In this previous post, the advantages of using high-density slurry in TBM operation has been explain. Then why aren’t all slurry TBM operations using high-density slurry?
Before we answer the question, these facts must be laid out:
The slurry is not only used for supporting face pressure, but also for muck-away
To prevent the solid suspension in the slurry-muck mixture from settling, a minimum speed (say 3m/s) of the mixture fluid must be maintained
High speed of the slurry-muck mixture leads to high abrasivity to the pipeline, causing more maintenance needs
Higher speed of the flow also requires more pressure and higher pumping capacity
The muck emerging from the pipes in high density slurry transport looks more like paste rather than fluid
Now let’s answer the original question:
More sophisticated pumping system required. More specialist pumps such as piston pumps, over centrifugal pumps are likely needed. These pumps are more expensive to build and operate and need more frequent maintenance, especially given that the muck is a very abrasive material.
More downtime and cost for maintenance. This is due to higher wear and tear to the pipeline and the pump.
More risk of blockage. Needless to explain further, paste-like material is more prone to ‘constipation’ of the pipeline than fluid-like material.
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