This assignment was locked Apr 28 at 11:59pm. At the start of a morning shift, a technician fitted regulators to two gas

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This assignment was locked Apr 28 at 11:59pm.
At the start of a morning shift, a technician fitted regulators
to two gas cylinders, one containing pure hydrogen sulphide
(H2S) and the other containing pure nitrogen
(N2) in order to calibrate some instruments using a gas
divider. He then proceeded to set up some analyzers for
calibration, but accidentally knocked over the H2S gas
cylinder. This resulted in the regulator becoming slightly loosen,
causing a small gas leak. The technician proceeded to spend the
next 10 hours calibrating instruments in the room on his own. The
room was glazed on one side. The technician had opened one of the
windows very slightly so that he could have the purge tubes from
the waste outlet of the analyzers discharged outside. The
technician was experiencing discomfort towards the end of the day,
he assumed it was due to waste gases from the analyzer re-entering
the room.
The volume of the room was 300 m3 and the
ventilation rate of the room with the window slightly open was 0.6
ACH. There was a mechanical fresh air supply to the room but no
mechanical exhaust. Assume that there were enough cylinder contents
for the gas leak to remain constant and that the gas escaped at the
rate of 0.66 cm3/sec (ml/sec).
The Labour Department of the Hong Kong Government produces
guidelines concerning the recommended exposure limits to certain
chemical substances. For Hydrogen sulphide (H2S) the
STEL is 15 ppm and the TWA is 10 ppm. Hydrogen sulphide
(H2S) is considered toxic if inhaled.
(a) What are STEL and TWA and what
conditions are applied to their interpretation? (6 marks)
(b) Explain the relationship
between the gas concentration expressed in
mg/m3 and the gas concentration expressed in ppm.
Hence calculate the equivalent STEL and TWA values expressed in
mg/m3.
State any
assumptions made. The pressure can be taken to be standard
atmospheric pressure and the temperature was 25°C. The molar volume
of a gas under such condition can be assumed as 24.45 litres. The
atomic weights of sulphur and hydrogen are 32 g/mol and 1 g/mol
respectively.
(c) What was the maximum
concentration (in ppm) of hydrogen sulphide in the room?
(d) Comment on the health and
safety procedures in force at the time of the incident. Make
recommendations that could be implemented as part of a health and
safety policy when undertaking similar tasks in the future.