Lime-softened water

Steam produced from demineralized water is free of carbonates. Steam produced from lime-softened water will be contaminated with carbonates that decompose in the boiler to carbon dioxide. As the steam condenses in a reboiler, the C02 accumulates as a noncondensable gas. This gas will be trapped mainly below the channel head pass partition baffle shown in Fig. 8.6. As the concentration of C02 increases, the C02 will be forced to dissolve in the water ... 

The dissolved solids, such as calcium carbonates, are removed by hot-lime softening or demineralization.1 Demineralized water (also called deionized water) typically has essentially all anions and cations removed by ion-exchange resin. Demineralized water is preferable to hot-lime-softened water as boiler feedwater for several reasons. 

For one thing, steam produced from hot-lime-softened water will have some amount of silicates. These silicates tend to deposit on the rotor blades of turbines, which use the motive steam as a source of energy. The silicate fouling of the turbine blades reduces the turbine s efficiency. But, more importantly, from an operator s point of view, the silicate deposits eventually break off of the blades. This unbalances the rotor. An unbalanced rotor is the fundamental cause of vibration, Vibrations lead to damage of the shaft bearings and seals. Eventually, vibrations will destroy the turbine s internal components. 

Also, the hot-lime-softened water has variable amounts of carbonate contamination. When boiler feedwater is converted to steam, the carbonate deposits will break down into carbon dioxide and hardness deposits. 

An operator working at the main boiler would open the boiler s blowdown valve. Some lime-softened water, about 10 percent of the boiler s total feedwater must be drained to maintain the total dis-solved-solids (TDS) content of the boiler s circulating water below a safe maximum. 

It is really the quality of the treated boiler feedwater that sets the blowdown rate. Deionized or demineralized water might require a 1 to 2 percent blowdown rate. Hot-lime-softened water might require a 10 to 20 percent blowdown rate. 

NOTE Where lime-softened water is used as a source of makeup, the initial pH of the makeup water is usually very high and may exceed pH 10 however, when this water mixes with bulk water and recirculates over the cooling tower, equilibrium with CO2 is quickly reestablished and the pH of the system drops, to be more in line with the proposed pH shown in Table 4.7. 

Sodium alumiaate is used ia the treatment of iadustrial and municipal water suppHes and the use of sodium alumiaate is approved ia the clarification of drinking water. The FDA approves the use of sodium alumiaate ia steam generation systems where the steam contacts food. One early use of sodium alumiaate was ia lime softening processes, where it iacreases the precipitation of ions contributing to hardness and improves suspended soHds removal from the treated water (17). Sodium alumiaate reacts with siHca to leave very low residual concentrations of siHca ia hot process water softeners. Sodium alumiaate is often used with other chemicals such as alum, ferric salts, clays, and polyelectrolytes, as a coagulant aid (18,19). 

Makeup. Makeup treatment depends extensively on the source water. Some steam systems use municipal water as a source. These systems may require dechlorination followed by reverse osmosis (qv) and ion exchange. Other systems use weUwater. In hard water areas, these systems include softening before further purification. Surface waters may require removal of suspended soHds by sedimentation (qv), coagulation, flocculation, and filtration. Calcium may be reduced by precipitation softening or lime softening. Organic contaminants can be removed by absorption on activated carbon. Details of makeup water treatment may be found in many handbooks (22—24) as well as in technical Hterature from water treatment chemical suppHers. 

Gold Lime Softening. Precipitation softening accompHshed at ambient temperatures is referred to as cold lime softening. When hydrated lime, Ca(OH)2, is added to the water being treated, the following reactions occur ... 

Factor Raw water Removal of calcium alkalinity cold-lime Lime—soda softening (cold) Lime—soda softening (hot) Lime softening (hot) ... 

The two principal methods of softening water for municipal purposes are addition of lime or lime-soda and ion exchange. The choice method depends upon such factors as the raw-water quaUty, the local cost of the softening chemicals, and means of disposing of waste streams. 

Lime and Lime-Soda Processes. The first softening plant in the early 1900s used the lime softening process with fill and draw units. Later, continuous-treatment units, which greatiy increased the amount of water that could be treated in a faciUty of given size, were developed. More than 1000 municipahties soften water. Most are in the Midwest and in Florida. However, concern for the adverse effect of soft water on cardiovascular disease (CVD) may limit the number of plants that introduce softening. 

Conductivity Expressed as micromhos, specific conductance Conductivity is the result of ionizable solids in solution high conductivity can increase the corrosive characteristics of a water Any process which decreases dissolved solids content will decrease conductivity examples are demineralization, lime softening... 

Chemical precipitation Precipitation induced by addition of chemicals the process of softening water by the addition of lime and soda ash as the precipitants. Chloramines Compounds formed by the reaction of hypochlorous acid (or aqueous chlorine) with ammonia. 

Conductance pS Due to ionizing solids in solution an increase in conductivity occurs resulting in corrosive water Reduce dissolved solids by lime softening or demineralization... 

Under hot BW conditions this reaction is reversible, leading to a serious risk of carbonate scale depositing on heat transfer surfaces. Consequently, many large water utilities and industries around the world continue to use the old established, but effective lime (calcium hydroxide) and soda ash (sodium carbonate) processes to soften water by precipitating out insoluble hardness salts. 

Some older plants still carry this softening process further by the addition of mono- or di-sodium phosphate and, although it lowers the hardness to only a few ppm, it ultimately results in additional carbonate alkalinity, which is undesirable. Additionally, the phosphate is too expensive and it is better to simply pass the filtered, lime-treated water through a BX softener. 

High levels of silica in the raw water supply can lead to serious risks of deposition in boilers, especially if cycles of concentration (COC) also are high. The incoming silica can be reduced by adsorption on magnesium hydroxide [Mg(OH)2] precipitate during lime-softening processes, or by the addition of magnesium hydroxide in a reaction tank, followed by filtration. 

Lime is used in drinking water treatment to control pH, soften water, and control turbidity. Lime, in combination with sodium carbonate, is used to precipitate the major bivalent... 

High levels of silica in the raw water supply can be reduced by adsorption of magnesium hydroxide [Mg(OH)2] precipitate during lime-softening processes. 

---