 ft/min 5 812 ft/min 7 953 ft/min
9 ⁷⁄₈ 7 5⁄8 8 946 ft/min 12 242 ft/min 5 682 ft/min 7 776 ft/min
9 ⁷⁄₈ 8 ⁵⁄₈ 15 232 ft/min 20 855 ft/min 9 675 ft/min 13 240 ft/min
1 900 cfm = 53.8 m3/min 1 000 ft = 304 m 5 000 ft/min = 25.4 m/s
When moving from sea level up to high altitudes the density of air will decrease, and the temperature will drop at a fairly uniform rate.
For example, an 800 HP diesel en- gine might have an altitude limit of 8,000 ft (2,438 m). At 15,000 ft (4,572 m), the power would be 21% less, or 632 HP. At full power assuming maxi- mum compressor and rotary head power, this unit would approach 90% load, or 720 HP. If this unit operated with mechanical or hydraulic drive systems that do not de-rate, there would be insufficient power to operate at this elevation. However, because the air compressor is working with lower density air, its power requirement is reduced by 1.5% per 1,000 ft (304 m). The 1,900 CFM compressor itself requires approximately 430 HP to pro- vide full flow and pressure. At 15,000 ft (4,572 m), the load drops by 22.5% to 333 HP, effectively cutting the total load from 720 HP to 623 HP, safely below the de-rated power.
For best results, it is preferable to go with a larger displacement engine with a higher output to offset the reduction due to altitude. However, it is not always possible to provide larger engines as the drills are sized to handle a particular size engine from the begin- ning and space is not available.
Another alternative is electric power. Electric motors do not rely on combus- tion for power, and are not subject to power loss due to lack of oxygen. However, there are two issues which can impact the operation of electrical components such as motors, starters and transformers. One is the decreased cooling capacity at altitude. Higher capacity components may need to be specified to achieve the same rating as lower altitudes, though the cold air at altitude generally ensures stand- ard components do not overheat. The second issue is that lower air density reduces the di-electrical strength of air gaps in components, thus reducing the insulation capacity of these components and increasing the risk of electrical breakdown, or flashover. At extremely high elevations, special electrical com- ponents may be required.
The decision to go with electric po- wer has to consider other factors like how electricity is generated and the demand for mobility of the rig. In
remote areas, the only alternative to produce electricity might be by diesel powered generators. In such a case, the power generation will also suffer from the power de-rating at high altitude, and a diesel powered rig can still be the best alternative if it can be config- ured to meet the required performance. Electric power may offer higher capac- ity, but this might be offset by the mobility and higher utilization of a diesel powered rig.