H2O Homework: Treating Water for Boilers
The liquid support systems that keep boilers boiling are the keys to boosting efficiency and maintainability. To effectively specify and commission these subsystems, leading engineers consider the effect of common problems like carryover, corrosion and other water concerns due to impurities and deposits-such as oxygen, silica and iron-found in hard water.
By C.C. SULLIVAN Editorial Director
The liquid support systems that keep boilers boiling are the keys to boosting efficiency and maintainability. To effectively specify and commission these subsystems, leading engineers consider the effect of common problems like carryover, corrosion and other water concerns due to impurities and deposits-such as oxygen, silica and iron-found in hard water. Hardness can precipitate and adhere to boiler metal as scale, building up over time and compromising efficiency.
issolve" hardness; the additives are now as prevalent as phosphates, which precipitate magnesium and calcium. Chelates are cleaner than phosphates but prone to overfeeding and thus corrosion, while polymer treatments are less likely to cause corrosion. To treat precipitated hardness, engineers can use sludge conditioners.
a softening. Other pretreatment alternatives include ion exchange and reverse osmosis.
In addition to the boiler systems, related subsystems must be considered. Water treatment these help prevent corrosion , which can introduce iron and copper compounds into heating systems, clogging deaerators, economizers and boilers.
arbonic acid, which is caused by CO2.
The main problems
A host of biological and chemical phenomena are the root causes of water-related boiler problems. For example, carryover is the contamination of steam with boiler-water solids. This can be caused by foam or mist on the water surface inside the boiler, or by priming or surging of boiler water.
The main enemy of good boiler operation is corrosion , although general corrosion in boilers is usually not a concern. Typical causes include low-pH water or dissolved oxygen and CO2in feedwater systems, as well as low or high alkalinity.
Treat water well
To remove impurities from boiler water, a number of approaches are employed. Clarification is the removal of suspended matter and color from water, usually by means of filters or settling basins. For finer particles and colloidal materials, coagulation and flocculation are ways to aggregate the substances into larger masses more suitable to extraction. Chemical precipitation , such as lime-soda softening, can turn dissolved metals into relatively insoluble reaction products.
Recently developed filtration methods offer sophisticated treatment for boiler water. Ion exchange uses resins to remove dissolved mineral ions from water, exchanging them for less pernicious ions. A cation exchange softener, for example, trades sodium ions for magnesium and calcium ions.
Another common technology is RO, or reverse osmosis : a semipermeable membrane is subjected to high-pressure flows of up to 900 psi, allowing dissolved solids and ions to pass through. While RO systems can be expensive, they are extremely effective at reducing the dissolved solids in raw water. This, in combination with other methods, is a key tool that consulting and staff engineers can employ to improve boiler performance and energy use.