Geothermal Steam Purity Modelling – Theory and Practice

Abstract

Geothermal steam purity modelling is considered by some to
be an esoteric exercise. Two experienced practitioners in
geothermal process engineering and geothermal chemistry
seek to demystify steam purity modelling for the next
generation of practitioners. This is done by explaining the
basic principles used (along with key assumptions) and how
modelling can assist steam separation and scrubbing pipeline
design. Verified by drain-pot sampling, it provides an
accurate measure of the overall efficiency of steam line
scrubbing and gives confidence in the level of steam purity
at the turbine - a key parameter operators are interested in.
Steam purity modelling considers initial steam separation
performance and quantifies heat and mass transfer within
steam lines in discrete stages, corresponding to each
successive condensate removal point as well as final
demisting prior to admission to the power plant. Chloride
ion is a representative dissolved impurity that is non-volatile
and remains entirely with the liquid phase in each stage; it is
present in steam lines due to minor amounts of brine carryover.
The removal of slightly volatile silica can also be
accounted for. Heat loss from pipelines can produce
significant amounts of steam condensate which is involved
in solution/mixing within each stage. If the condensation can
be reasonably estimated in any section, then it can be used as
a dilution-tracer to calculate the purity of the total steam flow
in that section.
Assumptions about turbulent mixing of steam and
brine/condensate and attainment of chemical equilibrium in
each stage are explained, along with guidance on acceptable
scrubbing velocity criteria, and the effectiveness of
condensate removal at drop pots.
Insights that may be gained from steam purity modelling are
discussed, and “proof” of the validity of modelling is
provided by comparing modelling predictions with
generalised (i.e. non-specific) geothermal steam system data.