Boiler feedwater treatment, also written boiler feed water treatment, is the removal of hardness, alkalinity, dissolved solids and oxygen from the makeup water and returned condensate that feed a steam boiler, followed by chemical conditioning, so that a pressure deaerator delivers about 7 ppb (0.007 mg/L) of oxygen and an oxygen scavenger removes the rest. Which steps remove what, and which chemicals go into the feedwater line?
Feedwater chemicals are one group of water treatment chemicals used in steam systems, each with its own feed point and pressure limit. Seven steps treat makeup and condensate, sulfite or hydrazine finishes oxygen removal by pressure band, and ASME targets and 21 CFR 173.310 set the limits.
The key figures table lists four values from the Veolia Water Technologies Handbook of Industrial Water Treatment.
| Key figure | Meaning |
|---|---|
| 7 ppb (0.005 cm³/L) oxygen | pressure-deaerator outlet guarantee (ch. 10) |
| 7.88 mg/L sodium sulfite per mg/L oxygen | theoretical scavenger demand (ch. 11) |
| 1,000 psig (69 bar) | where sulfite gives way to hydrazine or organic scavengers (ch. 11) |
| 0.300 mg/L to not detectable | ASME feedwater hardness as CaCO3, low to high pressure (ch. 13) |
Every value on this page is checked against primary sources; see our methodology and fact-checking process.
What Is Boiler Feedwater Treatment and Why Is It Necessary?#
Boiler feedwater treatment is necessary because a boiler concentrates everything its feedwater carries: hardness and silica deposit on tubes, oxygen and carbon dioxide corrode the feedwater train, and nearly complete oxygen removal needs both deaeration and a chemical scavenger (Veolia handbook, ch. 10).
Water at 15.6 °C (60 °F) and 1 atm holds about 8 ppm (mg/L) of oxygen (same handbook, ch. 11). What exactly is in the feedwater?
What Is Boiler Feedwater Made Of? Makeup Water and Returned Condensate#
Boiler feedwater is the blend of returned condensate and treated makeup water that the feed pumps deliver to the boiler, and its flow equals the steam produced plus the blowdown (handbook ch. 13). Feedwater has 2 sources.
- Treated makeup water: steps 1 to 4.
- Returned condensate: step 5.
What Does Untreated Feedwater Do? Scale, Oxygen Corrosion and Carryover#
Untreated feedwater causes 3 failures: scale on heated surfaces, oxygen corrosion in the feedwater train and economizer, and carryover of solids into the steam.
- Scale: calcium, magnesium, silica, iron.
- Oxygen corrosion: economizer and feed-line pitting.
- Carryover: solids in steam.
Solubility and saturation are explained under scale formation.
Solids in steam: boiler carryover, foaming and priming.
What Are the 7 Steps of Boiler Feedwater Treatment?#
Boiler feedwater treatment runs in 7 steps along the water path: clarification, softening, dealkalization or RO, demineralization, condensate polishing, deaeration and chemical conditioning.
The table follows feedwater from the makeup source to the boiler inlet.
| Step | What it removes or adds | Typical outlet or target | Source |
|---|---|---|---|
| 1. Clarification and filtration | suspended solids, color, iron | iron stabilized or removed ahead of softeners | DuPont AmberLite manual |
| 2. Softening (sodium zeolite, lime, hot process) | calcium, magnesium; silica (hot process with magnesium oxide) | below 2 ppm (handbook) or below 1 ppm as CaCO3 (counter-current, below about 5,000 ppm TDS); salt 90 to 240 g/L (5.6 to 15 lb/ft³) as 10 % brine, co-flow; hot process at 108 to 116 °C (227 to 240 °F) about 0.5 gr/gal (about 8.6 mg/L as CaCO3, calculated) | Veolia handbook ch. 7, 8; DuPont AmberLite manual |
| 3. Dealkalization or reverse osmosis | bicarbonate alkalinity (CO2 source); dissolved salts | about 90 % alkalinity removed (chloride-anion); degasser about 5 ppm or below 1 ppm CO2 (vacuum); RO rejects 95 to more than 99 % of salts | Veolia handbook ch. 8; DuPont AmberLite and FilmTec manuals |
| 4. Demineralization and mixed bed | all ions, silica | below 1 µS/cm and 10 to 20 ppb SiO2 (two-bed, counter-current); below 0.10 µS/cm and below 10 ppb SiO2 (mixed bed) | DuPont AmberLite manual and mixed-bed TechFact |
| 5. Condensate return and polishing | corrosion products, in-leakage | mixed-bed polishers above 50 % cation resin, 30 to 120 BV/h | DuPont mixed-bed TechFact |
| 6. Deaeration | dissolved oxygen, CO2 | 7 ppb (pressure deaerator guarantee); 330 to 650 ppb (vacuum); about 2 mg/L (atmospheric feedtank) | Veolia handbook ch. 10; Spirax Sarco |
| 7. Chemical conditioning | remaining oxygen; low pH; residual hardness | scavenger on oxygen demand; pH 9.2 to 9.6 (carbon steel); chelant only below 1.0 ppm hardness | Veolia handbook ch. 11, 12, 35 |
1. Clarification with Coagulants and Filtration#
Clarification and filtration remove suspended solids, color and iron from makeup water, because iron must be stabilized with polyphosphate or removed by oxidation and filtration ahead of a softener (DuPont AmberLite Water Conditioning Manual).
Alum, ferric salts and polymers are compared under coagulants for water treatment.
2. Softening by Ion Exchange, Lime or Hot Process#
Sodium zeolite softening exchanges calcium and magnesium for sodium and delivers below 2 ppm hardness (Handbook of Industrial Water Treatment, ch. 8), or below 1 ppm as CaCO3 with counter-current regeneration (step table above).
Resin types and capacities: ion exchange resins.
Regenerant doses: water softening resin and softener salt.
3. Dealkalization and Reverse Osmosis#
Dealkalization removes the bicarbonate that would otherwise leave as carbon dioxide: each ppm of bicarbonate alkalinity as CaCO3 releases 0.79 ppm CO2 into the steam, so 100 ppm becomes 79 ppm CO2 (handbook ch. 19). Removing about 90 % leaves 10 ppm, about 7.9 ppm CO2 (calculated).
Chloride-anion and weak acid cation units: dealkalization by ion exchange. RO, like a dealkalizer, cuts the CO2 load that limits amine feed where steam contacts food.
Recovery and staging: how reverse osmosis works.
4. Demineralization and Mixed-Bed Polishing#
Demineralization removes almost all dissolved ions in a cation, degasser and anion train, reaching below 1 µS/cm and 10 to 20 ppb silica with counter-current regeneration, and a mixed bed polishes the water to below 0.10 µS/cm and below 10 ppb silica. IAPWS TGD3-10(2015) asks utility makeup for conductivity and cation conductivity below 0.1 µS/cm and silica below 10 µg/kg.
Train configurations: demineralization by ion exchange.
Polisher regeneration: mixed bed ion exchange.
5. Condensate Return and Condensate Polishing#
Returned condensate is the purest feedwater source, and mixed-bed condensate polishers run with more than 50 % cation resin at 30 to 120 bed volumes per hour to strip the corrosion products and in-leakage it picks up.
Return-line pH control belongs to condensate system treatment.
6. Deaeration: Mechanical Oxygen Removal#
A pressure deaerator strips dissolved oxygen by heating the feedwater with steam and venting the gas, to a vendor-guaranteed 0.005 cm³/L (7 ppb), while a vacuum deaerator leaves 0.25 to 0.50 cm³/L (330 to 650 ppb) (handbook ch. 10). About 97 to 98 % of the oxygen leaves in the spray section.
The table compares outlet oxygen by deaeration option.
| Deaeration option | Operating point | Outlet oxygen | Source |
|---|---|---|---|
| Pressure deaerator | steam-heated, vented; tray, spray, jet-atomizing, film or bubbling design | 0.005 cm³/L (7 ppb), vendor guarantee | Veolia handbook ch. 10 |
| Pressurised deaerator | about 0.2 bar g (3 psig), 105 °C (221 °F); 10 to 20 min storage; preferred above 20 bar g (290 psig) | target below 20 ppb | Spirax Sarco |
| Vacuum deaerator | below atmospheric pressure | 0.25 to 0.50 cm³/L (330 to 650 ppb) | Veolia handbook ch. 10 |
| Atmospheric feedtank | 85 to 90 °C (185 to 194 °F) | about 2 mg/L | Spirax Sarco |
The 7 ppb guarantee and 20 ppb target are different bases. Tray, spray and vacuum designs: deaerators.
7. Chemical Conditioning of the Feedwater#
Chemical conditioning finishes what the deaerator starts: a scavenger removes the last oxygen, ammonia or an amine raises pH, and chelant, polymer or phosphate enters at its own feed point, fed in proportion to oxygen or hardness (handbook ch. 35).
Which Chemicals Are Used to Treat Boiler Feedwater?#
Boiler feedwater is treated with 3 groups of chemicals: oxygen scavengers such as sodium sulfite, hydrazine and N,N-diethylhydroxylamine (DEHA); pH chemicals such as ammonia, morpholine and cyclohexylamine; and internal-treatment chemicals such as EDTA and dispersant polymers.
The table lists each feedwater chemical with its pressure range, feed point and 21 CFR 173.310 status (eCFR as of 2026-10-01).
| Chemical | Function | Pressure range | Feed point | Theoretical ratio or control value | 21 CFR 173.310 status |
|---|---|---|---|---|---|
| Sodium sulfite | oxygen scavenger, catalyzed or uncatalyzed | below 1,000 psig (69 bar) | deaerator storage section or drop leg; never attemperation water | 7.88 mg/L per mg/L O2; practical about 10:1; boiler water above 20 ppm as SO3 | listed (c), no numeric limit |
| Sodium bisulfite | catalyzed bisulfite scavenger | below 1,000 psig (69 bar) | as sulfite | not stated | not listed |
| Hydrazine | scavenger, passivator, AVT(R) reducing agent | 1,000 psig (69 bar) and above | feedwater or condensate; acceptable in attemperation water | 1.0 theoretical; 1.5 to 2.0 practical; 0.05 to 0.1 mg/L excess | (d): zero in steam |
| Carbohydrazide | volatile scavenger, passivator | medium and high pressure | feedwater or deaerator storage | 1.41 (calculated) | not listed |
| DEHA | volatile scavenger, passivator | medium and high pressure | feedwater or condensate | 1.24 (calculated) | not listed |
| Sodium erythorbate | scavenger, passivator | low and medium pressure | feedwater | 12.4 (11.0 as erythorbic acid) (calculated) | not listed |
| Hydroquinone | scavenger, catalyst | start-up and shutdown | feedwater or condensate | 6.9 (calculated) | not listed |
| MEKO | volatile scavenger | hydrazine alternative | feedwater | slowest initial oxygen reaction (Sipilä et al. 2024 ranking) | not listed |
| Tannin (quebracho extract) | scavenger, sludge conditioner | low pressure | feedwater | not stated | listed (c), no numeric limit |
| Ammonia | volatile alkalizing agent | utility and HRSG cycles (AVT) | condensate or feedwater | IAPWS AVT(O) economizer-inlet pH 9.2 to 9.8 | ammonium hydroxide: 21 CFR 184.1139 |
| Neutralizing amines: morpholine, cyclohexylamine, DEAE, MOPA | condensate and feedwater pH | industrial | deaerator storage section, feedwater line or boiler; not the steam header | morpholine 2.0, cyclohexylamine 2.3, DEAE 2.6 ppm per ppm CO2 (Veolia handbook ch. 19) | in steam: morpholine 10, cyclohexylamine 10, DEAE 15 ppm; MOPA not listed |
| EDTA | chelant (internal treatment) | industrial chelant and polymer programs | beyond the feed pumps through a stainless-steel quill; never the drum | 3.80 mg/L Na4EDTA (active) per mg/L hardness as CaCO3 | tetrasodium EDTA listed (c), no limitation |
| Dispersant polymers (polyacrylate, polymaleic acid, AA/AMPS, phosphinocarboxylic acid), see scale inhibitors | dispersant, weak chelant | polymer-only where feedwater hardness is consistently very low | feedwater | not applicable | in feedwater: polymaleic acid 1 ppm as acid, AA/AMPS 20 ppm active, phosphinocarboxylic copolymer 1.5 ppm; sodium polyacrylate no limitation |
Theoretical ratios are calculated (100 % basis). Chemicals not listed in 21 CFR 173.310 carry no claim for food-contact steam.
Oxygen Scavengers: Sulfite Below 1,000 psig, Hydrazine and Organic Scavengers Above#
Sodium sulfite and catalyzed sodium bisulfite are the oxygen scavengers below 1,000 psig (69 bar); hydrazine or organic scavengers take over at 1,000 psig and above, where sulfite decomposes to sulfur dioxide and hydrogen sulfide (Veolia handbook, ch. 11). Catalyzed sulfite (usually cobalt) removed all oxygen in 25 seconds, uncatalyzed less than half.
Selection by pressure: oxygen scavengers for boiler water. Sipilä et al. (Corrosion Science 240, 112476, 2024) quote the speed order carbohydrazide > hydrazine ≈ erythorbic acid > DEHA >> MEKO.
Neutralizing Amines and Ammonia for Feedwater pH#
Ammonia and neutralizing amines raise feedwater pH toward 9.2 to 9.6 for carbon steel, or 8.8 to 9.2 with copper alloys (handbook ch. 11), and amines go to the deaerator storage, feedwater line or boiler, not the steam header (ch. 19).
Distribution ratios and basicity: neutralizing amines for condensate treatment. Hydro-Logic advises industrial plants not to feed ammonia into the deaerator tank, where the vent loses it.
Utility cycles follow IAPWS TGD3-10(2015): economizer-inlet pH 9.2 to 9.8 under all-volatile treatment (AVT).
Chelants, Polymers and Phosphate: Internal Treatment Fed with the Feedwater#
Chelant and polymer programs are fed into the feedwater beyond the feed pumps through a stainless-steel injection nozzle, never into the drum, and a chelant program keeps the boiler deposit-free only while feedwater hardness stays below 1.0 ppm (handbook ch. 12 and 35). Overfeed dissolves protective magnetite.
EDTA, NTA and newer chelants: chelating agents in water treatment.
Program control: all-polymer and chelant boiler programs. Orthophosphate goes to the drum, because it deposits calcium phosphate in a hard feed line.
Drum dosing and hideout: phosphate treatment programs for boilers.
Feed Points and Compatibility Rules for Feedwater Chemicals#
Five feed rules keep feedwater chemicals from reacting in the wrong place (handbook ch. 11 and 35). The five rules are listed below.
- Sulfite: deaerator storage or drop leg via an injection quill; never attemperation water.
- Chelant: beyond the feed pumps.
- Orthophosphate: to the drum.
- Hydrazine: alone (incompatible with organics, amines, nitrates).
- Catalyzed sulfite: alone, diluted with condensate or demineralized water.
Tank and pump materials: chemical feed systems.
Mixtures to avoid: water treatment chemical compatibility.
How Much Oxygen Scavenger? Dose Basis and a Worked Example#
Oxygen-scavenger feed is set on feedwater oxygen plus system demand, while chelant and phosphate feed follow hardness (handbook ch. 35), so the deaerator outlet decides most of the scavenger demand.
The worked example below calculates sulfite feed for two deaeration cases.
| Case | Feedwater O2 | Sodium sulfite, mg/L (theoretical 7.88:1) | Feed, kg/d (lb/day), theoretical | Feed at practical 10:1 |
|---|---|---|---|---|
| Atmospheric feedtank | about 2 mg/L | 15.8 | 7.6 (16.7) | 9.6 kg/d (21.2 lb/day) |
| Pressure deaerator | 7 ppb (0.007 mg/L) | 0.055 | 0.026 (0.058) | 0.034 kg/d (0.074 lb/day) |
Inputs: 20 m³/h (88 gpm, 0.127 MGD), 100 % Na2SO3; kg/d = mg/L × m³/d / 1,000; lb/day = mg/L × MGD × 8.34; no residual.
Other flows and products: dosage and feed rate calculator.
Nearly 95 % of feed systems use metering pumps: chemical dosing pumps.
How Is Boiler Feedwater Quality Monitored and Controlled?#
Boiler feedwater is monitored against pressure-based targets for oxygen, iron, copper, hardness and pH, tested at least once per shift, and checked at the deaerator itself (Veolia handbook, ch. 11 and 13).
Feedwater Targets by Pressure (ASME Consensus)#
ASME consensus feedwater targets tighten with pressure: dissolved oxygen before the scavenger falls from below 0.040 mg/L (40 ppb) up to 450 psig (31.0 bar) to below 0.007 mg/L (7 ppb) above it, and hardness falls from 0.300 mg/L as CaCO3 to not detectable above 1,000 psig (69 bar) (handbook ch. 13).
ASME targets for water-tube, drum-type boilers with a deaerator are listed by pressure.
| Drum pressure, psig (bar) | DO before scavenger, mg/L | Iron, mg/L | Copper, mg/L | Hardness, mg/L as CaCO3 | Feedwater pH |
|---|---|---|---|---|---|
| 0-300 (0-20.7) | < 0.040 | 0.100 | 0.050 | 0.300 | 7.5-10.0 |
| 301-450 (20.8-31.0) | < 0.040 | 0.050 | 0.025 | 0.300 | 7.5-10.0 |
| 451-600 (31.1-41.4) | < 0.007 | 0.030 | 0.020 | 0.200 | 7.5-10.0 |
| 601-750 (41.5-51.7) | < 0.007 | 0.025 | 0.020 | 0.200 | 7.5-10.0 |
| 751-900 (51.8-62.1) | < 0.007 | 0.020 | 0.015 | 0.100 | 7.5-10.0 |
| 901-1,000 (62.2-68.9) | < 0.007 | 0.020 | 0.015 | 0.100 | 8.5-9.5 |
| 1,001-1,500 (69.0-103.4) | < 0.007 | 0.010 | 0.010 | not detectable | 9.0-9.6 |
| 1,501-2,000 (103.5-137.9) | < 0.007 | 0.010 | 0.010 | not detectable | 9.0-9.6 |
Above 473.2 kW/m² (150,000 Btu/ft²·h), use the next band.
Boiler-water columns: boiler water limits by pressure.
Feedwater Tests, Sampling Lines and Continuous Oxygen Measurement#
Feedwater is tested for hardness, iron, copper, oxygen and pH at least once per shift, ideally with a continuous oxygen meter, from sample lines flowing at 1.5 to 1.8 m/s (5 to 6 ft/s) and 800 to 1,000 mL/min (handbook ch. 11). Oxygen follows ASTM D888-18(2026); low-conductivity pH follows ASTM D5464-25 at 25 °C (77 °F).
Methods and units: dissolved oxygen.
Boiler-water tests follow boiler water testing.
Checking the Deaerator and Controlling Scavenger Feed#
A deaerator is checked by stopping the scavenger feed briefly and measuring dissolved oxygen at the deaerator outlet and the feed-pump discharge (handbook ch. 10). Volatile-scavenger feed is judged against feedwater oxygen; IAPWS AVT(R) asks for economizer-inlet oxygen below 10 µg/kg.
What Goes Wrong in Boiler Feedwater Treatment?#
Feedwater treatment fails in 4 ways: pretreatment leaks hardness or silica, oxygen gets past the deaerator, returned condensate brings organics and corrosion products, or the scavenger leaves acidic or sulfur byproducts in the cycle.
Hardness and Silica Leakage from Pretreatment#
Hardness leakage starts when a softener is overrun, under-regenerated or fed water above about 5,000 ppm TDS (DuPont AmberLite manual), and it matters most on chelant programs that need feedwater hardness below 1.0 ppm (Veolia handbook, ch. 12). Feedwater hardness usually ranges from 0.01 to 2.0 ppm as CaCO3 with softened or demineralized makeup (same handbook).
Units and conversions: water hardness.
Oxygen In-Leakage, Economizer Pitting and Deaerator Cracking#
Feedwater oxygen of 3 to 10 ppb does not significantly damage economizers (handbook ch. 11), so readings above the ASME 7 ppb guideline (EPRI fossil guidance: below 5 ppb) point to deaerator trouble or air in-leakage. Deaerators themselves crack by corrosion fatigue near welds.
Contaminated Condensate: Organics and Corrosion Products#
Returned condensate can carry process organics: Brad Buecker (Chemical Processing, 2026-10-05) describes condensate with up to 200 mg/L TOC entering 550 psi (38 bar) boilers whose ASME feedwater guideline is below 0.5 mg/L TOC, with superheater failures every 1.5 to 2 years.
Measurement: total organic carbon (TOC).
Scavenger Byproducts: Sulfite Decomposition, Acetic Acid and CO2#
Every oxygen scavenger leaves a byproduct: sulfite adds sulfate (and SO2 and H2S above 1,000 psig), DEHA forms acetic acid that raises cation conductivity, carbohydrazide releases CO2 and hydrazine above about 135 °C (275 °F), and hydrazine forms ammonia from about 204 °C (400 °F).
Byproducts by scavenger: volatile oxygen scavengers and metal passivators.
Which Regulations and Standards Apply to Boiler Feedwater Chemicals?#
Three sets of rules touch feedwater chemicals: 21 CFR 173.310 where steam contacts food, ASME, IAPWS and EN quality guidance, and hazard classifications; NSF/ANSI/CAN 60 covers drinking-water additives and does not apply.
US and EU instruments are indexed under water treatment regulations and standards.
21 CFR 173.310: Feedwater Limits Where Steam Contacts Food#
21 CFR 173.310 caps 4 additives in feedwater where steam contacts food: trisodium NTA 5 ppm (not with milk products), AA/AMPS copolymer 20 ppm active, poly(acrylic acid-co-hypophosphite) 1.5 ppm and polymaleic acid 1 ppm as the acid (eCFR as of 2026-10-01). Hydrazine is "zero in steam"; carbohydrazide, DEHA, hydroquinone, MEKO, erythorbate and sodium bisulfite are not listed in 21 CFR 173.310.
The section was last amended by 88 FR 17710 (2023-03-24); NSF categories G6 and G7 are not an eligibility route. Every entry: 21 CFR 173.310 boiler water additives.
ASME, IAPWS, EN 12952-12 and EN 12953-10 Feedwater Guidance#
Feedwater quality is set by guidance, not by law: the ASME consensus documents CRTD-34 (1994, first published 1979) and CRTD-81 (2006) for industrial boilers, IAPWS TGD3-10(2015) for utility and HRSG cycles, and in Europe EN 12952-12 for water-tube and EN 12953-10:2003 for shell boilers.
Hazard Classifications of Feedwater Oxygen Scavengers#
Hydrazine carries the harmonised EU CLP Annex VI classification H226, H301, H311, H314, H317, H331, H350 (Carc. 1B), H400 and H410 (index 007-008-00-3) and is on the REACH Candidate List; the harmonised classification of hydroquinone includes Muta. 2 (H341) and Carc. 2 (H351), and that of MEKO includes Carc. 1B (H350), which is why organic alternatives and AVT(O) replace hydrazine in many cycles. DEHA has no Annex VI entry; most ECHA C&L notifications report H226, H312, H332, H335 and H411. Boiler-grade hydrazine is a 35 % solution; handling follows the supplier SDS.
How Does Feedwater Treatment Fit into the Full Boiler Water Treatment Program?#
Feedwater treatment is the first of three zones in a steam system: makeup and condensate before the boiler, the boiler water, and the steam and condensate after it.
All three zones: boiler water treatment.
Other systems: industrial water treatment.
Feedwater, Boiler Water and Condensate: Three Treatment Zones#
Phosphate, chelant and all-polymer boiler programs and amine condensate programs start where feedwater treatment ends, so their dose depends on the feedwater the 7 steps deliver.
Blowdown, Scale and Corrosion: Where Feedwater Quality Shows Up#
Feedwater quality sets the boiler blowdown rate, from below 1 % of feedwater with very high-quality feedwater to above 20 % with poor feedwater (Veolia handbook, ch. 13).
The math: boiler blowdown: calculation and control. Deposits: boiler scale.
Oxygen pitting: boiler corrosion. Idle boilers: boiler lay-up.
What is boiler feed water used for?#
Boiler feed water replaces the water that leaves the boiler as steam and as blowdown, so feedwater flow equals steam flow plus blowdown flow.
Why is boiler feed water pH high?#
Boiler feed water pH is held high, 9.2 to 9.6 for carbon steel and 8.8 to 9.2 with copper alloys (handbook ch. 11), because carbon steel corrodes least there and because ammonia or amines must neutralize the CO2 that makeup alkalinity releases.
Who Treats Boiler Feedwater? Program Providers and Equipment Suppliers#
Two kinds of independent providers and suppliers treat boiler feedwater: program providers that supply and control the chemicals, and equipment suppliers that build softeners, dealkalizers, RO and demineralization trains and deaerators.
Choosing a water treatment program provider.