Boiler TDS Explained: Total Dissolved Solids Control
TDS (Total Dissolved Solids) is the most important water quality parameter for boiler operation. This guide explains what TDS is, why it matters, how to measure it, and how to control it through blowdown.
What is TDS?
TDS (Total Dissolved Solids) is the total concentration of dissolved substances in water. In boiler water, these are primarily mineral salts — calcium, magnesium, sodium, chloride, sulphate, bicarbonate, and silica — that enter the boiler through the feedwater.
As water evaporates in the boiler to produce steam, the dissolved solids remain behind. Unlike the water (which leaves as steam), the solids accumulate. Over time, the concentration of solids in the boiler water increases — this is why blowdown is necessary to remove concentrated water and maintain TDS within safe limits.
Why TDS Matters
- Foaming and carryover: When TDS exceeds the limit for the operating pressure, the water foams. Foam reaches the steam outlet and carries water droplets (with dissolved solids) into the steam system, contaminating downstream equipment.
- Scale formation: High TDS means high concentrations of scale-forming minerals (calcium, magnesium, silica). These precipitate on heat transfer surfaces, reducing efficiency and causing tube overheating.
- Corrosion: High concentrations of certain ions — particularly chloride and hydroxide — accelerate corrosion of boiler steel. Chloride causes pitting; excessive hydroxide causes caustic embrittlement.
- Steam purity: High TDS directly reduces steam purity. Every kilogram of carryover water brings dissolved solids into the steam. For process applications (food, pharmaceutical, turbines), even trace contamination is unacceptable.
TDS Limits by Boiler Pressure
| Boiler Pressure | Max TDS (ppm) | Why Lower at Higher Pressure? |
|---|---|---|
| Low (0–4 bar) | 3,000–3,500 | Lower steam velocity, larger drum, more tolerance for solids |
| Medium (4–10 bar) | 2,500–3,000 | Moderate steam velocity, standard separation equipment |
| High (10–20 bar) | 2,000–2,500 | Higher steam velocity, more efficient separation needed |
| Very high (20–40 bar) | 500–1,500 | Very high steam velocity, small drum, carryover risk is severe |
| Super-critical (60+ bar) | Below 500 | Demineralised water required — any solids cause turbine damage |
How to Measure TDS
- Conductivity meter (most common): Dissolved solids increase water's electrical conductivity. A conductivity probe immersed in a cooled boiler water sample gives a reading in µS/cm, which is converted to TDS using a conversion factor (typically TDS ppm ≈ conductivity µS/cm × 0.5 for boiler water).
- In-line conductivity sensor: A sensor installed directly in the boiler blowdown line provides continuous TDS monitoring. Connected to an automatic blowdown controller, it maintains TDS at the setpoint without operator intervention.
- Gravimetric analysis (laboratory): The most accurate method — a sample is evaporated and the residue is weighed. Used for calibration of conductivity meters, not for routine monitoring.
How to Control TDS
TDS is controlled by blowdown — removing concentrated boiler water and replacing it with fresh feedwater. The required blowdown rate depends on the feedwater TDS and the boiler's TDS limit:
Example: Feedwater TDS = 300 ppm, boiler limit = 3,000 ppm:
- If feedwater TDS is high, more blowdown is needed — this wastes more energy. Lowering feedwater TDS (better softening, more condensate return, or RO) reduces the blowdown requirement.
- Automatic TDS-controlled blowdown maintains the TDS precisely at the setpoint, preventing both high-TDS carryover and excessive energy-wasting blowdown.
- Maximising condensate return is the most effective way to reduce TDS — returned condensate has near-zero TDS, diluting the feedwater.