Chemical Treatments for Waterside of Boiler Tubes

The feedwater is composed of makeup water (usually city water from outside boiler room/ process) and condensate (condensed steam returning to the boiler). The feedwater normally contains impurities, which can cause deposits and other related problems inside the boiler. Common impurities in water include alkalinity, silica, iron, dissolved oxygen and calcium and magnesium (hardness). Blowdown, a periodic or continuous water removal process, is used to limit the concentration of impurities in boiler water and to control the buildup of dissolved solid levels in the boiler. Blowdown is essential in addition to chemical treatments.

Boiler Waterside Fouling Scale is one of the most common deposit related problems. Scale is a buildup of solid material from the reactions between the impurities in water and tube metal, on the water-side tube surface. Scale acts as an insulator that reduces heat transfer, causing a decrease in boiler efficiency and excessive fuel consumption. More serious effects are overheating of tubes and potential tube failure (equipment damage). Fuel wasted due to scale may be approximately 2-5 percent depending on the scale thickness.

Oxygen attack is the most common causes of corrosion inside boilers. Dissolved oxygen in feedwater can become very aggressive when heated and reacts with the boiler’s internal surface to form corrosive components on the metal surface. Oxygen attack can cause further damage to steam drums, mud dams, boiler headers and condensate piping.

Acid attack is another common causes of corrosion. Acid attack happens when the pH of feedwater drops below 8.5. The carbonate alkalinity in the water is converted to carbon dioxide gas (CO₂) by the heat and pressure of the boilers. CO₂ is carried over in the steam. When the steam condenses, CO₂ dissolves in water to form carbonic acid (H₂CO₃) and reduces the pH of the condensate returning to the boilers. Acid attack may also impact condensate return piping throughout the facility.

Chemical Treatments

• Lime Softening and Soda Ash Quick or slaked lime (usually calcium hydroxide) is added to hard water to precipitate the calcium, magnesium and, to some extent, the silica in the water. Soda ash is added to precipitate non-bicarbonate hardness. The process typically takes place in a clarifier followed by a hydrogen cycle cation exchange and a hydroxide cycle anion exchange demineralization. Please see the basic boiler system schematic illustrated on page 3.

• Phosphate Mono-, di- or trisodium phosphate and sodium polyphosphate can be added to treat boiler feedwater. Phosphate buffers the water to minimize pH fluctuation. It also precipitates calcium or magnesium into a soft deposit rather than a hard scale. Additionally, it helps to promote the protective layer on boiler metal surfaces. However, phosphate forms sludge as it reacts with hardness; blowdown or other procedures should be established to remove the sludge during a routine boiler shutdown.

• Chelates Nitrilotriacetic acid (NTA) and ethylenediamine tetraacetic acid (EDTA) are the most commonly used chelates. Chelates combine with hardness in water to form soluble compounds. The compounds can then be eliminated by blowdown. The preferred feed location for chelates is downstream of the feedwater pump. A stainless steel injection quill is required. However, chelates treatment is not recommended for feedwater with high hardness concentration.

• Polymers Most polymers used in feedwater treatment are synthetic. They act like chelates but are not as effective. Some polymers are effective in controlling hardness deposits, while others are helpful in controlling iron deposits. Polymers are often combined with chelates for the most effective treatment.

• Oxygen Scavengers A deaerator removes most of the oxygen in feedwater; however, trace amounts are still present and can cause corrosion-related problems. Oxygen scavengers are added to the feedwater, preferably in the storage tank of the feedwater, to remove the trace mount of oxygen escaped from the deaerator. The most commonly used oxygen scavenger is sodium sulfite. Sodium sulfite is cheap, effective and can be easily measured in water.

• Neutralizing Amines Neutralizing amines are high pH chemicals that can be fed directly to the feedwater or the steam header to neutralize the carbonic acid formed in the condensate (acid attack). The three most commonly used neutralizing amines are morpholine, diethyleminoethanal (DEAE) and cyclohexylamine. Neutralizing amines cannot protect against oxygen attack; however, it helps keep oxygen less reactive by maintaining an alkaline pH.

• Filming Amines Filming amines are various chemicals that form a protective layer on the condensate piping to protect it from both oxygen and acid attack. The filming amines should be continuously fed into the steam header with an injection quill based on steam flow. The two most common filming amines are octadecylamine (ODA) and ethoxylated soya amine (ESA). Combining neutralizing and filming amine is a successful alternative to protect against both acid and oxygen attack. The basic boiler system schematic shown below illustrates the points of chemical addition for boiler water treatment.