Treating boiler feed water is essential for both high- and low-pressure boilers. Ensuring the correct treatment is implemented before problems such as fouling, scaling, and corrosion occur, will go a long way in avoiding costly replacements/upgrades down the line.
An efficient and well-designed boiler feed water treatment system should be able to:
- Efficiently treat boiler feed water and remove harmful impurities prior to entering the boiler
- Promote internal boiler chemistry control
- Maximize use of steam condensate
- Control return-line corrosion
- Avoid plant downtime and boiler failure
- Prolong equipment service life
Boiler feed water treatment system
As mentioned above, the exact components of a boiler feed water treatment system depend on the quality of water being drawn from in relation to the quality of water makeup needed for the specific boiler (according to the manufacturer’s recommendations), but in general, a basic boiler feed water treatment system typically includes some type of:
- Filtration and ultrafiltration
- Ion exchange/softening
- Membrane processes such as reverse osmosis and nanofiltration
- Coagulation/chemical precipitation
Depending on the impurities present in your water, any combination of these treatments might best suit your facility and make up your treatment system, and depending on the needs of your plant and process, these standard components are usually adequate. However, if your plant requires a system that provides a bit more customization, there might be some features or technologies you will need to add on.
What does a boiler feed water treatment system typically remove?
A boiler feed water treatment system might be made up of the technologies necessary to remove problematic dissolved solids, suspended solids, and organic material, including any number of the following:
- Iron: either soluble or insoluble, iron can deposit on 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: if not removed to low levels, especially in high-pressure boilers, silica can cause extremely hard scaling
- Calcium: can cause scaling in several forms depending on the chemistry of the boiler feed water (e.g. calcium silicate, calcium phosphate, etc.)
- Magnesium: if combined with phosphate, magnesium can stick to the interior of the boiler and coat tubes, attracting more solids and contributing to scale
- Aluminum: deposits as scale on the boiler interior and can react with silica to increase the likelihood of scaling
- Hardness: also causes deposits and scale on boiler parts and piping
- Dissolved gasses: chemical reactions due to the presence of dissolved gases such as oxygen and carbon dioxide can cause severe corrosion on boiler pipes and parts
How does a boiler feed water treatment system work?
Specific treatment processes vary depending on the requirements of the boiler and quality/chemistry of the feed and makeup water, but a typical boiler feed water treatment system will usually include the following steps:
Makeup water intake
Makeup water, or the water replacing evaporated or leaked water from the boiler, is first drawn from its source, whether raw water, city water, city-treated effluent, in-plant wastewater recycle (cooling tower blow down recycle), well water, or any other surface water source.
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, 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 that settle out. The most widely used coagulates are aluminum-based such as alum and poly aluminum chloride.
Sometimes a slight pH adjustment will help coagulate the particles, as well.
Filtration and ultra filtration
The next step is generally running through some type of 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. Depending on the type of filtration used, suspended particles can be removed down to under one micron.
Ion exchange softening
When pretreating boiler feed water, if there is high hardness complexed with bicarbonates, sulphates, chlorides, or nitrates, a softening resin can be used. This procedure uses a strong acid cation exchange process, whereby resin is charged with a sodium ion, and as the hardness comes through, it has a higher affinity for calcium, magnesium, and iron so it will grab that molecule and release the sodium molecule into the water.
After the softening process, some boiler feed water treatment systems will utilize dealkalization to reduce alkalinity/pH, an impurity in boiler feed water 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. Although it does not remove alkalinity 100%, it does remove the majority of it with what can be an easy-to-implement and economical process. Weak acid dealkalization only removes cations bound to bicarbonate, converting it to carbon dioxide (and therefore requiring degasification). It is a partial softening process that is also economical for adjusting the boiler feed water pH.
Reverse osmosis (RO) and nanofiltration (NF)
Reverse osmosis (RO) and nanofiltration (NF) are often used down the line in the boiler feed water treatment system process so most of the harmful impurities that can foul and clog the RO/NF membranes have been removed. Similar processes of separation, they both force pressurized water through semipermeable membranes, trapping contaminants such as bacteria, salts, organics, silica, and hardness, while allowing concentrated, purified water through. Not always required in boiler feed water treatment, these filtration units are used mostly with high-pressure boilers where concentration of suspended and dissolved solids needs to be extremely low.
Deaeration or degasification
At this point in the boiler feed water treatment process, any condensate being returned to the system will mix with the treated makeup water and enter the deaeration or degasification process. Any amount of gasses such as oxygen and carbon dioxide can be extremely corrosive to boiler equipment and piping when they attach to them, forming oxides and causing rust. Therefore, removing these gases to acceptable levels (nearly 100%) can be imperative to the service life and safety of the boiler system. There are several types of deaeration devices that come in a range of configurations depending on the manufacturer, but generally, you might use a tray- or spray-type deaerator for degasification or oxygen scavengers.
After the boiler feed water has been sufficiently purified according to the boiler manufacturers recommendation and other industry-wide regulations, the water is fed to the boiler where it is heated and used to generate steam. Pure steam is used in the facility, steam and condensate are lost, and condensate return is pumped back into the process to meet up with the pretreated makeup water to cycle through pretreatment again.