Feature 5 | SUBMARINE PROPULSION
leak detector would be designed to only whole submarine. limit for hydrogen. As it has already been
alarm if more than this volume of water Th e transfer pipework would be double proven that the lower fl ammability limit
was displaced and therefore the worst case walled, isolable in the HCWTs and RFC cannot be reached there is no danger of
undetected leak is 0.1m
3
, a trivial amount. compartments and with no fi ttings between hydrogen asphyxiation occurring in general
Should more hydrogen be released it the two. Internal pipework has been conditions. Furthermore, as hydrogen is
would be detected and the contents of minimised by positioning the HCWTs so buoyant the usual reaction to oxygen
the containment tank could be flushed underneath the RFC compartments and starvation is to collapse thus probably
overboard using the trim or ballast system. the fore-aft connection routed outside the lowering the head below the hydrogen
As a result the risk from the metal hydride pressure hull, under the casing. As a result cloud. The only exception to this could
tanks is considered negligible. the risk from the hydrogen pipework is be during a tank entry to the HCWTs for
Th e RFC compartments would normally considered negligible. Because hydrogen gas maintenance or inspection; under these
be sealed with the exception of a ventilation consists of such small energetic molecules circumstances normal confi ned-space entry
supply and exhaust, each of which would special pipework is required to avoid procedures would mitigate the asphyxiation
be fitted with flame suppressors and hydrogen embrittlement and welds and risk to tolerable levels.
atmosphere monitoring equipment. All seals must be extremely tight. Oxygen is a highly reactive gas and when
ignition sources would be properly sealed Should a hydrogen fire develop in a in a pure form can react extremely violently
and rated for gas-dangerous compartments. RFC compartment (the only credible place with fl ammable materials such as oil and
Should hydrogen be detected the RFC it could) the diffi culty would most likely grease. By citing the RFCs adjacent to the
would automatically shut down, isolation come in detection as hydrogen burns with oxygen compressors, leading the compressor
valves shut and a purge cycle started as with an almost invisible fl ame. It is worth noting discharge pipework immediately outside
the Royal Navy’s Gas Management Plants that submariners are experienced with the pressure hull and running the fore and
(GMPs). Ventilation would continue in equally hard-to-fi nd steam leaks. Should the aft connection under the casing the risk of
order to diff use the hydrogen throughout 2.1litres/s leak calculated above ignite on exit an internal oxygen leak is minimised as
the submarine; each RFC compartment from the leak site (before it has dispersed) it far as possible. However, if a leak should
is only 0.3% of the internal volume of the would produce 23.0kW. Th is would result in occur the risk of fi re would be mitigated
submarine, therefore even if a compartment a 2.2°C temperature rise per second which by maintaining the RFC compartments as
was 100% full of hydrogen the lower would be easily detected by the fi re detectors ‘clean’ spaces with no fl ammable materials
fl ammability limit submarine-wide would already fi tted throughout RN submarines. allowed within. Oxygen can also be toxic
not be reached. Existing CO/H burners The system would be automatically if the partial pressure exceeds about 0.5bar
2
would be suffi cient to remove any residual shutdown and a fi xed CO drench system (in standard atmosphere it is 0.2bar). As
2
hydrogen from the atmosphere. activated. With proper design the chances the tanks are stored external to the pressure
Were the detection system defective of other materials combusting (and hence hull a signifi cant leak into the submarine
a major leak would still be identifi ed by producing smoke) can be minimised, the is unlikely and even if an entire tank were
pressure drop or power loss, but a small only other result from such a fi re would discharged into the submarine the partial
pinhole leak might be missed by the control be the production of a small amount of pressure of oxygen would only increase to
systems. Assuming a fl ow meter comparison water vapour (1.7cc/s). It is the heat from 0.27bar.
system has an accuracy of 1% the trip setting a hydrogen fire that is the main danger, In the authors’ opinion the risk posed by
would be set at 5% to allow for transient followed by oxygen consumption - unlike a compressed oxygen is the greatest risk of the
induced system lag. The maximum hydrocarbon fi re where the main danger is design and a detailed investigation would
hydrogen fl ow rate in each half of the system the production of toxic gases such as carbon have to be carried out prior to any further
is 42litres/s, therefore a leak up to 2.1litres/s monoxide and soot, which can rapidly design work.
might go unnoticed. Each RFC compartment poison the confi ned volume of a submarine.
would contain 8.6m
3
free volume and a As a result the risk from a hydrogen fi re in the Increased redundancy
ventilation fl ow rate of 72litres/s assuming RFC compartment is considered negligible. The safety analysis concentrated on
the air is turned more than 30 times an Th e eff ects of oxygen starvation range disproving, mitigating or highlighting
hour (the value specifi ed within provisional from a reduction in coordination to coma potential problems; however, the safety
Lloyd’s Register rules for gas fuelled ships). and subsequent death within 40 seconds. advantages of the system must also be
As this is a far higher turnover of air than the For the purposes of this investigation it considered. Th ese comprise of a number
leak a dangerous hydrogen build-up would is assumed that 16% is the cut-off point of factors such as greater reliability and
not occur, provided the ventilation system below which injury may occur either from reduced temperatures. However, one of the
has been adequately designed to minimise cellular oxygen starvation or as a result of most signifi cant advantages is the provision
air pockets, particularly at the top of the reduced coordination and decision making. of a second Air Independent Propulsion
pressure hull where the H is likely to collect. For the oxygen concentration to fall by 4% (AIP) system.
2
Even without the fitted CO/H burners to 16% requires 20% of the compartment Submarines have an extremely low
2
it would take more than 11 days to reach volume to be displaced by hydrogen – reserve of buoyancy and with a nearly
the 4% lower fl ammability limit within the that is, fi ve times the lower fl ammability circular cross-section they are liable to roll
50 Warship Technology October 2008
WWT_Oct08_p42+43+44+46+47+50+51.indd Sec3:50T_Oct08_p42+43+44+46+47+50+51.indd Sec3:50 110/10/08 1:17:17 PM0/10/08 1:17:17 PM
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