Make Up Water
Since there is a constant loss of cycle water for one reason or another, it is always necessary to have a continual source of incoming water. Treating this water is the beginning of the power plant抯 cycle chemistry. Makeup treatment almost always consists of demineralization to remove dissolved impurities.
Other pretreatment equipment consists of softeners, clarifiers, and filters. On an increasing basis, membrane technology is being used along with ion exchangers for effective demineralization treatment. The overall goal of the demineralization treatment is to yield high purity water for use in the overall feedwater/condensate cycle
Ion exchange treatment will typically involve the use of at least a cation exchanger followed by an anion exchanger. Often times a mixed bed exchanger will follow these, with a vacuum degasifier somewhere in the series. Membrane treatment, either of the reverse osmosis (RO) or electrodialysis (ED) type, is a technique frequently utilized to yield a more efficient demineralizer system. This treatment is often upstream of the ion exchangers to reduce the dissolved solids, thus cutting back the load on the ion exchangers.
Makeup water is usually not directly added to the system; rather, it is stored in the makeup water storage tank, where enough water is available to plant operations for a short period of time if the need should arise. This water is continually monitored to ensure integrity and may be reprocessed through the demineralizers if necessary.
The condensate portion of the cycle includes the condenser, hotwell, and the condensate polishers. The condenser is cooled by water from the cooling towers in order to condense the steam into water, where it collects in the hotwell. Makeup water is also typically added to the hotwell or condensate storage tank. The mixture of makeup water and condensate is then transferred by a condensate pump to the condensate polisher system for further treatment.
Although the makeup water should be high purity water, the condensate may often contain some water hardness, corrosion products, and impurities, usually resulting from a condenser leak. The polishing treatment is necessary to prevent these corrosive products and impurities from building up in the cycle and causing problems in the boiler (fossil fuel plants), steam generator (nuclear reactor plants) or turbine.
A polishing treatment system is made up of combinations of filtration and ion exchange, although some nuclear reactor treatments use membrane technology. The filtration system must be adequate to effectively remove insoluble corrosion products. The ion exchange is necessary for removal of dissolved solids, although it can serve as a filter as well. Mixed-resin de-mineralizers are typically used. The system used is dependent upon the water requirements and characteristics.
After water exits the condensate polishers, it is usually delivered to high and low pressure heaters, as well as a mechanical de-aerator. These components increase the temperature substantially and lower the dissolved oxygen to acceptable levels. The water is now called feedwater.
The purpose of feedwater treatment in a power plant is to deliver a minimum level of contaminants and corrosion products to the boiler or nuclear reactor. It is considered to be the most important part of cycle chemistry. The cycle chemistry control of feedwater can vary extensively depending upon the type of boiler or reactor, operating pressure, and water characteristics at a plant.
The boiler's purpose is to convert water into steam. Most power plant boilers, regardless of operating pressure, are catagorized as either once-through or drum-type units. The type of boiler greatly affects the cycle chemistry control. A drum-type boiler has a drum where the water-steam mixture is separated. Since the majority of contaminants are retained in the water, they are removed from the cycle by blowdown. Condensate polishers are not usually used in plants where drum-type boilers are used, although the polishers are beneficial and should be used for optimal cycle chemistry.
Once-through boilers do not have a separating drum, so the steam/water mixture continues out of the boiler directly to a superheater. This allows impurities to affect components downstream of the boiler. Once-through boilers will thus have more stringent cycle chemistry control and will almost always utilize a condensate polisher.
Three separate types of feedwater treatment are typically used, primarily depending upon whether the boiler is a drum-type or a once-through unit, and secondly upon the existing metallurgy. If a drum-type boiler is used, either phosphate or all-volatile treatment is used. In once-through units, all-volatile treatment is utilized. However, a new type of oxygenated treatment is also being employed in both types of boiler units.
In plants where a drum type boiler is present, a coordinated phosphate/pH treatment is often utilized. This treatment is used to precipitate the hardness constituents of water and provide alkaline pH control, which will reduce boiler corrosion. This type of program maintains the sodium-to- phosphate molar ratio within a narrow range of about 2.1 to 2.9. This ratio must be maintained within this established control range to prevent formation of phosphoric acid (ratio below 2.1) or free sodium hydroxide (ratio above 2.9). The pH typically ranges anywhere from 8.4 to 10.6 depending upon the pressure of the boiler. Phosphate treatment offers excellent buffering protection against potentially corrosive contaminants.
The objective of All Volatile Treatment (AVT) is to provide a high pH, high purity, low oxygen environment to minimize the corrosion of metal surfaces. The usual materials of construction in a fossil plant drum or once-through boiler are carbon or low-alloy steel. In high temperature boiler systems (greater than 400o F), a protective metal oxide layer of magnetite (Fe3O4) forms on steel surfaces to prevent corrosion. However, cooler temperature, steel surfaces in the steam/water loop (primarily those in the condensate/feedwater cycle), remain active and vulnerable to corrosion. The AVT objective is accomplished by adding ammonia or morpholine to elevate the pH level to somewhere between 8.8 to 9.6, depending upon the metallurgy. Mechanical de-aerators and an oxygen scavenger such as hydrazine or sodium sulfite are used to lower the dissolved oxygen level to less than 7 ppb.
While elevated pH is the basis of AVT, a new trend in corrosion prevention known as Oxygenated Treatment (OT) uses oxygenated ultrapure water to minimize corrosion in the feedwater train. In plants using OT, oxygen is added to the system to form a protective oxidized layer of hematite (Fe2O3) on low temperature steam/water loop surfaces. With OT for once-through units, an oxygen level of 30-150 ppb is monitored across the whole plant cycle. The use of oxygen as a corrosion inhibitor allows satisfactory operation over a large pH range; therefore, a reduction in plant cycle pH down to a level of 8 to 8.5 (once-through boilers), or 9 to 9.5 (drum boilers), is possible. It must be noted that in order to use OT, the system must have all-ferrous metallurgy downstream of the condenser.
In pressurized water reactors (PWR), there are two separate loops, a primary and a secondary. The primary loop water is circulated through the reactor itself to become heated. The heat from the primary loop is then transferred to the secondary loop, which transforms this secondary feedwater into steam. This place where the heat transfer takes place in the PWR is known as the steam generator, and the water chemistry is very similar to that of a drum type boiler. The other type of nuclear reactor, a boiling-water reactor (BWR), has just one loop and the feedwater is converted to steam by contacting the reactor.
When steam is driven off the boiler drum, the chemicals and impurities in the water are left behind. The concentration of solids (scale-forming salts) will increase with every gallon of makeup water, and sludge buildup in the drum will reduce transfer of heat through the drum wall. Also, accumulated concentration of solids increase the danger of carryover into the steam lines. Solid material in the steam can damage steam driven equipment. To prevent (or at least minimize) the concentration of solids in the drum from building up as the steam is driven off, a small amount of water is continuously removed. This is called blowdown. A similar type of blowdown is done in nuclear reactor plants as well.
Since blowdown is typically inadequate, it is usually accompanied by addition of chemicals to control precipitation or condition sludge. Blowdown is wasteful since heated water is being effectively removed from the cycle; therefore, it is important to properly control the cycle chemistry such that blowdown is minimized. Current guidelines allow for operation with a minimum of 1% (100 cycles) of blowdown.
The ultimate purpose of maintaining a good water chemistry program is to ensure that highest quality steam is produced on a continual basis. Regardless of whether a plant produces steam based on a boiler or a nuclear reactor steam generator, the steam purity is essential. Steam purity for a given system is dependent upon the intended use for the steam. Considerations such as the type of boiler or reactor as well as the type of turbine greatly affect the limits on purity, as do component type and initial water purity.
Once the steam passes from the boiler or steam generator, it usually passes into a superheater where it is heated above the temperature at which it was produced in the boiler. This helps to improve the thermal range of the steam cycle and reduce downstream condensation. The superheated steam is then passed through the first turbine. The steam exiting this high pressure turbine is then usually sent back to a reheat superheater, where the steam is reheated to be sent to lower pressure turbine stages. Usually these lower stages will consist of a single low pressure turbine or a combination of an intermediate turbine followed by a low pressure turbine. After the final stage, the steam enters the condenser, where it changes back to water.
Any water that cannot be reused is some part of the plant, typically from the blowdown from the boiler and the cooling tower, will be discharged. Environmental standards must be met for all water released from a plant. Typically the dissolved oxygen must be raised to the parts-per-million (ppm) level and the pH must be neutralized to levels somewhere between 6 to 9 pH.
Many plants are being designed for zero discharge. This means that no water will be discharged. The water, whether from boiler or cooling tower blowdown, will in some way be recirculated, usually to the makeup water treatment system for the boiler cycle or occasionally for the cooling tower. If zero discharge is the goal, this recirculated water will also typically be monitored on a continual basis to determine water characteristics.
APPLICATION SAMPLING POINTS AND MEASUREMENTS
Makeup Water - Conductivity is almost always monitored continuously, as well as pH, ORP, and sodium ion depending upon the components in the makeup treatment system.
Cation conductivity is measured in the makeup water storage tank to ensure water integrity. Typically this water is at a minimum of 1 megohm-cm of resistivity (1 micromho/cm conductivity), with usual limits being 5 to 10 megohms-cm resistivity.
ORP may be monitored if some form of chlorination / de-chlorination exists, whether as a monitor of incoming water or as a controlled parameter to protect some types of reverse osmosis or de-ionization resins.
Specific and cation conductivity is typically measured in various places to determine efficiency of ion exchangers, softeners, and reverse osmosis systems.
pH or conductivity may be measured as part of the ion exchange regeneration cycle.
Sodium ion concentration may be measured at the inlet and outlet of the ion exchangers to determine process efficiency and the need for regeneration.
Condensate Water - pH, conductivity, sodium ion and dissolved oxygen are normally measured.
Specific and cation conductivity are typically continuously monitored at the inlet and outlet to the condensate polisher. These measurements are done to check the total dissolved solids level as well the process efficiency, water purity, and need for regeneration.
Dissolved oxygen is monitored continuously at the inlet and outlet of the condensate polisher to detect air leaks.
pH may be monitored continuously or periodically at the inlet and outlet of the condensate polisher to monitor for process leaks and to ensure that a scale-forming pH level does not occur.
Sodium ion concentration is measured at the inlet and outlet of the condensate polisher to determine process efficiency and the need for regeneration.
Specific and cation conductivity will be continuously monitored after the condensate pump discharge to determine overall water quality.
Specific and cation conductivity are continuously measured at the de-aerator inlet to monitor water purity. Conductivity is also usually measured at the inlet and/or outlet of the high and low pressure heaters.
Dissolved oxygen is measured at the inlet and outlet of the de-aerator to monitor the de-aerator efficiency. It is also typically measured at the inlets and/or outlets of the high and low pressure heaters to detect air leaks.
Conductivity is typically measured in condenser leak detection trays and/or hotwell zones to ensure that a condenser tube leak is detected significantly earlier than the condensate pump discharge.
Feedwater - Conductivity, pH, sodium ion and dissolved oxygen are measured on a continuous basis to ensure that requirements for water entering the boiler are met. Some of these measurements are also done for feedwater to a boiling-water-reactor (BWR) or pressurized water reactor (PWR).
Dissolved oxygen is measured in the final feedwater to ensure that no process leaks have occurred, as well as for feedforward control of any oxygen scavenger that may be used to lower the oxygen level further.
Specific and cation conductivity, and occasionally sodium, are continuously measured in the final feedwater to guarantee that the water meets the purity standards required for the boiler cycle.
pH is measured in the final feedwater to control the pH level in the boiler, preventing a scaling or corrosive condition.
Blowdown - Conductivity is measured to monitor the periodic or continuous blowdown in the boiler or cooling tower.
Specific conductivity determines the cycle of concentration, used to determine the need for blowdown.
pH is often measured to ensure that a scaling or corrosive condition does not exist.
Dissolved oxygen is often measured in blowdown water to detect corrosive oxygen levels.
Steam - Steam sampling, whether saturated or superheated, is difficult and therefore is often not done by many plants. However, obtaining information about steam purity is useful to detect contamination in steam lines.
Cation conductivity is often continuously measured in superheated, saturated, cold reheat, and hot reheat steam.
Sodium ion is continuously monitored in saturated steam to ensure that excess carryover is not occurring.
Cooling Tower Water - pH, conductivity and ORP are normally measured in cooling tower water treatment systems to minimize scale, corrosion and biological growth.
pH, typically controlled by sulfuric acid addition, should be monitored to control scaling or corrosive conditions.
ORP will be used to monitor the optimum amount of oxidizing biocides, such as chlorine, bromine, or ozone, which are added to cooling tower water to control microbiological fouling. ORP may also be used to alert personnel of process leaks in the cooling tower heat exchangers.
Conductivity is used to determine cooling tower blowdown.
Effluent Water - pH and dissolved oxygen should be measured on a continuous basis to ensure that environmental requirements are met.
Dissolved oxygen is measured to guarantee that oxygen levels in water meet environmental regulations. This may occasionally require some sort of aeration, which may use the dissolved oxygen measurement for control.
pH should be measured to ensure that levels in the water are safe for discharge. This may require neutralization, so the pH measurement is also used for control of reagent addition.
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Make Up Water