Circuit water

Copyright 2006 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-30997-7 

---

PWRs operate differendy from BWRs. In PWRs, no boiling takes place in the primary heat-transfer loop. Instead, only heating of highly pressurized water occurs. In a separate heat-exchanger vessel, heat is transferred from the pressurized water circuit to a secondary water circuit that operates at a lower pressure and therefore enables boiling. Because of thermal transfer limitations, ultimate steam conditions in PWR power plants ate similar to those in BWR plants. For this reason, materials used in nuclear plant steam turbines and piping must be more resistant to erosion and thermal stresses than those used in conventional units. 

Closed-Mill Concept. The closed-mill concept, or water circuit closure, has been studied by the pulp and paper industry for many years. In some parts of the paper manufacturing process, up to 98% of the water is recycled within the process, eg, the wet end of the paper machine. However, in the pulp mill, especiaUy kraft mills, effluents are produced owing to the need to purge from the system various metals that come in with the wood, as weU as organic by-products from the pulping process, additives, and especially chloride ions that originate in the bleach plant. 

Closed Air Circuit Water Cooled (CACW) The motor s interior hot air forms one part of the closed air circuit that is circulated by the motor s internal fans. A separate heat exchanger is mounted on top of the motor as the cooling water circuit. This forms the second cooling circuit. 

Water-Cooled motors, type CACW (cooling type ICW 37 A 81 or ICW 37A 91) (Section 1.16, Table 1.12) should be fitted with moisture detectors to provide an audio-visual alarm in the event of a leakage in the water circuit or a higher coolant temperature. 

Chilled water circuit Figure 13.49(c) is a How ditigram for the chilled water. 

Figure 13.49(b) Condenser water Figure 13.49(c) Chilled water circuit... 

Direct expansion is also possible wherein the refrigerant is used to chill the incoming air directly without the chilled water circuit. Ammonia, which is an excellent refrigerant, is used in this sort of application. Special alarm systems would have to be utilized to detect the loss of the refrigerant into the combustion air and to shut down and evacuate the refrigeration system. 

Chemical consumption Chemical consumption will be associated with boiler feed make-up water-treatment plant, dosing systems for feedwater and boiler system, treatment of cooling water circuits and effluent treatment. Typical chemical requirements for a thermal power plant are given in Table 15.9. 

Corrosion in the Steam/Water Circuit Due to Soiute Carryover... 

There is extensive literature dealing with the consequential corrosion in steam/water circuits after solute carryover. It is not the intention to discuss it here at any great length, but two examples are given to illustrate some aspects of the subject. 

As well as corrosion in oil and gas streams, there are other applications such as various water circuits and injection of treated sea-water into the reservoir. 

Miscellaneous There are many interesting applications that arise from time to time that are outside the main stream of industry described above. Examples include desalination plant reactor cooling water circuits automobile body corrosion in situ) marine (vessels, piling, harbour installations) aircraft (in situ) packaging and cavitation monitoring. 

There are several methods of providing a percentage hleed-off from the water circuit ... 

Water-cooled condensers can he fitted with a directly controlled water-regulating valve operated hy condenser pressure, or may have a three-way blending valve in the water circuit. 

Water flow switch, to stop the machine if flow stops in the chilled water circuit... 

The heat reclaim packaged unit system comprises water-cooled room units with reverse cycle valves in the refrigeration circuits. The water circuit is maintained at 21-26°C, and may be used as a heat source or sink, depending on whether the individual unit is heating or cooling. (See Figure 28.11.)... 

If the water circuit temperature rises above about 26°C, the cooling tower comes into operation to reject the surplus. If the circuit drops below 21°C, heat is taken from a boiler or other heat source to make up the deficiency. During mid-season operation within a large installation, many units may be cooling and many heating, so that energy rejected by the former can be used to the latter. With correct system adjustment, use of the boiler and tower can be minimized. 

In heating, ventilation, and air-conditioning (HVAC) system designs, the hydronic heating systems also may incorporate cooling or chilled water circuits, laid out as two-pipe or four-pipe systems. These terms typically refer to ... 

Corrosion anywhere within the steam-water circuits results in metal wastage and possible equipment failure. And if the boiler plant is not operated correctly or if the water chemistry is not maintained within certain control parameters, the generated steam may contain contaminants in a cause-and-effect process that ultimately affects the utilization of the steam, reducing quality in a number of areas, and increasing fuel, manufacturing, or maintenance costs. 

In addition to external conditioning processes and the need to provide internal chemical treatments to some or all steam-water circuits within the steam cycle, the scope of boiler water treatment includes, as mentioned earlier, the provision of suitable technical resources sufficient to control the steam-water chemistry within defined limits appropriate for the boiler plant under consideration. Because these steam-water control limits tend to narrow considerably with increase in boiler pressures (and heat-flux densities), suitable monitoring and control procedures may require implementing actions with knife-edge precision. 

Scales may originate at some distant point within the steam-water circuit and only deposit at a point of high heat transfer. Or they may simply originate and concentrate locally on boiler surfaces, especially if the boiler is highly rated. 

Deposits found in boilers and other steam-water circuits tend to originate from several possible sources, including the following ... 

Another source of deposits is materials in the steam-water circuits. Acting separately or collectively as foulants and contaminants, these can pass to the boiler and combine to form deposits that stifle heat transfer. Corrosion debris from corrosion processes occurring at other points in the steam-cycle system can certainly form deposits, and the corrosion product ferric oxide (in the form of... 

Foulants and contaminants may originate virtually anywhere in the overall steam-water circuit. Some may be derived from pre-boiler systems (such as an economizer or deaerator) or post-boiler systems (such as a steam trap or a condensate line), but they inevitably find their way into the boiler and cause significant damage and expense. 

The corrosion of steel or other metals in a boiler plant system takes place when an electrochemical cell is established. This occurs when two different metals (anode and cathode) are coupled together in water, which acts as the electrolyte in any steam-water circuit. 

The potential for corrosion as a result of the reactions of noncondensable gases present in steam-water circuits is a major area of risk. The dissolved oxygen (DO) content of MU water is recognized as a primary source of gas entering a boiler system, and effective deaeration of MU and FW is therefore critical. 

Uniform rates of corrosion such as general etch corrosion seldom occur in steam-water circuits. Rather, pitting, tuberculation, and other complex types of corrosion tend to predominate. These forms of corrosion often result directly or indirectly from reactions occurring in particular areas of the system where fouling and deposition may be present. 

---