Abstract

The depressurization of a pipeline containing a subcooled liquid at high pressure is an operation to be considered and analysed during pipeline design, in order to verify the relevant time trends of the hydraulic parameters, with particular focus on temperature. Usually, considering onshore pipeline systems, the blowdown operation is carried out for a section included between two MLV stations only, as a consequence of an emergency or a damage repair scenario; the inventory is discharged through vents or flares installed at the adjacent MLV stations. The types of fluids considered in this study are classified as HVP (High Vapour Pressure fluids), and include LPG, NGL, ethane rich mixtures, CO2 rich mixtures.

In this study, pipeline depressuring behaviour of HVP fluids is investigated by performing a sensitivity on the main governing parameters as fluid thermodynamic properties, heat transfer from environment, pipe wall thickness, pipeline elevation profile, as well as on the influence of discharge rates and discharge location configuration (one side, two-side).

The analysis is performed by using the commercial hydraulic simulation tool OLGA, with full consideration of possible two-phase flow regimes, liquid holdup distribution, heat transfer modelling including the transient soil thermal dynamics. The target of this study is to provide indications for the engineering analysis of such kind of pipelines.

Introduction

The analysis of the time evolution of an industrial scale onshore buried pipeline containing a HVP fluid during a potential depressurization operation, performed by using flaring or cold venting facilities with realistic discharge characteristics, and taking into consideration the effects of hilly terrains, pipe wall thickness and heat transfer from soil cannot be found in literature. On the other hand, published papers present simulation of short and small size pipe sections (length of 100 m, 328 ft) typical of testing facilities, compared with the reference blowdown tests of Isle of Grain [1], [2], where the duration of the fluid discharge experiments were few tens of seconds, or idealized long pipelines with completely flat profiles, homogenous two-phase flow and external heat transfer coefficients typical of offshore pipelines or adiabatic scenarios, subject to full-bore ruptures [2], [3].

In those models important factors influencing real onshore pipeline systems depressurization evolutions as long liquid residence time in dips, long time duration of the operation (can be order of some days), thermal heat capacity of the steel and soil thermal impact are not captured.

For HVP liquids, the physical behaviour during blowdown of pipeline sections of typical lengths of the order 20-40 km (12.4-24.8 miles) is similar. In fact, all exhibit a quite rapid initial depressurization following a thermodynamic expansion within the liquid phase region; then the fluid crosses the bubble point curve and flashes, and for a significant time the evolution is close to isothermal and isobaric, since the fluid behaves in a way very close to a one-component substance.

During this period characterized by long pressure and average temperature plateaux, the fluid progressively vaporizes and tends to achieve the full vapour phase condition, while flowing as a two-phase mixture.

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