Controlling TDS: Keeping Boiler Performance on Track

Keeping dissolved solids under control is critical to boiler efficiency, reliability and long-term performance.  STL Power & Process Controls Ltd is the official UK Channel Partner for CIRCOR-RTK and IGEMA GmbH, supplying RTK control valves and IGEMA conductivity electrodes for boiler TDS applications.

What is TDS?

Total Dissolved Solids are the minerals, salts, metals and other charged particles dissolved in water, typically measured in mg/L or parts per million (ppm).

TDS can originate from organic matter, industrial waste, rock, air and water-treatment processes, as well as metals such as copper and lead picked up from pipework.

Why control TDS?

As boiler water is converted into steam, the steam leaves the boiler while dissolved solids remain behind. Without effective control, TDS concentrations within the boiler rapidly increase, potentially affecting steam quality, efficiency and boiler integrity.

Excessive TDS can result in:

• Foaming and carryover: High TDS promotes foaming, allowing water droplets to enter the steam system. This can cause water hammer, valve erosion and damage to downstream pipework.
• Scale formation: Minerals can deposit on boiler tubes, creating an insulating layer that restricts heat transfer. Even a 3 mm layer of scale can increase fuel consumption by around 2–3%.
• Corrosion: Concentrated salts can attack internal metal surfaces, accelerating corrosion and reducing equipment life.

How is TDS measured?

TDS can be monitored indirectly through electrical conductivity. Dissolved ions carry electrical current between submerged electrodes; the higher the ion concentration, the higher the conductivity.

Electrical conductivity, measured in µS/cm, can be correlated to TDS in ppm using an appropriate K-factor, providing a continuous indication of boiler water quality.

Automated TDS control

An effective boiler TDS system operates as a continuous, closed-loop process, that automatically detecting rising conductivity and responding before excessive concentration becomes a problem. 

A typical automated TDS control system comprises of:

1. Conductivity probe – Continuously measures boiler-water conductivity, providing a direct indication of dissolved ion concentration.

2. TDS controller – Receives the conductivity signal, compensates for temperature effects and converts the measurement into the required conductivity or TDS value. The controller then determines whether the water has exceeded its configured limit.

3. Automatic blowdown valve – When TDS rises above the set point, the controller opens a motorised or pneumatic valve, discharging concentrated boiler water so it can be replaced by lower-TDS feedwater.

4. Optional cyclic bottom blowdown – A timer can provide periodic blowdown to help remove accumulated solids.

Take control of your boiler TDS

The right TDS control system can protect your boiler, improve steam quality and help reduce unnecessary energy and maintenance costs.

Talk to STL today to discuss your boiler TDS requirements and find a solution designed for reliable, efficient control.

*Auto TDS System with the TDS electrode mounted inside the boiler using a special T-Piece.

Installation variants of Auto TDS System include:

• In-boiler: TDS electrode mounted directly inside the boiler, with blowdown through a separate boiler connection.
• External: TDS electrode installed in the continuous blowdown line using a special mounting between the isolation valve and blowdown valve.

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