Mine Ventilation

Mine Gases & Gas Dilution

Firedamp, blackdamp, whitedamp and stinkdamp — their explosive/toxic limits and the steady-state airflow needed to dilute a gas make below statutory limits.

PART 1

Topic Breakdown & Traps

The Engineering Principle

The underground atmosphere is continuously contaminated by gases: methane (firedamp, CH₄) from coal seams is explosive between 5 % and 15 %, most violently around 9.5 %; carbon monoxide (whitedamp, CO) is acutely toxic with a TLV of only 50 ppm; carbon dioxide (blackdamp, CO₂) and hydrogen sulphide (stinkdamp, H₂S) are also regulated. We keep concentrations safe by dilution — supplying enough fresh air so that, at steady state, the gas make is swept away below its permissible limit. The governing idea is a simple volumetric mass balance: gas added per second must be diluted by the air swept past per second.

The Core Formula Matrix

Concentration ↔ ppm

1% by volume = 10, 000 ppm, C [fraction] = \frac{C [ % ]}{100}

Dilution airflow (steady state, dilute gas):

Q_{ai r} = \frac{q _{g}}{C - B}

q_{g}

= gas make rate

[m^{3} / s]

C

= maximum permissible concentration (fraction),

B

= concentration already in the intake air (fraction),

Q_{ai r}

= required fresh airflow

[m^{3} / s]

.

Gas carried by an air current:

q_{g} = Q_{ai r} C

.

Methane explosibility: Lower Explosive Limit (LEL) ≈ 5 %, Upper Explosive Limit (UEL) ≈ 15 %, most explosive ≈ 9.5 %.

Methane-in-air explosibility band: inert below 5 %, explosive 5–15 % (peak ≈ 9.5 %), fuel-rich above 15 %.

The ‘IIT Traps’

⚠
ppm ↔ % decimal slips. $50 ppm = 0.005%$ (divide by 10,000), not $0.05%$ or $0.5%$ . Misplacing this decimal is the single most common gas-question error.
⚠
**Forgetting the intake background $B$ .** If the intake air already carries some of the gas, the *available* dilution capacity is only $(C - B)$ , not $C$ . Using $C$ alone under-estimates the air needed.
⚠
Confusing the gases. Hydrogen's explosive range is ~4–75 % and CO's is ~12.5–74 %; methane is the narrow 5–15 % band. Mixing these ranges up is a classic trap.
⚠
Density direction. Methane (SG ≈ 0.55) layers along the roof; CO₂ and H₂S (heavier than air) collect in floor dips and standing water — sampling at the wrong height misreads the hazard.

PART 2

Progressive 3-Tier Question Suite

Q1BASIC1 Mark · MCQ

The 8-hour threshold limit value (TLV) for carbon monoxide is 50 ppm. Expressed as a percentage by volume, this is:

Q2MEDIUM2 Marks · NAT

A development heading liberates methane at

q_{g} = 0.05 m^{3} / s

. The intake air already contains

0.10%

CH₄. The minimum fresh-air quantity required to hold the general-body concentration at the statutory limit of

1.25%

is ______ m³/s. (Round off to two decimal places.)

m³/s

Q3HARD2 Marks · NAT

A return airway carries

Q = 50 m^{3} / s

of air (negligible methane at intake). To keep the methane concentration in this current at or below

0.75%

, the maximum permissible methane make into the airway is ______ m³/s. (Round off to two decimal places.)

m³/s