Boiler High Conductivity: Causes & Treatment
High conductivity in boiler water indicates excessive dissolved solids, which causes foaming, carryover, and unstable operation. Learn what causes high conductivity and how to bring it under control.
What is Conductivity?
Conductivity is a measure of the water's ability to conduct electricity, which is directly proportional to the concentration of dissolved ions (dissolved solids) in the water. In boiler water management, conductivity is used as a proxy for TDS (Total Dissolved Solids) because it's easier to measure continuously than TDS directly.
As water evaporates in the boiler, dissolved solids remain behind and concentrate. The longer the water stays in the boiler, the higher the conductivity. Without blowdown to remove concentrated water, conductivity rises until foaming and carryover occur.
Acceptable Conductivity Limits
| Boiler Pressure (bar g) | Max TDS (ppm) | Approx. Conductivity (µS/cm) |
|---|---|---|
| 0–2 | 3,500 | 7,000 |
| 2–4 | 3,000 | 6,000 |
| 4–10 | 2,500 | 5,000 |
| 10–20 | 2,000 | 4,000 |
| 20–40 | 1,500 | 3,000 |
| 40–60 | 1,000 | 2,000 |
| 60+ | 500 | 1,000 |
Values are approximate. Always follow the boiler manufacturer's recommendations.
Causes of High Conductivity
- Insufficient blowdown: The most common cause. If surface blowdown is not removing enough concentrated water, TDS (and conductivity) rises. Fix: increase blowdown rate or verify the automatic conductivity controller is functioning.
- High feedwater TDS: If the makeup water has high dissolved solids (hard water, poor softener performance, or seawater contamination), the feedwater brings more solids into the boiler. Fix: check the softener, consider RO or demineralisation.
- Low condensate return: Condensate has very low TDS. If the return rate drops (leaks, process dumping, or condensate system failure), more high-TDS makeup water is needed, raising conductivity. Fix: maximise condensate return.
- Excess chemical dosing: Treatment chemicals (sulfite, phosphate, amine) are dissolved solids — they add to the TDS load. Overdosing raises conductivity. Fix: dose based on test results, not on a fixed rate.
- Automatic controller failure: The conductivity sensor may be scaled, the blowdown valve may be stuck closed, or the controller may be misconfigured. Fix: clean the sensor, verify the valve operates, and check controller setpoints.
- Process contamination: If process equipment leaks (heat exchanger tube failure, cross-connection), process chemicals may enter the condensate and return to the boiler, raising conductivity. Fix: identify and isolate the contamination source.
Consequences of High Conductivity
- Foaming — stable bubbles form on the water surface as TDS increases
- Carryover — foam reaches the steam outlet, contaminating steam with water and dissolved solids
- Unstable water level — the gauge glass shows fluctuating levels as foam rises and falls
- Scale formation — high dissolved solids increase the rate of scale deposition on tubes
- Corrosion — high concentrations of certain ions (chloride, hydroxide) accelerate corrosion
- Steam purity issues — downstream equipment receives contaminated steam, causing quality problems in food, pharmaceutical, and textile processes
Treatment
- Increase blowdown immediately: Open the surface blowdown valve manually to bring conductivity below the limit. Once under control, verify the automatic controller is maintaining the correct setpoint.
- Clean the conductivity sensor: Scale deposits on the electrode cause false high readings. Remove and clean with a soft brush and dilute acid, then recalibrate.
- Check feedwater quality: Test the feedwater TDS. If it's high, investigate the softener, RO, or makeup water source.
- Maximise condensate return: Find and fix condensate losses. Every 10% increase in return rate dilutes the feedwater and reduces the blowdown needed.
- Review chemical dosing: Verify dosing rates match the water treatment supplier's recommendations. Overdosing adds unnecessary dissolved solids.
Pro tip: Install an automatic conductivity-controlled blowdown system. It continuously monitors boiler water conductivity and opens the blowdown valve only when the setpoint is exceeded — keeping TDS at the optimum level without wasting energy on excessive blowdown.