Transient Flow in Gas Transport
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
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
its simpler to account for some of the effects such as temperature etc.
Place, publisher, year, edition, pages
Institutt for energi- og prosessteknikk , 2011. , 125 p.
ntnudaim:5705, MTPROD produktutvikling og produksjon, Industriell mekanikk
IdentifiersURN: urn:nbn:no:ntnu:diva-12992Local ID: ntnudaim:5705OAI: oai:DiVA.org:ntnu-12992DiVA: diva2:429670
Ytrehus, Tor, Professor