Oxygen scavengers compared: DEHA vs sodium bisulfite or sulfite
A general-chemistry comparison of DEHA and sodium bisulfite or sulfite oxygen scavengers: volatility, condensate protection, solids contribution and how to decide.
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DEHA and sodium bisulfite or sulfite both remove dissolved oxygen from boiler water, but they behave differently. Sulfite is non-volatile and stays in the boiler water, adding dissolved solids. DEHA is volatile, travels with the steam and helps protect condensate lines, without adding solids. The right choice depends on steam use, boiler operating conditions and condensate system design.
Why does a boiler need an oxygen scavenger?
Even a good deaerator leaves a small residual of dissolved oxygen. At boiler temperatures that oxygen attacks feedwater lines, the economiser and boiler metal, and it causes pitting that is expensive to repair. A chemical oxygen scavenger removes what the mechanical step leaves behind and helps keep a protective oxide film on the steel.
How does sodium bisulfite or sulfite work?
Sulfite reacts with dissolved oxygen to form sulfate. The reaction is simple, fast with the right conditions and well understood, which is why it has long been a common choice. Sodium bisulfite is a closely related form that releases sulfite in the water. Our datasheets list a sodium bisulfite-based boiler oxygen scavenger.
- It is non-volatile, so it stays in the boiler water and protects the feedwater and boiler, but it does not travel in the steam.
- The reaction product, sulfate, adds to the dissolved solids, which raises conductivity and blowdown demand.
- At higher pressure and temperature, sulfite can break down into products that are acidic and can affect the steam and condensate, so its suitability depends on the pressure range.
- It is a familiar chemistry with simple residual tests.
How does DEHA work?
DEHA, diethylhydroxylamine, reacts with dissolved oxygen and also helps passivate metal surfaces. Our datasheets list a DEHA-based boiler oxygen scavenger. Its main difference is volatility: a share of the chemical goes forward with the steam into the condensate system.
- It is volatile, so it can provide some protection of condensate lines and not only the boiler.
- It does not add to the dissolved solids in the boiler water in the way that sulfite does.
- It helps form a protective film on metal surfaces.
- Its distribution in the steam system depends on the layout, and its residual is measured differently from sulfite.
- Local rules and the steam end use may affect the choice, which should be confirmed with the technical team.
What does volatility mean for condensate protection?
Condensate lines corrode when carbon dioxide forms carbonic acid, and when oxygen leaks in. A volatile scavenger reaches those lines because it leaves the boiler with the steam. A non-volatile one stays in the drum and does nothing for the condensate network. If condensate corrosion is a recurring problem, a volatile chemistry may help, but it does not replace a neutralizing amine for carbon dioxide control.
How do solids contribution and blowdown compare?
Every chemical that stays in the boiler water adds to its dissolved solids. Because sulfite ends up as sulfate, a sulfite programme steadily raises the conductivity of the boiler water and so calls for more blowdown. A programme that adds fewer solids can let the boiler run closer to its limit with less water and heat lost to blowdown.
The effect is small in some systems and noticeable in others. A system with high make-up, high duty or limited blowdown capacity feels it most.
Which should I choose?
Neither is better in every case. The decision follows the system. Use the questions below as a starting point and confirm with the technical team.
- Is condensate corrosion a problem, and is the condensate network long? This favours a volatile scavenger.
- Is the boiler pressure high, or are solids and blowdown a concern? This favours a low-solids programme.
- Is the steam used where residues matter? Confirm which chemicals are acceptable for that use.
- Is the boiler small and low in pressure, with simple needs? A sulfite programme can be perfectly adequate.
- Is catalysed sulfite already in use, and is it working? A change needs a reason and a monitoring plan.
Can the two be used together?
They are sometimes combined, with sulfite protecting the boiler and a volatile product protecting the condensate, but the chemistries must be checked for compatibility and the monitoring must follow both. A mixed programme is a design decision and not an ad hoc fix.
What should be monitored in either case?
- The scavenger residual in the boiler water or feedwater, using the test method for that product.
- Boiler water pH and conductivity, and alkalinity.
- Condensate pH, iron and copper.
- Dissolved oxygen at the deaerator outlet, to see how much the scavenger has to do.
- Inspection findings at each outage.
Treatment rate is set from the water analysis and operating conditions, and the residual is used to trim it. Our technical team can discuss this against your data.
What about steam end use and the handling of residues?
When steam contacts food, drink or other sensitive products, or is used for humidification or sterilisation, the question of which chemicals may go forward with the steam becomes important. A volatile treatment sends some chemical into the steam by design, and a non-volatile one does not. Confirm the site rules and the technical team recommendation before choosing, and do not assume a chemistry suits the end use.
How should a change from one scavenger to another be handled?
- Review the current monitoring results and inspection findings so the reason for the change is clear.
- Check compatibility with the other boiler chemicals and the materials in the system.
- Plan the monitoring for the changeover, including the new residual test.
- Inspect the boiler at the next outage to confirm there are no adverse effects.
What should I send for a technical review?
- The boiler type, operating pressure range, steam use and end use of the steam.
- Analyses of feedwater, boiler water and condensate.
- The current scavenger, how it is fed and the residual readings.
- Blowdown rate or conductivity control and any limits on blowdown.
- Condensate system layout and any history of corrosion or iron pick-up.
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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