I tend to agree with most of what has already been stated, however one can be "fooled" by ignoring important anions in the water, the balance of bicarbonate with carbonate, etc., how much silica is dissolved in the water (not merely suspended), all have a role to play. I have witnessed waters that are marginally stable with respect to scale indices still produce a scale that has nothing to do with calcium carbonate, and the water was not saturated with respect to magnesium silicate. The critter that forms is a complex ferric calcium-magnesium aluminosilicate, possibly arising from silt particles (as in clay) that were present in the water as SDI. A bugger to be rid of too, requires something like concrete remover if going the chemical route. I suspect cold scale removal by CO2 or Nitrogen systems (CONCO), might be very successful on scales such as this.
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The principle of cryo-blasting the scale is that it happens rapidly enough with dry ice powder or with liquid nitrogen, that the scale cools first, does not particularly transfer heat from the metal to cool it much, and therefore shrinks drastically first, and becomes a powder that the gases leaving the tube carry with the flow.
very interesting post by James Stewart. Can you please elaborate on cold scale removal that is mentioned. I am learning about this the first time. Thank you.