Low Pressure Boiler Make-up Water Treatment

High-purity makeup water and strict control of boiler water and steam chemistry are critical for high-pressure steam systems in power generation. Many industrial facilities, however, use low-pressure boilers, typically up to 600 psi, to produce process steam for things like heating reactors, evaporators, or building spaces. Although these systems require less rigorous treatment than high-pressure units, neglecting proper makeup water and condensate treatment can still cause significant corrosion and scaling.

Makeup Water for Low-Pressure Steam Boilers

The primary scaling risk in these boilers is calcium carbonate (CaCO₃) deposition, which forms when calcium ions react with bicarbonate alkalinity under heat, as demonstrated in basic boiler chemistry.

This is the same limescale found in household hot water systems and on heat exchanger tubes.

For many years, sodium zeolite softening has been the standard method for treating makeup water in industrial low-pressure boilers. In this process, water passes through ion exchange resin beds that replace hardness ions (calcium and magnesium) with sodium ions.

However, the softened water still contains other dissolved species such as alkalinity, chlorides, sulfates, and silica.

This approach offers clear benefits: it is simple, cost-effective compared to advanced methods such as reverse osmosis or demineralization required for high-pressure systems, and it reduces equipment and operating expenses.

However, this method has limitations. Sodium softening does not remove these other ions, which can cause additional problems:

• Bicarbonate alkalinity decomposes in the boiler to form carbon dioxide (CO₂), which volatilizes with steam and re-dissolves in condensate as carbonic acid. This lowers pH and promotes corrosion in return lines.

• Chlorides, and to a lesser extent sulfates, can concentrate beneath deposits—often iron oxides from condensate corrosion—and promote under-deposit corrosion, particularly in the presence of oxygen.

• Inadequate softener maintenance or operational upsets can result in hardness breakthrough, causing scale and corrosion that could be prevented with improved oversight.

Advanced Makeup Treatment Options

While sodium softening remains common, modern membrane technologies such as reverse osmosis (RO) have changed the game for producing cleaner makeup water. Single- or double-pass RO can reject 99% or more of dissolved solids, producing very low-TDS permeate. This reduces boiler treatment complexity, lowers blowdown requirements, and minimizes silica accumulation and alkalinity-related issues.

Successful RO operation requires proper pretreatment to remove suspended solids and the use of antiscalant chemicals to prevent membrane fouling. Accurate feedwater analysis and consultation with experts are essential for system design and chemical selection.

Higher-purity makeup water helps control silica-related scaling, which can result in persistent magnesium or calcium silicates if hydroxide alkalinity is insufficient. It also reduces the need for aggressive pH elevation, which can otherwise increase the risk of caustic corrosion under deposits.

Addressing Carbonic Acid Corrosion in Condensate Systems

Industries such as refining, petrochemicals, pharmaceuticals, and steel production often have extensive steam distribution and condensate return networks.

When makeup water relies solely on sodium softening, bicarbonate alkalinity enters the boiler, decomposes to CO₂, and is carried with the steam. In condensers and heat exchangers, this CO₂ dissolves to form carbonic acid, lowering condensate pH and corroding carbon steel piping.

In power plants, ammonia is often used to control pH, but its volatility makes it less suitable for low-pressure industrial systems. Instead, neutralizing amines are widely used to raise condensate pH and counteract acidity. Different amines have varying basicity and distribution ratios, so a single amine may not provide complete protection in complex systems.

Blended amine formulations offer broader protection throughout the steam and condensate system. Plant audits help customize treatment programs, which are often paired with automated feed and monitoring for improved reliability.

Condensate may also acquire process contaminants from leaks, which can require additional polishing before being returned to the system.

Boiler Water Treatment Approaches

Historically, tri-sodium phosphate (TSP) became the standard for drum boilers to maintain alkaline conditions, minimizing general carbon steel corrosion and precipitating hardness as soft sludges removable by blowdown. In industrial low-pressure units, TSP is often blended with di- or mono-sodium phosphate to prevent excess caustic (NaOH) formation, which can cause under-deposit caustic gouging.

Phosphate treatment is most effective when combined with sludge conditioners, which are water-soluble polymers that disperse solids, modify crystal formation, and aid in suspension and sequestration for easier removal. These polymers may be used alone if hardness ingress is minimal.

Chelants such as EDTA bind metals directly to keep them soluble, but they are rarely used today due to the risk of localized corrosion if misapplied. Their use requires excellent deaeration and precise control.

All-polymer programs have become more popular for low- to medium-pressure boilers, especially when reliable low-hardness feedwater is available. These programs offer deposit control without the sludge associated with phosphate or the risks of chelants.

The optimal internal treatment depends on factors such as boiler design, pressure, makeup water quality, condensate purity, and regulatory requirements (e.g., FDA limits in food and beverage applications). A thorough system evaluation ensures the most reliable and efficient choice.

H2 Global Solutions has the expertise to conduct a comprehensive boiler system audit for your facility. Our team will assess your specific requirements, select an appropriate treatment program, and customize products as needed. We are committed to delivering effective water treatment solutions, allowing you to focus on your core operations.

Contact us today!