Page 48 - International safety guide for oil tankers and terminals

INTERNATIONAL SAFETY GUIDE FOR OIL TANKERS AND TERMINALS

2.3.6.1 Sources of Hydrogen Sulphide (H S)
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Many crude oils come out of the well with high levels of H 2 S, but a
stabilisation process usually reduces this level before the crude oil is
delivered to the ship. However, the amount of stabilisation may be
temporarily reduced at times and a tanker may receive a cargo with an
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
H 2 S content higher than usual or expected. In addition, some crude oils
are never stabilised and always contain high levels of H 2 S.

H 2 S can also be encountered in refined products such as naphtha, fuel
oil, bunker fuels, bitumens and gas oils.

Cargo and bunker fuels should not be treated as free of H 2 S until after
they have been loaded and the absence of H 2 S has been confirmed by
both the results of monitoring and the relevant MSDS information.

2.3.6.2 Expected Concentrations
It is important to distinguish between concentrations of H 2 S in the
atmosphere, expressed in ppm by volume, and concentrations in liquid,
expressed in ppm by weight.

It is not possible to predict the likely vapour concentration from any given
liquid concentration but, as an example, a crude oil containing 70 ppm (by
weight) H 2 S has been shown to produce a concentration of 7,000 ppm
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(by volume) in the gas stream leaving the tank vent.

Precautions against high H 2 S concentrations are normally considered
necessary if the H 2 S content in the vapour phase is 5 ppm by volume
or above.

The effects of H 2 S at various increasing concentrations in air are shown
in Table 2.1.

The H 2 S concentration in vapour will vary greatly and is dependent upon
factors such as:
Liquid H S content.
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Amount of air circulation.
Temperature of air and liquid.
Liquid level in the tank.
Amount of agitation.

2.3.6.3 Exposure Limits
The TLV-TWA for H S is 5 ppm over a period of eight hours. However,
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working procedures should aim at ensuring that the lowest possible
gas concentrations are achieved in work locations.

2.3.6.4 Procedures for Handling Cargo and Bunkers
Containing H S
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The following precautions should be taken when handling all cargoes and
bunker fuels likely to contain hazardous concentrations of H S. They
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should also be taken when ballasting, cleaning or gas freeing tanks which
previously contained a cargo with an H S content. Practical guidance on
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operational measures that can be taken to minimise the risks associated
with loading cargoes containing H S is given in Section 11.1.9.
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14 © ICS/OCIMF/IAPH 2006