A Simple Model for Estimating Leak Rates from Flashing Liquid Inventories, in Safety Studies of LNG & FLNG Facilities
Authors: Buonfiglioli Marco , Cowan Ian 
Published: May 2013

A key part of any hazard consequence assessment study is the estimation of the leak rate following failure of part of the process plant. This is the source term that drives the dispersion of the process material, and possibly the subsequent fire and/or explosion if the material is flammable. This estimation is made more complex for LNG facilities, since it is usual for parts of the plant to contain large inventories of pressurized liquefied gas that will flash strongly when the pressure is relieved. Unlike sub-cooled liquid or pressurized vapor, the behavior of a flashing inventory is complex, and requires careful treatment in order that an accurate estimation is made for the inventory release rate. There are a range of modeling approaches that exist in the literature, but these are generally too complex to use in a typical risk assessment study, and so a typical approach taken by the industry to date seems to be to treat the release as a sub-cooled liquid outflow. This paper shows that this is unduly conservative, and presents instead an alternate simple approach that allows a much better assessment of the consequences from these types of process leaks.

Introduction

A regular part of the design of any new asset is the consideration of the hazards that can arise from process leaks, to both personnel and the plant itself. Typically, a range of safety studies are undertaken to understand the consequences of these leaks, which in the case of a LNG liquefaction plant include fire, explosion and cryogenic impairment, as well as potential asphyxiation risks. These consequences are often interpreted on a frequency basis, using information on expected leak frequency, ignition probability and environmental factors, in order to derive Design Accident Loads (e.g. the explosion over-pressure load that a structure is expected to see with a frequency of say 10–4/year) or as input to a Quantitative Risk Assessment (QRA).

The fire, explosion or dispersion consequences of a process leak are all highly dependent on the nature of the process leak fluid as it exits its pipe or vessel, i.e. is it a low speed vapor jet, an under-expanded vapor jet, a low speed liquid cascade, an atomizing liquid droplet spray, a flashing liquid spray etc. They are also directly dependent on the release rate, with larger release rates generally causing large hazards (there are some exceptions to this, for example a vapor leak into a semi-confined region where higher release rates may result in gas concentrations rising above the upper flammable limit). This parameter is the focus of this paper, since it is the bedrock of any consequence analysis.

As a simple example, consider a propane jet fire. The chart in Figure 1 shows that the flame length for the fire rises from 12m to 15.8m as the release rate is doubled from 1kg/s to 2kg/s, a 32% increase in hazard range.

Release rates for vapor or sub-cooled liquid releases are simple to calculate, since the fluid stream remains as a single phase as it depressurizes. More complex are two phase streams, i.e. streams that either contain a mixture of liquid and gas under pressure or a liquefied gas that starts to vaporize rapidly (" flash") as the pressure reduces. Precise calculation of the conditions at the breach are difficult and depend both on the process conditions of the stream inside the pipe or vessel, and the geometry of the case being studied. Further discussion is provided later in this paper. Release rates for such cases are generally intermediate between those for sub-cooled liquid releases and vapor releases.