All natural waters contain varying amounts of suspended and dissolved matter as well as dissolved gases. Likewise, the minerals in water suitable for drinking which are considered nutritional for the human body, cannot be handled as well by boilers. Whatever the source, impurities found in source water become an important consideration when used for steam generation. Feedwater must be treated to remove impurities to control deposition, carryover, and corrosion in the boiler system.
Maintaining good feedwater is an important and fundamental aspect of any steam turbine power plant. A plant that maintains good feedwater achieves the following three benefits:
Help to ensure maximum life out of its boilers, steam turbines, condensers, and pumps.
Reduce maintenance expenses.
Maintain optimal performance
On the otherhand, poor quality water gives poor quality steam. In the event that the boiler system shuts down, most processes within the plant will not be operable. For this reason, proper treatment measures are essential for boiler feedwater.
Because boiler systems are constructed primarily of carbon steel and the heat transfer medium is water, the potential for impurities deposit and corrosion is high. Deposits are seldom composed of one constituent alone, but are generally a mixture of various types of solid sediments, dissolved minerals, corrosion products like rust, and other water contaminants. The most problematic impurities to boilers and steam turbines are calcium and magnesium scale, silica, sodium, chloride, oxygen and iron. Oil and other process contaminants can form deposits as well and promote deposition of other impurities. Localized attack on metal can result in equipment failure or reduced product quality and therefore, removal of these impurities is aimed at preventing unplanned, forced shutdown or productivity decline.
For each plant operation, there is an optimum method of treatment. Many factors are involved in proper selection of feed water preparation and treatment. Principally, these are the requirements of the plant for safe and reliable operation at an economical treating cost. Depending on the quality of water, it may be subjected to one or more treatments, including precipitation/coagulation, lime-soda softening, ion exchange, deaeration, and reverse osmosis.
Siemens Water Technologies provides a range of products and services to ensure the quality and quantity of boiler feedwater remains consistent and reliable.
The first step in the process is often coagulation, or the addition of chemicals to reduce suspended solids, silt, turbidity and colloids of the raw water. Coagulation causes smaller, suspended solids to adhere to each other, making them heavier and forming larger solids which will settle out of the solution. Typical chemicals used to improve settling rates and efficiency include aluminum sulfate (alum), sodium aluminate, polyelectrolytes (polymer) and ferris sulfate.
Chemical precipitation sometimes follows and is a process in which chemical added reacts with dissolved minerals in the water. Precipitation methods are used in reducing dissolved hardness, alkalinity, and silica. The most common chemical precipitation method is lime-soda softening, which not only reduces hardness, but in so doing, alkalinity, total dissolved solids, and silica are also reduced. Additionally, a prior coagulation step may not be necessary with the lime-soda softening process. Instead, when added to the softening process, coagulants speed up settling of sludge by 25-50% while also contributing to softening reactions (particularly in reducing magnesium) and removing silica, which tends to be adsorbed on the floc produced by coagulation of sludge.
Filtration is also necessary for any water treatment process to work properly. Depending on the treatment process, a filtration step is necessary for pre-treatment before ion exchange treatment and reverse osmosis or post-treatment, following chemical precipitation using lime-softening. Filtration technologies from Siemens include both media systems as well as ultrafiltration with membrane-based systems. Removal of these remaining solids is of critical importance particularly for the prevention of fouling and/or contamination in either the ion exchange resin or reverses osmosis membrane-based systems which follow in the next treatment phase. When resins become coated with suspended matter, they produce shorter run lengths, loose their effectiveness and capacity to regenerate. Likewise, reverse osmosis membranes can get fouled, leading to reduced efficiency and shorter effective life.
Siemens provides ion exchange systems for softening and demineralization of the water stream. These systems feature ion exchange resins which have a certain capacity for removing ions from water and when their capacity is used up they have to be regenerated. Regeneration involves taking the vessel off line, treating it with concentrated solution of the regenerant and removing the ions from the resins. The ions are rinsed from the vessel and the resins are returned to service.
Siemens also offers reverse osmosis treatment systems in replacement of coagulation, precipitation and ion exchange treatment systems. RO involves separating water from a solution of dissolved solids by forcing the water through a semi-permeable membrane. The use of RO has grown rapidly in the industrial sector, but in no industry more quickly than power generation. Many plant operators are adopting reverse osmsosis membrane technology for cost and operational savings, and, the elimination of hazardous chemical handling and safety of personnel.
Boiler Feedwater Pretreatment Technologies for Your Industrial Process Water Applications:
Ion Exchange- softening
Boiler Feed Water “Polished” With Mobile Demineralizer Trailers
A refinery in Northern California was unable to meet its desired boiler feed water silica specifications on a consistent basis. The higher silica levels led to an excessive blow down requirement in the boiler system. The higher blow down rates were significantly affecting the refinery’s water, chemical, and natural gas costs. The plant was using reverse osmosis (RO) for silica treatment and TDS reduction. However, RO membranes are limited in the amount of silica they can reject, thus often creating an excess of undesired silica in the RO permeate.
Siemens Water Technologies brought in Mobile Demineralizer (MODI) trailers as a solution
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