After much investigation a fouled water line was identified as the likely problem. Cutting a sample out of the 4-in. line confirmed this. The inside of the pipe was coated with a hard layer of carbonates that reduced the effective line diameter to only 1 in. Multiple acid washings couldn’t remove the scale. Many water systems can contain carbonates, bicarbonates or sulfates. Calcium carbonates are particularly common. Calcium carbonate is only slightly water soluble. Unlike most solutes, its solubility drops with rising temperature. Varied terminology is used in evaluating solubility in ionic systems. When a solute reaches saturation, precipitation starts. Precipitation is the result of chemical equilibria in a heterogeneous system. The most common evaluation method assesses the system in terms of solubility products. With calcium carbonate the equilibrium expression is:CaCO3 (solid) ↔ Ca2+ (aqueous) + CO32- (aqueous) This indicates that solid calcium carbonate is in equilibrium with dissolved calcium carbonate as ions surrounded by water molecules. The solubility product (Ksp) is a type of equilibrium constant specifying where the equilibrium of zero solid formation lies. It’s the result of multiplying the ion concentrations at equilibrium: Ksp = [Ca2+][ CO32-] To evaluate a system you must calculate an ion product (Qsp). This, also often referred to as the ion activity product, is the result of multiplying the ion concentrations at actual conditions: Qsp = [Ca2+][ CO32-] Solubility data for particular compounds in water normally are given as solubility products. Evaluation of a specific system is a complex task. Solubility products vary strongly with either pressure or temperature or both for some compounds but can remain nearly constant for others. Trace components also matter. Other compounds in the system either can inhibit or increase the precipitation potential. Published data aren’t much help. They can conflict, have limited ranges of applicability or come from systems not allowed to go to equilibrium; data sometimes lack critical information on system composition and conditions. For a specific system, the conventional approach to investigate temperature effects is to develop a solubility index (SI) versus temperature curve. The solubility index is the ratio of the ion product to the solubility product: SI = Qsp/ Ksp
Figure 1. Getting into Hot Water: Tracing led to too hot a pipe, which
actually spurred scale buildup.