Boiler water chemical selection guide: oxygen, pH, scale and condensate
How to choose boiler water chemicals by function: oxygen scavengers, pH control, internal scale and corrosion inhibitors and neutralizing amines, driven by water analysis.
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Boiler water chemicals are chosen by function, not by brand: an oxygen scavenger to remove dissolved oxygen, a pH booster for alkalinity, an internal scale and corrosion inhibitor for the boiler water, and a neutralizing amine to protect steam and condensate lines. The water analysis, boiler type and steam use decide which are needed and how they are fed.
What are the jobs a boiler treatment programme must do?
A boiler has three zones that each fail in their own way. The feedwater system and economiser suffer oxygen corrosion. The boiler itself suffers scale and deposit formation, and caustic or acid attack when the pH is wrong. The steam and condensate system suffers carbonic acid corrosion. A programme is a set of chemicals, one for each risk.
How does the water analysis drive the selection?
Analysis of the make-up, feedwater, boiler water and condensate shows where the risks sit. Our datasheets state that treatment rates are set from the water analysis and the operating conditions, with monitoring of pH, conductivity and corrosion indicators.
- Make-up hardness and alkalinity, which drive scale and carbonate breakdown into carbon dioxide in the steam.
- Silica and iron, which form hard deposits and need blowdown and pre-treatment.
- Dissolved oxygen after the deaerator, which sets the oxygen scavenger demand.
- Boiler water pH and alkalinity, to judge whether a pH booster is needed.
- Condensate pH and iron and copper, to judge the need and the type of neutralizing amine.
- Conductivity of the boiler water, which fixes how much blowdown is needed.
When do I need an oxygen scavenger?
Dissolved oxygen that survives the deaerator attacks feedwater lines, pumps and the economiser, and it can pit boiler metal. An oxygen scavenger removes the residual oxygen chemically, and the choice depends on the pressure, the steam use and whether a non-volatile or volatile chemistry suits the system.
Our datasheets list boiler oxygen scavengers of two types: a DEHA-based product and a sodium bisulfite-based product. A separate guide compares the two in more detail.
When do I need a pH booster?
Boiler water needs a protective alkaline pH so that a stable oxide film forms on steel. If the make-up has low alkalinity, or if blowdown and deposit control leave the boiler water too neutral, a pH booster restores a protective alkalinity. Too much alkalinity causes carryover and caustic attack, so the pH is controlled inside a window and not pushed as high as possible.
What does a scale and corrosion inhibitor do inside the boiler?
Hardness that enters the boiler can deposit on heat-transfer surfaces and insulate them, which raises fuel use and can lead to overheating. An internal scale and corrosion inhibitor conditions the hardness and keeps deposits dispersed so they leave with blowdown, while supporting the protective film on the metal.
- It complements, and does not replace, softening and good deaeration.
- It works well on a controlled blowdown that removes the dispersed solids.
- It should be compatible with the oxygen scavenger and the amine in use.
- Its effect is checked by inspections, deposit weights and iron in the boiler water.
Why add a neutralizing amine for steam and condensate?
Carbonate and bicarbonate in the feedwater break down in the boiler to release carbon dioxide into the steam. When the steam condenses, the carbon dioxide dissolves and forms carbonic acid, which lowers condensate pH and corrodes the lines. A neutralizing amine travels with the steam and raises the condensate pH to a protective level.
Amines differ in volatility and distribution, so the choice depends on the length and layout of the condensate system. A datasheet product for steam and condensate is listed in our boiler category.
How do the steam end use and the boiler type change the choice?
- Steam that touches food, drink or medicines needs treatment chemicals that suit that contact. This is a point to confirm with the technical team and the site rules, not to assume.
- High-duty boilers need more careful control of solids and oxygen than low-duty ones.
- Long condensate networks favour amines with good distribution.
- Boilers with a lot of make-up need more attention to hardness and alkalinity.
- Intermittent operation raises the risk of oxygen attack during standby, which calls for a layup plan.
How should I plan monitoring?
- Boiler water pH, conductivity and alkalinity at a regular interval.
- Oxygen scavenger residual in the boiler or feedwater, as the product allows.
- Condensate pH and iron, to check that the amine is working.
- Hardness in the feedwater, to catch softener breakthrough early.
- Inspection of drums and tubes at each outage.
Treatment rates are set from the water analysis and the operating conditions, and adjusted using these results. Our technical team can discuss them against your data.
What about layup when a boiler is out of service?
A boiler that stands idle can corrode quickly if it is left wet and exposed to air. Whether it is kept dry or kept full with a protective treatment, the layup method should be chosen for how long the shutdown will last and how fast the boiler may be needed again. Plan it with the technical team before the shutdown, and record the condition of the boiler water at return to service.
How do I know the programme is working?
- Boiler water parameters stay inside the agreed window with steady trends.
- Condensate pH is protective and iron pick-up is low and stable.
- Inspection shows clean, dark, adherent metal surfaces and no hard deposits.
- Blowdown demand is steady, and fuel use per unit of steam does not creep upwards.
What should I send for a technical review?
- Analyses of make-up water, feedwater, boiler water and condensate.
- The boiler type, operating pressure range, steam use and whether steam contacts a product.
- The deaerator or other oxygen removal arrangement and the current chemicals fed.
- Blowdown method and control, and the proportion of make-up water.
- Inspection findings, deposit samples or corrosion evidence from the last outage.
Next step
Send your operating data and a recent water analysis to our technical team for a technical review.
Sources
- Advanced Chemicals Industrial product datasheets, for the product categories and monitoring practice they state.
- General industrial water-treatment practice, described qualitatively.
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