5.3 Internal Chemical Treatment: Phosphates, Chelants, Sulfite & Amines

5.3 Internal Chemical Treatment: Phosphates, Chelants, Sulfite & Amines

Quick Summary: External pretreatment appliances (softeners, deaerators, and RO systems) eliminate the vast majority of raw water contaminants, but trace hardness and residual dissolved oxygen always enter the boiler cycle. Internal chemical treatment introduces engineered compounds directly into feedwater or the boiler drum to scavenge residual oxygen, condition hardness into non-adherent sludge or soluble complexes, maintain protective alkaline buffering, and passivate metal surfaces. Low-pressure boilers utilize catalyzed sodium sulfite, but thermal breakdown into corrosive SO₂ above 900 psig mandates hydrazine or volatile organics in high-pressure plants. Hardness is controlled either by precipitating calcium as fluid hydroxyapatite sludge via phosphate programs or by binding ions into soluble ring complexes via chelants (EDTA/NTA), supported by polymeric dispersants and volatile amines.

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1. Chemical Oxygen Scavengers: Sulfite, Hydrazine & Organics

Mechanical deaeration heats feedwater to saturation and strips dissolved oxygen down to approximately 0.005 cc/liter (~7 ppb). To achieve the absolute zero dissolved oxygen required to eliminate localized pitting, chemical oxygen scavengers must be injected continuously into the deaerator storage drop leg or boiler feed pump suction line.

1. Sodium Sulfite ($Na_2SO_3$)

Sodium sulfite is the standard, most economical oxygen scavenger for low- and medium-pressure boilers operating below 900 psig. It reacts directly with dissolved oxygen to form harmless, soluble sodium sulfate ($Na_2SO_4$):

$2 \text{Na}_2\text{SO}_3 + \text{O}_2 \longrightarrow 2 \text{Na}_2\text{SO}_4$

• Stoichiometry and Operational Feeding: Chemically, 7.88 lb of pure sodium sulfite consumes 1 lb of oxygen. In industrial plant practice, operating engineers feed 8 to 10 ppm of commercial sodium sulfite for every 1.0 ppm of dissolved oxygen present in feedwater.
• Reserve Residual Target: To absorb sudden deaerator temperature fluctuations or load swings, a constant chemical reserve must be maintained in the boiler drum water:
  • Up to 300 psig: 30 to 60 ppm residual sulfite ($SO_3^{2-}$)
  • 300 to 600 psig: 20 to 40 ppm residual sulfite
  • 600 to 900 psig: 10 to 20 ppm residual sulfite
• Catalyzed Sulfite: At feedwater temperatures below 180°F, pure sodium sulfite reacts sluggishly with oxygen (taking several minutes). To ensure instantaneous reaction before water enters economizer tubes, chemical vendors add 0.1% cobalt chloride ($CoCl_2$) as a catalyst, cutting reaction time to fractions of a second.

Thermal Breakdown of Sulfite Above 900–1,000 psig

Sodium sulfite is strictly prohibited in high-pressure power boilers (>900–1,000 psig) due to thermal decomposition. Above 540°F (approx. 950 psig saturation temperature), sodium sulfite breaks down into corrosive acidic gases and caustic:

$\text{Na}_2\text{SO}_3 + \text{H}_2\text{O} + \Delta (\text{heat}) \longrightarrow 2 \text{NaOH} + \text{SO}_2\uparrow$ $4 \text{Na}_2\text{SO}_3 + 2 \text{H}_2\text{O} + \Delta (\text{heat}) \longrightarrow 3 \text{Na}_2\text{SO}_4 + 2 \text{NaOH} + \text{H}_2\text{S}\uparrow$

The liberated sulfur dioxide ($SO_2$) and hydrogen sulfide ($H_2S$) gases flash into the steam space, carry through the superheater, and dissolve into condensing turbine moisture, forming aggressive sulfurous acids that cause rapid stress corrosion cracking of steam turbine blades. Furthermore, converting sulfite to sodium sulfate adds heavy Total Dissolved Solids (TDS), drastically increasing blowdown requirements.

2. Hydrazine ($N_2H_4$)

Hydrazine is the premier oxygen scavenger for high-pressure utility and industrial power boilers (>900 to 3,000+ psig):

$\text{N}_2\text{H}_4 + \text{O}_2 \longrightarrow \text{N}_2\uparrow + 2 \text{H}_2\text{O}$

• Zero Added Solids: The reaction products of hydrazine and oxygen are pure, inert nitrogen gas ($N_2$) and pure water ($H_2O$). Unlike sodium sulfite, hydrazine adds zero dissolved solids to boiler water, eliminating solid-induced carryover and blowdown heat losses.
• Passivation of Metal Surfaces: Hydrazine directly reduces red ferric oxide rust into protective black magnetite ($Fe_3O_4$): $\text{N}_2\text{H}_4 + 6 \text{Fe}_2\text{O}_3 \longrightarrow 4 \text{Fe}_3\text{O}_4 + 2 \text{H}_2\text{O} + \text{N}_2\uparrow$
• High-Temperature Decomposition: Above 750°F (400°C), residual unreacted hydrazine decomposes into ammonia and nitrogen: $3 N_2H_4 \longrightarrow 4 NH_3 + N_2$. The volatile ammonia carries over with steam, safely buffering condensate pH against carbonic acid.
• Safety Hazard: Hydrazine is a volatile, toxic suspect carcinogen. It requires closed, automated injection systems to prevent operator exposure.

3. Organic Oxygen Scavengers

Modern plants increasingly utilize non-toxic volatile organic scavengers such as Diethylhydroxylamine (DEHA), Carbohydrazide, and Erythorbic acid (vitamin C derivative). They passivate metal, add negligible solids, volatilize with steam to protect condensate lines, and present no occupational carcinogen hazards.

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2. Hardness Control: Phosphate Precipitation vs. Chelant Programs

Even with 99% efficient sodium zeolite softening, raw water slipping into boiler feedwater contains trace hardness (0.5 to 2.0 ppm). Inside the steam drum, continuous evaporation concentrates this hardness 20 to 50 times. Stationary engineers employ two fundamentally different internal chemical philosophies to prevent crystalline scale: precipitation (phosphate) or solubilization (chelants).

                      INTERNAL HARDNESS CONTROL PHILOSOPHIES
                                        │
         ┌──────────────────────────────┴──────────────────────────────┐
         ▼                                                             ▼
   PHOSPHATE TREATMENT PROGRAM                                   CHELANT TREATMENT PROGRAM
   (Precipitation Chemistry)                                     (Solubilization Chemistry)
   - Feeds Sodium Orthophosphates (TSP, DSP, MSP)                 - Feeds EDTA or NTA organic chelants
   - Forces Ca²⁺ to precipitate as non-adherent                   - Binds Ca²⁺, Mg²⁺, Fe²⁺ into soluble
     HYDROXYAPATITE: Ca₁₀(PO₄)₆(OH)₂                               ring complexes (metal chelates)
   - Requires positive OH alkalinity                              - ZERO sludge formed; surfaces stay pristine
   - Sludge settles in mud drum -> Bottom Blowdown                - DANGER: Overfeed causes severe CHELANT GOUGING!

1. Orthophosphate Precipitation Programs

Phosphate treatment programs deliberately feed sodium orthophosphates into boiler water to force calcium to precipitate as a soft, non-adherent, flocculent sludge instead of hard crystalline scale.

Three orthophosphate salts are utilized, chosen based on the boiler water's natural alkalinity:

1. Monosodium Phosphate ($NaH_2PO_4$): Acidic. Lowers boiler pH while adding phosphate.
2. Disodium Phosphate ($Na_2HPO_4$): Nearly neutral buffer. Adds phosphate with minimal impact on pH.
3. Trisodium Phosphate ($Na_3PO_4$): Strongly alkaline. Adds phosphate while simultaneously boosting hydroxide alkalinity.

Formation of Non-Adherent Hydroxyapatite

When orthophosphate ions ($PO_4^{3-}$) encounter calcium ions ($Ca^{2+}$) in an alkaline environment (pH > 10.0), they react instantly to synthesize calcium hydroxyphosphate (hydroxyapatite):

$10 \text{Ca}^{2+} + 6 \text{PO}_4^{3-} + 2 \text{OH}^- \longrightarrow \text{Ca}_{10}(\text{PO}_4)_6(\text{OH})_2\downarrow$

Hydroxyapatite is a flocculent, snow-like precipitate. Because its crystal structure does not interlock, it possesses virtually zero adhesion to hot steel. Instead of baking into a ceramic scale, it remains suspended as microscopic particles in the circulating water and settles cleanly into the mud drum as soft mud, removed via bottom blowdown.

Magnesium Precipitation as Serpentine

Simultaneously, magnesium hardness ($Mg^{2+}$) reacts with natural boiler silica and hydroxide alkalinity to precipitate as serpentine (magnesium silicate / hydroxide):

$3 \text{Mg}^{2+} + 2 \text{SiO}_3^{2-} + 2 \text{OH}^- + \text{H}_2\text{O} \longrightarrow 3 \text{MgO}\cdot 2 \text{SiO}_2\cdot 2 \text{H}_2\text{O}\downarrow$

Serpentine is also soft and non-adherent. Notice a critical operating rule: silica is deliberately precipitated by magnesium. If magnesium is deficient, silica will bond with calcium to form rock-hard calcium silicate scale!

2. Chelant Treatment Programs (EDTA and NTA)

A radical alternative to precipitation is chelation (derived from the Greek word chele, meaning claw). Chelants are organic chemical compounds that grab multivalent metallic cations and lock them within a stable, water-soluble ring-shaped molecular structure:

• Primary Chelants Utilized: The two standard chelating agents in boiler engineering are Ethylenediaminetetraacetic Acid (EDTA) (usually fed as the tetrasodium salt, $Na_4EDTA$) and Nitrilotriacetic Acid (NTA) ($Na_3NTA$).
• Mechanism of Solubilization: Rather than reacting with hardness ions to form an insoluble precipitate (sludge) that must be conditioned and blown down, chelants form stable, soluble metal-chelate coordination complexes with calcium, magnesium, and iron:

$\text{Ca}^{2+} + \text{EDTA}^{4-} \longrightarrow [\text{Ca-EDTA}]^{2-} \quad (\text{Soluble in Water!})$ $\text{Mg}^{2+} + \text{EDTA}^{4-} \longrightarrow [\text{Mg-EDTA}]^{2-} \quad (\text{Soluble in Water!})$

• Engineering Advantages: Because the hardness ions remain completely dissolved in true solution, zero sludge is generated. Boiler heating surfaces, drum interiors, and generating tubes remain chemically polished down to bare, passivated metal. Heat transfer efficiency is maximized, and bottom blowdown requirements are dramatically curtailed.

Operational Constraints and the "Chelant Corrosion" Hazard

While chelant programs produce remarkably clean boilers, they require rigorous analytical control and carry severe metallurgical risks:

1. Pressure Limit: Chelants decompose thermally at high temperatures. EDTA is typically limited to boilers operating below 1,000 to 1,200 psig, while NTA is limited to boilers below 600 to 900 psig.
2. Feedwater Quality Requirement: Chelants are economically viable only in plants with low, highly consistent feedwater hardness (<1.0 to 2.0 ppm). If raw hardness spikes, chelant demand soars, making the treatment cost prohibitive.
3. Deaeration Dependency: Chelants must never be fed into water containing dissolved oxygen. In the presence of oxygen, chelants break down rapidly into organic acids that attack steel.
4. Catastrophic Chelant Gouging / Corrosion: If an operator overfeeds chelant, the unreacted "free chelant" ($EDTA^{4-}$) encounters no dissolved calcium or magnesium to bind. The chelant aggressively attacks and dissolves the protective magnetite layer ($Fe_3O_4$) and parent carbon steel, seeking out iron cations ($Fe^{2+}$ and $Fe^{3+}$). This produces catastrophic chelant gouging—smooth, deep, localized metal thinning, particularly in areas of high fluid velocity and turbulence such as steam drum internals, downcomer nozzles, and boiler tube entrances. Plants must maintain a strictly controlled "free chelant" residual of only 1 to 3 ppm.

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3. Sludge Conditioners & Polymeric Dispersants

In phosphate-treated boilers, hydroxyapatite and serpentine do not form hard scale, but they are insoluble solids. During peak firing conditions, intense heat can bake even soft phosphate sludge onto generating tube walls, forming a dense mud cake. Furthermore, heavy sludge can agglomerate into thick cakes in the mud drum, choking bottom blowdown lines.

To prevent this, water treatment programs incorporate chemical sludge conditioners:

   WITHOUT POLYMER CONDITIONER                    WITH POLYMER CONDITIONER
 (Crystals Interlock and Bake on Metal)       (Negative Charges Repel, Distorted Lattice)
┌───────────────────────────────────────┐   ┌───────────────────────────────────────┐
│         Hot Tube Metal Surface        │   │         Hot Tube Metal Surface        │
├───────────────────────────────────────┤   ├───────────────────────────────────────┤
│  [Ca]─[PO₄]─[Ca]─[PO₄]─[Ca]─[PO₄]     │   │     (-)          (-)         (-)      │
│    │    │     │    │     │    │       │   │  ╭───────╮    ╭───────╮   ╭───────╮   │
│  [PO₄]─[Ca]─[PO₄]─[Ca]─[PO₄]─[Ca]     │   │  │Crystal│    │Crystal│   │Crystal│   │
│ (Interlocking Rigid Scale Matrix)     │   │  ╰───────╯    ╰───────╯   ╰───────╯   │
│                                       │   │     (-)          (-)         (-)      │
│                                       │   │ (Electrostatic Repulsion: Particles   │
│                                       │   │  Float Freely to Mud Drum!)           │
└───────────────────────────────────────┘   └───────────────────────────────────────┘

1. Modern Synthetic Polymers (Dispersants)

Modern water treatment utilizes water-soluble synthetic polymers, including polyacrylates, polymethacrylates, maleic acid copolymers, and sulfonated styrenes. They operate via two distinct physical mechanisms:

• Crystal Lattice Distortion: Polymer chains adsorb onto the microscopic growing faces of hydroxyapatite crystals. This interrupts the symmetrical geometric growth of the crystal, creating misshapen, irregular, rounded particles that cannot mechanically interlock or bond to each other or to the steel tube wall.
• Electrostatic Repulsion (Dispersion): The polymer molecules impart a strong negative electrical charge to the surface of every suspended sludge and iron particle. Because like charges repel, the particles constantly push away from one another and from the negatively charged steel tube walls. The sludge remains in fluid, microscopic suspension, flowing cleanly with circulating water down into the mud drum for easy evacuation via bottom blowdown.

2. Natural Organic Conditioners (Tannins and Lignins)

Historically (and still utilized in older low-pressure firetube boilers), natural organic extracts—tannins, lignosulfonates, and starches—are fed as sludge conditioners. These complex organic molecules coat suspended mineral crystals with a slippery, gelatinous colloidal sheath, preventing them from coalescing into a solid mass.

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4. Alkalinity Control & The Hydroxide Buffer

Maintaining correct alkalinity in boiler water is mandatory to: (1) maintain the passive protective magnetite layer, (2) drive phosphate precipitation of calcium, and (3) precipitate magnesium as soft serpentine. Alkalinity is supplied by adding caustic soda (sodium hydroxide, $NaOH$) or soda ash (sodium carbonate, $Na_2CO_3$).

The Three Forms of Alkalinity

In water chemistry, alkalinity is measured by acid titration using two different chemical indicators, expressing all results as equivalent ppm $CaCO_3$:

                           THE ALKALINITY SPECTRUM
pH 14.0
   ▲
   │  [ Hydroxide / Caustic Alkalinity: OH⁻ ]
   │   Active above pH 10.0; required for magnetite passivity & serpentine sludge
   │
pH 8.3 ┼─────────────────────────────── Phenolphthalein Endpoint (P-Alkalinity)
   │  [ Carbonate Alkalinity: CO₃²⁻ ]
   │   Half titrated at pH 8.3; completely neutralized at pH 4.3
   │
pH 4.3 ┼─────────────────────────────── Methyl Orange Endpoint (M-Alkalinity / Total)
   │  [ Bicarbonate Alkalinity: HCO₃⁻ ]
   │   Completely neutralized at pH 4.3
   ▼
pH 0.0

1. Phenolphthalein Alkalinity (P-Alkalinity): Titrated with standard acid down to pH 8.3 using phenolphthalein indicator (turns from pink to colorless). Measures all hydroxide ($OH^-$) and half of carbonate ($CO_3^{2-}$).
2. Total / Methyl Orange Alkalinity (M-Alkalinity): Titrated with standard acid down to pH 4.3 using methyl orange or bromocresol green indicator (turns from yellow to orange/pink). Measures all hydroxide, all carbonate, and all bicarbonate ($HCO_3^-$).
3. Caustic / Hydroxide Alkalinity (OH-Alkalinity): Represents free hydroxide ions ($OH^-$) in the boiler water. Calculated directly from titration relationships: $\text{If } 2P > M: \quad \text{OH-Alkalinity} = 2P - M$

Statutory Plant Operating Rule: Boiler water must always maintain positive OH-alkalinity ($2P > M$). In industrial boilers, OH-alkalinity is typically maintained between 50 and 150 ppm as $CaCO_3$ (with total boiler water pH between 10.0 and 11.5). If OH-alkalinity drops to zero, magnesium will precipitate as magnesium phosphate—a sticky, adherent scale that destroys generating tubes.

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5. Antifoams & Carryover Control

Under high steaming rates, high concentrations of dissolved solids (TDS), suspended sludge, and excessive caustic alkalinity dramatically increase the surface tension of boiler water. Instead of steam bubbles breaking cleanly at the water surface, a tough, stubborn foam layer forms across the steam drum.

Hazards of Foaming and Carryover

When the foam layer rises toward the top of the steam drum, steam leaving the drum entrains foam, mist, and liquid droplets—a casualty known as carryover:

• Liquid water enters the superheater tubes, causing severe thermal shock and water hammer.
• Dissolved mineral salts carried by water droplets deposit onto hot superheater tubes, insulating them and causing superheater tube burnout.
• Mineral salts carry over into the steam turbine, impinging onto high-speed turbine blading, causing erosion, scale imbalance, and violent turbine vibration.

Antifoam Chemistry

To prevent foam formation, chemical treatment programs inject antifoaming agents—primarily polyalkylene glycols and polyamides—at minute dosages (typically 1 to 5 ppm). Antifoam molecules are surface-active agents that concentrate at the liquid-vapor bubble film. They radically weaken the localized surface tension of the bubble wall, causing micro-bubbles to coalesce into large bubbles that burst immediately upon reaching the water surface, guaranteeing clean steam disengagement.