Short summary
• Phosphate dosing — internal phosphate treatment: prevents scale and forms a protective film in the boiler drum/wet parts (used in drum-type boilers where phosphate program is specified).
• Hydrazine dosing — oxygen scavenger: removes dissolved O₂ to prevent pitting/corrosion and helps maintain a reducing environment in the feedwater/boiler system.
• Ammonia dosing — volatile pH control (AVT/volatile amine): raises/controls pH in condensate/feedwater to reduce corrosion in condensers, feedtrain and steam-exposed metal surfaces.
Detailed role & functioning
1) Phosphate dosing (internal phosphate treatment)
Purpose
• Controls scale formation and reduces corrosive attack by forming a protective iron phosphate layer on boiler surfaces (mainly for drum boilers with moderate pressures and for waters with hardness control).
• Helps manage carryover and solids chemistry in the boiler drum.
Where dosed
• To feedwater or directly to boiler drum/feedwater line so the phosphate is present in the drum water where it reacts and forms protective compounds.
How it works (in brief)
• Phosphate ions react with iron ions at metal surfaces to form a thin, adherent protective film (iron phosphate), which inhibits general corrosion and reduces under-deposit corrosion and localized attack.
Control & monitoring
• Phosphate analyzer, conductivity, TDS and boiler water tests (silica, alkalinity). Avoid “over-phosphating” — too high phosphate causes deposits/carryover and foaming.
Risks if misapplied
• Excess phosphate carries over into steam (contaminates turbine), can form hard deposits on heat-transfer surfaces, and can cause boiler foaming and carryover.
2) Hydrazine dosing (oxygen scavenger / reducing agent)
Purpose
• Chemically removes dissolved oxygen (O₂) remaining after mechanical deaeration to prevent oxygen pitting and general corrosion in feedwater/boiler metal.
• Helps maintain a mildly reducing chemistry which protects copper alloys and steel in the high-temperature circuit.
Where dosed
• Common practice: dose after deaerator in the feedwater line (any remaining oxygen is scavenged before the water enters boiler/high-heat surfaces). May also be added to the deaerator sparge/feed.
Main reaction (simplified)
• N₂H₄ + O₂ → N₂ + 2 H₂O (hydrazine reacts with oxygen to form nitrogen and water)
• At high temperatures hydrazine can also decompose to ammonia and nitrogen.
Control & monitoring
• DO (dissolved oxygen) meters, hydrazine residual checks (chemical test kits), feedforward control tied to condensate/boiler feed flow and DO.
• Metering pumps provide controlled feed rate proportional to flow or DO.
Safety & notes
• Hydrazine is toxic and potentially carcinogenic — strict safety, storage and handling rules required.
• Overdose is wasteful and creates excess ammonia (from decomposition), underdose leaves oxygen and risks pitting.
3) Ammonia dosing (volatile pH control / AVT)
Purpose
• Raises and controls the pH of the condensate/steam-water path using a volatile base (ammonia). A slightly alkaline pH reduces corrosion in condensers, feedwater heaters, piping and turbine wet areas.
• Used as part of All Volatile Treatment (AVT) or combined with oxygen scavengers (e.g., hydrazine) depending on plant chemistry strategy.
Where dosed
• Often dosed into condensate or condensate pump suction so the volatile ammonia travels with steam condensate and levels out pH throughout the steam/condensate/feedwater system.
How it works
• Ammonia is volatile with steam, so it partitions between steam and water and provides alkalinity throughout the system, neutralizing acidic species and inhibiting corrosion, especially of condensers (passivation of stainless/copper alloys).
Control & monitoring
• pH meters in condensate and feedwater, conductivity, and periodic chemical analyses. Dosing controlled by pH and flow-based metering pumps.
Risks
• Excess ammonia can harm metallurgy, affect lubricant chemistry (if present), and cause environmental problems if vented. Insufficient ammonia leaves the system acidic and prone to corrosion.
Plant chemistry strategies and interplay
• Two common approaches across plants:
• Phosphate program (internal treatment): For certain drum boilers — phosphate + hydrazine (oxygen scavenger) may be used together, with controlled phosphate concentrations in drum water.
• All-Volatile Treatment (AVT): Use volatile amines (ammonia or alternative amines) + oxygen scavenger (often hydrazine or alternative) — no phosphate. This is common in higher-pressure systems or systems favoring a volatile-only program.
• Choice depends on boiler type, pressure, metallurgy, feedwater quality and regulatory/environmental constraints.
Typical instrumentation & control loop (concept)
• Sensors: DO meter, pH sensors (condensate/feedwater), conductivity, phosphate analyzer (if used).
• Controller: PLC/DCS takes sensor input and modulates metering
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