There are two types of water treatment: mechanical and chemical. Mechanical water treatment often includes using softeners, deaerators, economizers, reverse osmosis and other means. Chemical water treatment involves utilizing oxygen scavengers, sludge conditioners, amines and pH buffers.
Mechanical water treatment: Water softeners remove the hardness inherent in all water sources, as untreated water hardness leads to scale. Proper water softener operation improves boiler heat transfer and efficiency, reduces thermal stress on the boiler and minimizes blow-off losses, resulting in less water, chemical and fuel consumption.
Boilers can also lose efficiency from scale. Scale is a buildup of dissolved impurities, such as calcium, magnesium, phosphate, silica and iron oxide. These impurities concentrate in the boiler water due to evaporation. Only pure water evaporates as steam, leaving dissolved minerals behind. Scale on heating surfaces reduces heat transfer and is detrimental to boiler efficiency.
There are several types of mechanical equipment used to modify the characteristics of water prior to feeding it into a boiler: dealkalizers, demineralizers, and reverse osmosis. Dealkalizers regenerate like softeners, and caustic is added to drive reactions. They also increase conductivity of makeup water while reducing alkalinity. Reverse osmosis uses pressure and a membrane to filter out hardness, alkalinity and other ions. Though it used to be expensive, reverse osmosis has now become more viable and often pays for itself in a short time period by reducing blowdown and chemical use.
Another critical mechanical treatment method is deaeration. Coldwater carries more dissolved oxygen, which leads to corrosion and is the main cause of boiler tube failure. While a chemical oxygen scavenger is always needed, it is best to remove as much oxygen as possible before the water enters the boiler. This can be done by raising the water temperature and venting off the oxygen that is less soluble at a higher temperature. Heating boiler makeup water reduces thermal stress, drives out dissolved oxygen and carbon dioxide, and improves boiler system operational efficiency.
The most effective method of removing oxygen is a pressurized deaerator, which removes dissolved gases and vents them to the atmosphere by increasing the water temperature to 220 degrees Fahrenheit or more. Dissolved oxygen can be reduced to 7 parts per billion.
Chemical water treatment: Even with mechanical treatment, a chemical scavenger is required to completely remove dissolved oxygen. Chemical treatment prevents corrosion in the feedwater system, boiler and condensate return system. The most common chemical oxygen scavengers are sodium sulfite and diethyl hydroxylamine.
Almost 90 percent of boiler corrosion occurs when a boiler is off-line or being taken off-line. Idle boilers and standby boilers do not circulate chemicals, so they need to be warmed, water needs to be circulated, and chemical residuals need to be maintained at least weekly.
Condensate lines should be protected by the addition of volatile amines to neutralize the acid and boost the pH of the condensate, preventing it from being corrosive. These amines can be added either to the boiler feedwater system to ensure good distribution or to the main steam header, when applicable.
One important aspect of controlling scale in a boiler is blowdown. This process is required to remove impurities left behind after evaporation and helps maintain balanced conductivity levels. It also helps to minimize carryover and preserve safe and reliable boiler operation, delivering dry, quality steam for the process.
It is important to maintain a proper, consistent water treatment program by using both mechanical and chemical treatments. This will help uphold system safety and reliability, improve performance, maximize operational efficiency, conserve resources and extend equipment life.