Haroun Mahgerefteh

A transient outflow model for pipeline puncture

Abstract This paper describes the development and validation of a highly efficient robust numerical simulation based on the method of characteristics for predicting release rates following the puncture of pipelines containing high pressure hydrocarbon mixtures. The model accounts for real fluid behaviour, radial and axial flow as well as the locality of puncture relative to the length of the pipeline. It is applicable to both isolated and un-isolated flows where pumping at the high-pressure end continues despite puncture. The results of the application of the model to the hypothetical puncture of a 16 km , 0.42 m dia. pressurised pipeline containing a condensable hydrocarbon mixture are presented as a case example. Simulated fluid velocity and pressure profiles are used to provide a pictorial timeline representation of the important post-puncture fluid dynamics phenomena within the pipeline, which ultimately govern the discharge process. These results indicate that the conventional outflow models treating the pipeline as a vessel discharging through an orifice are inappropriate especially during the early stages of depressurisation. Good agreement is obtained between the results of the simulation when compared to experimental data from the puncture of 100 m long LPG pipeline.

A numerical blowdown simulation incorporating cubic equations of state

Abstract The development of a numerical simulation, based on cubic equations of state for blowdown of vessels containing high pressure hydrocarbons is described. The model’s performance is evaluated by comparison with experimental data relating to the blowdown of a condensable hydrocarbon mixture in a full sized vessel at a starting pressure of 118 bar. Typical output data including time–temperature profiles for the vapour and liquid phases as well as the wetted and unwetted walls are reported. These are shown to be within ±5 K of experimental data. In general, it is found that the choice of the cubic equation of state has little effect on the data.

A Study of the Dynamic Response of Emergency Shutdown Valves Following Full Bore Rupture of Gas Pipelines

A numerical simulation based on the method of characteristics is employed to study the dynamic response of ball valves and check valves following full bore rupture of high pressure gas pipelines. The study, performed in conjunction with the hypothetical rupture of a 145 km pipeline containing methane at 133 bar, includes simulating the effects of valve proximity to the rupture plane and the delay in closure on the total amount of inventory released prior to pipeline isolation. The accompanying pressure oscillations and surges are also accounted for. The results are in turn used to recommend guidelines regarding the appropriate choice of emergency shutdown valve depending on the failure scenario.

Global sensitivity analysis of the impact of impurities on CO2 pipeline failure

This paper describes the testing, comparison and application of global sensitivity techniques for the study of the impact of the stream impurities on CO2 pipeline failure. Global sensitivity analysis through non-intrusive generalised polynomial chaos expansion with sparse grids is compared to more common techniques and is found to achieve superior convergence rate to crude Monte Carlo, quasi-Monte Carlo and EFAST for functions with up to a moderate level of “roughness”. This methodology is then applied to the hypothetical full bore rupture of a 1 km CO2 pipeline at 150 bara and 283.15 K. The sensitivity of the ensuing outflow to the composition of a quaternary mixture of CO2 with N2, CH4 and O2 as representative stream impurities. The results indicate that the outflow rate is highly sensitive to the composition during the early stages of depressurisation, where the effect of the impurities on phase equilibria has a significant impact on the outflow.

Predictive criteria for the optimisation of a vibrating-reed transducer

The optimisation of a vibrating-reed technique which is capable of measuring various properties such as fluid pressure, density, viscosity and mass under aggressive environments is described. The unit operates simply by utilising a stiff reed which is clamped securely at an intermediate point along its length. One end, mounted with an inert concentrated mass is exposed to the test environment, while the other is mechanically excited so that the system is sinusoidally vibrated at its first modal resonant frequency. Changes in the resonant conditions then reveal information about the physical properties of the fluid under test. The finite-element method of analysis adopted for a cylindrical reed reveals that optimum operating conditions in terms of output response are achieved with maximum reed diameter, minimum reed length and minimum mass ratio of the attached mass to that of the reed. The stability criteria on the other hand indicate that most stable operation corresponds to the clamping of the reed as far away as possible from its centre. The analysis also considers the effect of choosing different reed materials and shapes on the systems resolution and sensitivity. Most of the theoretical predictions are also confirmed by experimental evidence.

Thermodynamic interpolation for the simulation of two-phase flow of non-ideal mixtures

This paper describes the development and application of a technique for the rapid interpolation of thermodynamic properties of mixtures for the purposes of simulating two-phase flow. The technique is based on adaptive inverse interpolation and can be applied to any Equation of State and multicomponent mixture. Following analysis of its accuracy, the method is coupled with a two-phase flow model, based on the homogeneous equilibrium mixture assumption, and applied to the simulation of flows of carbon dioxide (CO2) rich mixtures. This coupled flow model is used to simulate the experimental decompression of binary and quinternary mixtures. It is found that the predictions are in good agreement with the experimental data and that the interpolation approach provides a flexible, robust means of obtaining thermodynamic properties for use in flow models.

A geometrically based grid refinement technique for multiphase flows

Abstract An adaptive mesh refinement technique developed for the solution of scalar problems is extended to the simulation of two-phase flow problems, as a means of reducing the computational runtime associated with such problems. The methodology, involving the adaptive partition of the domain into uniformly discretised regions, is extended to systems of equations without increase in algorithmic complexity. By application first to the simpler case of the Euler equations of gas dynamics, the technique is shown to handle shocks without loss of accuracy and to result in significant CPU runtime reductions of over 90%. Application to more complex two-phase flow problems, including the flashing flow during the decompression of a pipeline, also show dramatic increase in computational performance.

A novel vibrating reed technique for particle size measurement

Abstract When a container partly filled with a powder is vibrated in the transverse direction with a maximum acceleration greater than that due to gravity, the particles inside the container become effectively ‘fluidised’. The amount of frictional interactions transmitted from the powder to the container as a result of the vibration is then a function of the average particle size and its distribution. This paper describes the design and development of a novel vibrating reed technique operating on the basis of the above principle for the measurement of average particle sizes. Typical resolution in size is between ±8 and ±20 μm in the 20 to 900 μm size range particles. Preliminary results are also reported which establish the potential of the device for use for particle size distribution analysis.

Optimal Valve Spacing for Next Generation CO2 Pipelines

Pipeline transportation is considered as the primary mode of transporting CO2 for future carbon capture and storage (CCS) projects. The failure of such pipelines could lead to the release of a significant amount of inventory, which in high enough concentrations is toxic and presents a significant risk to life. To mitigate this hazard, emergency shutdown valves (ESDVs) are installed at regular intervals along the pipeline, to minimise the amount of inventory released in the event of failure. This paper presents a methodology and the required metrics for optimising valve spacing as a trade-off between the reduction in hazard against the cost of installation and maintenance.

On-line particulate emission monitor

Abstract The design and development of a coaxial capacitance transducer for on-line measurement of particulates in air is described. The systems performance is evaluated in response to changes in a number of operating parameters including relative humidity (8–78%), temperature (20–100 °C) and flow velocity (6.5–15 m/s). Important particulate characteristics investigated include electrical properties, density, mean size and shape. It is found that the effective dielectric constant, e eff , for all solids–gas dispersions tested is directly proportional to the solids concentration. However, in contrast to that for conducting powders, e eff for dispersions of insulating powders is found to be also dependent on the respective dielectric constant of the constituent solid particles, volumetric ratio as well as mean size. A ‘temperature capacitor coefficient’ is determined to account for the effect of temperature on the systems signal. Typical system particulate volumetric concentration resolution in the range 0–0.004% v/v is ±2×10 −4 %.