Water conditioning

Water IS a vital resource for industry and agriculture, as well as for domestic use. The ever-increasing demands for water are outstripping the supply in many places where abundant water was once taken for granted. It is therefore becoming increasingly important to recycle water whenever possible, to ensure that water returned to rivers or lakes is fit for reuse by someone else (as well as safe for aquatic life), and to learn how to make use of impure but abundant resources such as brackish water or even seawater.1-11 The value of the water treatment market has been estimated at 30 billion annually, worldwide. 

Therefore, the objectives of water treatment are (a) to prepare available water for use in boilers, chemical processes, and cooling systems, for laundry, and for domestic consumption,1-4 and (b) to clean up wastewa- 

Suspended matter down to bacterial size can best be removed by subsidence in settling ponds and by skimming off floating material such as grease, which may form a paste that coats heat-exchange surfaces. Filtration would, no doubt, do a better job (see later), but on a large scale it is not always cost-effective (besides, filters tend to clog). 

Filtration can remove fine suspended solids and microorganisms, and microfiltration membranes of cellulose acetate or polyamides are available that have pores 0.1-20 /xm in diameter. Clogging of such fine filters is an ever-present problem, and it is usual to pass the water through a coarser conventional filter first. Ultrafiltration with membranes having pores smaller than 0.1 fim requires application of pressures of a few bars to keep the membrane surface free of deposits, water flows parallel to the membrane surfaces, with only a small fraction passing through the membrane. The membranes typically consist of bundles of hollow cellulose acetate or polyamide fibers set in a plastic matrix. Ultrafiltration bears some resemblance to reverse osmosis technology, described in Section 14.4, with the major difference that reverse osmosis can remove dissolved matter, whereas ultrafiltration cannot. 

The most significant dissolved ionic solid in most natural freshwater is calcium carbonate (actually present mainly as the bicarbonate), and so, for the purpose of water treatment, it is convenient to express the concentration of any ionic solute as a CaCOs equivalent. The formula weight of the solute is divided by the valence and by the number of its cations—for CaCOs itself, this comes to 50.0—and the concentration (usually in mg kg-1, ppm) is converted to an equivalent concentration of CaC03 on this basis.2 

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From the technical and practical points of view, the efficient monitoring of rivers and water basins is a very difficult problem. This is due to the fact, that it is necessary not only to carry out a big number of water condition parameters measurements, but to follow the physical-chemical changes going on in the water as well. Such changes have a strong influence on the future state of the water basins. 

Commercial Item Description (CID), Sodium Chloride, Technical (Water) Conditioning Grade A-A-694 General Services Administration... 

Salt can be classified under five principal use categories, plus a catch-all classification that includes most industrial uses, as (/) food-grade, (2) agriculture, (2) highway, (4) water conditioning, (5) chemical, and 6) miscellaneous. 

Bet iHandbook of Industrial Water Conditions, Betz, Trevose, Pa., 1962. 

Chelant Control. Chelants are the prime additives in a solubilizing boiler water treatment program. Chelants have the abihty to complex many cations (hardness and heavy metals under boiler water conditions). They accomplish this by locking metals into a soluble organic ring stmcture. The chelated cations do not deposit in the boiler. When apphed with a dispersant, chelants produce clean waterside surfaces. 

Bet Handbook of Industrial Water Conditionings Niath Edition, Trevose, PA, 1991. 

S. T. PoweH, Water Conditioning for Industg McGraw-HiU Book Co., New York, 1954. 

Polyester (Textured or Filament) Dyed Under Pressure. The dyebath (50°C) is set with water conditioning chemicals as required, acetic acid to ca 5 pH, properly prepared disperse dyes, and 1—3 g carrier/L. The bath is mn for 10 minutes, then the temperature is raised at 2°C/min to 88°C and the equipment is sealed. Temperature is raised at l°C/min to 130°C, and the maximum temperature held for 1/2—1 h according to the fabric and depth of shade required. Cooling to 82°C is done at 1—2°C/min, the machine is depressurized, and the color sampled. The shade is corrected if needed. Slow cooling avoids shocking and setting creases into the fabric. Afterscour is done as needed. 

In order to predict tower performance it is necessary to know the required tower characteristics for fixed ambient and water conditions. 

Pitting can occur when normally protective corrosion-product or deposit layers are locally breached. Localized attack occurs during upsets or when protracted idle periods change water conditions abruptly. Regions adjacent to localized corrosion sites often remain... 

Corrosion of industrial alloys in alkaline waters is not as common or as severe as attack associated with acidic conditions. Caustic solutions produce little corrosion on steel, stainless steel, cast iron, nickel, and nickel alloys under most cooling water conditions. Ammonia produces wastage and cracking mainly on copper and copper alloys. Most other alloys are not attacked at cooling water temperatures. This is at least in part explained by inherent alloy corrosion behavior and the interaction of specific ions on the metal surface. Further, many dissolved minerals have normal pH solubility and thus deposit at faster rates when pH increases. Precipitated minerals such as phosphates, carbonates, and silicates, for example, tend to reduce corrosion on many alloys. 

Water condition (sandy, oily, etc.) and t pe (river, canal, harbor, sea). The contaminating chemicals and/or minerals should be identified. Type of water treatment. 

This latter type appears similar to some previously discussed, closely spaced structural packing, but is specifically designed for this application. Beyers [148] recommends film fill as the best choice if the water conditions of Table 9-49 are appropriate. For scaling or plugging water conditions, select splash fill. 

Betz Handbook of Industrial Water Conditioning, Betz Laboratories Inc., Trevose, Pennsylvania, 1980. 

Where a customer has suitable berthing facilities and satisfactory arrangements can be made, deliveries can be by either river or coastal tanker. River tankers currently in use are of 1860 tons capacity but may be larger if conditions permit. The customer is responsible for supplying oil hose and/or discharge arms capable of discharging the full contents of the barge under low-water conditions. 

Water-level controls continuously monitor the level of water in a steam boiler in order to control the flow of feedwater into the boiler and to protect against a low water condition which may expose the heating surfaces with consequent damage. The controls may be either float operated or conductivity probes. 

With water-level controls, it is important to check they are functioning correctly, and they will be operated daily to simulate low-water condition. On shell boilers, this is invariably a manual operation but may be motorized on watertube boilers. 

Shell boilers may be fitted with internal level controls. Here controls are mounted on the crown of the boiler with the floats or probes extending to the water surface through the steam space. To check the operating function of these, it is necessary to drop the water level in the boiler, or, alternatively, a separate electronic testing device can be fitted. With fully flooded hot water boilers, a single level control or switch is fitted to protect against low-water condition. 

The most frequent cause of damage and even explosion in boilers is a low-water condition. This will expose the heating surfaces, which ultimately overheat and rupture under the operating pressure. Experience has shown that since the introduction of controls for unattended automatic operation of boilers the accident rate has increased. Investigation invariably shows that lack of maintenance has been the main contributing factor. It is therefore imperative that personnel responsible for the running of the boiler plant be fully trained and conversant with its safe operation. 

For hot water boilers the burner controls will be similar but controlled by a combination of pressure and temperature signals. A single overriding level control will be fitted to the flooded boiler to protect against any accidental low-water condition. 

The golden rule, therefore, is to deal only with consultants, contractors, plant suppliers and water-conditioning experts whose experience and standing are known to be good. If there is any doubt, references should be sought and followed up. Water and effluent installations are a relatively minor cost item in any plant, but their failure can be disastrous. It is foolish to make false economies on so essential a service. 

Boilermakers now recognize that their heat transfer rates had become too high, which made control of the boiler water quality unacceptably critical. They have reverted to slightly lower heat transfer rates, but poor boiler water quality remains the main single cause of boiler failure. Good water-treatment plant and boiler-water conditioning and control are still vital not only to the boiler s performance but also to its integrity. 

Chul Kim, U. R. and van Rooyen, D., Strain rate and temperature effects on the stress corrosion cracking of Inconel 600 steam generator tubing in the (PWR) primary water conditions , Proc. 2nd Int. Conf. on Environmental Degradation of Materials in Nuclear Power Systems-VIalet Reactors, Monterey, USA, 9-12 Sept. 1985, American Nuclear Society, pp. 448-55 (1986)... 

Tubular Anodes Tubular anodes are supplied in diameters between 12-5 and 32 mm and have been designed for installations where water conditions on the plant under protection are known to be turbulent. The tubular anode has a number of holes drilled in the active portion of the anode and the nonactive portion is filled with sand to act as a damping agent. As in the case of rod anodes they are supplied complete with mounts ready for installation in the prepared bosses on the plant under protection. They are particularly suitable for internal protection of pump casings and internal protection of pipelines, carrying salt water or other low resistivity liquids. 

Overall, these features mean that chemical control standards are necessarily high. For example, supplementation of the water treatment plant by condensate polishing plant and periodic chemical cleaning are particularly important. In addition, before each period of operation, a clean-up of the cycle is applied to remove crud. Stringent attention must be paid to the feed-water conditioning. 

It is not the intention here to consider in detail the subject of boiler feed-water conditioning and treatment for nuclear plant, but the general principles may be noted. Essentially, the same objectives apply as in fossil-fuelled plant, embodied in the three aims to minimise corrosion, deposition and steam-carryover. Requirements are more stringent in nuclear plant because there is no possibility of repairing tubes which have failed, let alone those which have suffered either deposition or corrosion. Again, certain tubes in nuclear plant have very modest design corrosion allowances so that only minimal loss of thickness from any cause can be tolerated. 

However, the source of MU water for higher pressure steam-raising plants is important because it is generally the case that no matter the origin of the water, it will not be entirely suitable for the purpose and some form of water conditioning will be required. This conditioning requirement necessitates the provision of a (pre-boiler section) water pretreatment plant system (external treatment/external conditioning) to render the water fit for purpose. 

The selective utilization of external water conditioning equipment... 

Where MU requirements are high, irrespective of absolute boiler output, more sophisticated water conditioning is usually warranted than would otherwise be provided. Also, as boiler output and operating pressures increase and heat and mass transfer issues become more critical, so the demand for more exacting water treatment programs, monitoring, and control activities increases in importance. 

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