Boiler and Cooling Water Treatment: Chemistry, Equipment and Control Limits

Quick Answer

Industrial water systems fail in three ways — scale, corrosion and fouling — and water treatment exists to prevent all three at once, which requires controlling opposing chemistry. In boilers, ASME consensus operating practice limits silica, total alkalinity, specific conductance and total dissolved solids by drum pressure, tightening sharply as pressure rises: a boiler below 20 bar may run at 3,500 µS/cm conductance, while one above 60 bar is limited to a few hundred. Deaeration is non-negotiable: a properly operating deaerator takes dissolved oxygen from around 8-9 mg/L down to about 7 µg/L (0.007 mg/L), after which an oxygen scavenger removes the remainder — and oxygen pitting is responsible for more boiler tube failures than any other single mechanism. In cooling systems, cycles of concentration govern everything: raising cycles from 3 to 6 roughly halves make-up water and cuts blowdown by about 70%, but concentrates scaling species proportionally, so the limit is set by calcium carbonate, calcium sulphate and silica saturation, usually assessed with the Langelier Saturation Index and its refinements. Blowdown is not waste to be minimised to zero; it is the only mechanism that removes dissolved solids, and a system with inadequate blowdown scales, carries over or both.

A textile dyehouse in South Asia lost a 12 tonne/hour firetube boiler after four years. The failure was a furnace tube rupture. The post-mortem found 4 mm of hard scale on the fire-side-facing water surfaces and the tube metal had overheated and bulged before it tore.

Their treatment programme was a drum of phosphate dosed by a plunger pump at a rate set in 2019 and never changed, and a manual blowdown valve opened "every shift or so, for a bit." There was no softener on the make-up — the bore water was 240 mg/L as CaCO₃ hardness — and no conductivity measurement anywhere in the system.

Scale is an insulator. Metal under 4 mm of calcium carbonate scale cannot transfer heat to the water, so it runs at flame temperature instead of water temperature. The tube did not fail because the pressure was too high; it failed because the steel was too hot, which is what scale does.

Replacing the boiler cost about 310,000 USD plus eleven weeks of hired steam. A softener, a conductivity controller with an automatic blowdown valve, and a properly specified chemical programme would have cost about 28,000 USD installed, and would also have cut their fuel bill, because every millimetre of scale costs several percent of boiler efficiency.

Water treatment is the cheapest insurance in a plant and the most consistently neglected.

The Three Problems

Scale is the deposition of sparingly soluble salts on heat transfer surfaces. Calcium carbonate is the most common, and it has the awkward property of inverse solubility — it becomes less soluble as temperature rises, so it deposits preferentially on the hottest surfaces, which are exactly the surfaces that matter. Calcium sulphate, calcium phosphate, magnesium silicate and amorphous silica follow. Scale costs heat transfer, costs fuel, and in boilers causes overheating failures.

Corrosion is metal loss. In boiler systems the dominant mechanisms are oxygen pitting (localised, rapid, and the leading cause of tube failure), carbonic acid attack in condensate return lines from CO₂ released by carbonate decomposition, caustic attack where concentrated hydroxide accumulates under deposits, and acid attack from low pH. In cooling systems, general corrosion, galvanic corrosion at dissimilar metal joints, under-deposit corrosion and microbiologically influenced corrosion all operate.

Fouling is the accumulation of material that is not scale: suspended solids, corrosion products, process leakage, biological growth, airborne debris in cooling towers. Fouling reduces heat transfer, restricts flow and — critically — shelters the metal beneath it from the treatment chemicals in the bulk water, so under-deposit corrosion proceeds unchecked.

These interact. Deposits shelter corrosion. Corrosion products become deposits. Biofilm provides sites for both. Treating one in isolation makes the others worse — which is why water treatment is a programme rather than a chemical.

Boiler Water Treatment

Make-up water pretreatment

The treatment required scales with pressure.

Softening (sodium cycle ion exchange) replaces calcium and magnesium with sodium, eliminating hardness scale. Regenerated with brine. Cheap, robust, the foundation of low-pressure boiler treatment. It does not remove alkalinity, silica or total dissolved solids.

Dealkalisation removes bicarbonate alkalinity, which otherwise decomposes in the boiler to release CO₂ into the steam, acidifying the condensate. Either chloride-anion exchange or weak-acid cation exchange.

Reverse osmosis removes 95-99% of dissolved solids, most silica and most organics. Reduces blowdown requirement dramatically because the boiler water starts much purer. Needs antiscalant dosing, cartridge filtration and in many cases dechlorination ahead of the membranes.

Demineralisation (ion exchange) produces high-purity water. Mixed-bed or two-bed cation-anion. Essential above about 60 bar, where silica carryover into the turbine becomes the controlling issue.

Deaeration removes dissolved oxygen and CO₂ mechanically. Water heated to within a few degrees of saturation at the deaerator pressure, with counter-current steam scrubbing, reduces oxygen from around 8-9 mg/L at ambient to approximately 7 µg/L — the classic performance figure for a tray or spray-tray deaerator. Mechanical deaeration does the heavy lifting; chemical scavenging polishes.

Internal treatment chemistry

Oxygen scavengers remove the residual oxygen the deaerator leaves.

• Sulphite (sodium sulphite, bisulphite) — fast, inexpensive, adds dissolved solids, and decomposes above about 60 bar to form SO₂ and H₂S. Standard below that.
• Hydrazine — adds no solids, passivates metal surfaces, but is a suspected carcinogen and is restricted or banned in many jurisdictions.
• Carbohydrazide, DEHA, erythorbate — volatile or low-solids alternatives that protect the condensate system as well as the boiler.

Scale control

• Phosphate (coordinated or congruent phosphate) — precipitates residual calcium as a non-adherent hydroxyapatite sludge removed by blowdown. Requires alkalinity control to keep the precipitate in the right form.
• Chelants (EDTA, NTA) — sequester hardness in solution. Effective but corrosive if overfed, and degrade above about 60 bar.
• Polymer dispersants — keep precipitated solids suspended and non-adherent. Used alongside phosphate; modern programmes rely heavily on them.
• All-volatile treatment (AVT) — ammonia and an oxygen scavenger only, no solids. For high-pressure and once-through boilers with demineralised feedwater.

Alkalinity and pH control — boiler water pH is typically held at 10.5-11.5 to passivate steel and keep phosphate chemistry in the right region. Caustic soda or the natural alkalinity from feedwater provides it. Too low and the steel corrodes; too high and caustic embrittlement and caustic gouging become risks, particularly under deposits or where steam blanketing concentrates the hydroxide.

Condensate treatment

• Neutralising amines (morpholine, cyclohexylamine, DEAE) raise condensate pH to neutralise carbonic acid. Different amines have different distribution ratios between steam and water, so blends are used to protect both near and far parts of the condensate system.
• Filming amines (octadecylamine) form a hydrophobic barrier film on metal surfaces.
• Condensate return is the most valuable water in the plant — hot, pure and already treated. Protecting it pays for itself in fuel, make-up water and chemicals.

ASME consensus control limits

Limits tighten sharply with drum pressure. The widely used ASME consensus operating practice values for watertube boilers with drum-type construction are approximately:

Figures are indicative of the published consensus bands and should be confirmed against the current ASME guidance and the boiler maker's own requirements, which may be stricter. The pattern is what matters: every parameter tightens as pressure rises, and above about 60 bar the margins become narrow enough that reliable instrumentation and automatic control are not optional.

Silica is the parameter most often ignored and most damaging in high-pressure plants. Silica volatilises into the steam at high pressure and deposits in the turbine, where it cannot be removed without an outage. Silica in the boiler water is controlled by feedwater quality and blowdown, not by chemicals.

Blowdown

Blowdown is the only mechanism that removes dissolved solids from a boiler. Steam leaves as pure water; everything dissolved in the feedwater stays behind and concentrates.

Continuous blowdown from the steam drum surface, controlled on conductivity, removes dissolved solids. Should be automated with a conductivity controller and modulating valve.

Intermittent bottom blowdown from the mud drum or lowest point, short and sharp, removes settled sludge. Typically a few seconds every shift. Long bottom blowdowns waste large amounts of energy and water and can disturb drum level.

Blowdown rate calculation:

Blowdown % = (feedwater TDS × 100) / (maximum allowable boiler water TDS − feedwater TDS)

For feedwater at 60 mg/L TDS and a boiler limit of 2,000 mg/L, blowdown is about 3.1% of feedwater. For feedwater at 400 mg/L — poorly softened or high-TDS make-up — the same limit requires 25% blowdown, which is an enormous energy and water loss and usually unaffordable. This is the economic case for better pretreatment: cutting feedwater TDS cuts blowdown proportionally.

Blowdown heat recovery — blowdown leaves at saturation temperature and carries significant energy. A flash vessel recovers low-pressure steam, and a blowdown heat exchanger preheats make-up water. On a boiler with 5% blowdown at 10 bar, recovery typically pays back in well under two years.

Cooling Water Treatment

Cycles of concentration

In an evaporative cooling tower, water evaporates and leaves dissolved solids behind. Cycles of concentration (CoC) is the ratio of dissolved solids in the circulating water to that in the make-up, and in practice is measured as the ratio of conductivities or of a conservative ion such as chloride.

CoC = make-up volume / blowdown volume = circulating conductivity / make-up conductivity

The water balance:

Make-up = evaporation + blowdown + drift

Blowdown = evaporation / (CoC − 1)

The consequence is dramatic at low cycles and flattens quickly:

Going from 2 to 4 cycles cuts make-up by a third and blowdown by two-thirds. Going from 6 to 10 saves comparatively little water but roughly doubles the concentration of every scaling species. The useful range for most systems is 4 to 8 cycles, and the limit is set by which species saturates first.

Scaling indices

Langelier Saturation Index (LSI) = pH − pH_s, where pH_s is the pH at calcium carbonate saturation, calculated from calcium hardness, total alkalinity, total dissolved solids and temperature.

• LSI < 0 — undersaturated, corrosive tendency
• LSI = 0 — at equilibrium
• LSI > 0 — supersaturated, scaling tendency
• LSI above about +0.5 to +1.0 — scaling without inhibitor; modern inhibitors permit controlled operation at LSI +2.0 to +2.8 in well-managed systems

Ryznar Stability Index (RSI) = 2pH_s − pH. Below 6 scaling, above 7 corrosive.

Puckorius Scaling Index (PSI) uses equilibrium pH derived from alkalinity rather than measured pH, which is more realistic in cooling towers where CO₂ stripping elevates measured pH.

Other saturation limits to check independently of LSI:

• Calcium sulphate — gypsum scale. The product of calcium and sulphate concentrations must stay below the solubility product. Governs where make-up is high in sulphate, or where sulphuric acid is used for pH control.
• Silica — amorphous silica is generally limited to about 150-180 mg/L as SiO₂ in the circulating water, and magnesium silicate to a magnesium × silica product limit. Silica scale is very hard and essentially impossible to remove chemically. In high-silica make-up water, silica often sets the cycle limit before calcium carbonate does — a frequent surprise in Middle Eastern and North African groundwater.
• Calcium phosphate — where phosphate-based inhibitors are used, calcium phosphate itself can scale. A real constraint in high-calcium, high-pH systems.

Cooling water chemical programmes

Scale inhibitors

• Phosphonates (HEDP, PBTC, AMP) — threshold inhibitors that disrupt crystal growth at sub-stoichiometric doses. PBTC tolerates chlorine better than HEDP.
• Polymers (polyacrylate, polymaleic, sulphonated copolymers) — disperse precipitated solids and inhibit crystal growth. Sulphonated copolymers also tolerate high calcium and iron.
• Polyphosphates — inhibit scale and provide some corrosion protection, but revert to orthophosphate and can contribute to calcium phosphate scaling and to nutrient loading.

Corrosion inhibitors

• Zinc — effective cathodic inhibitor, but discharge-restricted in many jurisdictions
• Molybdate — effective anodic inhibitor, expensive, environmentally better regarded
• Orthophosphate — anodic inhibitor forming a protective iron phosphate film; needs careful control against calcium phosphate scaling
• Azoles (tolyltriazole, benzotriazole) — specific to copper and copper alloys, essential wherever admiralty brass or copper tubing is present
• All-organic programmes — phosphonate and polymer based, no heavy metals, now standard where discharge regulations are strict

Microbiological control

• Oxidising biocides — chlorine, bromine (stable at higher pH than chlorine, which makes it preferable in alkaline cooling water), chlorine dioxide, stabilised hypochlorite. Continuous low-level or intermittent slug dosing, controlled on ORP.
• Non-oxidising biocides — isothiazolinone, glutaraldehyde, DBNPA, quaternary ammonium compounds. Alternated with oxidisers to prevent resistance and to penetrate biofilm.
• Biodispersants — surfactants that penetrate and lift biofilm so biocides reach the organisms underneath.
• Legionella control is a specific duty with its own regulatory regime in many countries, requiring a written risk assessment, temperature and chemical control, drift elimination, and routine sampling. Cooling towers are a recognised Legionella transmission route and this cannot be treated as a sub-case of general microbiological control.

Biofilm is thermally far more resistant than scale for a given thickness — a biofilm less than a millimetre thick can impair heat transfer as much as several millimetres of calcium carbonate. Microbiological control is a heat transfer issue, not only a hygiene issue.

Side-stream filtration

Cooling towers are air scrubbers: they wash dust, pollen, insects and airborne solids out of very large volumes of air and into the water. A side-stream filter treating 1-5% of the circulating flow continuously — sand, multimedia, bag, cartridge or centrifugal separator — removes suspended solids and markedly reduces under-deposit corrosion and biofouling. Underspecified far more often than it is overspecified.

Monitoring and Control

A treatment programme without measurement is a hope.

Minimum boiler monitoring

• Continuous conductivity on boiler water with automatic blowdown control
• Feedwater and condensate conductivity
• Boiler water pH, phosphate, alkalinity (daily or continuous)
• Feedwater dissolved oxygen, continuous above about 40 bar
• Silica on medium and high-pressure plants
• Feedwater iron and copper periodically — rising iron indicates condensate system corrosion
• Deaerator outlet temperature and pressure

Minimum cooling monitoring

• Continuous conductivity with automatic blowdown control
• ORP for oxidising biocide control
• pH, calcium, alkalinity, chloride, silica, iron
• Inhibitor residual, by the appropriate method for the product
• Corrosion coupons or online corrosion monitoring, with mild steel and copper alloy coupons matching system metallurgy
• Dip slides or ATP for microbiological count
• Legionella sampling to the applicable regulatory schedule
• Heat exchanger approach temperatures as a fouling indicator

Automatic blowdown control pays for itself quickly in both systems. Manual blowdown is either too little — and the system scales — or too much, which wastes water, chemicals and, in boilers, substantial heat.

Common Specification Mistakes

1. No softener on boiler make-up with hard water. The textile dyehouse example: 4 mm of scale, a ruptured furnace tube, 310,000 USD.

   Prevention: Specify pretreatment against the actual make-up water analysis and the boiler pressure, with duplex softeners so regeneration never interrupts supply.

2. Manual blowdown. Either inadequate, so solids concentrate and scale, or excessive, which wastes water and heat.

   Prevention: Specify conductivity-controlled automatic continuous blowdown plus timed intermittent bottom blowdown.

3. Chemical dose rate set once and never adjusted. Load changes, make-up quality changes, and condensate return fraction changes all alter the required dose.

   Prevention: Control dosing on a measured parameter — conductivity, residual, feed rate — not on a fixed pump setting, and review the programme against test results at defined intervals.

4. Treating cooling water on LSI alone and ignoring silica. Many Middle Eastern and North African groundwaters are high in silica, which sets the cycle limit well before calcium carbonate does.

   Prevention: Check calcium carbonate, calcium sulphate, silica and magnesium silicate saturation independently at the intended cycles, and set cycles against whichever saturates first.

5. Sulphite oxygen scavenger above about 60 bar. It decomposes to SO₂ and H₂S, which attack the steam and condensate system.

   Prevention: Use a volatile or low-solids scavenger above 60 bar, and all-volatile treatment on high-pressure and once-through units.

6. No deaerator, or a deaerator running below its design temperature. Oxygen pitting proceeds regardless of how much scavenger is dosed.

   Prevention: Verify the deaerator operates at its design pressure and temperature with adequate venting, and measure dissolved oxygen at the outlet rather than assuming it.

7. No azole in a cooling system containing copper alloys. Copper corrodes, and dissolved copper plates onto steel, creating aggressive galvanic cells.

   Prevention: Include a copper corrosion inhibitor wherever admiralty brass, copper or cupronickel is present anywhere in the circuit.

8. No side-stream filtration on a cooling tower in a dusty location. Suspended solids accumulate, deposit, and shelter under-deposit corrosion.

   Prevention: Specify side-stream filtration at 1-5% of circulating flow, sized against local airborne dust loading.

9. Oxidising biocide used alone, continuously. Biofilm develops resistance and shelters organisms from the oxidiser.

   Prevention: Alternate oxidising and non-oxidising biocides, include a biodispersant, and verify with dip slides or ATP rather than assuming.

10. No condensate treatment. Carbonic acid from carbonate decomposition thins condensate lines, and the iron returns to the boiler as deposit-forming iron oxide.

    Prevention: Dealkalise the make-up or dose neutralising amine blends, and monitor condensate pH and iron.

11. No blowdown heat recovery. Boiler blowdown at saturation temperature carries significant energy straight to drain.

    Prevention: Specify a flash vessel and blowdown heat exchanger on any boiler with meaningful continuous blowdown.

12. Treating a high-TDS make-up with chemicals instead of improving pretreatment. The required blowdown becomes so high that fuel and water costs dwarf the pretreatment capital.

    Prevention: Calculate the blowdown percentage from feedwater TDS and the boiler limit, cost the resulting heat and water loss, and compare against RO or demineralisation capital.

Supply from Kasko Makine

Kasko Demir Çelik Makine supplies water treatment equipment for boiler, cooling and process water systems:

Pretreatment

• Multimedia, sand, activated carbon and cartridge filters
• Duplex and triplex water softeners with brine systems
• Dealkalisers
• Reverse osmosis systems, single and two-pass, with membrane housings and antiscalant dosing
• Mixed-bed and two-bed demineralisers
• Degassers and decarbonators
• Ultrafiltration skids

Boiler plant equipment

• Spray-tray and spray-type deaerators with storage sections
• Feedwater tanks and condensate receivers
• Blowdown flash vessels and blowdown heat exchangers
• Boiler feed pumps, multistage
• Continuous blowdown valves and conductivity-controlled automatic blowdown systems
• Sampling coolers and sample panels
• Chemical dosing skids with metering pumps, tanks, agitators and injection quills
• Conductivity, pH and oxygen analysers with controllers

Cooling system equipment

• Side-stream filters: multimedia, bag, cartridge and centrifugal separators
• Automatic blowdown control valves with conductivity controllers
• Biocide and inhibitor dosing skids with ORP control
• Corrosion coupon racks and monitoring assemblies
• Cooling tower components: fill, drift eliminators, nozzles, basin strainers
• Circulating pumps and strainers
• Plate and shell-and-tube heat exchangers

Associated supply

• Piping, valves and fittings in carbon steel, stainless steel, HDPE and PVC
• Steam traps and condensate equipment
• Instrumentation: flow, level, pressure, temperature
• Tanks and vessels in carbon steel, stainless and GRP

Engineering support

Send a full make-up water analysis (hardness, alkalinity, chloride, sulphate, silica, iron, TDS, pH), boiler pressure and capacity, condensate return percentage, cooling system circulating flow and evaporation rate, system metallurgy and local discharge limits. We will return a treatment scheme with pretreatment selection, calculated blowdown rate or achievable cycles of concentration with the limiting species identified, equipment sizing, and the monitoring and control instrumentation needed to operate it.

Certification

Material certificates to EN 10204 3.1 for pressure components, ASME Section VIII and PED documentation for pressure vessels, NSF or equivalent certification for potable-contact components where required, hydrostatic test certificates, performance test reports for deaerators, and functional test records for dosing and control skids.

Logistics

Standard softeners, filters and dosing skids generally ship in 4-8 weeks. RO and demineralisation plants typically 10-16 weeks. Deaerators and pressure vessels 12-20 weeks depending on size and certification. Shipping from Istanbul by road to Europe, the Caucasus and Iraq, and by sea to Gulf, African and Asian destinations.

Send your water analysis and system data and we will return a treatment scheme with equipment sizing within five working days. Reach us at info@kaskomakine.com or WhatsApp +90 (537) 521 1399.

Continue Reading: Water and Steam Series

• Steam Traps: Types, Selection and Failure
• Wastewater Treatment Equipment
• Heat Exchangers: Complete Guide
• Shell & Tube vs Plate Heat Exchangers

Frequently Asked Questions

Q: What are cycles of concentration in cooling water?
A: Cycles of concentration is the ratio of dissolved solids in the circulating cooling water to that in the make-up, measured in practice as the ratio of conductivities. Blowdown equals evaporation divided by cycles minus one, so raising cycles from 2 to 4 cuts blowdown by two-thirds and make-up by a third. The practical range is usually 4 to 8 cycles, limited by whichever scaling species — calcium carbonate, calcium sulphate or silica — saturates first.

Q: How much blowdown does a boiler need?
A: Blowdown percentage equals feedwater TDS multiplied by 100, divided by the difference between the maximum allowable boiler water TDS and the feedwater TDS. For 60 mg/L feedwater and a 2,000 mg/L boiler limit, that is about 3.1% of feedwater. Poor pretreatment giving 400 mg/L feedwater would require about 25% blowdown at the same limit, which is why improving pretreatment is usually cheaper than paying for the heat and water lost in blowdown.

Q: Why is dissolved oxygen so damaging in boilers?
A: Dissolved oxygen causes localised pitting corrosion rather than general metal loss, so it concentrates attack and perforates tubes quickly, making it the leading cause of boiler tube failure. A properly operating deaerator reduces oxygen from around 8 to 9 mg/L at ambient to approximately 7 µg/L, and a chemical scavenger removes the remainder. Chemical scavenging alone cannot compensate for a deaerator running below its design temperature.

Q: What is the Langelier Saturation Index?
A: LSI is the measured pH minus the pH at which calcium carbonate is saturated, calculated from calcium hardness, total alkalinity, total dissolved solids and temperature. A negative LSI indicates corrosive, undersaturated water and a positive value indicates a scaling tendency. Untreated water scales above about +0.5 to +1.0, while modern threshold inhibitors allow controlled operation at LSI around +2.0 to +2.8.

Q: Why does scale cause boiler tube failures?
A: Scale is a thermal insulator, so metal under a scale layer cannot transfer its heat into the water and instead rises toward flame temperature. The steel loses strength as it overheats, bulges under pressure and ruptures. The pressure has not changed; the metal temperature has. A few millimetres of calcium carbonate is enough to cause this, and the same insulation also costs several percent of boiler efficiency.

Q: How often should cooling tower biocide be changed?
A: Oxidising biocides such as chlorine or bromine used continuously allow biofilm to develop tolerance and shelter organisms from the oxidiser. Good practice is to alternate oxidising and non-oxidising biocides, add a biodispersant to penetrate and lift biofilm, and verify results with dip slides or ATP testing rather than assuming effectiveness. Systems subject to Legionella regulations have their own mandatory sampling and control schedule.

Q: Does silica limit cooling water cycles of concentration?
A: Frequently, yes. Amorphous silica is generally limited to about 150 to 180 mg/L as SiO2 in circulating water, with magnesium silicate limited by a magnesium times silica product. Many Middle Eastern, North African and Central Asian groundwaters are high in silica, so silica saturation is reached before calcium carbonate and sets the cycle limit. Silica scale is extremely hard and cannot practically be removed chemically, so it must be prevented.