Diesel Exhaust Overexposure Behind a Ventilation Short Circuit
An underground sublevel stoping mine is accessed by a decline and ventilated by a single 2 × 110 kW axial main fan at the portal, rated 55 m³/s at 1,800 Pa. The diesel fleet is three 250 kW LHDs and two 160 kW haul trucks. On the deepest level, airflow measured at the working face is 6 m³/s, dry-bulb temperature is 31 °C at 95% relative humidity, and a 15-minute sample returns 4.5 ppm NO₂ against a 3 ppm occupational limit. The return airway passes two stopes that were backfilled and have since collapsed, and the failed backfill now blocks roughly half the return drift. A booster fan rated 25 m³/s at 1,200 Pa was installed in the 3.0 m × 3.0 m access drift serving this level; a vane anemometer reads 12 m³/s at the booster inlet and 18 m³/s twenty metres downstream, with visible dust recirculating at the level junction. Production insists on running all three LHDs.
1. Quantify the airflow the face actually needs, explain why installing a second fan will not solve this, and rank the corrective actions.
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Requirement. Using the conventional planning factor of 0.05 m³/s per kW of installed diesel power at the face: two LHDs working one heading = 500 kW × 0.05 = 25 m³/s. The whole fleet requires 1,070 kW × 0.05 = 53.5 m³/s, essentially the entire mine airflow from the main fan. That is the strategic point - the fleet is oversized relative to the ventilation system, and no incremental fan will close a 25 m³/s gap.
Why the face gets 6 m³/s. The fan is not necessarily failing. A partly blocked return drift raises system resistance, which moving the operating point along the fan curve toward higher pressure and lower flow. Check fan static pressure and motor current against the curve: if pressure is above the 1,800 Pa rating, the problem is resistance, not the fan. The collapsed stopes almost certainly opened a direct leakage path between fresh and return, so the 49 m³/s that never reaches the face is short-circuiting through old workings - not lost through the fan.
Ranked actions.
- Seal the collapsed stopes with bulkheads designed for the local pressure differential; this is the largest single leak and the cheapest flow to recover.
- Clear the return drift so the fan returns to its design operating point.
- Repair or replace doors and airlocks on level accesses, then measure branch flows; balance with regulators only after the network is tight.
- Until face airflow exceeds 25 m³/s, restrict the level to one LHD and enforce cab filtration plus personal NO₂ monitoring.
Adding fans before sealing leaks adds flow to the leak, which is the trap this mine has already fallen into.
2. Evaluate the booster fan installation. Is it the right fix, and what hazards does it introduce?
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Verdict: it is masking the problem and creating a new one.
The numbers given are diagnostic: 12 m³/s at the inlet, 18 m³/s twenty metres downstream, and still 6 m³/s at the face. Six cubic metres per second has been added to a branch that is not delivering air to the working place. The only places that flow can go are the leak paths - the collapsed stopes and the doorways - or back around the junction, which is precisely what the visible dust recirculation indicates.
Mechanics. In a network dominated by a low-resistance leak, the system resistance curve is flat. An added fan simply shifts the operating point to a higher total flow, most of which follows the path of least resistance. The booster's own operating point will also sit well off its rated 25 m³/s at 1,200 Pa; the fan curve and the branch curve intersect wherever they intersect, not at the nameplate point.
Hazards.
- Recirculation of NO₂, CO and dust is a direct overexposure mechanism. The 4.5 ppm reading is plausibly a recirculation artefact rather than pure engine output, and it means the sample understates cumulative exposure in the zone.
- Dust recirculation raises silica exposure and fouls engine radiators, pushing diesel temperatures and emissions up.
- Personnel downstream may believe flow means fresh air, which erodes the reliability of every future ventilation reading.
Recommendation. Isolate or remove the booster. If the level genuinely needs a dedicated air supply, install it on the fresh-air side with ducting to the face, with intake positioned so contaminated air cannot re-enter. Treat the booster as an interim measure only, documented in the ventilation plan, with continuous NO₂ monitoring at the face and a withdrawal trigger. The permanent fix is sealing and re-balancing.
3. Write the exposure-control plan for the diesel fleet over the next two weeks, including what monitoring proves it is working.
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Immediate (days 1–3).
- Withdraw personnel from the level while NO₂ exceeds the limit; a 15-minute sample at 4.5 ppm against a 3 ppm limit is a strong indicator of overexposure, and short-term peak limits apply regardless of the 8-hour average framing.
- One diesel unit per heading maximum. The arithmetic is unforgiving: 250 kW demands 12.5 m³/s and 6 m³/s is available, so even a single unit is marginal.
- Seal the collapsed stope accesses and repair doors - this is the intervention that changes the number.
Days 4–14.
- Tune engines to manufacturer specification and verify with a smoke/opacity test; a poorly tuned engine can double NO₂ output per kW.
- Evaluate retrofit controls (DPF plus SCR or NOx adsorber) on the two units that work the deepest level, with a documented maintenance interval - a loaded DPF raises backpressure, heat and fuel consumption if neglected.
- Ventilation-on-demand: idle-time shutdown, and staging so that trucks do not queue in the same drift as loaders.
- Re-entry procedure after blasting tied to measured clearance, not to a fixed clock.
Proof of control.
- Face airflow above 25 m³/s measured at the working place, with a documented traverse.
- Personal and fixed-point NO₂/CO monitoring with data logging, sampled over full shifts rather than 15-minute grabs; trend against unit hours.
- Branch airflow survey confirming the leakage path is closed: the sum of face flows should approach fan flow minus planned leakage.
- Baseline personal exposure records retained for the workforce, since this event may generate a regulatory review.