Manhole safety indication: A safety lamp flame intensifies or ignites near the upper portion of a manhole. Which contaminant is most likely present?

Introduction / Context:
Methane (CH₄) is lighter than air and tends to accumulate near the crown of sewers and the upper part of manholes. A safety lamp (or flame tube) flaring or igniting at the top region suggests a combustible gas concentrated aloft—commonly methane in sewer atmospheres.

Given Data / Assumptions:

• Flame behavior is observed in the upper manhole zone.
• Environment is a sewer where CH₄ generation is possible.
• No strong petroleum sources indicated.

Concept / Approach:

Because CH₄ has a relative density < 1, it stratifies above air; thus flames may enlarge or ignite near the top. While petrol vapours are heavier-than-air and tend to settle lower, methane signals hazard at the top. Always use intrinsically safe gas detectors rather than open flames for real testing.

Step-by-Step Solution:

Verification / Alternative check:

Use calibrated LEL and CH₄ sensors; continuous monitoring during entry confirms safe conditions.

Why Other Options Are Wrong:

CO₂ is non-flammable; H₂S, though toxic, is not typically concentrated at the top and is dangerous at low ppm; petrol vapours usually accumulate low due to higher density.

Common Pitfalls:

Using a flame test (unsafe); ignoring stratification; failing to ventilate adequately.

Final Answer:

Methane gas