Abstract

The assessment of the level of risk associated with operating a sour gas pipeline requires the determination of the atmospheric release-rate in the event of a rupture. A common assumption of currently available release-rate models is that the emergency shut-down valves close at the instant of the rupture. In the case of a small rupture, the time required for valve activation and closure is appreciable. The current models may therefore significantly underpredict the release-rate and cumulative emission volume, leading to an optimistic assessment of the potential risk. In this work, a pipeline depressurization model is presented which specifically accounts for emergency shut-down valve activation and closure time.

Introduction

The licensing and operation of a sour gas pipeline system requires an assessment of the public safety hazard associated with an atmospheric sour gas release in the event of an accidental pipeline rupture. Risk assessment includes an evaluation of the downwind ground level contaminant concentrations based on an atmospheric dispersion model coupled with a model describing the transient gas emission rate from the depressurizing pipeline. Sour gas pipelines are equipped with emergency shut-down (ESD) valves located at regular intervals. In the event of a line failure, the valves immediately upstream and downsteam of the leak sense the reduction in line pressure and isolate the damaged section of line, thereby limiting the volume of gas emitted. A realistic pipeline release-rate model should therefore include the effects of ESD valve activation and closure.

LITERATURE

Several gas pipeline depressurization models have been reported in the engineering literature for predicting the release rates from high pressure sour gas pipeline ruptures. Picard and Bishnoi1 developed a comprehensive one-dimensional non-isothermal decompression model in which the dynamic gas flow equations were rigorously solved by a method of characteristics. The model is based on the thermodynamics of real-fluid behavior, and accounts for heat transfer, viscous dissipation, and phase change effects.

The most wide-spread release-rate model in current use is the so-called "double exponential model" of Wilson2, developed from the earlier work of Bell3. The model is based on the thermodynamics of onedimensional isothermal quasi-steady flow of an ideal gas. The transient release rate is expressed as the sum of two exponential terms, the first representing the transient from the sonic expansion wave moving along the pipe, and the second representing the friction dominated gas expansion. The model can be expressed as4:Equations (Available in full paper)

Wilson's model does not account for heat transfer, viscous dissipation or phase change effects. Picard and Bishnoi1 compared the results of their model with those of Wilson's model and determined that the simple double exponential formulation yields conservative estimates of be considered constant, allowing the equation to be reduced to: Equation (Available in full paper)

where C is a constant that depends only on the transmission line geometry, gas gravity and temperature. In the model pipeline system under consideration, the flow of gas between the pipe segment and gathering system is determined from: Equation (Available in full paper)

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