Boiler and HRSG feedwater in power plants is expected to be virtually free of hardness—and ion‑exchange softeners deliver when sized and regenerated correctly. Here’s how conventional systems work, how to size them, and why disciplined regeneration keeps turbines and boilers out of trouble.
In power generation, even trace calcium and magnesium are a liability. International boiler standards (ASME/ABMA) effectively require “virtually zero” hardness in feedwater; APAVE tables show allowable CaCO₃ hardness ≤0.3 mg/L for low‑pressure boilers, dropping to 0.05 mg/L or “not detectable” at high pressures (bqua.com). An Indonesian PLTU manual warns plainly that “hardness is the main cause of deposits on heat exchangers, boilers and pipes,” with fouling and efficiency losses to match (id.scribd.com).
The risk is not just in the boiler. Dissolved solids, including hardness and silica, carry into turbine stages and deposit on blades—reducing efficiency and causing vibration or cracking (pttensor.com). In practice, HRSG/boiler feedwater is softened (or demineralized) to very low hardness—e.g., under 0.1 mg/L CaCO₃—before turbine or boiler inlet to avoid unscheduled outages and costly scale removal.
Sodium‑cycle ion exchange softening
Sodium‑cycle ion exchange is the classic approach to hardness removal, and industrial softeners such as a softener remain the workhorse. A strongly‑acidic cation resin in the sodium form (R–Na) exchanges its Na⁺ for Ca²⁺ and Mg²⁺. Mechanism: 2 R–Na + CaCl₂ → R₂–Ca + 2 NaCl; during regeneration, high Na⁺ brine reverses the exchange: R₂–Ca + 2 NaCl → 2 R–Na + CaCl₂. The resin itself—see ion exchange resins—is the active medium that drives both service and regeneration cycles.
Operators follow a four‑step regeneration sequence when a softener’s effluent hardness creeps up or on a timer (waterworld.com) (waterworld.com):
Backwash lifts particulates and expands the bed by roughly 50% at about 6 gpm/ft² for 10–15 minutes (waterworld.com). Brine soak introduces an 8–12% NaCl solution (typically ~10%) for ~30 minutes; salt dosage typically runs 6–15 lb NaCl per ft³ of resin (waterworld.com) (complete-water.com). Slow (displacement) rinse gently pushes out concentrated brine. Fast rinse finishes at ≥1–1.5 gpm/ft³ to clear residual brine and re‑compact the bed (waterworld.com), with specifications also citing ≥1 gpm/ft³ as a typical fast‑rinse rate.
Two‑bed systems using strong‑acid and strong‑base resins with acid/caustic regeneration are deployed for demineralization, but for hardness‑only softening a single cation bed regenerated with NaCl is sufficient. For full demineralization context, see demineralizers. A complete range of cation and anion systems is cataloged under ion exchange.
Capacity, salt use, and cycle economics
Softener sizing is often expressed in “grains”—where 1 grain equals 1/7000 lb of CaCO₃. At ~10 lb salt/ft³ regeneration, a commercial Grade I cation resin typically delivers ~25,000–30,000 grains per ft³ (≈1.8–2.0 kg CaCO₃) capacity (swtwater.com). In practice, “high‑capacity” softeners are rated 32,000–48,000 grains per ft³ by using more salt or deeper beds.
Salt efficiency is the design tradeoff. Industry practice ranges from ~6 lb/ft³ (lower capacity per cycle) to 15 lb/ft³ (higher capacity) (waterworld.com) (complete-water.com). Typical regeneration might consume on the order of 10–20 gallons of water and 10–20 kg of salt per ft³ of resin each cycle. Monitoring (for example, with conductivity or hardness probes) ensures regeneration is triggered before breakthrough.
Service velocity guidance sits around 5–50 BV/hour (≈20–200 m/h), where 1 BV (bed volume) is the volume of resin itself—this frames service‑flow expectations and rinse design (dardel.info). Run time between regenerations ranges from hours to days, depending on plant operation.
Sizing for uninterrupted soft water
Designers match resin volume to hardness load so that run time is comfortably longer than regeneration time. Consider a 100 m³/h stream at 150 mg/L as CaCO₃ (≈9 gpg): hardness load is 15 kg CaCO₃ per hour, or 360 kg per day. One cubic meter of resin (≈35 ft³) at ~30,000 grains/ft³ capacity—about 60 kg per regeneration—removes only ~60 kg per cycle, so multiple cubic meters or more frequent regens are required. Large plants therefore deploy multiple trains; several trains of 5–10 ft³ (0.14–0.28 m³) each, cycled daily or every two days, is illustrative.
To make production continuous, a duplex arrangement is common—one vessel online while the other regenerates. Automatic changeover keeps soft water flowing as one unit finishes and the other begins regeneration (3‑m.hr). The same duplex schematic shows why this is favored for 24/7 duty; a downstream surge tank often buffers the brief swap. Undersized systems will regenerate nearly continuously—wasting salt and risking hardness breakthrough—while oversized systems cut salt use and maintenance. Commentary notes that moving from a “24,000‑grain” to a “32,000‑grain” unit can materially reduce salt use and improve reliability (complete-water.com).
Regeneration discipline and upkeep
Regeneration is not optional; it is the core of softening performance. Operating practice is to schedule by throughput or time—often nightly or between shifts—so resin never lingers exhausted. Daily effluent‑hardness checks confirm that any breakthrough only appears at the end of the cycle, not during service. Resin is replaced periodically as capacity fades due to age or fouling.
Preventative tasks—valve service, brine‑line cleaning, flow‑controller checks—keep regeneration efficient. Guidance from CWS water treatment sources ties these basics to lower salt use and stable output (complete-water.com).
What the standards and field data imply
The standards and plant experience align: hardness must be minimized. APAVE tables cited earlier set <0.3 mg/L as CaCO₃ for low pressure, nearing 0.05 mg/L or “not detectable” for high pressure (bqua.com). Indonesian PLTU guidance stresses hardness as a primary depositor (id.scribd.com), and turbine‑stage deposition and vibration risks are documented (pttensor.com). Properly sized sodium‑cycle softeners, regenerated at appropriate salt doses, reduce hardness to essentially zero during service and support that standard.
Sources: Industry manuals and studies of boiler pretreatment, chemical engineering texts, and power‑plant guidelines. See cited references for detailed figures and design data (bqua.com) (id.scribd.com) (waterworld.com) (3‑m.hr) (swtwater.com) (complete-water.com) (dardel.info).
