4th CCPS Global Summit on Process Safety Okayama, Japan
Pressure Safety Valves (PSV) on Liquefied Natural Gas (LNG) storage tanks are designed for high relieving rates in excess of 100 tons/hr. Typically, multiple PSVs are installed on the tank roof and the relieved gases are vented directly to atmosphere.
Ignition of PSV release can result in high levels of thermal radiation on the tank roof with potential for damage. Therefore, PSV vent pipes are elevated to reduce the thermal radiation impact on the tank roof to below 32kW/m2 for concrete roofs, and below 15kW/m2 for steel tank roofs as per BS EN 1473. The thermal radiation analysis is commonly performed using empirical consequence assessment models such as PHAST.
The empirical flame and thermal radiation models are considered well developed for a single flame modelling from an ignited PSV. However, if there are multiple PSVs relieving in proximity where a significant interaction of the flames is expected, the current practice is to either continue using the single flame model by combining the multiple releases into a hypothetical release from an equivalent single PSV or model multiple PSV releases with no flame interaction effects whatsoever. Both these methods give very different results suggesting that the confidence in the conclusion of the analysis.
An initiative, jointly developed by IRESC and Hong Kong University of Science and Technology (HKUST), has been taken to avoid these modelling uncertainties in future. The research involves CFD modelling of the multiple flames effects, with a view to developing a simple correlation or model that can be used to solve this problem. Some selected findings of this research are presented in this paper so as to highlight the main issues in thermal radiation design of LNG tanks.