Too Many or Not Enough? a Methodology for Fire and Gas Detector Layouts at LNG Facilities

NFPA 59A Standard for the Production, Storage, and Handling of Liquefied Natural Gas (LNG) requires facilities to have the equipment necessary for the detection and control of fires, leaks, and spills of hazardous materials, yet provides no requirements or guidance on the location of the detectors. Literature available in the public domain outlines key factors to consider in developing layouts for flame detectors (e.g. size of fire to be detected, fuel involved, sensitivity and field of view of detector, including obstructions, etc.) and gas detectors (e.g. pressure, temperature, density of the released material, topography, air movement and temperature effects, location of potential ignition sources, etc.), but a methodology to use this information in developing a detector layout is not clearly defined; instead, generic and non-quantifiable terminology such as “quick and reliable” is often used. Further, available guidance repeatedly calls for gas detectors to be placed in proximity to potential leak sources, which is often interpreted as placing point gas detectors right next to likely leak locations. This attempt at detecting the leak instead of the resultant gas cloud could result in a failure to detect the release in many cases, depending on factors such as release orientation and wind direction. It is also difficult to detect pressurized releases close to the source, where the cross-sectional area of the jet is quite small.

The lack of a consistent approach to developing gas detector layouts for land-based LNG facilities and of a systematic method for regulators to evaluate these designs are of particular interest to the Department of Transportation (DOT) Pipeline and Hazardous Materials Safety Administration (PHMSA), which regulates numerous LNG facilities in the United States. PHMSA permits applicants to use a 10-minute design spill duration if the process design includes acceptable detection, isolation, and shutdown, however, applicants are also permitted to evaluate a release duration shorter than 10 minutes based on demonstrable surveillance, shutdown and isolation design. Since there is currently no standard for the design of hazard detection systems at LNG facilities, it follows that there is no consistent methodology for demonstrating or evaluating successful detection, isolation, and shutdown provisions.

Blue Engineering and Consulting is working on a DOT-PHMSA sponsored project to develop a risk-based approach and criteria for hazard detector layouts at LNG facilities. This project builds upon performance-based design principles outlined in NFPA 72 National Fire Alarm and Signaling Code and the International Society of Automation (ISA) technical report 84.00.07 Guidance on the Evaluation of Fire, Combustible Gas, and Toxic Gas System Effectiveness. The methodology divides the LNG facility into Detection Areas based on the hazards present and the plant layout, establishes performance targets for each Detection Area, identifies the appropriate hazard scenarios to evaluate the detector layout, and optimizes the detector layout. This paper will describe and demonstrate the proposed methodology.