That facility uses two parallel pumps to transfer water from a sump to a storage tank upstream of its treatment unit (Figure 1). Local climate conditions made freeze protection on the water line mandatory. The normal operating temperature of the line was 140°F.
Standard practice was to wash the water line with an acid treatment on a scheduled basis. Over time, maintenance requirements on the pumps kept rising. Even with constant pump maintenance, their capacity seemed to keep dropping. Indeed, the sump level control valve (on the pump discharge) was wide open fairly rapidly — but pump capacity still was insufficient. Yet, field investigation showed the pumps’ discharge pressure met performance specifications.
Figure 1. Getting into Hot Water: Tracing led to too hot a pipe, which
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 2. Which curve is Right? Using “good” data, two methods
Researchers have extensively studied calcium carbonate systems. Nevertheless, even this system suffers from enormous variations in data interpretation. Figure 2 plots SI versus temperature for an industrial system reasonably similar to the one depicted in Figure 1 with data developed using two methods. Values have been restricted to the claimed ranges of validity for each method. Precipitation and deposit formation starts at 151°F, according to Method A, but at 188°F, according to Method B. Both of these predictions are based on “good” data. Even so, major differences result.
At the 140°F operating temperature only slight scaling would be expected. Variations in water composition could create periods when scale precipitation would occur. In theory, low ion-product concentrations should allow deposits to re-dissolve. In practice, kinetics, surface areas and contaminants matter. Removal of calcium carbonate scale in industrial systems requires mechanical cleaning or acid. With the relatively low saturation index at 140°F, an occasional acid treatment should have kept the line clean.
But is 140°F the important temperature here? The line was heat traced for freeze protection. Convenience drove the tracing selection. The plant had extensive experience with steam tracing and so decided to trace the line with steam. The closest (and cheapest) steam source came from a 250-psig steam header at 406°F.
Even accounting for thermal resistance in the tracing line, between the tracing and the pipe, and the pipe wall, temperatures significantly exceed 140°F at the pipe against the steam tracing points. Steam tracing is the major cause of the carbonate deposit.
The solution requires using “low temperature” tracing. There are three major approaches: steam tracing with insulation to reduce the temperature; electric tracing; or non-steam tracing — often using glycol. The plant preferred to remain with steam and so opted for a proprietary system of steam tracing with integral insulation.
This problem gives us both theoretical and practical insights into a complex system.
Ionic equilibrium thermodynamics is complicated. Data often contradict each other. Accuracy of solubility predictions can suffer greatly based on data quality. Incomplete information on composition and conditions and undocumented assumptions can make data analysis extremely difficult.
For water systems with calcium carbonates, high-temperature heat tracing can induce deposits on the inside of the pipe, cutting plant capacity and increasing maintenance requirements. So, consider heat tracing that just gets to the minimum temperature needed. Using conventional steam tracing may have unintended consequences.
Andrew Sloley is a contributing editor to Chemical Processing. You can e-mail him at ASloley@putman.net.