N-Class Fire Resistant Divisions in U.S. Naval Ships

Abstract

Abstract The U.S. Navy has now developed an N-Class division system to classify fire resistant boundaries in accordance with MIL-STD-3020, fire resistance on U.S. naval surface ships. The N-Class division is analogous to the commercial International Maritime Organization (IMO) system (e.g., A-Class) division. The key difference is that N-Class divisions are designed to have satisfactory fire resistance when exposed to hydrocarbon pool fire (UL 1709 fire exposure) following a shock event. In addition, N-Class is applicable to fire resistant divisions constructed from polymer composites structures when they meet the requirements of DDS-078-1. MIL-STD-3020 requires a hydrocarbon pool fire exposure (UL-1709 fire exposure) which provides a minimum average total heat flux of 188 kW per square meter (kW/m2) (60,000 British thermal unit per square foot/hour (Btu/ft2∣h)) within the first 5 min of the test exposure. At all times after 5 min, the average temperature within the furnace shall be maintained at a minimum of 1038 °C (1900 °F) and a maximum of 1149 °C (2100 °F) for the duration of the test. These fire conditions simulate a post-flashover fire scenario for shipboard applications. Performance is defined as the time period during which assemblies will continue to perform their intended function when subjected to fire exposure. The minimum duration of fire exposure is 30 min. Shock testing prior to fire resistance test is performed to ensure that fire resistance of divisions, with fire insulation, penetrations, and associated attachment methods, is not degraded in a combat environment. Shock test specimen size of 1219 by 3048 mm (4 by 10 ft) was selected to reduce the cost of shipping a full size fire test specimen of 3048 by 3048 mm (10 by 10 ft) or greater to the fire test laboratory. A full scale fire resistance test is still required for qualification purposes after successful completion of the shock test prior to fire resistance test. MIL-STD-3020 also permits AN-Class fire resistant divisions when fire threat is from common combustibles (Class A) and where liquid flammable (Class B) fire threat is non-existent. AN-Class divisions are those divisions formed by bulkheads and decks that are designed to protect against structural failure and prevent the passage of flame or hot gases when exposed to IMO Resolution A.754(18) fire exposure after shock testing. For all AN-Class divisions, including those with penetrations, the fire test duration is a minimum of 60 min. This paper will discuss fire resistance test results conducted using steel, aluminum, and composite substrates.