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Transient Flow in Gas Transport
Norwegian University of Science and Technology, Faculty of Engineering Science and Technology, Department of Energy and Process Engineering.
2011 (English)MasteroppgaveStudent thesis
Abstract [en]
Transport of natural gas to continental Europe and UK is a large portion of Norwegian petroleum industry. The gas is mainly transported undersea in large-scale transport pipelines. Amount of transported gas is currently close to maximum capacity of the pipeline network, and as a consequence the gas transport must be careful planned so that the optimal capacity can be utilized. An important tool in this planning is the use of computational method to predict the flow. Accurate computational tools is therefore of great value when predicting the pressures and flow rates in transient cases such as opening of a valve or shut down of a flow. This report is a part of a major research project initiated by Gassco, for better flow-predictions models in natural gas pipelines. A computational model based on the method of characteristics has been developed. In this report the main focus is on the solution of the energy equation and introduction of this equation to an already existing code solving for pressure and mass flux. The method is verified using measured values of pressure at the inlet. Since much of the uncertainty is related to the transients, this report focuses on transient cases. The old program solving the characteristic equations using an isothermal assumption actually proves surprisingly accurate, and the additional solution of temperature does not significantly improve the results. The method however does not provide satisfactory results at the larger transients. If large temperature gradients are imposed on the solver we see instabilities in the flow and it affects the solution of the parameters. The Joule Thomson effect that we have in our solution also results in a much higher drop of temperature than what can be measured, in case of pressure drop at the inlet. From the results we also see that the coefficient that is supposed to correct friction factor for additional drag effects, also should be a function of pressure and/or Reynold number. If such a correlation would provide more accurate results in the transient has not been debated, but more accurate correlation of friction depending on flow rate would probably give a more accurate result. Also worth noticing is that the method does not have a clear convergence, or reduction of error as the number of calculation points increases. It gives smaller extreme values, but average error is not reduced significantly. This is probably a result of the reduced effect of missing convective-term as the grid has a finer resolution and time-step decreases and the effect of loss of velocity in the characteristic becomes small. As a simple tool for calculation of gas transport in pipelines, the isothermal method of characteristics proves to give surprisingly accurate results. However, for more complex systems, i.e. including the temperature and variable properties such as compressibility and density, finite difference methods are more versatile. Finite difference methods can be done implicit, giving a more stable solver, and itÂ’s simpler to account for some of the effects such as temperature etc. iv
Place, publisher, year, edition, pages
Institutt for energi- og prosessteknikk , 2011. , 125 p.
Keyword [no]
ntnudaim:5705, MTPROD produktutvikling og produksjon, Industriell mekanikk
URN: urn:nbn:no:ntnu:diva-12992Local ID: ntnudaim:5705OAI: diva2:429670
Available from: 2011-07-05 Created: 2011-07-05

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