Boiler oxygen scavenger — why it is essential
Short answer: Dissolved oxygen in boiler feed water causes pitting corrosion — small deep pits that penetrate tube walls far faster than general corrosion. An oxygen scavenger chemically removes that dissolved oxygen. Without it, boiler tubes and the condensate system corrode from the inside.
See our ZeroScale boiler oxygen scavenger.
Why dissolved oxygen is so damaging
Oxygen dissolved in water attacks steel at specific points rather than uniformly. The result is pitting — narrow, deep holes rather than general thinning.
- A pit can penetrate a tube wall while the surrounding metal is still sound
- Failure is sudden rather than gradual
- Inspection can miss pits because overall wall thickness looks acceptable
Where oxygen enters
- Make-up water — fresh water carries dissolved oxygen
- Condensate return — air leaks into the system where it runs below atmospheric pressure
- Pump glands and fittings on the suction side
- Feed tank exposed to air
The more make-up water a system takes, the more oxygen enters — which is one reason condensate recovery is worthwhile.
Two stages of oxygen removal
1. Mechanical — deaeration
Heating the feed water reduces the solubility of oxygen, so much of it is driven off. A deaerator or a heated feed tank removes the bulk of the oxygen.
2. Chemical — oxygen scavenger
Mechanical deaeration never removes it all. The scavenger deals with the remainder, and provides protection when the deaerator is not performing perfectly.
Both are needed. Chemical dosing alone on cold make-up water means a very high chemical consumption; deaeration alone leaves residual oxygen.
Where to dose
Into the feed tank or deaerator storage section, so the chemical has contact time before the water reaches the boiler.
Dosing directly at the boiler inlet gives too little reaction time, and the oxygen will already have passed through the feed line.
What to monitor
| Parameter | Why |
|---|---|
| Feed water temperature | Higher temperature means less dissolved oxygen |
| Residual scavenger in boiler water | Confirms sufficient dose |
| Boiler water pH | Should stay alkaline — see our alkalinity builder |
| Condensate pH and iron | Indicates corrosion in the return system |
| Make-up water quantity | More make-up means more oxygen entering |
Dosing considerations
- Dose is based on the dissolved oxygen present plus a residual, so it depends heavily on feed water temperature
- Cold make-up water needs far more chemical than deaerated hot water
- Maintain a measurable residual in the boiler water — that is the confirmation the dose is adequate
- Use a dosing pump rated for the boiler feed line pressure — see our pump selection guide
The complete boiler treatment
Oxygen scavenging is one part of a boiler programme. The others are:
- Hardness removal — softening the feed, see our softener guide
- Scale control — boiler antiscalant
- Alkalinity and pH — alkalinity builder
- Condensate protection — condensate corrosion inhibitor
- Blowdown control — using conductivity, see our conductivity guide
The full range is on our boiler chemicals page.
Signs that oxygen control is inadequate
- Iron levels rising in the boiler water or condensate
- Pitting found during inspection
- Reddish deposits in the feed system
- Repeated tube failures at similar locations
- No measurable scavenger residual in the boiler water
Safety
- Handle with gloves and eye protection
- Store away from other chemicals and label clearly
- Keep the dosing tank covered
- Follow the product safety data sheet
Send us your boiler pressure, feed water analysis and make-up quantity, and we will recommend the correct programme.
Boiler treatment programme
Send your boiler pressure, feed water analysis and make-up quantity, and we will recommend the full chemical programme.
💬 WhatsApp par poochhein 📞 9910646957Frequently asked questions
Why is dissolved oxygen so damaging in a boiler?
It causes pitting corrosion, which produces narrow deep holes rather than general thinning. A pit can penetrate a tube wall while the surrounding metal still looks sound, so failure is sudden and inspection can miss it.
Where does oxygen enter a boiler system?
Mainly through make-up water, which carries dissolved oxygen, and through air leaking into the condensate return where it runs below atmospheric pressure. Pump glands and open feed tanks also admit air.
Is a deaerator enough on its own?
No. Mechanical deaeration removes the bulk of the oxygen but never all of it, and it may not perform perfectly at all times. A chemical scavenger deals with the remainder and provides protection when the deaerator underperforms.
Where should the oxygen scavenger be dosed?
Into the feed tank or deaerator storage section, so there is contact time before the water reaches the boiler. Dosing at the boiler inlet gives too little reaction time.
How do I know the dose is adequate?
By maintaining a measurable scavenger residual in the boiler water. Rising iron levels in boiler water or condensate indicate that oxygen control is inadequate.