17.21.1 Uncontrolled pressure loss from the cargo
can cause "sublimation" and the cargo will change from the liquid
to the solid state. The precise "triple point" temperature of a particular
carbon dioxide cargo shall be supplied before loading the cargo, and
will depend on the purity of that cargo, and this shall be taken into
account when cargo instrumentation is adjusted. The set pressure for
the alarms and automatic actions described in this section shall be
set to at least 0.05 MPa above the triple point for the specific cargo
being carried. The "triple point" for pure carbon dioxide occurs at
0.5 MPa gauge and -54.4°C.
17.21.2 There is a potential for the cargo to
solidify in the event that a cargo tank relief valve, fitted in accordance
with 8.2, fails in the open position. To avoid this, a means of isolating
the cargo tank safety valves shall be provided and the requirements
of 8.2.9.2 do not apply when carrying this carbon dioxide. Discharge
piping from safety relief valves shall be designed so they remain
free from obstructions that could cause clogging. Protective screens
shall not be fitted to the outlets of relief valve discharge piping,
so the requirements of 8.2.15 do not apply.
17.21.3 Discharge piping from safety relief valves
are not required to comply with 8.2.10, but shall be designed so they
remain free from obstructions that could cause clogging. Protective
screens shall not be fitted to the outlets of relief valve discharge
piping, so the requirements of 8.2.15 do not apply.
17.21.4 Cargo tanks shall be continuously monitored
for low pressure when a carbon dioxide cargo is carried. An audible
and visual alarm shall be given at the cargo control position and
on the bridge. If the cargo tank pressure continues to fall to within
0.05 MPa of the "triple point" for the particular cargo, the monitoring
system shall automatically close all cargo manifold liquid and vapour
valves and stop all cargo compressors and cargo pumps. The emergency
shutdown system required by 18.10 may be used for this purpose.
17.21.5 All materials used in cargo tanks and
cargo piping system shall be suitable for the lowest temperature that
may occur in service, which is defined as the saturation temperature
of the carbon dioxide cargo at the set pressure of the automatic safety
system described in 17.21.1.
17.21.6 Cargo hold spaces, cargo compressor rooms
and other enclosed spaces where carbon dioxide could accumulate shall
be fitted with continuous monitoring for carbon dioxide build-up.
This fixed gas detection system replaces the requirements of 13.6,
and hold spaces shall be monitored permanently even if the ship has
type C cargo containment.