design considerations for aluminum hull structures - Ship Structure ...

-69-cargo holds. This is assumed to be based on the premise that if a fire didstart the provision of two fire hoses would permit sufficient fire control,together with the fact that steel does not lose its inherent strength inthe face of an average fire. For some cargoes and arrangements, these fixedsystems may not be effective.The ignition temperatures of the dusts of the various types of bulkcargo vary approximately between 6S0 degrees F to 9000 degrees F. Thus apotential fire in the cargo spaces can not be disregarded. In view of themaximum temperature restriction of aluminum, this warrants the installationof a detection system within the cargo holds. A temperature rise sensingsystem would probably prove to be the most satisfactory.In addition to the conventional cargo hold fire extinguishing system,an inerting system is recommended for all cargo spaces. This system couldutilize either nitrogen or carbon dioxide, and would be activated whenpotentially dangerous cargoes are carried. such systems are presentlyincorporated in a number of oil tankers to reduce the risk of explosion.The cargo holds must be gas-freed prior to unloading cargo, so that menmay safely enter the hold, thereby requiring the installation of largesuction fans serving the holds. In this regard, nitrogen offers an advantagein that it is slightly lighter than air and would tend to risenaturally from the hold when the hatch covers are open. However, thistendency of gas to rise would necessitate special techniques to maintain asatisfactory dist~ibution of nitrogen throughout the cargo, such as circulatingfans or continual bleeding of additional nitrogen at the lowestlevel of the hold. Carbon dioxide, being heavier than air, would be moredifficult with regard to gas-freeing the hold, but would satisfactorilydistribute itself throughout the cargo without resupply or recirculation.Neither gas would appear to be harmful to the range of cargoes beingconsidered.There are a number of umknowns concerning an inerting system such asthat proposed~ which preclude an evaluation of its cost. These include suchfactors as the required concentration of inerting gas to maintain a satisfacto~level of fire protection, the residual coricentration which couldremain after gas-freeing the hold, recirculation requirements and so forth.The solution to these problems is beyond the scope of the present study, butwarrants further consideration.An alternative which might be preferable to an inerting system wouldinclude a high-capacity carbon dioxide smothering system in conjunction withfire detecting equ!cpment of improved sensitivity. This system would incorporatethe following features:o Sufficient quantities of carbon dioxide to selectively flood a~hold, or the engine room, with a high concentration of gas.o A gas delive~ system which would insure rapid flooding of thespaces and even distribution throughout the space.o A detection system sensitive to rate of temperature rise and toultra-violet emissions from open flame.