Boiler water treatment is the chemical program that keeps steam and hot-water boilers free of scale, corrosion and carryover: 29 chemicals in 10 functions, from oxygen scavengers to neutralizing amines, dosed to limits that tighten as pressure rises. Feedwater may carry 0.040 mg/L (40 ppb) of oxygen before scavenging in a boiler below 450 psig (31.0 bar) but only 0.007 mg/L (7 ppb) above it, so which program fits which boiler?
External treatment softens or demineralizes makeup and deaerates feedwater; internal treatment conditions boiler water, steam and condensate with scavengers, phosphates, chelants, polymers, caustic soda, amines and antifoams, plus cleaning and lay-up chemicals. Across those 10 functions, the boiler program draws on 8 of the 16 families of water treatment chemicals.
Sulfite with phosphate, chelant or polymer serves most boilers below 1,000 psig (69 bar); high-pressure and once-through units move to trisodium phosphate, caustic or all-volatile treatment. The ASME consensus cuts boiler-water silica from 150 to 1 mg/L across 8 pressure bands, and 21 CFR 173.310 caps 4 amines at 3 to 15 ppm in food-contact steam.
Every value on this page is checked against primary sources (eCFR, IAPWS, ASME consensus tables, CEN, PubChem, NIOSH); see our methodology and fact-checking process.
What Is Boiler Water Treatment?#
Boiler water treatment is the conditioning of makeup water, feedwater, boiler water and condensate with pretreatment and dosed chemicals so that a boiler makes steam or hot water without scale, corrosion or carryover. Boiler water itself is the concentrated water in the drum or shell; it carries the treatment residuals and up to 3,500 µS/cm of conductance below 300 psig (20.7 bar) under the ASME consensus. Boiler water concentrates continuously as steam leaves, so its chemistry is tested at least once per shift.
Boilers are 1 of 7 application programs on this site, each with its own chemistry and limits; cooling, membrane, closed-loop and wastewater programs are compared under industrial water treatment.
What goes wrong in an untreated boiler?#
An untreated boiler fails in 3 ways: scale and deposits insulate heated tubes, dissolved oxygen and concentrated caustic corrode steel, and foaming or priming carries boiler water into the steam. Scale forms where waterwall heat flux occasionally exceeds 631 kW/m² (200,000 Btu/ft²·h), and tubes overheat from about 399 °C (750 °F), according to the Veolia Water Technologies Handbook. Beneath porous iron oxide, caustic concentrates up to 100,000 ppm while the bulk holds 5 to 10 ppm. Carryover ends in the turbine, where deposits can cut efficiency by 5 % and capacity by 20 %.
Where in the cycle does each problem start? The table below compares the 3 failure modes by origin, cause and chemical answer.
| Problem | Where it starts | Cause | Chemical answer | Key number |
|---|---|---|---|---|
| Boiler scale | heated tubes | hardness, silica, iron | phosphate, chelant, polymer | heat flux above 631 kW/m² (200,000 Btu/ft²·h) |
| Boiler corrosion | feedwater train, tubes, condensate | oxygen, concentrated caustic, CO2 | oxygen scavenger, pH control, amines | caustic up to 100,000 ppm under deposits |
| Boiler carryover, foaming and priming | drum, separators | foaming, dissolved solids, silica | antifoam, blowdown, silica limits | 10 to 30 ppb steam solids for turbines |
Which boilers does a treatment program cover?#
A boiler program covers 4 classes of boiler: hot-water and low-pressure steam heating boilers at about 93 °C (200 °F) or 1 bar (15 psig), industrial steam boilers below 1,000 psig (69 bar), high-pressure drum boilers above that, and once-through or heat-recovery steam generators. The 4 classes are listed below from low to high pressure.
- Heating boilers: 200 to 400 ppm P-alkalinity as CaCO3; high-temperature hot water up to 34 bar (500 psig).
- Industrial steam boilers: sulfite with a phosphate, chelant or polymer program.
- High-pressure drum boilers: trisodium phosphate, caustic or all-volatile treatment.
- Once-through and HRSG units: volatile treatment only, because supercritical units tolerate no dissolved solids.
Pressure decides the program. Residential central-heating boilers are not covered.
External Treatment: Makeup Water and Feedwater#
External treatment removes hardness, alkalinity, silica and oxygen before water reaches the boiler, so the internal program only handles the traces that pretreatment leaves. Nearly complete oxygen removal takes both stages: mechanical deaeration first, then a chemical scavenger. Makeup quality also sets the internal doses, because hardness drives phosphate and chelant feed and alkalinity turns into carbon dioxide in the steam.
Makeup pretreatment: softening, dealkalization and demineralization#
Makeup pretreatment for boilers starts with sodium-cycle softening, which holds effluent below 2 ppm hardness, or below 1 ppm as CaCO3 with counter-current regeneration, using 90 to 240 g/L (5.6 to 15 lb/ft³) of salt in co-flow designs. The 2 ppm figure comes from the Veolia Water Technologies Handbook; the counter-current leakage, the salt dose and the mixed-bed ratio below are DuPont AmberLite resin data.
Strong-acid cation, weak-acid cation and strong-base anion resins carry most makeup duty; resin types, capacity and regeneration are compared under ion exchange resins for water treatment.
High-pressure cycles need demineralized makeup: IAPWS sets conductivity and cation conductivity below 0.1 µS/cm and silica below 10 µg/kg, and condensate-polishing mixed beds carry more than 50 % cation resin.
Alkalinity is the main source of carbon dioxide in steam. Salt-splitting dealkalizers cut alkalinity by about 90 %, and weak-acid cation units leave service at 10 % alkalinity leakage, per the same handbook; both designs are described under dealkalization by ion exchange.
Deaeration and the feedwater train#
Deaeration removes most dissolved oxygen mechanically: a pressure deaerator is guaranteed to reach 0.005 cm³/L (7 ppb), while a vacuum deaerator leaves 330 to 650 ppb, so a chemical scavenger finishes the job. Water at 15.6 °C (60 °F) holds about 8 mg/L of oxygen, and 97 to 98 % of the incoming oxygen leaves in the spray section of a deaerator. The full feedwater path from makeup to economizer is covered under boiler feedwater treatment.
The table below compares 4 deaeration options by operating point and the oxygen they leave in feedwater.
| Unit | Operating point | Oxygen left | Source |
|---|---|---|---|
| Atmospheric feedtank | 85 to 90 °C (185 to 194 °F) | about 2 mg/L | Spirax Sarco, pressurised deaerators |
| Pressurized deaerator | about 0.2 bar g (3 psig), 105 °C (221 °F) | below 20 ppb | Spirax Sarco, pressurised deaerators |
| Pressure deaerator, maker guarantee | deaerator design pressure | below 0.005 cm³/L (7 ppb) | Veolia handbook, chapter 10 |
| Vacuum deaerator | below atmospheric pressure | 0.25 to 0.50 cm³/L (330 to 650 ppb) | Veolia handbook, chapter 10 |
Pressurized deaerators are the usual choice for plants above 20 bar g (290 psig). An atmospheric feedtank leaves about 100 times more oxygen than a pressurized deaerator (2 mg/L against 20 ppb), and every mg/L of that oxygen consumes scavenger. Tray, spray and vacuum designs are compared under deaerators.
The Boiler Chemical Program: 10 Chemical Functions#
A boiler chemical program has 10 functions: oxygen scavenging, phosphate scale control, chelant and polymer control, alkalinity control, condensate neutralization, condensate filming, volatile treatment for high-purity cycles, foam control, and the off-line operations of chemical cleaning and lay-up. The table below maps the 10 functions along the water path, from the deaerator to the condensate return and the idle boiler.
| # | Function | Chemicals | Fed to | Family hub | Program node |
|---|---|---|---|---|---|
| 1 | Oxygen scavenging | sodium sulfite, hydrazine, carbohydrazide, DEHA, erythorbate, hydroquinone, MEKO, tannin | deaerator storage | Oxygen scavengers for boiler water | feedwater (deaeration section) |
| 2 | Phosphate control | tri-, di- and monosodium phosphate, SHMP, STPP | drum | Phosphates in water treatment | Phosphate treatment programs for boilers |
| 3 | Chelant and polymer control | EDTA, NTA, polyacrylate, polymaleic acid, AA/AMPS, PCA, HEDP | feedwater beyond the pumps | Chelating agents in water treatment | All-polymer and chelant boiler programs |
| 4 | Alkalinity and pH | sodium hydroxide | boiler | pH adjustment chemicals | none |
| 5 | Condensate neutralization | morpholine, cyclohexylamine, DEAE, MOPA, AMP, ethanolamine, ammonia | deaerator storage, feedwater or boiler | Neutralizing amines for condensate treatment | Condensate system treatment |
| 6 | Condensate filming | octadecylamine (also oleylamine, OLDA) | steam header | Filming amines | condensate (row 5) |
| 7 | Volatile treatment | ammonia, amines, hydrazine or oxygen | condensate or feedwater | amines (row 5) | All-volatile treatment (AVT) and oxygenated treatment |
| 8 | Foam control | polyglycol antifoams | feedwater or drum | Defoamers and antifoams | carryover (problems table) |
| 9 | Chemical cleaning | inhibited HCl, citric and EDTA solvents, TSP boil-out | off-line | Descaling and chemical cleaning agents | Boiler chemical cleaning and descaling |
| 10 | Lay-up | sodium sulfite, caustic, hydrazine, hydroquinone, nitrogen, desiccants | idle boiler | oxygen scavengers (row 1) | Boiler lay-up |
1. Oxygen scavengers#
Oxygen scavengers remove the oxygen a deaerator leaves: in theory 7.88 mg/L of sodium sulfite or 1.0 mg/L of hydrazine reacts with each mg/L of oxygen, and in practice about 10 kg of sulfite per kg of oxygen (10 lb per lb) is fed. Sulfite and catalyzed bisulfite serve boilers below 1,000 psig (69 bar) with a boiler-water residual above 20 mg/L as SO3, according to the Veolia Water Technologies Handbook; catalyzed sulfite removed all oxygen in 25 s, uncatalyzed sulfite less than 50 %. Hydrazine or organic scavengers take over at 1,000 psig and above.
Among the hydrazine alternatives, a 2024 VTT study in Corrosion Science ranks reaction speed as carbohydrazide first, hydrazine and erythorbic acid next, then DEHA, with MEKO last; their passivation chemistry is compared under volatile oxygen scavengers.
The table below compares the 8 scavengers by ratio in mg/L per mg/L O2, pressure range and food-steam listing.
| Scavenger | Theoretical ratio | Practical dose or target | Pressure use | 21 CFR 173.310 |
|---|---|---|---|---|
| Sodium sulfite | 7.88 | about 10 kg per kg O2; residual above 20 mg/L as SO3 | below 1,000 psig (69 bar) | listed |
| Hydrazine | 1.0 | 1.5 to 2.0 parts per part O2; feedwater excess 0.05 to 0.1 mg/L | 1,000 psig (69 bar) and above; AVT(R) | zero in steam |
| Carbohydrazide | 1.41 (calculated) | no verified practical dose | medium and high pressure | not listed |
| DEHA | 1.24 (calculated) | no verified practical dose | medium and high pressure, condensate | not listed |
| Hydroquinone | 6.9 (calculated) | lay-up 200 to 400 ppm | feedwater, start-up, lay-up | not listed |
| Sodium erythorbate | 12.4 (calculated; 11.0 as erythorbic acid) | no verified practical dose | low and medium pressure | not listed |
| MEKO | not stated | slowest initial reaction | feedwater | not listed |
| Tannin | not stated | sludge conditioner | low pressure | listed |
2. Phosphate programs#
Phosphate programs precipitate the last traces of hardness as a fluid calcium phosphate sludge, which forms non-adherent at a boiler-water pH of 11.0 to 12.0 and leaves through bottom blowdown. Polyphosphates such as SHMP and STPP revert rapidly to orthophosphate. Coordinated programs with monosodium or disodium phosphate below a 3:1 sodium-to-phosphate ratio caused acid phosphate corrosion, so IAPWS TGD4-11(2015) allows trisodium phosphate only, at Na:PO4 from 3:1 up to TSP plus 1 mg/kg NaOH and at least 0.3 mg/kg PO4, up to 19.3 MPa (2,800 psi). Phosphate hideout appears from about 10.0 to 10.5 MPa (1,450 to 1,500 psi).
3. Chelant and polymer programs#
Chelant programs keep hardness in solution: EDTA and NTA, introduced in the early 1960s, eliminate deposition when feedwater hardness stays below 1.0 ppm, and stoichiometric demand is 3.80 mg/L of Na4EDTA (active) per mg/L of hardness as CaCO3. Chelant and polymer programs run up to about 900 psig (62 bar), and overfeed dissolves protective magnetite. Polymer with phosphate cuts iron deposition by up to 95 %.
All-polymer programs need consistently very low feedwater hardness, and iron dispersants extend to 1,000 to 1,750 psig (69 to 121 bar). Polyacrylate, polymaleic and AA/AMPS chemistries are compared under polymer scale inhibitors and dispersants.
4. Alkalinity and pH control#
Alkalinity control keeps boiler water alkaline enough to protect steel, usually with caustic soda, while the ASME consensus caps total alkalinity at 350 mg/L as CaCO3 below 300 psig (20.7 bar) and 100 mg/L at 901 to 1,000 psig (62.2 to 68.9 bar). Carbon steel corrodes least at a feedwater pH of 9.2 to 9.6, or 8.8 to 9.2 in mixed copper and iron systems. Free caustic that concentrates under deposits gouges tubes, and sodium nitrate guards low-pressure boilers against caustic embrittlement.
5. Neutralizing amines for condensate#
Neutralizing amines raise condensate pH by neutralizing carbonic acid from makeup alkalinity: each ppm of bicarbonate alkalinity as CaCO3 releases 0.79 ppm of CO2 into the steam, and 2.0 to 2.6 ppm of amine neutralizes each ppm of CO2. One ppm of CO2 drops pure water to pH 5.49; complete neutralization needs condensate above pH 8.3, and most systems run at 7.5 to 8.5. Morpholine suits average pressures of at least 50 psig (3.4 bar), cyclohexylamine lower-pressure lines.
Worked example (inputs: makeup alkalinity 100 ppm as CaCO3; 0.79 ppm CO2 per ppm bicarbonate and 0.35 per ppm carbonate alkalinity): bicarbonate alkalinity releases 79 ppm CO2, carbonate alkalinity 35 ppm, because carbonate decomposes only about 80 %.
Morpholine, with a distribution ratio of about 0.4, stays in the first condensate; cyclohexylamine at 4.0 travels to remote lines. The ratio varies with pressure and pH, as explained under amine distribution ratio. The table below compares 8 amines by capacity, distribution ratio and food-steam cap.
| Amine | Capacity, ppm per ppm CO2 | Distribution ratio (atmospheric) | System fit | 21 CFR 173.310 |
|---|---|---|---|---|
| Morpholine | 2.0 | 0.4 | short systems, average pressure at least 3.4 bar (50 psig) | 10 ppm in steam |
| Cyclohexylamine | 2.3 | 4.0 | long, branched, low-pressure lines | 10 ppm in steam |
| DEAE | 2.6 | 1.7 | mixed pressure | 15 ppm in steam |
| MOPA | 2.03 (theoretical) | 1.0 (brine-pot test) | condensate, HRSG | not listed |
| AMP | 2.03 (theoretical) | 0.3 (brine-pot test) | first condensate | not listed |
| Ethanolamine | 1.39 (theoretical) | favors the water phase | HRSG, utility | not listed |
| Ammonia | 0.39 (theoretical) | 10.0 | utility AVT | not listed |
| Octadecylamine (filming) | n/a | n/a | steam header, 1 to 3 ppm | 3 ppm in steam |
Theoretical values assume 1 mol of amine per mol of CO2; atmospheric distribution ratios come from patent US 5,368,775 and brine-pot values from patent US 4,192,844.
6. Filming amines#
Filming amines protect condensate lines with a hydrophobic film instead of raising pH; IAPWS defines 3 of them, octadecylamine (ODA), oleylamine and oleyl propylenediamine, and ODA is fed at 1 to 3 ppm for a 0.1 to 0.5 ppm residual in returned condensate. ODA goes directly to the steam header, and corrosion coupons, not pH, are the primary monitor of a filming program. A combined neutralizing and filming program cut feedwater iron from 0.5 to 0.05 ppm in 2 months in a case printed in the same handbook.
7. All-volatile and oxygenated treatment#
All-volatile treatment adds only ammonia or amines, and sometimes a reducing agent, so high-purity cycles carry no solids: IAPWS sets economizer-inlet pH 9.2 to 9.8 for AVT(O) and 8.6 to 9.8 for oxygenated treatment in all-ferrous cycles. AVT(O) runs at 10 to 20 µg/kg of oxygen; AVT(R), for mixed metallurgy only, holds pH 9.1 to 9.3 and oxygen below 10 µg/kg. Oxygenated treatment doses 30 to 50 µg/kg in drum units and 30 to 150 µg/kg in once-through units, with feedwater cation conductivity below 0.15 µS/cm. IAPWS TGD3-10(2015) favors ammonia for high-pressure turbines.
8. Antifoams#
Antifoams protect steam purity where alkalinity, dissolved solids or saponified oil make boiler water foam, and they let a boiler hold higher concentrations with less blowdown. Drum internals separate at least 99.97 % of the 15 to 20 kg of water that circulates per kg of steam (15 to 20 lb per lb). Polyglycol antifoams are listed in 21 CFR 173.310; silicone antifoams are not listed in 21 CFR 173.310.
9. Chemical cleaning#
Chemical cleaning removes deposits the program could not prevent, most often with 5 % inhibited hydrochloric acid plus complexors, with ammonium bifluoride for silicate scale and thiourea where copper is present. Phosphoric, sulfuric and sulfamic acids serve units below 2,000 psig (137.9 bar). Before start-up, alkaline cleaning alone may suffice below 900 psig (62 bar), as a boil-out with 0.1 to 1 % trisodium phosphate and a surfactant. On-line chelant cleanup is possible in service, with inspections usually every 90 days.
10. Lay-up chemicals#
Lay-up chemicals protect an idle boiler, because corrosion can start within hours: wet lay-up holds at least 200 ppm sodium sulfite as SO3 with 400 ppm P-alkalinity as CaCO3, or at least 200 ppm hydrazine at pH 10.0. Hydroquinone serves at about 200 ppm in passivated systems and at least 400 ppm in new ones. Dry lay-up suits outages of a month or more, with quicklime at 0.96 kg/m³ (6 lb/100 ft³) as desiccant or a nitrogen blanket at 0.34 bar (5 psig).
Which Boiler Program for Which Pressure?#
Boiler pressure decides the program: phosphate, chelant and polymer programs with sulfite serve most boilers below 1,000 psig (69 bar), while high-pressure and once-through units move to trisodium phosphate, caustic or all-volatile treatment. A plant should choose from drum pressure first, then adjust to makeup quality. The table below compares the programs by boiler class from low to high pressure.
| Boiler class | Pressure | Internal program | Scavenger | Steam and condensate |
|---|---|---|---|---|
| Hot-water and low-pressure steam | about 1 bar (15 psig); HTHW to 34 bar (500 psig) | 200 to 400 ppm P-alkalinity as CaCO3, polymer | sulfite | high return |
| Industrial steam | below 69 bar (1,000 psig) | phosphate at pH 11.0 to 12.0; chelant and polymer to about 62 bar (900 psig); all-polymer | sulfite, catalyzed bisulfite | neutralizing and filming amines |
| High-pressure drum | 69 bar (1,000 psig) and above | TSP-only to 193 bar (2,800 psi); caustic to 165 bar (2,400 psi); AVT | hydrazine, organics or none (AVT(O)) | ammonia or amines |
| Once-through, supercritical, HRSG | all | AVT(O), AVT(R), OT | none, reducing agent or oxygen | ammonia, ethanolamine |
Hot-water and low-pressure steam boilers#
Hot-water and low-pressure steam boilers run the simplest programs: sulfite, 200 to 400 ppm P-alkalinity as CaCO3 and a polymer, with most water returning as condensate or loop water. Hot-water units operate near 93 °C (200 °F) and low-pressure steam boilers near 1 bar (15 psig). High-temperature hot-water systems reach 34 bar (500 psig), usually run at 2.4 to 24 bar (35 to 350 psig) and stay pressurized with nitrogen; they use sulfite, pH control and polymer.
Industrial steam boilers below 1,000 psig (69 bar)#
Industrial steam boilers below 1,000 psig (69 bar) choose between phosphate precipitation at pH 11.0 to 12.0, chelant and polymer up to about 900 psig (62 bar), or all-polymer where feedwater hardness stays very low, each with sulfite and neutralizing or filming amines. Chelant programs should run only where feedwater hardness stays below 1.0 ppm, since overfeed attacks magnetite. Phosphate sludge leaves through bottom blowdown, so blowdown control is part of a phosphate program. Above 450 psig (31.0 bar), the ASME oxygen value drops from 0.040 to 0.007 mg/L.
High-pressure drum boilers from 1,000 psig (69 bar)#
High-pressure drum boilers from 1,000 psig (69 bar) drop sulfite, which decomposes to SO2 and H2S, and run trisodium phosphate to 19.3 MPa (2,800 psi), caustic treatment to 16.5 MPa (2,400 psi) or all-volatile treatment. Silica becomes the steam-purity limit: vaporous silica carryover can start from 400 psig (27.6 bar) and rarely matters below 900 psig (62.1 bar), which is why the ASME silica limit falls to 2 mg/L at 1,001 to 1,500 psig. Feedwater must reach the no-detectable-hardness limit.
Once-through, supercritical and HRSG units#
Once-through and supercritical units tolerate no dissolved solids, so they run all-volatile or oxygenated treatment only, and multi-pressure HRSGs follow the IAPWS preference for AVT(O) without a reducing agent where feedwater systems are all-ferrous. Ammonia, ethanolamine or other amines set the pH. Ethanolamine stays in the water phase, while ammonia, about 10 times more volatile at low pressure, leaves with the steam. Flow-accelerated corrosion is worst below pH 9.4 at 121 to 232 °C (250 to 450 °F), the temperature range of HRSG low-pressure circuits, so pH control drives these programs.
Boiler Water Limits by Pressure#
Boiler water limits tighten with pressure: the ASME consensus for industrial water-tube drum boilers cuts boiler-water silica from 150 mg/L below 300 psig (20.7 bar) to 1 mg/L at 1,501 to 2,000 psig (103.5 to 137.9 bar), and conductance from 3,500 to 100 µS/cm. The table below compares all 8 ASME bands in mg/L (ppm); a deaerator is assumed and oxygen is measured before the scavenger.
| Drum pressure, psig (bar) | FW O2 | FW Fe | FW Cu | FW hardness as CaCO3 | FW pH | BW SiO2 | BW total alkalinity as CaCO3 | BW conductance, µS/cm |
|---|---|---|---|---|---|---|---|---|
| 0 to 300 (0 to 20.7) | < 0.040 | 0.100 | 0.050 | 0.300 | 7.5 to 10.0 | ≤ 150 | < 350 | < 3,500 |
| 301 to 450 (20.8 to 31.0) | < 0.040 | 0.050 | 0.025 | 0.300 | 7.5 to 10.0 | ≤ 90 | < 300 | < 3,000 |
| 451 to 600 (31.1 to 41.4) | < 0.007 | 0.030 | 0.020 | 0.200 | 7.5 to 10.0 | ≤ 40 | < 250 | < 2,500 |
| 601 to 750 (41.5 to 51.7) | < 0.007 | 0.025 | 0.020 | 0.200 | 7.5 to 10.0 | ≤ 30 | < 200 | < 2,000 |
| 751 to 900 (51.8 to 62.1) | < 0.007 | 0.020 | 0.015 | 0.100 | 7.5 to 10.0 | ≤ 20 | < 150 | < 1,500 |
| 901 to 1,000 (62.2 to 68.9) | < 0.007 | 0.020 | 0.015 | 0.100 | 8.5 to 9.5 | ≤ 8 | < 100 | < 1,000 |
| 1,001 to 1,500 (69.0 to 103.4) | < 0.007 | 0.010 | 0.010 | not detectable | 9.0 to 9.6 | ≤ 2 | not detectable | ≤ 150 |
| 1,501 to 2,000 (103.5 to 137.9) | < 0.007 | 0.010 | 0.010 | not detectable | 9.0 to 9.6 | ≤ 1 | not detectable | ≤ 100 |
Source: ASME consensus (CRTD-34) as printed in the Veolia Water Technologies Handbook, chapter 13; conductance unneutralized.
Above a heat flux of 473.2 kW/m² (150,000 Btu/ft²·h), the next higher pressure range applies. Sampling points, EN values and footnotes are on boiler water limits by pressure.
Feedwater limits: oxygen, iron, copper, hardness and pH#
ASME feedwater limits fall from 0.040 mg/L (40 ppb) oxygen, 0.100 mg/L iron and 0.300 mg/L hardness as CaCO3 below 300 psig (20.7 bar) to 0.007 mg/L oxygen, 0.010 mg/L iron and no detectable hardness above 1,000 psig (69 bar). Feedwater pH narrows from 7.5 to 10.0 to 9.0 to 9.6 above 1,000 psig. EPRI sets below 5 ppb oxygen for fossil units. ASME caps feedwater total organic carbon below 0.5 mg/L, yet in a case reported in Chemical Processing (2026-10-05) contaminated condensate returned up to 200 mg/L TOC.
Long sample lines distort oxygen readings, because the scavenger keeps reacting or air leaks in. Measurement of dissolved oxygen in boiler systems follows ASTM D888-18(2026).
Boiler water limits: silica, alkalinity and conductance#
Boiler-water limits protect steam purity: silica falls from 150 to 1 mg/L across the 8 ASME bands, total alkalinity from 350 mg/L as CaCO3 to not detectable, and unneutralized conductance from 3,500 to 100 µS/cm. Silica controls at high pressure because it volatilizes into steam from about 400 psig (27.6 bar). Conductance tracks dissolved solids: in boiler water, 1 µS/cm corresponds to 0.55 to 0.90 ppm of TDS, and hydroxide is neutralized with gallic or boric acid where conductance is used for control.
Silica analysis distinguishes the dissolved and the particulate share, and reactive and colloidal silica in water are measured and limited separately.
European and IAPWS limits#
European shell boilers follow EN 12953-10:2003 and water-tube boilers EN 12952-12, while power and HRSG cycles follow IAPWS guidance such as TGD3-10(2015) for volatile treatment and TGD4-11(2015) for phosphate and caustic treatment. EN 12953-10:2003 (CEN/TC 269, published 2003-09-16) is under revision, with no new edition as of 2026-10-20. The EWT code summary sorts European boilers into high-TDS (feedwater above 30 µS/cm, 3 to 10 % blowdown), low-TDS (1 to 2 %) and no-TDS modes (cation conductivity below 0.2 µS/cm, 0.5 to 1 %). VGB-S-010-T-00 is a further European code, and the IAPWS technical guidance documents are free to read.
How Is a Boiler Water Program Monitored and Tested?#
A boiler program is monitored in 3 layers: shift tests of feedwater and boiler water, continuous oxygen and conductivity signals that drive blowdown and feed, and condensate pH and corrosion coupons in the return system.
Which tests, where and how often?#
Boiler water is tested at least once per shift: feedwater for hardness, iron, copper, oxygen and pH, with a continuous oxygen meter recommended, and boiler water for phosphate, P-alkalinity or pH, sulfite and conductivity. Sample lines run continuously at 1.5 to 1.8 m/s (5 to 6 ft/s) and 800 to 1,000 mL/min, according to the Veolia Water Technologies Handbook. pH of feedwater and condensate follows ASTM D5464-25 for low-conductivity water and is reported at 25 °C, and dissolved oxygen follows ASTM D888-18(2026). Volatile scavenger feed is checked against feedwater oxygen, not against a boiler residual. The table below lists the tests by stream.
| Stream | Tests | Frequency or mode |
|---|---|---|
| Softener effluent | hardness | per shift |
| Feedwater | hardness, iron, copper, oxygen, pH | once per shift; oxygen continuous |
| Boiler water | phosphate (if used), P-alkalinity or pH, sulfite (if used), conductivity | once per shift; conductivity continuous |
| Condensate | pH near and remote, iron and copper, corrosion coupons | per program |
| Sample lines | 1.5 to 1.8 m/s (5 to 6 ft/s), 800 to 1,000 mL/min | continuous flow |
Test kits, frequencies and sample conditioning are detailed under boiler water testing.
Blowdown control: percent blowdown and cycles#
Boiler blowdown is a percent of feedwater, not of steam: blowdown divided by feedwater times 100, or feedwater chloride divided by boiler-water chloride times 100, from below 1 % with very pure feedwater to above 20 % with poor feedwater. Automatic control holds conductivity within 5 % of the set point and uses about 20 % less blowdown than manual control.
Worked example (inputs: steam 2,000,000 lb/day (907 t/d), 50 cycles of concentration, blowdown = steam / (cycles - 1)): the boiler blows down 2,000,000 / 49 = 40,816 lb/day (18.5 t/d), which is 2 % of feedwater. Calculation and control are covered under boiler blowdown.
Dose calculation and feed points#
Boiler chemical feed rests on 2 bases: phosphate and chelant feed follow feedwater hardness, while oxygen-scavenger feed follows feedwater oxygen plus the chemical demand of the system. The 4 feed-point rules for boiler chemicals are listed below.
- Oxygen scavengers: the deaerator storage section, fed continuously.
- Chelants: the feedwater beyond the feed pumps, never the drum.
- Orthophosphate: directly to the drum, because it deposits in feed lines.
- Amines: neutralizing amines to the deaerator storage, feedwater or boiler, never the steam header; filming amines to the steam header.
A dose in mg/L becomes a feed rate through flow and product strength: feed (lb/day) equals dose (mg/L) times flow (MGD) times 8.34, divided by purity. The chemical dosage calculator turns a mg/L target into kg/d, lb/day and a pump setting.
Which Regulations Apply to Boiler Water Chemicals?#
Boiler water chemicals face 3 layers of rules: 21 CFR 173.310 where steam contacts food, hazard classification and process-safety thresholds at the plant, and discharge limits for cleaning wastes at steam-electric plants. Boiler chemicals are not drinking-water additives: NSF/ANSI/CAN 60 covers chemicals dosed into drinking water, and boiler use falls outside its scope.
21 CFR 173.310 for steam that contacts food#
21 CFR 173.310 lists the boiler water additives allowed in steam that contacts food and caps 6 of them: cyclohexylamine at 10 ppm, DEAE at 15 ppm, morpholine at 10 ppm and octadecylamine at 3 ppm in steam, hydrazine at zero in steam, and trisodium NTA at 5 ppm in feedwater. These caps exclude steam that contacts milk products, and the values follow the eCFR text of 21 CFR 173.310 as of 2026-10-01 (last amended 2023-03-24, 88 FR 17710).
Listed oxygen scavengers are sodium sulfite, sodium metabisulfite, tannin and cobalt sulfate as catalyst. Carbohydrazide, DEHA, hydroquinone, MEKO, sodium erythorbate, MOPA, AMP and ethanolamine are not listed in 21 CFR 173.310, and NSF nonfood categories G6 and G7 are registration categories, not a listing. The table below gives every numeric cap.
| Substance | Limit | Basis |
|---|---|---|
| Cyclohexylamine | ≤ 10 ppm | in steam; not for steam contacting milk products |
| DEAE | ≤ 15 ppm | in steam; not for steam contacting milk products |
| Morpholine | ≤ 10 ppm | in steam; not for steam contacting milk products |
| Octadecylamine | ≤ 3 ppm | in steam; not for steam contacting milk products |
| Hydrazine | zero | in steam |
| Trisodium NTA | ≤ 5 ppm | in feedwater; not for steam contacting milk products |
| AA/AMPS copolymer (Mw ≥ 9,900, Mn ≥ 5,700) | ≤ 20 ppm active | in feedwater |
| Poly(acrylic acid-co-hypophosphite), sodium salt | ≤ 1.5 ppm | in feedwater |
| Polymaleic acid and sodium salt | ≤ 1 ppm as acid | in feedwater |
| Sorbitol anhydride esters | ≤ 15 mg/kg each | in steam |
The full list of 21 CFR 173.310 boiler water additives, with the general conditions of the section, sits on the standards page.
Hazard classification and process safety#
Hydrazine is the most hazardous boiler chemical: it carries a harmonised EU classification as Carc. 1B (H350) and sits on the REACH Candidate List; boiler grade is a 35 % solution, and organic scavengers replace it in many plants. Hydroquinone carries harmonised H341 and H351, MEKO Carc. 1B (H350) and cyclohexylamine H361f. NIOSH sets recommended exposure limits of 10 ppm for cyclohexylamine and DEAE, 20 ppm for morpholine and a 0.03 ppm 2-hour ceiling for hydrazine.
Anhydrous ammonia for volatile treatment reaches the same 10,000 lb (4,536 kg) threshold under EPA RMP (40 CFR 68.130) and OSHA PSM (29 CFR 1910.119); plant thresholds are listed under EPA RMP and OSHA PSM.
Discharge of boiler blowdown and cleaning wastes#
Boiler cleaning wastes at steam-electric plants are chemical metal cleaning wastes under 40 CFR 423.11(c) and must meet 1.0 mg/L copper and 1.0 mg/L iron under 40 CFR 423.13(e). Both values apply as daily maximum and 30-day average. These federal effluent guidelines bind steam-electric power plants only; other plants discharge blowdown and cleaning wastes under their own permit or pretreatment limits, covered under NPDES permits.
Complete List of Boiler Water Treatment Chemicals: All 29 in One Table#
The complete list of boiler water treatment chemicals below gives all 29 chemicals with their CAS number, function, pressure or system range, key ratio and 21 CFR 173.310 status. Rows follow the water path: 8 oxygen scavengers, 7 neutralizing and alkalizing amines, 1 filming amine, 5 phosphates, caustic soda, 2 chelants, and 5 polymers and phosphonates. Pretreatment chemicals and cleaning acids are not counted.
| # | Chemical | CAS | Function | Pressure or system | Key ratio or limit | 21 CFR 173.310 |
|---|---|---|---|---|---|---|
| 1 | Sodium sulfite | 7757-83-7 | oxygen scavenger | below 1,000 psig (69 bar) | 7.88 mg/L per mg/L O2 | listed |
| 2 | Hydrazine | 302-01-2 (hydrate 7803-57-8) | scavenger, passivator | 1,000 psig (69 bar) and above | 1.0 mg/L per mg/L O2 | zero in steam |
| 3 | Carbohydrazide | 497-18-7 | scavenger, passivator | medium, high pressure | 1.41 (calculated) | not listed |
| 4 | DEHA (diethylhydroxylamine) | 3710-84-7 | scavenger, passivator | medium, high pressure | 1.24 (calculated) | not listed |
| 5 | Sodium erythorbate | 6381-77-7 (acid 89-65-6) | scavenger | low, medium pressure | 12.4 (calculated) | not listed |
| 6 | Hydroquinone | 123-31-9 | scavenger, catalyst | feedwater, lay-up | 6.9 (calculated) | not listed |
| 7 | MEKO (methyl ethyl ketoxime) | 96-29-7 | scavenger | feedwater | not stated | not listed |
| 8 | Tannin (quebracho extract) | none (mixture) | sludge conditioner, scavenger | low pressure | not stated | listed |
| 9 | Morpholine | 110-91-8 | neutralizing amine | short systems | 2.0 ppm per ppm CO2 | 10 ppm in steam |
| 10 | Cyclohexylamine | 108-91-8 | neutralizing amine | long low-pressure lines | 2.3 ppm per ppm CO2 | 10 ppm in steam |
| 11 | DEAE (diethylaminoethanol) | 100-37-8 | neutralizing amine | mixed pressure | 2.6 ppm per ppm CO2 | 15 ppm in steam |
| 12 | MOPA (methoxypropylamine) | 5332-73-0 | neutralizing amine | condensate, HRSG | 2.03 (theoretical) | not listed |
| 13 | AMP (2-amino-2-methyl-1-propanol) | 124-68-5 | neutralizing amine | first condensate | 2.03 (theoretical) | not listed |
| 14 | Ethanolamine (ETA) | 141-43-5 | alkalizing amine | HRSG, utility | 1.39 (theoretical) | not listed |
| 15 | Ammonia | 7664-41-7 | alkalizing agent | AVT | 0.39 (theoretical) | not listed |
| 16 | Octadecylamine (ODA) | 124-30-1 | filming amine | steam, condensate | 1 to 3 ppm dose | 3 ppm in steam |
| 17 | Trisodium phosphate (TSP) | 7601-54-9 | phosphate program | to 19.3 MPa (2,800 psi) | PO4 at least 0.3 mg/kg | listed |
| 18 | Disodium phosphate | 7558-79-4 | phosphate program | precipitation programs | pH 11.0 to 12.0 | listed |
| 19 | Monosodium phosphate | 7558-80-7 | phosphate (historic) | below 3:1 caused acid phosphate corrosion | n/a | listed |
| 20 | Sodium hexametaphosphate (SHMP) | 68915-31-1 | formulation phosphate | reverts to orthophosphate | n/a | listed |
| 21 | Sodium tripolyphosphate (STPP) | 7758-29-4 | formulation phosphate | reverts to orthophosphate | n/a | listed |
| 22 | Sodium hydroxide (caustic soda) | 1310-73-2 | alkalinity, caustic treatment | to 16.5 MPa (2,400 psi) | ASME alkalinity caps | listed |
| 23 | EDTA | 60-00-4 (Na4EDTA 64-02-8) | chelant | to about 62 bar (900 psig) | 3.80 mg/L Na4EDTA per mg/L hardness (calculated) | Na4EDTA listed |
| 24 | NTA | 139-13-9 (Na3NTA 5064-31-3) | chelant (historic) | feedwater | n/a | 5 ppm in feedwater |
| 25 | Polyacrylic acid and sodium polyacrylate | 9003-01-4 (salt 9003-04-7) | dispersant | sludge control | n/a | sodium salt listed |
| 26 | Polymaleic acid (HPMA) | 26099-09-2 | dispersant | feedwater | n/a | 1 ppm as acid in feedwater |
| 27 | AA/AMPS copolymer | 40623-75-4 (salt 37350-42-8) | dispersant | feedwater | n/a | 20 ppm active |
| 28 | Phosphinocarboxylic acid (PCA) | 71050-62-9 | dispersant | feedwater | n/a | 1.5 ppm in feedwater |
| 29 | HEDP | 2809-21-4 | scale inhibitor | feedwater | n/a | listed, no numeric limit |
Every other chemical is filterable by CAS, function and system in the water treatment chemical database.
How Is a Boiler Water Treatment Program Bought?#
A boiler water treatment program is bought in 1 of 5 service models: full service with chemicals, feed equipment, visits, testing and reporting; chemical supply with technical service; chemical-only supply; monitoring-led service; or a performance-based contract.
How to compare providers is covered under water treatment companies.
A program quote rests on 7 inputs: boiler type and pressure in psig or bar, steam rate in lb/h or t/h, condensate return in percent, pretreatment, deaerator type, feedwater and boiler-water analyses, and the current provider with its contract end date.
A proposal states the scope by system, the products with SDS, the dose basis (as product or as active), a KPI table with method and frequency, the visit frequency, equipment ownership and return, and the price basis with escalation. Program prices depend on steam rate, makeup quality and service scope, so they come from a quote.
Grand View Research estimated the global industrial water treatment chemicals market at USD 18.2 billion in 2025 (analyst estimate), and boiler programs are one share of it. Chemical distributors and manufacturers are listed under water treatment chemical suppliers.
Are Boiler Treatment Chemicals Safe for Steam, People and Equipment?#
Boiler treatment chemicals are safe for steam and equipment when they stay inside their limits, such as 10 ppm of morpholine in food-contact steam, and they need SDS-based handling at the plant, where hydrazine, the amines and caustic soda are the main hazards. The questions below cover the most common follow-ups.
What chemicals are used to treat boiler water?#
Boiler water is treated with 7 groups of chemicals: oxygen scavengers such as sodium sulfite, phosphates such as trisodium phosphate, chelants such as EDTA, polymers such as sodium polyacrylate, caustic soda, neutralizing amines such as morpholine, and filming amines such as octadecylamine. Antifoams, cleaning acids and lay-up chemicals add the off-line and steam-purity functions, for 29 chemicals in total.
Is boiler water safe to drink?#
No: boiler water is not drinking water; it is concentrated to as much as 3,500 µS/cm and carries treatment chemicals such as sulfite above 20 mg/L as SO3, and boiler chemicals fall outside the scope of NSF/ANSI/CAN 60 for drinking-water additives. Boiler water can also carry hydrazine, amines, phosphate and caustic alkalinity, depending on the program. Condensed steam is a separate stream, and its amine content in food plants is capped under 21 CFR 173.310.
Why have many boilers moved away from hydrazine?#
Hydrazine is classified Carc. 1B (H350) under EU CLP and listed as an SVHC, so many plants use carbohydrazide, DEHA, hydroquinone or erythorbate, or follow the IAPWS preference for AVT(O) with no reducing agent in all-ferrous cycles. The substitutes bring their own chemistry: carbohydrazide partly hydrolyzes to hydrazine and CO2 above 135 °C (275 °F), according to patent US 4,269,717, and DEHA forms acetic acid that raises cation conductivity during transients.
Can hot-water boilers use the same program as steam boilers?#
No: hot-water boilers recirculate the same water at about 93 °C (200 °F), or up to 34 bar (500 psig) in high-temperature hot water, so they need a closed-loop program of sulfite, pH control and polymer rather than blowdown-driven steam chemistry.
Loop inhibitors and limits are covered under hot water heating system treatment.
Which boiler chemicals must never be mixed or fed together?#
4 feed rules prevent reactions in boiler feed systems, and they are listed below.
- Hydrazine is fed alone.
- Catalyzed sulfite is fed alone and diluted only with condensate or demineralized water.
- Orthophosphate never enters a hard feedwater line.
- Sulfite never goes into attemperation water.
The complete set of incompatible pairs is listed under water treatment chemical compatibility.
Is sodium bisulfite listed in 21 CFR 173.310?#
No: sodium bisulfite is not listed in 21 CFR 173.310, while sodium sulfite (neutral or alkaline) and sodium metabisulfite are listed (eCFR text as of 2026-10-01).