Boiler Water Conductivity: Preventing Foaming and Carryover with Surface Blowdown

However well the feed water to a steam boiler is treated, it always contains some dissolved matter. As water evaporates, these substances do not leave with the steam; they stay in the boiler and their concentration keeps rising. As dissolved salts increase, boiler water conductivity rises too, making it the most practical indicator of how concentrated the boiler water has become.

This article looks at the problems caused by high conductivity, how surface blowdown keeps it under control and how energy losses during blowdown can be minimised.

Conductivity and TDS

TDS (Total Dissolved Solids) is the total amount of salts dissolved in the water. Because these salts make water electrically conductive, conductivity is measured instead of TDS directly. As a rough guide, TDS (ppm) can be estimated by multiplying conductivity at 25 °C (µS/cm) by about 0.7, although the factor depends on the water chemistry. Conductivity also varies with temperature, so measurements must be temperature compensated.

Risks of High Conductivity

• Foaming: As the concentration of dissolved matter rises, a stable layer of foam forms on the water surface.
• Carryover: Foam and water droplets are carried into the steam main with the steam. Wet, contaminated steam causes erosion, deposits and failures in control valves, steam traps, heat exchangers and turbines, and can affect product quality.
• Unstable level readings: Foaming can make the water level read incorrectly and cause nuisance alarms.

Scale on boiler tubes, on the other hand, is mainly caused by hardness salts (calcium and magnesium). It is prevented by softening the feed water, chemical treatment and removing settled sludge by bottom blowdown. Surface blowdown controls the level of dissolved solids (TDS).

What Is Surface Blowdown?

Surface blowdown is the controlled removal of some boiler water from just below the normal water level. The high-conductivity water removed is replaced by lower-conductivity feed water, keeping boiler water conductivity within the desired range. Water is taken near the surface because dissolved matter and foam concentrate in this zone, where evaporation is most intense.

Saving Energy with Controlled Blowdown

Blowdown water leaves at boiler pressure and saturation temperature. Too much blowdown wastes heated, treated water; too little increases the risk of foaming and carryover. To strike the right balance:

• Automatic conductivity control: A conductivity probe fitted to the boiler measures the water continuously; the controller opens the blowdown valve when the set point is exceeded and closes it when the value falls, so only the necessary amount is blown down.
• Heat recovery: Blowdown water can be led to a flash vessel so the flash steam is used in the feed tank, while the heat of the remaining hot water can preheat the make-up water through a heat exchanger.

Equipment for the Surface Blowdown Line

Equipment in the blowdown line must withstand high temperature, pressure and wear caused by flashing. ORFA offers the following for surface blowdown applications:

• Conductivity probe and controller: Continuously measure boiler water conductivity and manage blowdown.
• Blowdown control valve: Adjusts blowdown flow in response to the controller signal.
• Strainer and auxiliary valves: Protect the control valve from solid particles and allow the line to be isolated for maintenance.

Frequently Asked Questions

Conclusion

Keeping boiler water conductivity under control is the key to preventing foaming and carryover, maintaining steam quality and limiting energy losses. Automatic surface blowdown and heat recovery protect the boiler and cut operating costs. See our Boiler Surface and Bottom Blowdown Systems page for our solutions.