Boiler failure is a major concern for heating and power plants, and the corrosion associated with the boiler's water quality is known to be the main cause. The total cost of boiler tube failures in power plants is estimated to be about $5 billion a year (note that this figure is from 2009, so is likely to be much higher.) To ensure the long-term safe operation of a boiler, the water's quality needs to be continually maintained within the tolerance requirements of the boiler.
Boiler Feedwater Impurities
Water sources contain impurities that, at certain levels, can be very harmful. At high temperatures, even trace amounts of certain substances in feedwater can cause severe damage to equipment and pipes in a short amount of time. Problematic dissolved solids or suspended solids, oxygen, carbon dioxide and organic matter can cause scaling, fouling and corrosion. For example:
- Iron can deposit on internal boiler parts and tubes, damage downstream equipment and affect the quality of certain manufacturing processes.
- Copper can cause deposits to settle in high-pressure turbines, decreasing their efficiency and requiring costly cleaning or equipment change-outs.
- Silica can cause extremely hard scaling, especially in high-pressure boilers.
- Calcium can cause scaling in several forms depending on the chemistry of the boiler feedwater.
- Magnesium can stick to the interior of the boiler and coated tubes, attracting more solids and contributing to scale, especially if combined with phosphate.
- Aluminum can deposit as scale on the boiler's interior and can react with silica to further increase the likelihood of scaling.
- Hardness causes deposits and scale on boiler parts and piping.
- Dissolved gasses such as oxygen and carbon dioxide can cause severe corrosion on boiler pipes and parts.
The most common type of corrosion is pitting attack due to dissolved oxygen. This is characterized by small but deep holes that penetrate tube walls and cause eventual failure. Factors affecting the severity include dissolved oxygen concentration, pH and temperature. (Related reading: How to Effectively Recognize, Prevent and Treat Pitting Corrosion.)
To prevent corrosion, boiler water needs to be somewhat alkaline and not acidic. Improper acidity and slightly alkaline water can dissolve metal and the protective film on metal surfaces.
Acidic attack occurs if the pH falls below 8.5. In this condition, materials are susceptible to surface thinning caused by etching. Any stressed area would be the most vulnerable in this type of attack.
Caustic attack occurs if the pH is above 12.9, and is often encountered at high temperatures involving phosphate-treated boilers. Deposits in high heat transfer areas can strip magnetite film from the base material.
Foaming Carry-Over
Foaming carry-over is caused by normal boiler impurities becoming too concentrated, over-treatment with chemicals or an accidental introduction of organic matter (grease or oil) into the boiler feedwater or condensate return. The foam inside the boiler is easily forced out of the boiler steam outlet and can cause major damage. Water hammer in a steam line, erosion in steam piping, and boiler low water events are examples.
Figure 1. Video describing boiler operation.
Boiler Feedwater Treatment Systems
A boiler feedwater treatment system is made up of several individual technologies that address the feedwater's condition to meet the requirements of the specific boiler. The treatment system is made up of the components necessary to remove impurities that are always present regardless of the source. A basic boiler feedwater treatment system typically includes some type of coagulation, filtration, ion exchange, dealkalination, deaeration and membrane processes such as reverse osmosis. These components are described in the following sections. (Related reading: How to Avoid Chelant-Based Corrosion in a Boiler Water Pipe.)
Coagulation and chemical precipitation
After all the large objects are removed from the original water source, various chemicals are added to a reaction tank to remove the bulk suspended solids and other various contaminants. This process starts off with an assortment of mixing reactors, which are typically one or two reactors that add specific chemicals to take out all the finer particles in the water by combining them into heavier particles via coagulation that eventually precipitate and settle out.
Filtration and ultra-filtration
The next step is generally running the water through filtration to remove any suspended particles such as sediment, turbidity and certain types of organic matter. It is often useful to do this early on in the process, as the removal of suspended solids upstream can help protect membranes and ion exchange resins from fouling later on in the pretreatment process.
Ion exchange softening
A softening resin can be used to address high hardness containing bicarbonates, sulfates, chlorides or nitrates. This procedure uses a strong acid cation exchange process, whereby resin is charged with a sodium ion. Hard water passes through a softener, and the calcium and magnesium trade places with sodium ions. Sodium ions are held loosely and are replaced easily by calcium and magnesium ions. During this process, “free” sodium ions are released into the water.
Dealkalization
Dealkalization can reduce the alkalinity or pH, which is an impurity that can cause foaming, corrosion and embrittlement. Sodium chloride dealkalization uses a strong anion exchange resin to replace bicarbonate, sulfate and nitrate for chloride anions.
Reverse osmosis and nanofiltration
Reverse osmosis (RO) and nanofiltration (NF) force pressurized water through semipermeable membranes, trapping contaminants such as bacteria, salts, organics, silica and hardness, while allowing concentrated, purified water through. These filtration units are used mostly with high-pressure boilers where the concentration of suspended and dissolved solids needs to be kept extremely low.
Deaeration or degasification
Condensate being returned will mix with the treated makeup water and enter a deaeration or degasification process. Any trace oxygen and carbon dioxide can be extremely corrosive to boiler equipment and piping. Therefore, removing these gases to acceptable levels is imperative to maximize the service life and safety of the boiler system. There are several types of deaeration devices that come in a range of configurations, but a tray- or spray-type deaerator for degasification or oxygen scavengers are the most common types.
High-Pressure and Low-Pressure Boilers
The feedwater's quality requirements depend on pressure and flow rate. For certain pressures, there is a maximum level of contaminants, and as the pressure increases it becomes more critical for thorough water treatment that yields higher quality water.
Low-pressure boilers (600 PSI and lower) and water with a low amount of total dissolved solids (TDS) typically only require simple filtration and water softening. As the water chemistry might dictate, or as the pressure increases, a dealkalizer may be needed.
High-pressure boilers (600 PSI and higher) usually require some type of demineralization, ion exchange or resin-based sandwich or mixed-bed polishing devices. Reverse osmosis used in combination with a polishing technology is a very popular where extremely high purity water is required.
Feedwater Testing and Quality Control
Preventing and minimizing corrosion processes in boilers is very important in the thermal power industry. Feedwater quality should be maintained at a level corresponding to the design standards, followed by noting any fluctuations in the measured parameters There are sensitive online and offline analytical instruments that can be used to measure the key diagnostic parameters that are crucial for boiler safety and performance.
Knowing where the feedwater source quality stands in relation to the water quality required for a specific boiler is essential, because inadequate water treatment can lead to scaling, corrosion and fouling of the boiler and downstream equipment.