17.18.1 Methyl acetylene-propadiene
mixtures should be suitably stabilized for transport. Additionally,
upper limits of temperature and pressure during the refrigeration
should be specified for the mixtures.
17.18.2 Examples of acceptable,
stabilized compositions are:
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.1
Composition 1
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.1.1 maximum methyl acetylene to propadiene
molar ratio of 3 to 1;
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.1.2 maximum combined concentration of methyl
acetylene and propadiene of 65 mol per cent;
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.1.3 minimum combined concentration of propane,
butane, and isobutane of 24 mol per cent, of which at least one third (on a
molar basis) must be butanes and one third propane; and
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.1.4 maximum combined concentration of
propylene and butadiene of 10 mol per cent.
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.2
Composition 2
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.2.1 maximum methyl acetylene and propadiene
combined concentration of 30 mol per cent;
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.2.2 maximum methyl acetylene concentration of
20 mol per cent;
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.2.3 maximum propadiene concentration of 20 mol
per cent;
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.2.4 maximum propylene concentration of 45 mol
per cent;
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.2.5 maximum butadiene and butylenes combined
concentration of 2 mol per cent;
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.2.6 minimum saturated C4
hydrocarbon concentration of 4 mol per cent; and
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.2.7 minimum propane concentration of 25 mol
per cent.
17.18.3 Other compositions
may be accepted provided the stability of the mixture is demonstrated
to the satisfaction of the Administration.
17.18.4 A ship carrying methyl acetylene-propadiene
mixtures should preferably have an indirect refrigeration system as specified in 7.2.4.2. Alternatively, a ship not provided with indirect
refrigeration may utilize direct vapour compression refrigeration subject to pressure
and temperature limitations depending on the composition. For the example compositions
given in 17.18.2, the following features should be provided:
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.1 A vapour compressor that does not raise the
temperature and pressure of the vapour above 60°C and 17.5 bar gauge during its
operation, and that does not allow vapour to stagnate in the compressor while it
continues to run.
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.2 Discharge piping from each compressor stage or
each cylinder in the same stage of a reciprocating compressor should have:
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.2.1 two temperature-actuated shutdown switches
set to operate at 60°C or less;
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.2.2 a pressure-actuated shutdown switch set to
operate at 17.5 bar gauge or less; and
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.2.3 a safety relief valve set to relieve at
18.0 bar gauge or less.
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.3 The relief valve required by 17.18.4.2.3 should vent to a mast meeting the requirements
of 8.2.9, 8.2.10, 8.2.13 and 8.2.14 and should not relieve into the compressor suction
line.
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.4 An alarm that sounds in the cargo control position
and in the navigating bridge when a high-pressure switch, or a high-temperature
switch operates.
17.18.5 The piping system,
including the cargo refrigeration system, for tanks to be loaded with
methyl acetylene-propadiene mixtures should be either independent
(as defined in 1.3.20) or separate
(as defined in 1.3.32) from piping
and refrigeration systems for other tanks. This segregation applies
to all liquid and vapour vent lines and any other possible connections,
such as common inert gas supply lines.