Boilers make steam from feedwater, and most feedwater has impurities that accelerate corrosion in the boiler chamber — how much of any contaminant found in feedwater depends on the water source. Freshwater comes from many sources, such as lakes, rivers or wells. When water makes steam, its impurities are important for consideration.

With the trend toward high-pressure boilers, the successful operation of power plants demands that water be pretreated to remove impurities. This practice reduces corrosion, carryover and deposits. When water is of poor quality, so is the steam, and downtime from corrosion problems and routine maintenance increase.

According to the American Society of Mechanical Engineers (ASME), plant operators who want control over the deposition and corrosion in the boiler must have in place an effective plan for the effective monitoring of makeup water, condensate, feedwater and boiler water qualities.

Without sound online monitoring and control of feed water, a greater likelihood exists for increased costs and added failure of boiler components. But even with the best systems in place, the complete elimination of feedwater contaminants is impossible.

The following are four factors that need ongoing surveillance:

1. Dissolved oxygen

When oxygen contaminates steam condensate, one result is the potential for inefficient or improper feedwater aeration. The usual suspects for oxygen contamination are leaking heat exchangers; leakage of air at pump seals, flanges and receivers; or ingress into systems that run under vacuum. Other serious problems caused by dissolved oxygen include localized pitting deterioration that in turn might cause failure of critical parts of the steam system which is often sudden.

A popular solution is dosing boiler feedwater with the oxygen-scavenging chemical hydrazine. Boiler operators can measure the efficiency of hydrazine or similar chemicals by measuring for dissolved oxygen at the boiler inlet or economizer. By the monitoring of dissolved oxygen, operators can adjust the dosing to support acceptable levels.

Since oxygen levels change rapidly, an analyzer that has a quick response is necessary.

2. Hydrazine

When using hydrazine to control dissolved oxygen, it is normal for it to convert into nitrogen and water. However, when used at high pressure and high temperatures, it also makes ammonia. Ammonia increases the pH of the feed water, which in turn lowers the possibility of acidic corrosion.

Another welcome feature of hydrazine is its reaction with soft-haematite layers found on the boiler tubes. The reaction creates a layer of hard protective magnetite that help protect the tubes from corrosion.

Monitoring hydrazine at the feedwater inlet measures for the correct dosage. Using too much is an unnecessary cost, too little will not correctly control dissolved oxygen and magnetite will not form.

3. Sodium

The abundance of sodium is the primary cause of several types of boiler corrosion, making it one of the critical measures for power plant boilers.

When sodium undergoes hydrolysis, it becomes sodium hydroxide. Sodium hydroxide causes the iron found in boilers to dissolve, turning the sodium hydroxide into sodium ferroate. Under hydrolysis, the sodium ferroate regenerates as sodium hydroxide. When this cycle occurs unchecked, boiler parts such as joint and bends are under ceaseless bouts. This makes them brittle and subject to the threat of cracks and leaks.

When steam carries sodium, it allows for buildup on other important parts including the steam turbines. Because of the many dangers related to sodium contamination operators need measurements from points in both the generation and distribution loops. Places to watch include:

  • Polishing plant outlet
  • Saturated and superheated steam distribution loops
  • Condenser extractor pump
  • Water treatment plant

4. Silica

Silica is responsible for the creation of hard and dense scale within the boilers and turbines of power generation plants. The scale is nearly impossible to remove and reduces the thermal efficiency by as much as 28 percent.

The only way to control silica is through constant monitoring at several system points including:

  • Superheater and condenser outlets
  • Boiler feed water
  • Boiler drums
  • Demineralization plant

Online monitoring systems allow a single operator using one computer dashboard to effectively monitor these and other water parameters such as chloride, phosphate and ammonia.

Treating feedwater properly is a key part of operating and maintaining a boiler system.