A boiler TDS control solution combines measurement, calculation and control to keep total dissolved solids (TDS) in boiler water just below the permitted limit: blow down exactly what is needed – no excess blowdown wasting fuel and water, no under-blowdown causing foaming and scale. Combined with blowdown heat recovery, it significantly cuts boiler operating costs. This guide covers the calculations, manual vs automatic blowdown and the equipment required to European standards.
What is TDS? Converting TDS and conductivity
TDS (Total Dissolved Solids) is the total of dissolved minerals, salts and metals in water, in mg/L (= ppm). In boilers TDS is measured indirectly via conductivity (µS/cm):
- TDS (ppm) ≈ k × conductivity (µS/cm), with k ≈ 0.5–0.7 depending on water composition – determine it on site.
- Shell boilers typically run at 2,000–3,500 ppm TDS, always below the boiler manufacturer’s limit and water-quality standards.
| TDS (ppm) | Conductivity (µS/cm, k = 0.5) | Relative density |
|---|---|---|
| 0 | 0 | 1.00000 |
| 1,000 | 2,000 | 1.00091 |
| 2,000 | 4,000 | 1.00182 |
| 3,000 | 6,000 | 1.00273 |
| 4,000 | 8,000 | 1.00364 |
Why temperature compensation matters
Conductivity rises about 2 % per °C: ET = E25 × [1 + α (T − 25)], α ≈ 0.02/°C. Boiler water of 5,000 µS/cm (25 °C) at 10 bar g (≈ 184 °C): ET = 5,000 × [1 + 0.02 × (184 − 25)] ≈ 20,900 µS/cm. Without compensation the reading is about 4× too high and drifts with boiler pressure – so boiler TDS probes must have an integrated temperature sensor.
Why is a boiler TDS control solution needed?
- Steam quality: high TDS causes foaming and carryover – wet steam contaminating products and equipment.
- Scale and corrosion: deposits on heat-transfer surfaces, headers, valves and traps increase fuel use and shorten life.
- False level readings: foam disturbs gauge glasses and level probes.
- Blowdown cost: every kg blown down is treated water heated to saturation – excess blowdown burns money.
Calculating the blowdown rate
Br = Q × F / (B − F) — Br: blowdown (kg/h); Q: steam output (kg/h); F: feedwater TDS (ppm); B: boiler water TDS to maintain (ppm).
| Example | Value |
|---|---|
| Boiler pressure | 12 bar g |
| Steam output Q | 10,000 kg/h |
| Feedwater TDS F | 250 ppm |
| Boiler TDS B | 2,500 ppm |
| Blowdown Br | 10,000 × 250 / (2,500 − 250) ≈ 1,111 kg/h (≈ 11 % of steam output) |
According to the US Department of Energy, blowdown typically ranges from 1 % to 8 % of feedwater flow, and can reach 20 % with high-solids make-up water (DOE Steam Tip Sheet #23). Reducing feedwater TDS (softening, RO, more condensate return) is the most effective way to reduce blowdown.
Manual vs automatic TDS blowdown
Manual: saw-tooth TDS profile; average must be kept low for safety → more blowdown than necessary.
Automatic: TDS held close to set point → minimum blowdown, stable steam quality.
The DOE notes that automatic conductivity-based blowdown control typically offers a simple payback of 1–3 years; for a 100,000 lb/h boiler, cutting blowdown by 1,000 lb/h saves about US$31,400 per year in fuel, water and chemicals (DOE reference conditions) (source).
Blowdown heat recovery
Blowdown leaves at boiler saturation temperature (about 184 °C at 10 bar g). Much of this heat can be recovered:
- Flash vessel: reduces blowdown pressure so part of it flashes to low-pressure steam for the feed tank or deaerator. Worked example: blowdown at 10 bar g flashed to 0.5 bar g produces about 14 % flash steam.
- Blowdown heat exchanger: the remaining hot water preheats make-up water before discharge and lowers effluent temperature.
- The DOE recommends heat recovery for boilers with continuous blowdown above 5 % of steam rate; flash plus heat-exchanger systems can recover around 90 % of blowdown heat (DOE Steam Tip Sheet #10).
ISE TDS control solution components
| Equipment | Function / data |
|---|---|
| TDS sensor (conductivity probe) | Continuous measurement, integrated temperature compensation, PN40, up to 32 bar g / 239 °C |
| TDS controller | Set point, hysteresis, MIN/MAX alarms, 4–20 mA, Modbus RTU |
| Automatic TDS blowdown valve | DN15–DN25, PN40, electric or pneumatic |
| Sample cooler | Sampling for checks and periodic calibration |
| Flash vessel + heat exchanger | Blowdown heat recovery (optional) |
| Condensate monitoring | Detects contamination before return (optional) |
Details: boiler TDS sensor (Gestra, ViRA) • automatic TDS blowdown system • condensate contamination control.
Reference standards
- EN 12953-10 (shell boilers) and EN 12952-12 (water-tube boilers): feedwater and boiler water quality.
- Pressure equipment per PED 2014/68/EU; CE-marked controllers (EMC 2014/30/EU, LVD 2014/35/EU).
- Unattended operation requires type-tested conductivity limiters per EN 12953-9 / EN 12952-11.
Implementation
- Survey the boiler, measure feedwater and boiler TDS, estimate current blowdown.
- Calculate optimum blowdown, set point and heat-recovery potential.
- Supply and install probe, valve and controller; integrate with PLC/SCADA.
- Calibrate against boiler water samples, commission and train operators.
FAQ
How is a TDS control solution different from a TDS blowdown system?
The blowdown system is the equipment (probe, controller, valve). The solution also covers blowdown calculation, set-point optimisation, feedwater improvement and heat recovery – optimising total system cost.
What TDS set point should be used?
Typically 2,000–3,500 ppm for shell boilers, but always per the boiler manufacturer and EN 12953-10; ISE determines it during the survey.
Contact ISE about boiler TDS control
EQUIPMENT INDUSTRIAL SOLUTION CO., LTD (ISE) – Ho Chi Minh City, Vietnam – surveys, calculates and supplies complete TDS control and blowdown heat-recovery solutions.
Contact us: Sales +84 909 617 185 – Technical +84 979 117 185 – Email: [email protected]
