Boiler feedwater

Steam is generated from boiler feedwater. For example, raw water drawn from the Mississippi River has three objectionable contaminants  

The particulate matter (sand and silt) is removed by flowing upward through a gravel-and-sand filter bed. The coarser gravel is in the lower part of the bed. The filter beds are backflushed periodically with clear water to clean off the accumulated mud. 

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. 

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. 

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Figure 6.25a shows the same grand composite curve with two levels of saturated steam used as a hot utility. The steam system in Fig. 6.25a shows the low-pressure steam being desuperheated by injection of boiler feedwater after pressure reduction to maintain saturated conditions. Figure 6.256 shows again the same grand composite curve but with hot oil used as a hot utility. 

The output from the turbine might be superheated or partially condensed, as is the case in Fig. 6.32. If the exhaust steam is to be used for process heating, ideally it should be close to saturated conditions. If the exhaust steam is significantly superheated, it can be desuperheated by direct injection of boiler feedwater, which vaporizes and cools the steam. However, if saturated steam is fed to a steam main, with significant potential for heat losses from the main, then it is desirable to retain some superheat rather than desuperheat the steam to saturated conditions. If saturated steam is fed to the main, then heat losses will cause excessive condensation in the main, which is not desirable. On the other hand, if the exhaust steam from the turbine is partially condensed, the condensate is separated and the steam used for heating. 

Taking the heat capacity of water to be 4.3 kJ kg K , heat duty on boiler feedwater preheating is... 

The gaseous reactor product is cooled first by boiler feedwater before entering a cooling water condenser. The cooling duty provided by the boiler... 

There are two esdsting steam mains. These are high-pressure steam at 41 bar superheated to 270°C and medium-pressure steam at 10 bar saturated at 180°C. Boiler feedwater is available at 80°C and cooling water at 25°C to be returned at 30°C. 

Figure 13.8 The grand composite curve for the whole process apparently requires only high-pressure steam generation from boiler feedwater.

Exampie A.3.1 The pressures for three steam mains have been set to the conditions given in Table A.l. Medium- and low-pressure steam are generated by expanding high-pressure steam through a steam turbine with an isentropic efficiency of 80 percent. The cost of fuel is 4.00 GJ and the cost of electricity is 0.07 h. Boiler feedwater is available at 100°C with a heat capacity... 

From steam tables, the outlet temperature is 251°C, which is superheated by 67°C. Although steam for process heating is preferred at saturated conditions, it is not desirable in this case to desuperheat by boiler feedwater injection to bring to saturated conditions. If saturated steam is fed to the main, then the heat losses from the main will cause a large amount of condensation in the main, which is undesirable. Hence it is better to feed steam to the main with some superheat to avoid condensation in the main. 

The plant wastewater containing NH and urea is subjected to a desorption—hydrolysis operation to recover almost all the NH and urea. In some plants, this water can then be used for boiler feedwater. 

Carbon produced by these latter reactions is formed in the catalyst pores, making it much more difficult to remove, and potentially causing physical breakage. Operating steam to carbon ratios are chosen above the minimum required in order to make carbon formation by these reactions thermodynamically impossible (3). Steam is another potential source of contaminants. Chemicals from the boiler feedwater or the cooling system are poisons to the reformer catalyst, so steam quality must be carefully monitored. 

Selection of the high pressure steam conditions is an economic optimisation based on energy savings and equipment costs. Heat recovery iato the high pressure system is usually available from the process ia the secondary reformer and ammonia converter effluents, and the flue gas ia the reformer convection section. Recovery is ia the form of latent, superheat, or high pressure boiler feedwater sensible heat. Low level heat recovery is limited by the operating conditions of the deaerator. 

Process condensate from reforming operations is commonly treated by steam stripping. The stripper is operated at a sufficiently high pressure to allow the overhead stripping steam to be used as part of the reformer steam requirement (71). Contaminants removed from the process condensate are reformed to extinction, so disposal to the environment is thereby avoided. This system not only reduces atmospheric emissions, but contributes to the overall efficiency of the process by recovering condensate suitable for boiler feedwater make-up because the process is a net water consumer. 

Fixed investment includes cooling tower, boiler feedwater treatment, raw water ammonia storage as minimum off-sites requirement. 

Gas leaving the converter is normally cooled to 180—250°C using boiler feedwater in an "economizer." This increases overall plant energy recovery and improves SO absorption by lowering the process gas temperature entering the absorption tower. The process gas is not cooled to a lower temperature to avoid the possibiUty of corrosion from condensing sulfuric acid originating from trace water in the gas stream. In some cases, a gas cooler is used instead of an economizer. 

Constmction of new power plants in the coal region of the western United States presents serious problems in states whose laws dictate zero effluent. In these plants, cooling-tower water withdrawn from rivers cannot be returned to them. In these situations, cooling-tower effluent is purified by distillation (vapor-compression plants have predominated) and by a combination of distillation and membrane technology. The converted water then is used as boiler feedwater the plant blowdown (effluent) is evaporated from open-air lined pools, and pool sediment is periodically buried back in the coal mine with the flue ashes. 

CT = clay treater CC = cumene column and BFW = boiler feedwater. 

Boiler Economizers. Heat exchangers that use boiler flue gases to preheat the boiler feedwater are termed boiler economisers. 

Inorganic compounds Monosodium phosphate mixed with boric acid and ethyl carbonate, disodium phosphate sodium aliiminate, bentonite and other solids Distillation instant coffee boiler feedwater sugar extraction... 

The range of applications for magnesium anodes includes the internal protection of boilers, feedwater tanks, filter tanks, coolers, pipe heat exchangers and condensers. They are mainly used in conjunction with coatings and where impressed current equipment is too expensive or cannot be installed. 

To remove these undesirable but unavoidable salt deposits, a signi-fieant amount (about 1 % of mass flow) of boiler feedwater at 80-90°C is periodieally injeeted. Injeetion nozzles are typieally loeated in the inlet seetion, in the return ehannels, and in the labyrinth seals on the diseharge side of nitrous gas eompressors. This periodie flushing of the eompressor with water does have eertain disadvantages ... 

This approach will provide a number aecurate enough for initial planning. For detailed design, the proeess engineer should work elosely with the meehanieal engineer and/or vendor representative involved to set exact requirements, ineluding orifice type and size for the minimum flow line. Also, a eooler may be required in the minimum flow line or it may need to be routed to a vessel. For boiler feedwater pumps, a special stepped type orifice is often used to eontrol flashing. 

Knox has provided the following graphs for estimating the required vent steam from boiler feedwater deaerators. Vent steam rate depends upon the type of deaerator (spray or tray type) and the percentage of makeup water (in contrast to returning condensate). Low makeup water rates require relatively lower steam vent rates, but there is a minimum rate required to remove CO2 from the returning condensate. 

Process Steam Generation. Steam generated in the process sections of the plant may be at the highest plant pressure level or an intermediate level. Also, the process area may have fired boilers, waste heat boilers, or both. There may be crossties between utility and process generated steam levels. Enough controls must be provided to balance far-ranging steam systems and protect the most critical units in the event of boiler feedwater shortage situations. 

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