When it comes to choosing technology for boiler feed water treatment, knowing the feed water source quality in relation to the water quality required for your specific boiler is essential, as inadequate water treatment can lead to the scaling, corrosion, and fouling of the boiler and downstream equipment. We’ve mentioned this before in some of our previous boiler feed water treatment articles, but this bears repeating as some of these issues, if neglected, can be pretty serious.
Although the water quality and makeup quantity is a complex calculation that needs to be analyzed by a boiler feed water treatment expert, there are some common characteristics for boilers and their recommended feed water treatments that can indicate different technologies that might be useful in your system.
Here’s how to be sure you choose the best boiler feed water treatment technologies for your plant:
Know the quality of water feeding your boiler
Since water absorbs more heat than any other inorganic substance, it is often used to create energy. However, when water is being used in a boiler to create a large amount energy in the form of steam, any impurities in the water can be a detriment to the boiler itself and equipment down the line. For this reason, it’s essential to know the impurities present in the water and treat them accordingly.
Boiler feed water is a combination of the boiler makeup water (what is required to replace any lost water in the boiler due to evaporation or water loss in blowdown and processing steam) and condensate return water (the distilled water created when the boiler is producing steam that condensates on the internal areas of the boiler):
Boiler makeup water
Depending on how often your boiler is blown down (rid of any impurities that occur as a result of the steam-making process) or how much water is lost to evaporation and steam generation, the quantity of makeup water needed to replenish this loss might fluctuate.
Pretreatment of makeup water is important, though, especially in the case of higher-pressure boilers that require extremely pure water.
Choosing your boiler makeup water source critical in determining the treatment options that will go into the makeup of your system. These sources might include city water, city-treated effluent, in-plant wastewater recycle (cooling tower blowdown recycle), well water, or any other surface water source.
Some common impurities include:
- Dissolved gasses
- Total dissolved solids
- Suspended solids and organic material
The presence of these contaminants can cause a scale to form inside the boiler pipes and parts. This is a hard deposit that can decrease the efficiency of the boiler, promote local overheating, and be extremely damaging to the system. Others promote corrosion, fouling, and loss of steam purity and need to be removed to maintain the integrity of the system.
Some common treatments to remove these types of contaminants include:
- Coagulation/chemical precipitation
- filtration and ultrafiltration
- ion exchange/softening
- membrane processes such as reverse osmosis and nanofiltration
So, again, depending on the impurities present in your water, any combination of these treatments might best suit your facility and makeup your treatment system.
Condensate return water
When steam is produced inside the boiler, the water particles collect and condense, then are recycled and used as part of the boiler feed water again. Technically, the condensate that the steam-making process produces is distilled, pure water, but dissolved gases such as oxygen and carbon dioxide are sometimes present. The chemical reactions due to the presence of these dissolved gases can cause severe corrosion on boiler pipes and parts.
The gases are typically removed with deminerization, advanced deaeration devices, or chemical scavengers, so if you have these present, chances are your boiler feed water system will incorporate some form of these technologies.
Know the quality of water needed for your boiler
The quality of feed water needed for your individual boiler depends on many factors, but the primary element to consider is the pressure at which you need to run your boiler in relation to the amount of water you need to process per day and how fast (this is your required peak gallons per minute, or GPM). For certain pressures, there is a maximum level of contaminants to you can feed into the boiler, and as you increase the pressure in your boiler, 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. Typically the technology used for lower-pressure boilers includes simple filtration to make sure no dirt gets into the boiler and a water softener to take out the hardness. As the water chemistry might dictate, or as the pressure increases, you might use a water softener in addition to a dealkalizer for a lower alkalinity feed.
- High-pressure boilers (600 PSI and higher). Treating your feed water for a higher-pressure boiler usually requires some type of demineralization, ion exchange, or electrodeionization (EDI) polishing. Resin-based sandwich or mixed-bed polishing devices can also be used, and these technologies can be permanent (regenerable in place) or portable (requiring an exchange service from an outside provider). Reverse Osmosis is a very popular technology for power plants when used in combination with a polishing technology. They are typically used on high-pressure boilers in power plants or refineries where extremely high purity water is desired.
Also note that boiler/turbine manufacturers each have their own requirements for water quality, so be sure to check with your manufacturer what their recommendations are.
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