Dissolved oxygen is the quiet killer of steam systems. It doesn’t announce itself. It pits feedwater lines, eats economizer tubes, and corrodes boiler internals from the inside — and by the time you see the leak, the damage has been happening for months. Your deaerator (DA) is the single most important defense against it. The problem is that a DA can be failing at its one job while looking completely normal from the outside.
The only way to know whether your deaerator is actually doing its job is to test the feedwater for dissolved oxygen. Here’s why that test matters more than almost any other check in the boiler room, how to run it correctly, and what the number is really telling you.
Why oxygen is the enemy
Oxygen dissolved in hot water is aggressively corrosive to carbon steel. It attacks in a particularly nasty way — not as uniform thinning, but as localized pitting. A pit concentrates the attack in one tiny spot, which is how a feedwater line or an economizer tube can develop a through-wall leak while the surrounding metal still looks healthy. Every downstream component — the feedwater piping, the economizer, the boiler’s own heating surfaces — is on the line.
That’s why the deaerator exists: to mechanically strip dissolved oxygen (and CO₂) out of the feedwater before it ever reaches the boiler.
What a healthy deaerator does
A properly operating DA removes oxygen mechanically, using heat and surface area:
- It heats the feedwater to saturation temperature for its operating pressure. Water can only hold very little dissolved gas at saturation — the heat is what drives the oxygen out of solution.
- It breaks the water into a thin film or fine spray (through spray valves, nozzles, and trays) to expose maximum surface area so the gases can escape.
- It vents the liberated gases to atmosphere through a continuous, visible vent plume.
Done right, mechanical deaeration brings dissolved oxygen down into the single-digit parts-per-billion range — a well-run DA is typically expected to hit around 7 ppb (0.005 cc/L) or better at the storage outlet. Your oxygen scavenger chemical is then there to mop up the trace residual, not to do the heavy lifting. If the scavenger is carrying the load, something mechanical is wrong.
The test: how to pull a dissolved-oxygen sample correctly
The test itself is simple, but how you take the sample determines whether the number means anything. Get the sampling wrong and you’ll either contaminate the sample with air or flash off the very gas you’re trying to measure.
- Sample from the right point. Pull from the DA storage-section outlet / boiler feedwater — that’s the water actually headed to the boiler, and it’s what you care about.
- Cool the sample properly. Run it through a sample cooler and let it come down before it hits the meter. Hot water at or near saturation will flash as pressure drops, and flashing blows off dissolved oxygen — giving you a falsely good reading. Never sample into an open cup and never let it flash.
- Avoid air contamination. Let the sample line run to flush it, use a closed/overflow sampling method, and don’t let the sample sit exposed to room air before you read it. Ambient air will contaminate a low-ppb reading fast.
- Measure it. Use a dissolved-oxygen meter or a CHEMet-style ampoule test kit rated for the low-ppb range. Both work; the meter is faster for routine trending, the ampoules are a good field check.
- Sample under realistic conditions. Test while the plant is running at a representative load. A DA can look fine at steady state and fall apart at high turndown or during rapid load swings — so check it under the conditions that actually stress it.
Reading the number — what oxygen is telling you
Here’s the diagnostic value, and it’s the whole point: a high dissolved-oxygen reading points to a mechanical problem in the deaerator, not a chemical one. Before you touch the chemical program, troubleshoot the DA itself:
- Is the storage section at saturation temperature for its pressure? A cold or under-pressure DA physically cannot deaerate. This is the number-one cause of high DO. Check the temperature against the steam tables for your operating pressure.
- Is the vent plume steady and visible? No plume means the liberated gases have nowhere to go — they stay in the water. A vent that’s been throttled shut “to save steam” is a classic self-inflicted failure.
- Is the spray valve / nozzle spraying a proper cone, or dribbling? This is where oxygen testing earns its keep.
- Are the trays intact and seated, or collapsed, fouled, or fallen out of position?
How the oxygen test fingerprints internal damage
You can’t see inside a running deaerator. You can’t shut it down and open it up every time you’re curious. The dissolved-oxygen test is how you look inside without opening it.
The spray nozzles and trays exist to create surface area. When a spray nozzle erodes, cracks, or plugs, its spray pattern collapses from a fine cone to a dribble — and the water that dribbles through never gets properly exposed. When trays collapse, corrode, foul, or shift out of position, the water sheets past instead of cascading across the surface area it’s supposed to. In both cases the DA loses contact area, and its ability to strip oxygen drops.
None of that is visible from the catwalk. The temperature might look right. The vent might look right. But the dissolved-oxygen number climbs — because the internals aren’t doing their job. A rising DO trend, with the DA otherwise “looking fine,” is often the first and only early signal that a nozzle or tray has failed internally. It’s a window into a sealed vessel.
Trend it — don’t just spot-check it
A single reading tells you where you are today. A trend tells you where you’re headed. Log your dissolved-oxygen readings over time — date, load, storage temperature, and DO. A slow, steady climb is your early warning that internals are degrading, giving you time to plan an inspection and repair on your schedule instead of discovering it the hard way when a feedwater line springs a leak.
Don’t chase oxygen with chemicals
When DO comes back high, the tempting fix is to feed more oxygen scavenger. Resist it. More chemical will knock the residual oxygen down and hide the symptom — but the mechanical failure is still there, you’re now spending more on chemistry every month, and the deaerator continues to degrade behind the mask. Worse, some scavengers are only effective at proper feedwater temperature, so a cold DA undermines the chemical program too. Fix the deaerator, then let the scavenger go back to its real job: cleanup, not rescue.
The DA health checklist
If you want a deaerator that actually protects your boiler, these are the conditions to keep true. Walk this list on a schedule — most of it takes minutes.
Operating conditions (check every round)
- ☐ Storage section held at saturation temperature for its operating pressure (verify against the steam tables — this is the #1 driver of good deaeration).
- ☐ Continuous, visible vent plume — never throttle or valve the vent shut to “save steam.”
- ☐ Steam supply / pressure-reducing valve holding the DA at stable operating pressure.
- ☐ Correct water level maintained — not so high it floods the spray/tray section, not so low it uncovers the pump suction.
- ☐ Incoming make-up and returned condensate blended and preheated — avoid dumping big slugs of cold water that quench the storage section.
Mechanical / internals (inspect on a scheduled outage)
- ☐ Spray valve / nozzle producing a proper spray cone, not a dribble — clean or replace worn nozzles.
- ☐ Trays intact, seated, and clean — no collapsed, fouled, corroded, or displaced trays.
- ☐ Vent orifice/valve clear and correctly sized (not plugged, not oversized).
- ☐ Level control and transfer/feed pumps working; pump suction protected by proper temperature and level (guards against flashing and NPSH problems).
- ☐ Storage tank interior reasonably free of sludge and scale — clean out as needed.
- ☐ Relief/safety valve on the DA tested and current.
Water chemistry
- ☐ Oxygen scavenger fed downstream of the DA, into the feedwater line, metered to demand — let the DA do the mechanical removal first.
- ☐ Feedwater temperature high enough for the scavenger to react effectively.
Monitoring & records
- ☐ Dissolved oxygen tested and trended on a schedule (target single-digit ppb at the storage outlet).
- ☐ Storage temperature, pressure, and vent condition logged on rounds.
- ☐ Internal inspection scheduled (annually or at each outage) to lay eyes on nozzles and trays.
Keep this list true and the oxygen number takes care of itself. Let one item slip — a throttled vent, a cold storage section, a worn nozzle — and the dissolved-oxygen test will usually catch it before your boiler does.
The bottom line
Your deaerator’s whole purpose is to keep oxygen away from your boiler, and dissolved-oxygen testing is the only honest report card it gets. Sample it correctly, read the number as a diagnostic of the DA’s mechanical health, and trend it so a failing nozzle or tray shows up as a rising line on a chart — not as a corroded boiler two years from now. It’s one of the cheapest, highest-leverage habits in the entire boiler room.
At Trap Energy Solutions, dissolved-oxygen testing and trending is part of how we evaluate feedwater systems and protect your boiler — it’s built into our Steam System CRES Score assessment. If you’re seeing corrosion you can’t explain, or you just want to know whether your deaerator is actually earning its keep, ask us for a CRES Score. We’ll test it, trend it, and tell you honestly what your feedwater is doing.