High Pressure Relief Systems in LNG Receiving Terminals – A Safety Case for HP Flare
Sunny Choi, Venkatesh Sourirajan, Gary Li
Worldwide demand for liquefied natural gas (LNG) has been steadily increasing, leading to the design and construction of larger receiving terminals in recent years. While the safety record of the LNG industry has been good, it may be beneficial to review some of the standard design safety features, with regard to their adequacy given the increased scale of the new facilities. In particular, unlike other industries where hydrocarbon vents are connected to flare system, it is common practice to route pressure safety valve (PSV) discharges on vaporizers to the atmosphere directly. The relief loads from one single vaporizer PSV has increased from 50t/hr in the past to the latest 300t/hr. As a result, it has become challenging to dispose of such a large quantity of flammable gas through atmospheric vents in a safe manner. Furthermore, during events such as blockage of the send-out pipeline, simultaneous relief of PSVs from multiple vaporizers may increase the total combined discharge rate to more than 1,000t/hr. High thermal radiation arising from the ignition of such a release may lead to damage of nearby equipment, piping and structures as well as personnel injury or fatality. To account for such effects, some designers have transformed the PSV discharge from simple tailpipes to multiple vent stacks with elevations of more than 20m.

The objective of this paper is to assess the hazards (possible consequences and likelihood) due to a flammable gas release from LNG vaporizer PSVs to the atmosphere with respect to personnel safety, asset damage and plant unavailability. The result can be used to evaluate the potential costs and benefits of introducing a high pressure flare system taking into account the associated capital and operating costs (e.g. extra land, pipework, structure). Apart from additional flare system, several other alternate mitigation measures are also proposed for consideration.