H2O Homework: Treating Water for Boilers

By C.C. SULLIVAN Editorial Director -- Consulting-Specifying Engineer, 11/1/2000

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.

•  Low-pressure boilers. For low-pressure boilers where most condensate is returned, internal chemical treatment of feedwater may suffice to prevent hardness from adhering and inject enough alkalinity to mitigate boiler corrosion. New scale-conditioning chemicals include chelates and polymers that "dissolve" 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.

•  Medium- to high-pressure boilers. External pretreatment and internal treatment are both required to effectively operate moderate- and high-pressure boilers and to prevent tube failures. Dissolved solids in raw and make-up water are conditioned by filtration and chemical treatment, such as lime-soda 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.

•  Condensate systems. In steam and condensate systems, the contamination of condensate by oil, chemicals, carbon dioxide and oxygen can cause corrosion. A number of mechanical and chemical approaches are employed: Sodium sulfite chemically removes oxygen, and "volatile amines" neutralize corrosive carbonic acid, which is caused by CO2.

•  Boiler feedwater. To improve feedwater quality and thus boiler efficiency, oxygen and impurities must be removed from water ahead of the deaerator and feedwater pumps.

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.