How they work
An effective boiler feed water treatment system works by both removing harmful impurities prior to entering the boiler as well as controlling the acidity and conductivity of the water. While treatment trains vary, a typical system will consist of primary treatment and possibly polishing depending on the boiler pressure, steam use, and chemistry of the boiler feed and makeup water.
As boilers are used, they lose water to steam consumption, loss of condensate return, and leaks. This water must be replaced with what is known as makeup water. Makeup water may be drawn from a treated city supply or a raw water treatment system.
The stream is typically filtered through one or more filtration units for removal of sediment, turbidity, and organic material.
IX is often used for removing hardness from boiler feed water, including bicarbonates, sulfates, chlorides, and nitrates.
Alkalinity can cause foaming and carryover in boilers, as well as corrosion in piping. Therefore, boiler feed streams are often treated with strong anion IX, or weak acid IX followed by degasification, which serve to remove bicarbonate, sulfate, and nitrate ions, reducing pH.
- Reverse osmosis (RO) and nanofiltration (NF)
RO and NF are not always used for boiler feed water treatment, however, they can be useful for removal of bacteria, salts, organics, silica and hardness. RO and NF are both types of membrane filtration, meaning that they employ a semi-permeable membrane to capture any contaminants too large to fit through their pores, while allowing water molecules to flow through.
- Primary ion exchange (IX)
For large volumes of water or high pressure boilers, deionizers may be used instead of membrane filtration.
- Deaeration or degasification
Deaeration/degasification is the removal of dissolved oxygen and carbon dioxide from the liquid stream, which is important for preventing corrosion.
Depending on the boiler requirements, polishing technologies may be required. This step can follow RO or primary IX. Typical polishing technologies include mixed bed deionization (DI), electrodeionization (EDI), or offsite regenerable DI.
Following all treatment steps, the boiler feed water is piped to the boiler, where it is heated to form steam. The condensate can then be combined with treated makeup water, and the cycle begins again.
Although these steps represent common boiler feed water system trains, it is important to understand that an individual boiler’s unique makeup/chemistry is an extremely complex calculation that will dictate the technologies needed.