Plant steam , demineralization

Pressure vessels and appurtenances should be constructed of stainless steel or other corrosion-resistant materials. Ideally, these steam generators should receive hot demineralized FW to minimize chemical treatment requirements. Alternatively, where a main boiler plant is installed, 100% steam condensate provides a good source of FW. In practice, it is very difficult to accurately control the correct amount of chemical feed. Chemicals are typically restricted to potable grade, deposit control agents such as polyacrylates, and other materials listed under the Code of Federal Regulations, CFR 21 173.310, or National Sanitary Foundation (NSF International) approval system. These boilers may be electrically heated or gas-fired. 

Neutralizing capacity functionality is important in lower pressure plants because it is common for some carbon dioxide derived from alkalinity breakdown to be carried over into the steam. It is less important in high-pressure units where demineralized water (or water of similar high purity) is employed because the FW is assumed to be essentially free of alkalinity. 

The absorber consists usually of a single stainless-steel tower, although two or more may be necessary for low-pressure plants. Another kind of absorber used is a drum absorber. In general, the cool gases along with additional air are fed to the bottom of the tower, and demineralized water is fed to the top of the tower. Instead of demineralized water, clean condensed steam can be used. Nitric acid is taken out from the bottom, whereas tail gases leave at the top of the tower. The absorption towers can be packed with sieve plates, bubble cap plates, or turbo-grids. 

Utilities. The utility requirements for a process are obtained from both a material and energy balance. The unit costs for each utility may be obtained from plant expense sheets, the accounting department, or a utility superintendent. These costs often increase continually, so they should be reviewed frequently. Utilities are usually steam (high-, medium-, and low-pressure) and their associated unit costs, electricity, natural gas, cooling tower water, and treated or city water. Sometimes instrument air, demineralized water, and refrigeration are considered utilities if they come from a central source and are not tied to a given process. 

Conductivity measurement with contact electrodes is used in many different process analytical applications. In all of them the main interest is to detect the presence of ionic species in the sample. The effectiveness of distillation and demineralization units is normally controlled by continuous measurement of the conductivity of the outgoing stream. Any defect in the operation of the unit can immediately be seen in the increase in conductivity. The feeding water to steam turbines has to be as clean as possible in order to avoid corrosion in the system that operates under high pressure and at elevated temperatures. The conductivities of feeding water as well as the condensed steam are also continuously monitored by conductivity measurement. Increased conductivity is an immediate indication of corrosion in the system or leakage in the cooling system. This is of extreme importance in nuclear power plants where among other risks corrosion products may also become activated. 

The balance-of-plant comprises the steam lines from the steam generators, the steam turbines and the alternating electrical generator, the condenser, various moisture separators and equipment to achieve de-aeration, demineralization, oxygen scavenging, reheating, and pH control of the feedwater returned to the steam generator. 

The steam produced in the reactor pressure vessel is directed to the high-pressure part of the turbine downstream, the condensate formed on the high-pressure turbine is separated and directed to the heater drains. The residual steam powers the low-pressure part of the turbine and, finally, is completely condensed in the main condenser. The main condensate is purified in the condensate demineralizers where corrosion products and ionic impurities are retained. The feedwater then is recycled via preheaters to the reactor pressure vessel. In older BWR plants, all the condensates are purified in the condensate polishing system in the newer, forward-pumped plants only the main condensate is purified there, while the heater drains are directly pumped to the feedwater tank. In some of the BWR plants the feed-water tank is equipped with an electromagnetic filter for removal of suspended corrosion products (mainly iron oxides). 

The demineralized (DM) water make up requirement of a 300 MW(e) AHWR is about 350 m /d. An additional requirement of about 150 m /d.of fresh water for drinking and other purposes is envisaged. It is therefore proposed to set up a 500 m /d low temperature multi effect distillation (LT-MED) seawater desalination plant utilizing low pressure steam from the turbine to meet the DM water requirements. Figure XI-8 provides a schematic flow sheet of the desalination plant of AHWR. 

Process condensate collected from the front end of the plant is condensed steam and therefore quite clean except for traces of soKds and a small amount of dissolved gases. The process condensate is stripped with steam and then sent to a demineralizer unit, where the solids are removed from the water so it can be recycled as boiler feedwater. The volatile by-products are carried with the steam into the reformer, where they are reprocessed. 

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