Boiler Chemical Dosing and the Daily Water Tests
External treatment — softening, RO, deaeration — removes most of the trouble before water reaches the boiler. Chemical dosing deals with what is left inside it. The daily water tests are how the boiler house knows whether either is working, before the damage shows.
You'll learn
- Name the main boiler water chemicals and what each does
- Know where each is dosed
- Run and interpret the routine water tests
- Act on an out-of-range result
Before this: Scale and corrosion, TDS control and blowdown
Why chemicals are still needed
Even well-treated feedwater carries traces of hardness, oxygen and dissolved salts. Inside the boiler those traces concentrate. Chemicals added in small, controlled amounts keep the remaining hardness from sticking as scale, mop up the last of the oxygen, and hold the water at the right alkalinity to protect the steel.
Chemicals are not a substitute for a softener or a deaerator. They are sized to handle small residuals; fed into untreated water, they are overwhelmed and simply add to the dissolved solids that blowdown has to remove.
The chemicals and where they go
| Chemical type | What it does | Usually dosed to |
|---|---|---|
| Oxygen scavenger (e.g. sulfite-based) | Reacts with the last dissolved oxygen | Feed tank or deaerator outlet |
| Phosphate and polymer conditioners | Turn residual hardness into a soft sludge that blowdown removes, and keep it dispersed | Feedwater or boiler |
| Alkalinity / pH builder | Keeps boiler water alkaline to protect steel | Feedwater or boiler |
| Neutralizing amines | Raise condensate pH to stop carbonic acid corrosion in return lines | Feedwater, carried with the steam |
| Antifoam | Suppresses foaming where TDS or contamination causes it | Boiler |
Products and doses are specific to the water and the boiler; follow the treatment supplier’s program.
Chemicals are fed by dosing pumps, ideally proportional to feedwater flow, so the dose follows the load. Slug-dosing by the bucket gives concentrations that swing too high, then too low.
The routine tests
| Test | Where | What it tells you |
|---|---|---|
| Hardness | Softener outlet and feedwater | Whether the softener is working; any hardness means trouble |
| TDS / conductivity | Boiler water | Whether blowdown is right |
| pH and alkalinity | Boiler water and feedwater | Whether the steel is protected |
| Sulfite (or scavenger) residual | Boiler water | Whether oxygen is being fully removed |
| Phosphate residual | Boiler water | Whether the conditioning dose is right |
| Condensate pH and conductivity | Return line | Corrosion in the steam system; process contamination |
Limits come from the boiler manufacturer, the treatment program and the applicable standard for the boiler’s pressure. Record every result in the log so that trends are visible.
Reading the results
- Hardness at the softener outlet: regenerate or repair the softener now; scale is forming.
- Boiler TDS high: increase blowdown; check whether feedwater TDS has risen or condensate return has fallen.
- Sulfite residual low or zero: oxygen is getting through. Check deaerator temperature and venting, and the dosing pump.
- Condensate conductivity rising suddenly: process contamination — a leaking heat exchanger letting product into the condensate. Dump the condensate until the source is found; contaminated condensate can foul or foam the boiler.
- Alkalinity too high: risk of foaming and caustic attack; review dosing and blowdown.
Key takeaways
- Chemicals treat residual hardness, oxygen and pH inside the boiler; they do not replace external treatment.
- Dose proportionally to feed flow, at the right point for each chemical.
- Test hardness, TDS, pH/alkalinity, scavenger and conditioner residuals, and condensate, routinely.
- Act on every out-of-range result and watch the trend in the log.