Overview
For boilers and cooling water systems alkalinity is a critical control parameter. It can cause corrosion, caustic metal embrittlement, and carry-over if the working range is exceeded. It has a considerable impact on chemical treatments for scale and corrosion inhibition in cooling water systems.
Objectives of the Boiler Water Testing
1. To keep track of the condition of the boiler water.
2. To regulate the boiler's chemical dosage.
3. To keep the boiler in good working order in the event of probable saltwater contamination.
What is Boiler Alkalinity?
Alkalinity is an issue in steam-generating boilers because it decomposes into carbon dioxide, which is carried away with the steam. When steam condenses, carbon dioxide is converted to carbonic acid, which is corrosive to steam lines.
The presence of alkalinity in a water sample can be caused by a variety of factors. The existence of Bicarbonate, Carbonate, and Hydroxide adds to the alkalinity of water for the purpose of simplicity.
Alkalinity may be found in water!
The amount is determined by the water's source, whether surface water (lakes and rivers) or groundwater (well water). Alkalinity can occur in water in three basic forms, depending on the pH: carbonate (CO3), bicarbonate (HCO3), or hydroxide (OH). The sum of these three kinds is total alkalinity.
P, M, or T alkalinity is determined by titrating a water sample with a standard acid to a certain pH and recording the results as P, M, or T alkalinity.
-P alkalinity is titrated to a pH of 8.3,
-M alkalinity to a pH of 4.6, and
-T alkalinity to a pH of 4.5 using phenolphthalein and total alkalinity indicator.
Why alkalinity is important in boiler feed water?
The alkalinity of the boiler water (carbonate and hydroxide) must be controlled since high amounts can cause the porcelain insulators used as lead-through bushings to bring electrical power into the boiler to be attacked. The total alkalinity should be kept below 400 ppm if porcelain insulators are utilized.
Special high alumina insulators are chosen if alkalinity is greater than 400 ppm. Because high alumina insulators are less prone to alkaline attack than porcelain, total alkalinity can be maintained up to 600 ppm. The pH, which should be between 8.5 and 10.5, is mostly determined by the ultimate alkalinity.
Boiler water treatment
The pH and alkalinity levels of boiler water are critical issues that impact the operation and maintenance of industrial boiler systems and steam raising plants.
The necessary water conditions must be maintained at all times to guarantee that such boiler systems are well maintained and run at maximum efficiency. This may be accomplished by utilizing alkalinity builders that are particularly intended to manage pH in boiler water systems.
To avoid scale, other deposits, and corrosion within the boiler, make-up waste and boiler water must be properly treated. The lack of sufficient exterior and interior treatment might cause operating disruptions or even complete boiler failure. When an option is available, pretreatment outside the boiler is always preferred and more dependable than pretreatment within the boiler.
The alkalinity can be removed and/or decreased in several ways:
a) Acid feed (if the industry is prepared to handle acid);
b) Degasifier;
c) Lime-soda softening;
d) Hydrogen cycle ion exchange/soft water split-stream makeup; and
e) Dealkalization with strong base anion ion exchange resin.
Because strong base anion resin does not reduce TDS, you may want to investigate reverse osmosis or deionization, depending on the operating pressure, to lower both the alkalinity and the total dissolved solids. Many of these systems may not be cost-effective depending on the size of the boiler and the operating pressure.
Conclusion
To guarantee that feed water and boiler water are within specification, representative samples must be examined on a regular basis.
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