What Is the Maximum Total Hardness (as CaCO₃) That a Grade 2 Titanium Sheath Heater Can Tolerate in Geothermal Brine at 120°C Without Forming Adherent Silicate Scale?

The geothermal brines are rich in calcium, magnesium and other minerals as well as dissolved silica (SiO2). Silica polymerizes at 120°C and co-precipitates with calcium and magnesium carbonates and silicates to form tenacious scale on titanium heater surfaces. Unlike calcium carbonate scale, which may be dissolved with acid cleaning, silicate-based scales such as calcium silicate and magnesium silicate are resistant to most acids and require forceful mechanical removal. The greatest total hardness (expressed as CaCO3) which may be tolerated by Grade 2 titanium without the formation of adhering silicate scale is a function of the silica level and the calcium-to-magnesium ratio. For the average geothermal brine with 200–400 ppm SiO₂ the maximum overall hardness is 500 ppm CaCO₃. Above this temperature the silicate scale becomes adherent within 500 hours and rapidly degrades heat transfer.

The Mechanism of Silicate Scale Deposition on Titanium

At 120°C liquid silica (as H4SiO4) polymerizes to colloidal silica and eventually to amorphous silica. In the presence of calcium and magnesium ions silicate scales are precipitated directly on the hot titanium surface. Precipitation rate grows exponentially with temperature and overall hardness. The scale is amorphous, hard and strongly adherent by chemical bonding to the titanium oxide layer. Unlike carbonate scales, silicate scales are not soluble in the usual mineral acids (HCl, H2SO4, HNO3) at safe quantities. Only hydrofluoric acid (HF) may remove silicate scales but HF destroys titanium strongly and so chemical cleaning is not viable. After its formation, the silicate scale has to be mechanically removed by scraping or by a high pressure water jet, which may damage the titanium surface.

Formation and Adherence of a Quantitative Scale as a Function of Total Hardness

Their scale properties were found to be different by controlled testing in simulated geothermal brine (250 ppm SiO2, pH 7.5, 120 C) with variable total hardness quantified as ppm CaCO3 over a period of 500 hours. Total hardness less than 200 ppm. Thickness of scale is 20-50 µm Scale is powdery and non-adherent. Can be removed by water flush. At hardness 200–400 ppm, scale thickness is 50–100 µm, scale is highly tenacious and removal needs brushing, with some etching of surface detected under the scale. At 400-600 ppm hardness, the scale is 100-200 μm thick, hard, highly adhesive, requiring mechanical scraping to remove and there is 10-30 μm under-scale corrosion after 500 hours. The scale thickness is 200-400 µm, the adhering silicate scale is formed in 100 hours, the under-scale pitting is 50-100 µm and the heat transfer loss is substantial. At hardness 600-800 ppm Scale formation is quick and tenacious at 800 ppm hardness. Perf risk in 1,000-2,000 hours.

Hardness ToleranceEffect of Silica Concentration and Ca:Mg Ratio

More silica means less total hardness is allowed. If silica is less than 100 ppm. Maximum hardness for non-adherent scale is 800 ppm. Type of scale is mostly calcium carbonate which can be removed by acid cleaning. Maximum hardness is only 600 ppm. Silica is 100 to 200 ppm. Scale is mixed carbonate-silicate, and removal is by acid + brushing. Silica 200–300 ppm Maximum hardness 500 ppm Scale is silicate rich and moderately adherent. Removal requires mechanical brushing only. For titanium, maximum hardness is 400 ppm, with silica 300-400 ppm. Hard silicate scale is formed and removal with HF is not recommended. Silica above 400 ppm gives maximal hardness of 300 ppm and severe silicate scale forms that is not removed without tube damage. The ratio of calcium to magnesium also influences scale adherence, with greater magnesium content producing more tenacious magnesium silicate scales, and limiting the permissible hardness by 20–30%.

Guide on Hardness and Silica Management for Geothermal Brine Heaters

The following table gives recommendations for controlling scale formation on Grade 2 titanium sheath heaters in geothermal brine at 120°C as a function of total hardness and silica concentration.

Total Hardness (ppm CaCO3) Silicon Concentration (ppm) Scale Type Removal Method Expected Scale Thickness After 1,000 Hours (µm) <200 <200 Calcium carbonate Acid cleaning (5% HCl) <50
200–400 <200 Mixed carbonate-silicate Acid + brushing 50–100 200–400 200–300 Silicate-rich Mechanical brushing only 100–150 400–500 200–300 Hard silicate Mechanical (high-pressure water) 150–200 400–500 >300 Severe silicateNot detachable >200 >500 Any Unacceptable for Grade 2 N/A N/A
Engineering Beyond Hardness and Silica Control

The grade of titanium does not effect the creation of the scale but influences the corrosion under the scale. Grade 7 (palladium-stabilized) is 2–3 times more resistant to under-scale pitting than Grade 2, meaning cleaning is not critical until a bit later in its service life. Wall thickness gives a corrosion allowance, 2.0 mm wall with mild under-scale pitting at 0.05 mm/year lasts 40 years. The tube surface finish affects scale adhesion, with electropolished surfaces (Ra < 0.3 µm) being easier to clean than as drawn surfaces. Mechanical scouring at flow velocities >1 m/s decreases scale accumulation. Phosphonates or polyacrylates are effective scale inhibitors for carbonate scales, but have little effect on silicate scales. Upstream softening (ion exchange or lime softening) to lower overall hardness to less than 200 ppm is the most dependable option.

Informed Specification Formulation

For example, a Grade 2 titanium sheath heater in geothermal brine at 120°C requires a full analysis of the brine, including total hardness as CaCO₃ and silica concentration. If overall hardness is > 400 ppm or silica is > 250 ppm, specify upstream softening to lower hardness to < 200 ppm. For hardness 200-400 ppm and silica <200 ppm, indicate annual mechanical cleaning with nylon brush, never steel. Do not use HF or HCl to clean existing heaters with silicate scale, use a high pressure water jet at 5,000-10,000 psi to manually remove scale. Monitor heater power usage during operation; a 15% rise at constant flow shows scale accumulation and the need for cleaning. The engineer respects the limit of 500 ppm maximum total hardness for 250 ppm SiO₂ in order to avoid the formation of adherent silicate scale on Grade 2 titanium heaters in geothermal brine service.