A deaerator is the pressure vessel in a steam plant that heats boiler feedwater with steam to its boiling point, strips out the dissolved oxygen and carbon dioxide that would corrode the boiler and condensate system, and stores the treated water for the feed pumps; Paul Industries installs, pipes, starts up, inspects, repairs and replaces deaerators. We do not manufacture them. We set the vessel, run the steam, water, vent, overflow and pump suction piping, commission the controls, and keep the unit working through its life. This page explains how it works, the types, pressure and sizing, the piping and controls, cracking and inspection, and how to troubleshoot one that is not doing its job.
What is a deaerator?
It is a mechanical oxygen removal system for boiler feedwater, built as a pressure vessel with three parts: a deaeration section, where incoming water meets steam; a deaerator storage tank beneath it, which holds the deaerated water at saturation; and a vent that releases the stripped gases. Plant people call the whole unit the DA, and the boiler deaerator usually sits on a structural stand above the boiler feed pumps. It does three jobs at once: removing corrosive gases, preheating feedwater so the boiler sees less thermal shock, and mixing returned condensate with treated makeup in one place.
Mechanical deaeration of water is cheaper and more reliable than removing the same oxygen chemically. A scavenger removes oxygen by reacting with it, at a chemical cost that rises with every part per million, while the DA does the bulk of the work with steam the plant already makes. Boiler feed water deaeration is therefore one stage in the treatment train rather than a substitute for it; the rest of the train, from softening and RO to internal chemistry and blowdown, is covered on our boiler water treatment page.
How does a deaerator work?
It heats the water to the saturation temperature for the vessel pressure, where gases are almost insoluble, and scrubs the released gas away with steam. The deaerator working principle rests on Henry’s law: the amount of a gas that stays dissolved is proportional to its partial pressure above the water. At saturation the space above the water is nearly all steam, so the partial pressures of oxygen and carbon dioxide fall close to zero and the gases come out of solution.
Heating alone is not enough. The water has to be broken into thin films or small droplets and agitated, so the gas has a short path to escape, and the released gas has to be carried away before it can redissolve. Incoming water is sprayed or cascaded through the deaeration section while steam flows through it, parallel, across or counter to the water. Most of the steam condenses as it heats the water; the US Department of Energy notes that a small fraction, usually 5 to 14 percent, has to be vented to carry the stripped gases out. The deaerated water falls into the storage section under a blanket of steam that keeps air away from it.
How low does deaeration take oxygen?
A pressurized unit in good order removes essentially all the dissolved carbon dioxide and takes oxygen down to single-digit parts per billion. Equipment specifications for tray units commonly carry a guarantee of 0.005 cc per liter, about 7 parts per billion, with no measurable carbon dioxide. DOE guidance is that most boilers above 200 psi need 5 ppb or lower, that low-pressure boilers can tolerate up to 43 ppb, and that holding 5 ppb in any boiler extends equipment life at little or no cost. For comparison, a vented atmospheric feed tank at 185 to 195 F leaves around 2 parts per million, several hundred times more.
The deaerator oxygen level that matters is the one at the storage outlet, under normal load and at the worst conditions: coldest makeup, highest makeup fraction and fastest load change. The small residual is then removed by a scavenger fed downstream. That deaerator chemical dosing point is usually in the storage section or the pump suction, so the scavenger has time to react before the water reaches the economizer.
Types of deaerators
The types differ in how they bring water and steam into contact, and the choice comes down to load range, headroom, available steam and how low the oxygen must go.
| Type | How water meets steam | Strengths | Watch for |
|---|---|---|---|
| Tray type deaerator | Water cascades over stacked perforated or slotted trays while steam rises through them | Wide load range; strong oxygen removal; common in larger industrial and utility plants | Taller vessel; trays can corrode, shift or plug and must be inspected |
| Spray type deaerator | Spring-loaded nozzles spray water into a steam space, then a scrubbing section finishes the stripping | Compact, fewer internals, common in packaged industrial units | Turndown limited by nozzle pressure; nozzles wear and stick |
| Spray tray type deaerator | Spray section preheats and strips most gas, a tray section finishes it | Combines spray heating with tray polishing | More internals to inspect and maintain |
| Vacuum deaerator | Packed column or spray under vacuum from an ejector or vacuum pump, at lower temperature | Works where steam is scarce or water must stay cool | Any air leak into the vacuum side adds oxygen back |
| Atmospheric feed tank (not true deaeration) | Vented tank heated by sparged steam below boiling | Low cost, simple | Removes only part of the oxygen; heavy scavenger demand |
Deaerator trays are where a tray unit’s performance is won or lost: a collapsed or plugged tray lets water channel through without being stripped. Vacuum units are more common in process water service, such as beverage and brewing plants, than in boiler rooms.
Spray vs tray type deaerator
A tray unit holds performance over a wider load range; a spray unit is more compact and has fewer internals. Spray nozzles need water pressure to atomize, and one major supplier puts typical spray turndown at about 5 to 1, below which the nozzles do not break the water up properly. A tray stack keeps working at low flow because the water simply cascades more slowly. Plants with deep load swings, high makeup fractions or very low oxygen targets lean toward trays; smaller plants with steady loads and limited headroom often choose spray units. Either type, specified correctly and kept at steady pressure, can meet the oxygen limits most industrial boilers need.
Deaerator tank vs condensate tank
A condensate tank, or receiver, collects returned condensate at roughly atmospheric pressure and pumps it on; it does not remove dissolved gas. The deaerator tank is the pressurized storage section of the DA, holding water at saturation under a steam blanket. Many plants have both, and some add a third vessel: in a deaerator and surge tank arrangement, the surge tank takes intermittent condensate returns and supplies the DA at a steady rate, so slugs of returning condensate do not upset its pressure and level. A heated, vented feed tank is the cheaper alternative on small plants, at the cost of more oxygen and more chemical.
Deaerator operating pressure and temperature
Pressure and temperature are locked together: the water in the storage section sits at the saturation temperature for the vessel pressure. Most industrial units run at a few psig, and DOE gives an overall range of about 215 F to more than 350 F across industrial and power service.
| Vessel pressure | Approximate saturation temperature | Typical application |
|---|---|---|
| 0 psig | 212 F | Atmospheric reference; vented feed tanks run below this |
| 3 psig | 221 F | Low-pressure packaged deaerators |
| 5 psig | 227 F | Common industrial set point |
| 15 psig | 250 F | Higher-pressure industrial units |
| Higher, with turbine extraction steam | Above 300 F | Power plants and large cogeneration |
Steady pressure is what makes the unit work. Deaerator pressure control uses a dedicated pressure-reducing valve on the heating steam, modulating to hold set point, because a sudden rise in pressure from a burst of flash steam leaves the water below the new saturation temperature, and a sudden fall flashes it, and DOE notes that pressure spikes can put oxygen back into the stored water. Our saturated steam table gives the temperature for any set point. Larger plants supply the DA from turbine extraction or a low-pressure header and keep a reducing station from a higher header as backup; that deaerator pegging steam piping holds the vessel at pressure when the normal supply falls away at low load. We run those lines under ASME B31.1; see steam piping.
The vessel needs protection on both sides. A deaerator safety valve, sized for the maximum steam that can enter through a failed-open reducing valve, guards against overpressure. A deaerator vacuum breaker guards the other way: when the steam supply is shut off, the steam inside condenses and pulls a vacuum that the shell may not be designed for. The deaerator design pressure is set above the operating point with margin for those upsets.
Sizing and steam consumption
Deaerator sizing starts from the maximum feedwater flow, which is the boiler steam rate plus blowdown, at the lowest expected inlet temperature and the highest makeup fraction, because cold, oxygen-rich makeup sets both the heating and the venting load. Storage is specified in minutes of feedwater flow at full load, commonly at least 10 minutes and up to about 20, enough to ride through an interruption in makeup or condensate.
The deaerator steam consumption calculation is a heat balance, made at the coldest expected inlet water. Steam must heat the incoming water to saturation, plus supply the vented steam, less any flash steam brought in by hot condensate or failed traps. A worked example: water entering at 150 F and leaving at 227 F gains about 77 Btu per pound. Steam from a 50 psig header gives up about 984 Btu per pound as it condenses into water at 227 F, so the heating steam is roughly 77 divided by 984, about 8 percent of the water flow, before the vent. Raise the condensate return or its temperature and the steam demand falls; past a certain point the unit can be overheated by flash steam alone, and surplus flash has to be handled elsewhere.
The deaerator vent rate is set by the manufacturer, and one major supplier puts typical recommendations at 0.5 to 2 pounds of steam and gas mixture per 1,000 pounds per hour of capacity. The deaerator vent is usually a fixed orifice or a valve locked part open, and it should always show a steady plume. A deaerator vent condenser, where fitted, uses incoming cold water to condense most of that steam and recover its heat while still letting the gases out.
Piping, controls and installation
A complete deaerator system is more than the vessel: it is the makeup and condensate inlets, heating steam supply and reducing station, vent, overflow, level and pressure controls, safety and vacuum relief, sampling, chemical feed, and the suction piping to the boiler feed pumps.
- Level. Deaerator level control modulates the makeup valve to hold storage level, with high and low alarms and a low-level trip for the feed pumps. Modulating control is preferred over on and off, because slugs of cold makeup upset the pressure. A deaerator overflow trap or overflow valve drains excess water from a high level without blowing steam to the drain.
- Inlets. Condensate return to deaerator inlets enters with makeup through the spray or tray distribution, never as a jet straight into the storage section. Hot, high-pressure condensate that flashes is brought in where its flash steam can do useful heating. On spray units each deaerator spray valve needs adequate inlet water pressure to atomize properly.
- Feed pump suction. The stored water is at its boiling point, so the DA is elevated to give the feed pumps the net positive suction head they need, and the suction line is short, generously sized and free of high points. Our NPSH calculator checks the margin. See also steam traps for the trap and condensate side.
- Setting the vessel. A DA with full storage is heavy and sits high, so the stand, anchors and rigging plan come first; our millwright and rigging crews set the vessel before the pipefitters connect it.
Every pipe connected to the vessel is pressure tested before startup; see hydrostatic testing. Startup then proves the controls: pressure held through load swings, level control stable, vent plume steady, and outlet oxygen measured.
How are deaerators inspected for cracking?
Deaerator cracking is the main structural risk, and internal inspection of the welds is how it is found. After catastrophic storage vessel failures in 1982 and 1983, some of them fatal, industry inspections found cracking in 30 to 50 percent of the industrial deaerators examined. The cracks concentrate at circumferential seams, especially where they meet longitudinal seams, and at nozzle welds that see thermal shock. The National Board attributes them to residual welding stress, thermal and pressure cycling, corrosion pits and corrosion fatigue, with slightly hardened heat-affected zones and weld defects contributing. NACE SP0590, the standard practice for preventing, detecting and correcting this cracking, grew out of that work.
| Inspection finding | Suggested interval to next internal inspection |
|---|---|
| New vessel | First inspection within one to two years of service |
| Cracks found and repaired | One year or less |
| Cracks found, not repaired | One to two years |
| No cracks, no repairs | Three to five years |
A proper deaerator inspection is made from inside the vessel: the welds are cleaned to bare metal, examined visually with attention to the seam intersections, and then tested over their full length with wet fluorescent magnetic particle examination by qualified personnel. Wall thickness is measured where corrosion is suspected. Deaerator tank corrosion shows up as oxygen pitting near the water line and inlets, and as flow-accelerated wall loss near steam nozzles and high-velocity regions. The deaerator internals are checked at the same time: trays, spray nozzles and their springs, baffles, vent connections and the vent condenser. Repairs to the pressure boundary are made under the jurisdiction’s rules, usually the National Board Inspection Code, and are followed by examination and, where required, a pressure test.
Operation is half of prevention. The same guidance warns against restricted steam supply with cold feedwater, running above or below the unit’s effective capacity, sharp pressure and temperature swings, water hammer and high oxygen, all of which are control and piping problems as much as vessel problems.
Deaerator problems and troubleshooting
Most faults show up as high outlet oxygen, unstable pressure or level, or a cold storage section, and most have a mechanical cause.
| Symptom | Likely causes | What to check |
|---|---|---|
| High outlet oxygen | Vent throttled, trays damaged or plugged, spray nozzles stuck or worn, unit overloaded, air leaking in at pump seals or suction joints | Vent plume, outlet temperature against saturation, internals, load against rating |
| Storage water below saturation temperature | Steam supply undersized or valve not opening, cold makeup surges | Reducing valve capacity and tuning, makeup control |
| Pressure swings | Flash steam slugs from condensate, poorly tuned pressure control, failed-open traps feeding the vessel | Condensate inlet arrangement, controller tuning, trap survey |
| Level hunting or overflowing | On-off makeup control, sticking valve, intermittent condensate returns | Modulating control, valve condition, surge capacity |
| Feed pump cavitation | Low level, pressure drops, restricted suction, insufficient elevation | Suction strainer, NPSH margin, pressure stability |
| Water hammer in the vessel or piping | Cold water meeting steam in a flooded line, wet steam supply | Inlet piping, drains on the steam supply |
Deaerator performance testing confirms the result: dissolved oxygen at the storage outlet by a low-range colorimetric test or an online analyzer, the storage temperature compared with saturation temperature at the measured pressure, and a check that the vent is open. DOE recommends continuous dissolved oxygen monitoring, because it catches operating practices that hurt removal before they damage the boiler. Routine deaerator maintenance is simple but often skipped: keep the vent open, keep pressure and level instruments calibrated, test the safety valve on schedule, inspect spray valves and trays at every outage, and look after the traps and condensate system that feed the vessel.
Repair, retrofit and replacement
Most units are repaired, not replaced: trays, nozzles, valves, vent condensers and controls can all be renewed inside the existing shell. Deaerator tank replacement becomes the better choice when inspection finds extensive cracking or wall loss, when the plant’s steam load has grown past the vessel’s capacity, or when an old feed tank is upgraded to a pressurized unit. Replacement is planned around the boiler outage: the new vessel and piping are prefabricated, the old unit is rigged out and the new one set, piped, tested and started up in one window. During the outage a temporary feed arrangement keeps the boilers running where the plant cannot shut down.
What Paul Industries does
Paul Industries installs, pipes, starts up, inspects and services deaerators for industrial steam plants: setting new and replacement vessels, heating steam supply and reducing stations, makeup, condensate, vent, overflow and feed pump suction piping, level and pressure controls, safety and vacuum relief, sampling and chemical feed points, internal repairs to trays, spray valves and vent condensers, and startup and performance checks. We work alongside the boiler and chemical program under our plant utilities and plant water treatment systems work, maintain deaerators under preventive maintenance, and respond to failures through emergency repair. We supply the feedwater side for pharmaceutical clean steam systems and refinery steam plants on our oil and gas page. Request a quote for a deaerator installation, inspection, repair or replacement.
Standards referenced: ASME BPVC Section VIII Division 1 · NACE SP0590 · National Board Inspection Code · ASME B31.1
Frequently asked questions
What is a DA tank for a boiler?
DA stands for deaerator. It is the pressurized storage section of the deaerator, holding feedwater at saturation temperature under a steam blanket, directly above the suction of the boiler feed pumps. In everyday use, people call the whole vessel the da tank.
What does a deaerator do?
It removes dissolved oxygen and carbon dioxide from boiler feedwater by heating it with steam to saturation and venting the released gases. It also preheats the feedwater, mixes condensate with makeup, and stores water at the feed pump suction.
Where is the deaerator located in a boiler room?
High above the boiler feed pumps, usually on a steel stand or an upper floor. Because the stored water is at its boiling point, the height gives the pumps the static head they need to avoid cavitation.
How much steam should a deaerator release through its vent?
Enough to carry out the stripped gases continuously, which is a small fraction of the heating steam. Manufacturers set the rate; in the field the check is a steady, visible plume, confirmed by outlet oxygen. Closing the vent to save steam is the most common cause of poor performance.
Is a deaerator tank a pressure vessel?
Yes. Pressurized deaerators are generally built and stamped as pressure vessels under ASME Section VIII Division 1, so they carry a nameplate design pressure and temperature, need overpressure protection, and are inspected and repaired under the jurisdiction's pressure vessel rules.
When is a pressurized deaerator worth it over a heated feed tank?
As steam rate, boiler pressure or makeup fraction rises. A vented feed tank leaves oxygen in the parts per million range, so scavenger cost and corrosion risk climb with load; a pressurized unit takes oxygen to parts per billion and usually repays itself in chemical savings and fewer feedwater line and economizer failures.
Does a deaerator eliminate the need for an oxygen scavenger?
No. A good deaerator leaves only a few parts per billion of oxygen, and a small scavenger feed removes that residual and covers upsets. What the deaerator does is cut the scavenger demand to a fraction of what it would otherwise be.
How do you check that a deaerator is working?
Measure dissolved oxygen at the storage outlet, compare the storage water temperature with the saturation temperature for the vessel pressure, and look for a steady vent plume. A storage temperature below saturation or a missing plume points to a problem before the oxygen test confirms it.
What causes deaerator pressure swings?
Usually flash steam from slugs of hot condensate, failed-open steam traps feeding the vessel, an undersized or poorly tuned steam reducing valve, or on-off makeup control dumping cold water in. Each swing can put oxygen back into the stored water.
Why does a deaerator need a vacuum breaker?
When the steam supply is shut off, the steam in the vessel condenses and pulls a vacuum. A vessel not rated for full vacuum can buckle, so a vacuum breaker admits air to relieve it, and the unit is then vented and restarted before feedwater is used.
What goes wrong with the trays in a tray unit?
Trays can corrode, crack, come loose from their supports or plug with scale and debris. Water then channels through without being stripped and outlet oxygen climbs. Trays are inspected at each internal inspection and replaced individually or as a stack.
How often should a deaerator be inspected?
Industry guidance is a first internal inspection within one to two years of service, then every one to five years depending on what was found, with the shortest intervals where cracks have been repaired. Jurisdictional pressure vessel inspections run alongside that schedule.
Should all condensate go to the deaerator?
Clean condensate should, because it is hot, low in dissolved solids and already treated. Condensate that may be contaminated by a process leak should pass through conductivity monitoring with an automatic dump first, and intermittent returns are best smoothed by a surge tank.
Do you manufacture deaerators?
No. We install, pipe, commission, inspect, repair and replace deaerators supplied by equipment manufacturers, and we can help specify the unit and coordinate its delivery and setting.
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