How a steam boiler concentrates dissolved solids, why it needs blowdown, and what the controller does to keep boiler water within its limits.
Boilers are regulated pressure vessels. Work on a boiler, its blowdown, or its sample lines only when isolated, depressurized, and cooled, and never defeat a low-water cutoff or safety valve. See Pressurized Systems.
A boiler turns water into steam. The steam leaves almost pure, so every dissolved solid that came in with the feedwater stays behind in the boiler — and the boiler water gets steadily more concentrated.
| Flow | What it is |
|---|---|
| Feedwater | Everything entering the boiler: fresh make-up plus condensate returned from the steam system |
| Steam | Leaves the dissolved solids behind |
| Condensate return | Condensed steam coming back. Nearly pure, so the more is returned, the lower the feedwater’s dissolved solids |
| Blowdown | Boiler water deliberately drained to remove concentrated solids — the control lever |
| Surface (continuous) blowdown | Bottom blowdown | |
|---|---|---|
| Where | Just below the water surface, from a skimmer line | From the lowest point of the boiler or mud drum |
| Removes | Dissolved solids, which are most concentrated near the surface | Settled sludge and suspended solids |
| How | Automatically, on conductivity — this is what the controller does | Short manual or timed blasts, as the boiler manufacturer specifies |
The controller manages surface blowdown. Bottom blowdown is still needed on most boilers; follow the boiler manufacturer’s schedule.
| Job | Why | How, on the InteliHive |
|---|---|---|
| Surface blowdown on conductivity | Holds boiler water dissolved solids below the limit where it foams, carries over, or deposits | The Boiler setup profile — Setpoint Control, Rising, with timed sampling. See Sampling and Blowdown Control |
| Feed oxygen scavenger and internal treatment | Dissolved oxygen pits steel; hardness forms scale | Volume Dosing on a feedwater or make-up meter, or Percent Timer |
| Stop when the boiler is off | A cold, idle boiler needs no blowdown and no feed | An interlock on a dry contact from the boiler’s operating controls: closed when the boiler is firing |
| Page | Covers |
|---|---|
| TDS and Blowdown | Cycles in a boiler, how much to blow down, choosing a conductivity limit, worked examples |
| Commissioning a Boiler | Step by step, from planning to hand-over, on the Boiler profile |
| Sampling and Blowdown Control | Why boiler water is hard to measure, the three sampling modes, the probe installation, calibration, and troubleshooting |