Walter Louis Fluid Technologies
Industrial Water Treatment Newsletter, July, 2012
Boiler or steam production is one of the most advanced applications for water treatment technology. Boiler system operators and engineers strive for the highest efficiency possilbe from their equipment, and they’ve learned that poor water quality has a direct impact on boiler efficiencey and longevity. An understanding of basic boiler water treatment is critical
Some boilers return a portion of the condensed unused steam or condensate return to the feedwater. Since most boilers use more water than the condensate return can provide, the water added to meet the boiler’s feedwater usage is referred to a makeup water. Boiler feedwater consists of condensate return and makeup water.
Carryover is a boiler industry term for a condition where boiler water is found in the steam: its usually caused by foaming or a mechanical problem such as water escaping around a damaged baffle. Any carryover found in the steam will degrade steam quality.
Scale deposits from calcium carbonate are common in low-pressure boilers and are a threat to any water-using process equipment. Scale deposits form on boiler tubes and interfere with the transfer of heat through the tubes, which can cause tube metal failure and, ultimately, severe damage or failure of the boiler. The maximum safe range for tube metal for low-pressure boilers is 900 to 1000 degrees Fahrenheit (480-540 C)
Scale deposits seriously disrupt the relationship intended to keep boiler tubes from failing by forming layers between the water and the tube walls. Given the operating heat, some residual solids will be present in the boiler despite your best efforts to treat the water. This is one of the purposes of boiler blowdown.
Corrosion is another major concern. Most water treatment professionals recognize corrosion as metal returning to its ore form. Corrosion can attack metals such as iron over a large area or produce very small pinpoint holes, retuning iron to iron oxide.
Besides the boiler system, corrosion can also effect the pre-boiler systems and condensate lines. Corrosion results from low pH, stresses, or excessive amounts of dissolved oxygen and carbon dioxide in the water. Where there is excessive dissolved oxygen in the feedwater, corrosion is actually accelerated by low pH and high water temperatures. Where there is excessive carbon dioxide the the steamlines, corrosion will occur in these areas.
Corrosion prevention consists of removing dissolved oxygen from the feedwater, maintaining slightly alkaline feedwater and keeping the boiler and tubes clean. It also includes neutralizing the carbon dioxide in the steamlines with a volatile amine.
Water Treatment Processes
Basic boiler water treatment consists of three recognized methods: internal, external, and blowdown
Internal Conditioning. When properly applied, will react with the feed water hardness, control the effects of corrosion, remove dissolved oxygen, and prevent carryover. Internal water treatment is acheived gy using specialized chemicals in either the boiler or the boiler feed water lines. In some cases, internal and external treatments are used on boiler feedwaters. Boiler operators use blowdown in conjunction with internal treatments.
External Conditioning. Several processes can improve external water quality, including precipitation, flocculation and clarification, ion exhange, dearation, evaporation, and reverse osmosis (RO).
Chemical precipitation. Used to reduce dissolved hardness. A common form is lime-soda softening.
Deaeration. Removes the dissolved oxygen prior to the boiler feedwater entering the boiler system. In this process, the boiler feedwater is heated with steam. A portion of the steam is vented which then carries off most of the dissolved ocygen from the feedwater.
Evaporation. This unit is used when steam is abundant as a heat source. The process involves preheating the boiler feedwater into a relatively pure vapor form. The process works well where the dissolved solids in the feedwaters are very high.
Ion Exchange. This process softens the feedwater and has been used extensively with very positive results. In boiler water treatment total hardness and total dissolved solids need to be reduced.
Demineralization. Done with ion exhange using a cation and an anion resin and produces the highest quality water.
RO. RO lowers cost associated with operating the DI plant by substantially lowering the TDS prior to the demin system, which lengthens periods between regeneration and/or tank exchanges and increases the DI resin’s life expectancy.
Controlling the boiler is of utmost importance in any steam generation energy saving program. Below are some ways to improve boiler efficiencies.