Fire and Gas Detection and Suppression Systems for LNG Facilities (Click on the title to view the complete article)
Planning for functional safety requires an understanding of process hazards, relevant industry standards and local codes, and available product and system solutions.
Liquefied natural gas (LNG) is 600 times denser than the gas form, making the economics to transport LNG more attractive (and more feasible) than transporting natural gas in pipelines over great distances across oceans. There are several proprietary processes used to make LNG, all of which involve refrigerating the gas and then expanding it to turn into a cryogenic liquid. Inherent in these processes are the risks associated with spills and leaks as well as other process hazards.
Non-flammable as a cryogenic liquid, when LNG warms, it vaporizes (re-gases) and may form a flammable mixture upon reaching a concentration of between 5% and 15% methane-to-air. If LNG should leak to the environment, it will quickly vaporize and form a rising cloud of methane gas, without leaving a residue. The hazard is created when the cloud forms a flammable concentration that presents a risk for ignition. If the leak occurs within a confined area, there is a potential for ignition with explosion. The heat release rate from an LNG pool fire is approximately 60% greater than that of a gasoline pool fire of the same size.
There are also non-flammable hazards in LNG facilities due to the presence of refrigerants and other chemicals used in the gas treatment process.
It is the job of the facility’s fire and gas detection system to detect these hazards and take appropriate action.
The required coverage of hazards by a flame and gas detection system needs to be specified. This should include specifying the appropriate technology for the detection of the hazard and the required coverage of each hazard. For example, infrared (IR) gas detectors will not detect hydrogen sulfide. The correct mechanical and electrical properties of the detectors must also be carefully selected to ensure that they are appropriate for use in the environment and location in which they are to be mounted.
3D models help document a fire and gas system design, and provide calculations for the coverage afforded by a particular detection scheme to ensure that coverage targets are attained. 3D mapping is also valuable for showing individual detectors’ obstructed fields of view.
Control and integration
Any effective fire and gas detection and suppression system must be capable of interfacing with and integrating flame, gas and smoke detectors, fire
suppression devices and notification appliances.
The fire/gas and suppression control system must be able to:
• Reliably detect hazards and provide appropriate audible and visual alarms and location of each hazard
• Be available during all plant conditions, e.g., plant shutdowns, turnarounds, maintenance
• Integrate seamlessly to other plant control systems, which provide control of mitigation actions such as starting water pumps, opening deluge valves and closing heating, ventilating and air conditioning inlet dampers
• Route digital outputs to the emergency shutdown system in order to isolate and shut down process equipment
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