Two major hazards of LNG spills are vapor hazard and fire hazard. The vapor hazard is the flammable LNG vapor cloud at the ground due to the dense gas behavior. The fire hazard is the thermal radiation of an LNG pool fire. High expansion foam has been proven to be effective to mitigate vapor hazard and fire hazard. The work conducted by MKOPSC involves experiments in the lab, wind tunnel and open field.
The lab tests required building a foam generator and a foam test apparatus. The foam generator is an improved version based on the schematic design in NFPA 11. The new features of the foam generator allow and facilitate the study of foam application on an LNG pool. Liquid nitrogen (LN2) was used in the lab tests as a safe analogue of LNG. This work aims to study the physical interaction between LNG vapor and foam, including the heat transfer between LNG vapor and foam, the formation of channels for vapor to escape, and foam breaking rate.
A small scale experiment has been performed to study the blanketing effect on LNG spill, in which LN2 was used due to its similar thermal properties and the associated benefits in terms of safety and financial flexibility. The foam blanket exerts two effects that lead to opposite results: Foam blanket blocks convection and radiation to reduce heat input for LN2 vaporization, while water drainage induced by convection, radiation and direct contact with LN2 adds extra heat to vaporize LN2. The performance of foam on eliminating heat input from convection and radiation, and the additional heat input due to water drainage were evaluated based on the vaporization rate measurement using a balance. The analysis of the experimental data concluded that high expansion foam is effective for reducing LN2 vaporization by blocking convection and radiation; the water drainage due to direct contact of foam with LN2 at the initial stage does add extra heat, but it is less significant compared with the reduction of convection and radiation. A correlation between heat inputs with and without foam effects was proposed, and reduction factor was determined for this specific experiment.
The experimental work conducted in Brayton Fire Training Field (BFTF) by MKOPSC aims to examine the performance of high expansion foam to mitigate the vapor hazard of an LNG spill, and provide a guidance for industrial practice based on a study of various parameters. The experiments were conducted in pits up to a scale of 6.5m x 10m. The performance of high expansion foam to reduce the vapor hazard was evaluated by the vapor concentration in the downwind direction at the ground level. More than 100 thermocouples were placed at various elevations in the LNG pool, in the foam blanket and above foam blanket to study the thermal effects of high expansion foam on LNG system. The experimental results indicate a significant reduction of vapor concentration in the downwind direction, and the “vapor exclusion zone” (lower flammable limit distance) was determined with the foam application. The elevated vapor temperature above the foam blanket indicates a warming effect of foam. A minimum effective foam depth of 0.64m was determined based on the ability of foam to increase the vapor buoyancy to a positive level, which can be used to guide industrial application of high expansion foam for LNG vapor hazard mitigation.
An LNG pool fire may cause a thermal hazard to personnel, properties and the environment. The experiments performed at BFTF by MKOPSC aims to examine the effectiveness of high expansion foam on reducing the thermal hazard of an LNG pool fire, and provide a guidance of the design and operation parameters for the industrial application. The experiments were conducted in pits up to a scale of 6.5m x 10m. The radiant heat of the LNG pool fire was monitored at the surrounding area and above the pits. With the same manner as the vapor dispersion study with high expansion foam, temperature was measured to provide extra information to understand the effects of high expansion foam to suppress the pool fire. The “thermal exclusion zone” (5 kW/m2 radiation distance) was determined experimentally to guide facility siting in the LNG plant. The control time is defined as the time required for 90% radiant heat reduction after high expansion foam application at a certain distance, and it was used as a criterion to determine the effective foam application rate. This work concluded that high expansion foam application reduces the “thermal exclusion zone” by 50%. A foam application rate of 10 L min-1 m-2 was recommended to mitigation thermal hazard of pool fire.