Development of a silica scaling test rig.
Thesis DisciplineMechanical Engineering
Degree GrantorUniversity of Canterbury
Degree NameMaster of Engineering
One of the most significant problems faced in the geothermal power industry is that of scaling due to amorphous silica. The silica can deposit out of super-saturated brine in monomeric form and as colloidal particles. Deposition can occur at problematic rates on pipe surfaces and in the rocks of the re-injection wells. Currently there are a number of methods for controlling deposition but the fundamental mechanisms that govern the transport and attachment of silica are not well understood.
Many field experiments on silica scaling have been conducted but, due to differences in brine chemistry and operational conditions, the results are hard to compare. Many laboratory experiments have also been performed but these are difficult to correlate with the field experiments. Previous research has found that hydrodynamics are important for the deposition of colloidal particles and inertial impaction was proposed to be the dominant transport mechanism.
These results were analysed and, in contradiction, the dominant transport mechanism of the particles was theoretically expected to be that of diffusion. A series of experiments were planned that could test the effect of hydrodynamics on colloidal silica deposition in cylindrical pipe flow. Three parameters were to be varied in the experiment: particle size (10nm and 100nm), Reynolds number (750 - 23,600) and viscous boundary layer thickness (0.06 - 0.38mm).
To perform this experimentation, a Silica Scaling Test Rig was designed, built and commissioned. A method for producing synthetic brine was developed which can provide sols that are stable for at least one month and have a particle size of 10-20nm. Silica deposition has successfully been obtained in three preliminary experiments using the rig.
Without the exclusion of air from the rig significant corrosion occurs in the mild steel test piece. Protrusions that were likely to be silica deposits were found to be co-located with the corrosion, suggesting that one process promotes the other. Neither deposition nor corrosion was found on the pipe’s weld seam and heat affected zone.
Corrosion was prevented using an oxygen exclusion system and two amorphous silica deposition structures were observed: a flat plate-like structure and a globular structure that consisted of 1-5μm diameter globules that built up on each other. Other field and laboratory experiments have produced globular structures similar to those found in this research.
To perform the planned experimentation, future work is required: the silica deposition rate must be increased, colloidal silica sol stability must be improved, and some modifications must be made to the rig.