≤ 7 µg/L (0.005 cm³/L)
Dissolved O₂ at outlet
Boiler auxiliaries · Feedwater system
Deaerator — thermal deaeration of boiler feedwater
A deaerator heats boiler feedwater to saturation with steam so dissolved oxygen falls below 7 µg/L (0.005 cm³/L), the level at which pitting corrosion of economizer and boiler tubes effectively stops. Arrow Energy Co., Ltd. supplies spray-tray and spray-scrubber deaerators, sized for 10–20 minutes of storage at MCR, for new boilers and retrofits.
0.5–1 % of deaerator capacity
Vent steam rate
10–20 min at MCR
Storage volume
0.2–1.5 barg
Typical operating pressure
01 — Function
What a deaerator does, and the two ways to build one
Heat the water to saturation; the gases have to leave.
A deaerator is a direct-contact heat exchanger: incoming feedwater — condensate return plus cold make-up — is mixed with low-pressure steam until it reaches saturation temperature at the vessel pressure, typically 105–130 °C at 0.2–1.5 barg. At saturation, the solubility of oxygen and carbon dioxide in water approaches zero (Henry's law: gas solubility falls as the partial pressure of the gas above the liquid falls, and above saturated water that partial pressure is essentially all steam). The stripped gases are carried out of a vent at the top; deaerated water drains to a storage tank below, from which the boiler feed pumps take suction. In the boiler island covered by our boiler pressure parts and auxiliaries scope, the deaerator sits between the condensate/make-up system upstream and the feed pumps and economizer downstream — and it is the last line of defence for both.
Two internals arrangements dominate:
- Spray-tray type. Water is sprayed through spring-loaded nozzles into the steam space for primary heating, then cascades over a stack of perforated trays where thin films and long residence time finish the gas stripping. Tolerant of load swings and off-design inlet temperature; the tray stack keeps working even if a spray nozzle degrades. The usual choice for boiler duty from about 20 t/h upward.
- Spray-scrubber type. Sprays do the primary heating, then the water is driven through a steam scrubber section where fresh pegging steam agitates it violently. Shorter vessel, lower cost, but stripping performance is more sensitive to turndown because scrubbing intensity falls with steam flow.
Both reach ≤ 7 µg/L dissolved O₂ at design conditions. The difference shows at 40–60 % load and during cold make-up surges, where tray units hold their guarantee window with more margin.
02 — Why 7 µg/L
Oxygen pitting: the failure the deaerator exists to prevent
Pitting is local, fast and invisible until a tube leaks.
Dissolved oxygen in hot feedwater does not corrode carbon steel uniformly — it pits. At a break in the magnetite layer, a small anodic site dissolves iron (Fe → Fe²⁺ + 2e⁻) while the surrounding oxygenated surface acts as a large cathode (O₂ + 2H₂O + 4e⁻ → 4OH⁻). The area ratio concentrates the entire corrosion current onto the pit, so penetration rates of 1–2 mm/year are possible in water that would barely mark the tube in the absence of oxygen. Pits perforate economizer inlet headers and tube bends first, because that is where the coldest, most oxygen-rich water meets the hottest metal.
The industry limit for pressure-deaerated feedwater is 7 µg/L (0.005 cm³/L) dissolved O₂ at the deaerator outlet. Below this level, oxygen scavenger dosing is a polishing step of a few cm³/h of solution; above it, the scavenger is consumed on bulk oxygen, residuals disappear, and pitting resumes. A deaerator drifting from 7 to 50 µg/L raises no alarm on its own — the damage is discovered two or three years later as economizer tube failures. This is why outlet dissolved-oxygen measurement, or at minimum a monthly grab-sample colourimetric test, belongs on the boiler instrumentation list.
03 — Steam & vent
Pegging steam and the vent that must never be shut
Stripping only works if the gases have an exit.
How is the deaerator vent rate set?
The vent orifice or valve is set to pass roughly 0.5–1 % of the deaerator's rated capacity as steam, continuously. That purge carries the stripped oxygen and CO₂ out of the vessel. A visible steam plume of about 0.5–1 m at the vent is the classic field check; no plume means gases are accumulating and outlet oxygen is climbing.
Pegging steam — the steam admitted below the water line or into the scrubbing section to hold vessel pressure — comes from a low-pressure header or a PRDS steam-conditioning station letting down from a higher-pressure main. Its control valve holds deaerator pressure constant; the heating duty is whatever the enthalpy balance demands as condensate return fraction and make-up temperature swing. Two operating errors dominate deaerator underperformance in the plants we audit: vents throttled shut to "save steam" (saving perhaps 0.3 t/h and buying an economizer retube), and pressure setpoint ramped down at part load, which drops saturation temperature and re-dissolves oxygen in the storage tank.
A deaerator vent passing 1 % of a 60 t/h plant's feedwater flow is about 0.6 t/h of steam. That is the operating cost of oxygen control. Recovering it with a vent condenser on the make-up line is legitimate; closing the vent is not.
04 — NPSH
Elevation: the deaerator protects the feed pumps
Saturated water gives the pump zero vapour-pressure margin — height is all you have.
Why does a deaerator sit at the top of the boiler house?
Because its water is at saturation, the vessel pressure and the water's vapour pressure cancel in the NPSH equation. The only net positive suction head available to the feed pump is the static height of the water column minus suction-pipe friction. Typical installations put the storage-tank floor 10–15 m above pump centreline.
The arithmetic, for water at saturation:
- NPSH available
- Z − hf (elevation minus friction; pressure and vapour-pressure terms cancel)
- Tank floor elevation Z
- 12.0 m above pump centreline
- Suction-line friction hf
- 1.5 m at design flow
- NPSH available
- 10.5 m
- Pump NPSH required
- 6.0 m (from pump curve)
- Margin
- 4.5 m
The margin is not luxury. When boiler demand rises suddenly, pegging-steam supply lags and deaerator pressure sags; the stored water is momentarily hotter than saturation at the new, lower pressure, and flashes in the downcomer. Each 0.1 bar of transient undershoot at ~110 °C costs roughly 1 m of effective suction head. A 4–5 m margin absorbs the excursion; a 1 m margin cavitates the pump — audible as gravel in the casing, measurable as eroded impeller vanes at the next overhaul. In retrofit work, where structure height is fixed, we recover margin instead by oversizing the suction line (halving velocity cuts friction to a quarter) and by slowing the pressure-control response so undershoot stays within about 0.05 bar.
05 — Sizing & control
Storage sizing and the level–pressure control interaction
Two loops share one vessel; tune them apart.
Storage volume is sized for 10–20 minutes of feedwater at maximum continuous rating. For a 60 t/h bagasse-fired boiler that is 10–20 m³ of effective (not geometric) volume between low-level trip and overflow. The lower end suits plants with steady condensate return; sugar mills with batch pans and fluctuating return favour 15–20 minutes. Undersized storage converts every condensate upset into a make-up surge of cold, aerated water that the heating section must absorb — and outlet oxygen shows it.
Two control loops act on the vessel and they interact: the level loop admits make-up water (cold, oxygenated), and the pressure loop admits pegging steam. A slug of make-up depresses temperature, which condenses steam, which drops pressure, which opens the steam valve — so a fast level loop drives the pressure loop into oscillation. Standard practice is to tune level slow (it is an inventory, not a quality variable; ±20 % swing over minutes is acceptable) and pressure fast, and to bring make-up in through the vent condenser or spray section rather than dumping it into the storage tank. On retrofits we frequently find both loops tuned fast, fighting each other on a 2–4 minute cycle that shows up as feed-pump suction pressure oscillation.
| Parameter | Spray-tray | Spray-scrubber |
|---|---|---|
| Outlet dissolved O₂ (design point) | ≤ 7 µg/L | ≤ 7 µg/L |
| O₂ performance at 40–60 % load | Holds with margin | Degrades; needs pegging-steam bias |
| Operating pressure, typical | 0.2–1.5 barg | 0.2–1.5 barg |
| Turndown on inlet water temperature | Wide (trays insensitive) | Narrower (scrubbing intensity falls) |
| Vent rate | 0.5–1 % of capacity | 0.5–1 % of capacity |
| Vessel height for equal duty | Taller (tray stack) | Shorter |
| Storage tank sizing | 10–20 min at MCR | 10–20 min at MCR |
06 — Integrity
Weld cracking, inspection, and retrofit versus new supply
The vessel that protects the boiler needs its own inspection regime.
Deaerator shells operate barely above atmospheric pressure, which tempts owners to treat them as tanks. They are pressure vessels with a documented cracking history: every make-up surge and every start-up quenches the shell locally, and the resulting thermal-fatigue stress, working on weld residual stress in an oxygenated water environment, initiates corrosion-fatigue cracks at and adjacent to circumferential welds — typically at the head-to-shell seam and below the water line. Failures in the 1980s were severe enough that the National Board Inspection Code (NBIC, NB-23) now recommends periodic internal inspection of deaerators with wet fluorescent magnetic-particle examination (WFMT) of internal weld seams. Cracks grow over years, so an inspection tied to the major-outage cycle — every 2–3 years for a sugar-mill unit that cycles seasonally — catches them at repairable depth. Repair is grinding and controlled rewelding to the original code of construction (ASME Section VIII Division 1 for the vessel shell in most export supply), followed by re-examination; this falls within our pressure-part fabrication and repair scope.
On the supply side, the decision is retrofit versus new vessel. Internals — spray nozzles, tray stacks, scrubber sections — wear and corrode independently of the shell, and a shell that passes WFMT and thickness survey will usually accept a new internals set: restored O₂ performance for roughly a third of the cost of a new vessel, executed inside a normal outage. We re-engineer internals from a dimensional survey where the OEM is gone, the same approach as our boiler spares reverse-engineering practice. A new deaerator is the right call when the shell shows crack colonies, when storage is undersized for the current boiler rating, or when a capacity increase moves the whole feedwater train — in which case elevation, suction piping and pump NPSH are re-checked together, not the vessel alone. Guaranteed outlet oxygen figures are stated per project after the technical assessment, on a stated basis (µg/L, load range, make-up fraction and temperature).
FAQ
Engineering questions, answered
What dissolved oxygen level should a deaerator achieve?
A properly working pressure deaerator delivers feedwater with 7 µg/L dissolved oxygen or less, equivalent to 0.005 cm³/L. Above that level, oxygen pits carbon-steel economizer and boiler tubes at the water line and under deposits. Chemical scavenger then only polishes the residual, not the bulk load.
Why does a deaerator vent steam continuously?
Stripped oxygen and carbon dioxide must leave the vessel, and they only leave carried in a purge of steam. The vent is typically set to pass 0.5–1 % of deaerator capacity. Throttling the vent to save steam traps the gases, and outlet oxygen climbs back above 7 µg/L within hours.
How high must a deaerator be installed above the boiler feed pump?
The stored water is at saturation, so tank pressure and vapour pressure cancel in the NPSH calculation. Available NPSH is static elevation minus suction-line friction. With a pump requiring 6 m NPSH and about 1.5 m friction, the tank floor needs roughly 10–12 m elevation to keep a safe margin during pressure transients.
How big should the deaerator storage tank be?
Standard practice is 10–20 minutes of water storage at maximum continuous rating. For a 60 t/h boiler that means roughly 10–20 m³ of effective volume. The storage rides through condensate-return upsets and gives operators time to act on a feedwater failure before drum level forces a trip.
Why do deaerators need periodic internal inspection?
Deaerator shells crack at and near welds from thermal cycling and oxygen attack, a failure pattern documented widely enough that the National Board Inspection Code calls for periodic internal inspection with wet fluorescent MT of weld seams. Cracks grow slowly, so inspection on outage cycles catches them before leakage.
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