A work crew opens a manhole and tests the air in the enclosed compartment below.

Question

A work crew opens a manhole and tests the air in the enclosed compartment below. They detect 29 mg/m3 of toxic H2S gas. It is 15°C inside the compartment. The maximum safe exposure level is 14 mg/m3 . To reduce the vapor concentrations to safe levels, the crew sets up a blower for ventilation. The volume of the enclosed space is 160 m3 and the blower creates an air flow-through rate of 10 m3 /min. If the inflow air is contaminant-free, H2S is assumed conservative, and the enclosed space is small enough to assume completely mixed conditions apply, how long must the blower run before it is safe for the crew to enter the manhole?

Explanation / Answer

from given data,

V = 160 cu.m

Q = 10 cu.m/min

T = 15C

H2S 29mg/cu.m but allowed 14 mg/cu.m

For complete removal 29 mg/cu.m takes 16 min (160/10)

then for achieving 14 mg/cu.m the inflow air should be pumped for (14/29)*16 = 7.72 min approx 8 min for attaining the permitted level for safe operation.

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