Raw Water Quality: What Is in the Water Before It Reaches the Boiler
Every boiler problem that starts on the water side starts with what was in the water to begin with. Borewell, river and municipal water all carry dissolved minerals and gases that a boiler concentrates as it turns water into steam. Knowing what is in the raw water decides what treatment the boiler needs.
You'll learn
- Name the impurities in raw water that matter to a boiler
- Understand TDS, hardness and alkalinity and their units
- See why a boiler concentrates impurities
- Read a raw water analysis
Before this: What counts as a boiler
Water is never just water
Rain picks up gases from the air; ground and river water dissolve minerals from the rock and soil they pass through. By the time water reaches a plant it carries calcium and magnesium salts, sodium salts, silica, iron, dissolved oxygen and carbon dioxide, and sometimes suspended mud and organic matter. Indian borewell water is often high in dissolved solids and hardness.
A boiler boils water away and sends it out as nearly pure steam. Almost everything dissolved in the water stays behind in the boiler. Feed a boiler water with a little impurity every hour and the boiler water steadily becomes more concentrated, until something is done about it.
The measurements that matter
| Parameter | What it is | Effect in a boiler |
|---|---|---|
| TDS (total dissolved solids) | All dissolved salts, in ppm or mg/L | Concentrates in the boiler; drives blowdown, foaming and carryover |
| Hardness | Calcium and magnesium, as mg/L CaCO₃ | Forms hard scale on heating surfaces |
| Alkalinity | Bicarbonate, carbonate, hydroxide | Breaks down to CO₂ in steam (corrodes condensate lines); high levels cause foaming |
| Silica | Dissolved SiO₂ | Very hard, insulating scale; can carry over with steam at higher pressure |
| Iron | Dissolved and suspended iron | Deposits on tubes; sign of corrosion upstream |
| pH | Acidity or alkalinity | Low pH corrodes; boiler water is kept alkaline |
| Dissolved oxygen | Gas from air contact | Pitting corrosion of feed system and boiler |
ppm (parts per million) and mg/L are effectively the same for water. Hardness and alkalinity are usually expressed “as CaCO₃”, a common basis that lets different salts be added together.
TDS can be estimated quickly from electrical conductivity, because dissolved salts conduct electricity. A conductivity meter in µS/cm, multiplied by a factor that depends on the water (often around 0.6–0.7), gives an approximate TDS. Boiler house tests and automatic blowdown controllers rely on it.
From analysis to treatment
Each impurity has its own treatment, and the raw water analysis decides which are needed: hardness is removed by softening; dissolved solids by reverse osmosis or demineralization; oxygen by deaeration and chemical scavenging; suspended solids by filtration. Higher-pressure boilers tolerate less of everything, because concentration factors are higher and deposits do more damage at higher heat flux.
- Water quality changes with the season: borewells rise and fall, rivers run muddy in the monsoon. Test at least seasonally, not once at commissioning.
- Sample correctly: from a flowing tap after flushing, in a clean bottle, tested promptly. Dissolved gases and pH change in a standing sample.
- Treat for the worst case: a softener sized for average hardness exhausts early when the hardness peaks.
Key takeaways
- Raw water carries hardness, dissolved solids, alkalinity, silica, iron and dissolved gases.
- A boiler concentrates everything dissolved in its feedwater.
- Hardness and alkalinity are expressed as mg/L CaCO₃; TDS is estimated from conductivity.
- Treatment follows the analysis; test seasonally.