Boiler water treatment is the removal and control of the impurities in boiler feedwater that cause scale, corrosion and carryover, and Paul Industries designs, installs and services the boiler water treatment equipment and deaerators that do it, from makeup softeners and reverse osmosis to deaerator vessels, blowdown systems and condensate return. We are the mechanical side of the job: the vessels, piping, pumps, controls and feed equipment. The chemical supply itself stays with the owner’s chemical supplier, and this page explains the chemistry at the level an engineer needs to specify and run the equipment.
What is boiler water treatment?
It is everything done to water between the raw supply and the steam drum so the boiler can run clean and intact. A complete boiler water treatment system has three layers. External treatment conditions the makeup water before it reaches the boiler: filtration, softening, reverse osmosis or dealkalization. Mechanical deaeration strips dissolved oxygen and carbon dioxide. Internal treatment conditions the water inside the boiler and condensate system with small, controlled chemical additions, and blowdown removes the solids that concentrate as water boils off. Each layer covers what the one before it cannot remove completely.
Why boilers need treatment: scale, corrosion and carryover
Water leaves the boiler as steam and leaves its dissolved solids behind, so every impurity in the feedwater concentrates in the boiler. Three failures follow if nothing controls them.
- Scale. Calcium, magnesium and silica deposit on the hottest tube surfaces. Scale insulates the metal from the water, so the tube runs hotter, fuel use rises and, in severe cases, the tube overheats and bulges or ruptures.
- Corrosion. Dissolved oxygen pits steel, especially in the feedwater system and economizer, and carbon dioxide forms carbonic acid in condensate lines. Boiler corrosion prevention starts with removing oxygen mechanically and then scavenging the trace that remains.
- Carryover. High dissolved solids, alkalinity, oil or suspended matter make boiler water foam, and boiler carryover sends water and solids into the steam, fouling control valves, turbines and heat exchangers downstream.
Treatment steps from raw water to steam
The boiler water treatment process follows the water: raw water to makeup treatment, makeup and returned condensate into the deaerator, deaerated feedwater pumped to the boiler with internal treatment added, and blowdown and steam leaving.
| Stage | Equipment | What it removes or controls |
|---|---|---|
| Pretreatment | Multimedia or cartridge filters, carbon where chlorine would harm RO membranes | Suspended solids, iron, chlorine |
| Softening | Sodium-cycle ion exchange softeners, usually duplex | Calcium and magnesium hardness |
| Dealkalization or RO | Chloride-cycle dealkalizer, or reverse osmosis | Alkalinity (and with RO, most dissolved solids and silica) |
| Deaeration | Pressurized deaerator with storage section | Dissolved oxygen and carbon dioxide |
| Internal conditioning | Chemical feed pumps and injection quills | Residual oxygen, residual hardness, pH and condensate corrosion |
| Blowdown | Continuous surface blowdown with conductivity control, bottom blowdown valves, flash tank, heat exchanger | Concentrated dissolved solids and settled sludge |
| Condensate return and polishing | Receivers, pumps, filters, polishers, conductivity monitoring | Iron corrosion products and contamination from process leaks |
A boiler feedwater treatment system works as one train, so it is designed as one: a change at any stage moves load onto the next, as when a softener leaking hardness fouls the RO membranes behind it, or a deaerator running below pressure leaves the oxygen scavenger to do work it cannot keep up with.
Makeup pretreatment: softening, dealkalization and RO
External treatment of boiler feed water is the equipment that removes impurities before the water enters the boiler, and it is where most of the mechanical scope sits. Boiler feedwater treatment almost always starts with softening, which exchanges calcium and magnesium for sodium so the hardness that would scale the tubes never arrives; see water softeners for boiler makeup. Softening does not reduce dissolved solids or alkalinity, so where the raw water is high in either, a further step follows.
A chloride-cycle dealkalizer, an anion exchanger regenerated with salt, removes most of the bicarbonate alkalinity after the softener. That matters because bicarbonate breaks down in the boiler into carbon dioxide, which leaves with the steam and becomes carbonic acid in the condensate. Reverse osmosis goes further, rejecting most of the dissolved solids and silica as well as alkalinity, which cuts blowdown sharply; it is usually run on softened water, or with an antiscalant, to protect the membranes. Higher-pressure boilers and those feeding turbines may add deionization or mixed-bed polishing after RO. See commercial reverse osmosis systems, RO skids and RO versus DI water.
Deaeration: removing oxygen before the boiler
Deaeration is the mechanical step that removes dissolved oxygen and carbon dioxide from feedwater before internal chemistry takes over. A pressurized deaerator heats the mixed makeup and returned condensate with steam to its saturation temperature, where those gases are almost insoluble, vents them, and stores the deaerated water at the suction of the boiler feed pumps. The US Department of Energy notes that most boilers above 200 psi need dissolved oxygen at 5 parts per billion or lower to prevent corrosion, that low-pressure boilers may tolerate up to 43 parts per billion, and that holding 5 parts per billion extends equipment life at little cost; a well-run deaerator also removes essentially all dissolved carbon dioxide.
What is a deaerator, physically? It is a steam-heated pressure vessel set high above the boiler feed pumps, with a deaeration section on top and a deaerator storage tank underneath, and operators call the whole unit the DA. The answer to a question new operators often ask, what is a DA tank for a boiler, is that lower section: it holds the treated water at its boiling point under a steam blanket and gives the feed pumps their reserve and suction head. How does a deaerator work? Steam brings the water up to saturation for the vessel pressure, the dissolved gases come out of solution, and a small, continuous flow of steam leaving through the vent carries them away. Deaerator operating pressure and temperature move together: about 227 F at a common 5 psig set point, and roughly 215 F to more than 350 F across industrial and power plants, according to DOE.
The common deaerator types in boiler rooms are tray, spray and spray-tray units. In a tray unit the deaerator trays break the water into thin films as it cascades down through rising steam; a spray unit atomizes it through spring-loaded nozzles and is more compact. Vacuum deaerators strip gas at low temperature, with an ejector or vacuum pump holding the vacuum, and are more common in process water service than in boiler rooms. Smaller plants often run a heated, vented boiler feed water tank instead, which removes only part of the oxygen and leaves more work for the chemical scavenger. On deaerator tank vs condensate tank, the two are not interchangeable: a condensate receiver collects returns near atmospheric pressure and pumps them on without stripping any gas.
For the water treatment program, what matters is that the deaerator holds steady pressure, keeps its vent open and is checked with a dissolved oxygen test at its outlet, because every part per billion it fails to remove becomes scavenger demand downstream. These are the checks that tie the DA to the chemistry.
| Check | What goes wrong | Effect on the water program |
|---|---|---|
| Pressure | Deaerator steam comes through a dedicated pressure-reducing valve; an undersized or badly tuned valve lets pressure swing | Water falls below saturation, or a pressure spike puts oxygen back in; scavenger demand rises |
| Vent | The deaerator vent is throttled to save steam | Stripped oxygen and carbon dioxide stay with the feedwater |
| Level | Deaerator level control runs on and off instead of modulating the makeup valve, or the deaerator overflow trap fails and blows steam to drain | Slugs of cold makeup upset pressure and let oxygen through; lost steam and heat |
| Internals | Trays or spray nozzles plugged, collapsed or stuck | Water channels through unstripped; high outlet oxygen |
| Capacity | Steam load has grown past the original deaerator sizing, which is based on peak feedwater flow (steam plus blowdown) at the coldest inlet water | Oxygen slips through at peak load and in winter |
| Vessel | Deaerator cracking at weld seams, found by internal wet fluorescent magnetic particle testing under NACE SP0590 | A safety issue rather than a chemistry one; inspections are planned into the boiler outage |
Where a plant still runs a vented feed tank, moving to a pressurized deaerator for boiler feedwater cuts the scavenger dose and protects the economizer. A full comparison of the types of deaerators, with piping, inspection and troubleshooting, is on our deaerators page.
Boiler water chemistry and internal treatment
Inside the boiler, small chemical additions finish what the equipment started. Internal treatment of boiler feed water has four jobs: scavenge residual oxygen, keep residual hardness from scaling, hold the water alkaline, and protect the condensate system.
Oxygen scavengers react with the oxygen the deaerator leaves. Sodium sulfite is the common choice in lower-pressure boilers; theoretically it takes about 7.9 parts of pure sodium sulfite to remove one part of oxygen, and more in practice. Sulfite breaks down at high pressure, so higher-pressure boilers use volatile scavengers. The use of hydrazine in boiler water treatment has declined because of its toxicity, and alternatives such as carbohydrazide, DEHA and erythorbate are now common.
Hardness that slips past the softener is dealt with in one of three ways. The role of phosphate in boiler water treatment is to precipitate calcium as a soft sludge that bottom blowdown removes instead of a hard scale; chelants keep hardness in solution; polymers disperse sludge and iron so they leave with the blowdown. The condensate side is handled separately: the purpose of an amine in boiler water treatment is to protect return lines, either neutralizing amines that raise condensate pH against carbonic acid or filming amines that coat the metal.
The boiler water treatment chemicals that do all this are dosed by feed pumps against measured flows and residuals, which is where the equipment and chemistry meet. A boiler water treatment program therefore has three owners: the equipment that removes impurities, the chemistry that conditions what remains, and the testing that proves both are working. We install and maintain the feed pumps, tanks, injection points, sample coolers and analyzers; the chemical supply and dosage targets come from the owner’s chemical supplier.
What is boiler blowdown?
Blowdown is the deliberate removal of boiler water to hold dissolved and suspended solids below their limits, because they concentrate as steam leaves. Surface blowdown, usually continuous, skims dissolved solids from just below the water surface under conductivity control. Bottom or mud blowdown is a short, intermittent opening of valves at the lowest point to remove settled sludge, typically a few seconds every several hours. DOE puts typical blowdown at 4 to 8 percent of feedwater flow, and up to 10 percent where makeup is high in solids.
How to calculate boiler blowdown: cycles of concentration is the boiler water concentration divided by the feedwater concentration, measured as conductivity, silica, chloride or alkalinity, and blowdown as a share of feedwater is one divided by the cycles. At 20 cycles blowdown is 5 percent of feedwater; at 10 cycles it is 10 percent. Better makeup treatment, such as RO, allows more cycles and less blowdown.
Blowdown is boiler water at full temperature, so it carries energy. DOE suggests heat recovery for any boiler with continuous blowdown over 5 percent of the steam rate: a flash tank sends low-pressure flash steam to the deaerator, and a heat exchanger warms the incoming makeup with the remaining hot water. Building drainage codes generally limit discharges to 140 F, so blowdown is cooled in a blowdown separator or tank before it reaches the drain.
Condensate return and what is condensate polishing?
Returned condensate is the best feedwater a plant has: hot, low in dissolved solids and already treated. DOE notes that it typically returns at 130 to 225 F against 50 to 60 F makeup, so every pound returned saves fuel, water, chemicals and blowdown. Condensate polishing is the treatment that keeps that return clean enough to reuse: cartridge or magnetic filters remove iron corrosion products, and ion exchange polishers remove hardness or other contaminants that leak in from process heat exchangers. Conductivity monitoring on each major return, with an automatic dump valve, keeps a leak in a process exchanger from contaminating the boiler. Condensate piping, traps and flash recovery are covered on our steam and condensate distribution page.
Water quality limits and testing
The reference most plants use for ASME boiler water treatment guidelines is the ASME consensus document on operating practices for feedwater and boiler water in industrial and institutional boilers. It sets feedwater and boiler water targets for hardness, dissolved oxygen, iron, copper, pH, silica, alkalinity and conductivity, by boiler type and operating pressure, with limits tightening as pressure rises. Boiler feed water limits for a specific plant also come from the boiler manufacturer, whose warranty depends on them, and from steam purity needs downstream: a turbine or a clean steam generator needs tighter control than a plant heating boiler. ASME Boiler Code Sections VI and VII give recommended care and operating practice for heating and power boilers.
| Test | Where sampled | Why | Typical frequency |
|---|---|---|---|
| Hardness | Softener outlet, feedwater | Catches softener exhaustion before scale forms | Each shift or daily, plus online monitor |
| Conductivity or TDS | Boiler water, condensate returns | Controls blowdown; detects process leaks into condensate | Continuous, with daily confirmation |
| Dissolved oxygen | Deaerator outlet | Proves the deaerator is working | Periodic, or continuous on larger plants |
| Scavenger residual | Boiler water | Confirms residual oxygen is being removed | Daily |
| pH and alkalinity | Boiler water, condensate | Scale and corrosion control; carbonic acid in returns | Daily for boiler water, weekly or more for condensate |
| Iron | Condensate, feedwater | Measures condensate system corrosion | Weekly to monthly |
Steam boiler vs hot water boiler
The difference is makeup. A steam boiler loses water continuously as steam, condensate losses and blowdown, so steam boiler water treatment is an ongoing process of makeup treatment, deaeration, chemistry and blowdown. A hot water boiler is a closed loop that should need almost no makeup once filled. Hot water boiler water treatment is therefore mostly done once: clean and flush the system, fill with softened water, and charge a boiler corrosion inhibitor, typically nitrite or molybdate based, then test periodically. A makeup water meter on a closed loop is the most useful instrument on it, because rising makeup means a leak, and each fill brings in fresh oxygen and hardness. ASME Section IV covers heating boilers up to 15 psig steam and up to 160 psig and 250 F hot water; larger units are Section I power boilers.
Boiler water problems and their usual causes
Most boiler water problems trace back to a piece of equipment, not to the chemistry itself, so the first check is mechanical.
| Symptom | Usual cause | Where to look |
|---|---|---|
| Scale on tubes, rising stack temperature | Hardness reaching the boiler | Softener exhaustion, regeneration settings, bypass valves left open |
| Foaming and carryover | High dissolved solids or alkalinity, oil or process contamination | Blowdown control, makeup quality, condensate contamination |
| Pitting in feedwater lines and economizer | Dissolved oxygen | Deaerator pressure and vent, scavenger feed, air leaks on pump suction |
| Grooving and thinning in condensate lines | Carbonic acid from feedwater alkalinity | Dealkalizer or RO performance, amine feed, condensate pH |
| Blowdown rate creeping up | Poorer makeup quality or lower condensate return | Pretreatment, return rate, failed traps and lost condensate |
| Iron in feedwater | Corrosion in the condensate system | Condensate pH, filters and polishers, oxygen entering at receivers |
Each row points to equipment we install and maintain, which is why a water analysis that drifts out of range is often fixed with a valve, a controller or a softener repair rather than a change of chemical. The table also works as an audit plan: walk the train from raw water to condensate return, confirm that each stage is doing its job with a test at its outlet, and only then adjust dosage targets with the chemical supplier.
Industrial vs commercial boilers, and choosing a contractor
Industrial boiler water treatment usually means process steam at medium or high pressure, low condensate return in some plants and steam that touches product or turbines, so it leans on RO, deaeration and tight blowdown control. Commercial boiler water treatment in hospitals, campuses and large buildings is usually low-pressure steam or hot water with high condensate return, where softening, a feed tank or deaerator and a simple chemical program are enough.
When comparing boiler water treatment companies, separate the two scopes. Chemical suppliers provide treatment products, dosage targets and routine testing. A mechanical contractor installs, repairs and maintains the equipment those programs depend on. Many plant problems blamed on chemistry are mechanical: a softener that regenerates on a timer instead of on demand, a vent valve on the deaerator choked down to save steam, a failed blowdown controller or a condensate return that was never connected. Our boiler water treatment services cover that equipment side.
What Paul Industries does
Paul Industries designs, installs, starts up, services and repairs boiler makeup and feedwater systems: filters, softeners, dealkalizers, RO and deionization, pressurized deaerator vessels and feedwater tanks, feed pumps, chemical feed skids and injection points, sample coolers and analyzers, continuous and bottom blowdown systems with flash tanks and heat recovery, and condensate return, receivers, polishing and monitoring. We work under plant utilities and plant water treatment systems, maintain the equipment under preventive maintenance, and supply the feedwater side for pharmaceutical clean steam systems and clean steam generators. Refinery and gas plant steam systems are covered on our oil and gas page, and blowdown heat exchangers on our heat exchangers page. We do not sell treatment chemicals or run chemical supply programs. Request a quote for a deaerator, makeup system or boiler feedwater upgrade.
Standards referenced: ASME Consensus on Operating Practices for the Control of Feedwater and Boiler Water Chemistry in Industrial and Institutional Boilers · ASME BPVC Section I · ASME BPVC Section IV · ASME BPVC Section VI · ASME BPVC Section VII · ASME BPVC Section VIII Division 1 · ASME B31.1 · NACE SP0590
Frequently asked questions
Do boilers need water treatment?
Yes. Every steam boiler needs at least softened makeup, oxygen control and blowdown, because impurities concentrate as water boils off. A closed hot water boiler needs much less, but still needs a clean fill, an inhibitor and a check on makeup, since every top-up brings in oxygen and hardness.
How often should boiler water be tested?
Most industrial plants test boiler water and softener outlet hardness every shift or daily, run conductivity continuously for blowdown control, and check condensate pH and iron weekly or more often. Larger and higher-pressure plants add continuous dissolved oxygen, hardness and conductivity analyzers.
Which water treatment process is best for boiler water?
It depends on the raw water and the boiler pressure. Softening is the minimum for almost any steam boiler. Add a dealkalizer where alkalinity is high, or RO where dissolved solids or silica are high or blowdown is costly. Higher-pressure boilers and turbine steam need RO with deionization or mixed-bed polishing.
What is boiler feed water?
Boiler feedwater is the water pumped into the boiler: returned condensate plus treated makeup, normally deaerated. Boiler water is the water inside the boiler, which is more concentrated because steam leaves its dissolved solids behind.
How much oxygen can a boiler tolerate?
DOE guidance is 5 parts per billion or lower for most boilers above 200 psi, with up to 43 parts per billion tolerated in low-pressure boilers. Holding 5 parts per billion in any boiler extends equipment life, and a pressurized deaerator plus a scavenger residual is how plants get there.
Can boiler blowdown go straight to the drain?
Not at boiler temperature. Plumbing and boiler rules generally limit discharge to building drainage to 140 F, so blowdown passes through a blowdown separator or tank, often with cooling water, or better through a flash tank and heat exchanger that recover its heat first.
Is blowdown heat recovery worth it?
Usually, where continuous blowdown exceeds about 5 percent of the steam rate. DOE figures show a boiler at 150 psig with 6 percent blowdown can recover about 1.7 million Btu per hour for every 100,000 pounds per hour of steam, using a flash tank to the deaerator and a makeup water heat exchanger.
Does reverse osmosis replace a softener for boiler makeup?
Usually not. Most boiler RO systems run on softened water so hardness does not scale the membranes, though some use antiscalant instead. RO then removes most of the dissolved solids, silica and alkalinity that a softener leaves, which allows more cycles and less blowdown.
What does a dealkalizer do in a boiler system?
A chloride-cycle dealkalizer exchanges bicarbonate and carbonate alkalinity for chloride after the softener. Less alkalinity means less carbon dioxide released in the boiler, so less carbonic acid in the condensate, fewer amines needed and less blowdown driven by alkalinity.
What happens if process contamination gets into the condensate return?
Oil, sugar, product or cooling water from a leaking exchanger can foul the boiler, cause foaming and carryover, and drive pH out of range. Conductivity or specialty analyzers on each return with an automatic dump valve divert contaminated condensate before it reaches the deaerator.
Why does a closed hot water loop need a makeup meter?
A closed loop should need almost no makeup, so any steady makeup flow means a leak. Each top-up dilutes the inhibitor and adds oxygen and hardness, so finding the leak protects the boiler and the piping.
Do you supply boiler treatment chemicals?
No. Paul Industries installs and services the equipment, including chemical feed pumps, skids and injection points. Chemical products and dosage targets come from the plant's chemical supplier, and we coordinate with them on feed points, sampling and controls.
What is cycles of concentration in a boiler?
It is the ratio of a dissolved impurity in the boiler water to the same impurity in the feedwater, usually measured as conductivity. Higher cycles mean less blowdown, but the ceiling is set by whichever limit is reached first for that boiler pressure, often silica, alkalinity or total dissolved solids, so better makeup treatment is what allows higher cycles.
Should a boiler makeup softener be duplex?
For most plants that make steam continuously, yes. Two tanks alternate, so one is always in service while the other regenerates, and regeneration is triggered by metered volume rather than a timer. A single tank either sends hard water to the boiler or stops the makeup supply while it regenerates.
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