Steam boilers feedwater treatment

The condensing steam turbine has a relatively low thermal efficiency because about two-thirds of the steam enthalpy is lost to cooling water in the condenser. Expensive boiler feedwater treatment is required to remove chlorides, salts, and silicates, which can be deposited on the blades causing premature failure. The blades are already under erosion conditions because of water drops present in the condensing steam. Even with these disadvantages, the condensing turbine is still selected, especially in a process that requires very large compressor drivers and relatively low amounts of process steam. 

Figure 23.2 shows a schematic representation of a boiler feedwater treatment system. Raw water from a reservoir, river, lake, borehole or a seawater desalination plant is fed to the steam system. However, it needs to be treated before it can be used for steam generation. The treatment required depends both on the quality of the raw water and the requirements of the utility system. The principal problems with raw water are1,2 ... 

The deaerated treated boiler feedwater then enters the boiler. Evaporation takes place in the boiler and the steam generated is fed to the steam system. Solids not removed by the boiler feedwater treatment build up in the boiler, along with products of corrosion. These are removed from the boiler by taking a blowdown (purge) from the boiler. The steam from the boiler goes to the... 

As discussed under boiler feedwater treatment, boiler blowdown is required to prevent the build up of solids in the boiler that would otherwise cause fouling and corrosion in the boiler. Carry over of solids from the boiler to the steam system via tiny water droplets should also be avoided. Total dissolved solids (TDS) and silica (SiC>2), as measured by the conductivity of water, are both important to be controlled in the boiler3. Dissolved solids carried over from the boiler will be a problem to all components of the steam system. Silica is a particular problem because of its damaging effect on steam turbines, particularly the low-pressure section of steam turbines where some condensation can occur. Blowdown... 

There are many types of steam boilers, depending on the steam pressure, steam output and fuel type. Blowdown is required to remove the dissolved solids not removed in the boiler feedwater treatment. The efficiency of the boiler depends on its load. 

Improving the energy efficiency to reduce steam demand and hence reduce the wastewater generated by the steam system through boiler blowdown, boiler feedwater treatment and condensate loss (see Chapter 23). 

Increasing condensate return for steam systems to reduce makeup water requirements, reduce aqueous waste from boiler feedwater treatment and boiler blowdown (see Chapter 23). 

Waste from steam systems. If steam is used as hot utility, then inefficiencies in the steam system itself cause utility waste. Figure 23.2 shows a schematic representation of a boiler feedwater treatment system. The constant loss of condensate from the steam system means that there must be a constant makeup with freshwater. This makeup causes utility waste ... 

A final step in boiler feedwater treatment consists of pH adjustment as a further aid to corrosion control. Usually the pH is adjusted to a range of 10 to 11 with trisodium phosphate (or combinations of caustic with sufficient mono- or disodium phosphate to form trisodium phosphate upon inadvertent evaporation of the water). This "coordinated phosphate" treatment is intended to preclude the environmental cracking of steel by free sodium hydroxide (caustic embrittlement), a catastrophic form of corrosion described in Chap. 6. Caustic carryover with the steam can present severe corrosion problems (Fig. 8.16). 

Steam systems. Copper and copper alloys resist attack by pure steam, but if carbon dioxide, oxygen, or ammonia is present, condensates can be quite corrosive to copper alloys. Modem power utility boiler feedwater treatments commonly include the addition of organic amines to inhibit the corrosion of iron components of the system by scavenging oxygen and increasing the pH of the feedwater. These chemicals tend to release ammonia, which can be corrosive to some copper alloys. 

The choice of boiler steam inlet conditions is usually dictated by the desire to achieve maximum output from the process steam flow. This requires high boiler steam pressure and temperature. However, there are practical considerations to observe. Above 40 bar more exacting feedwater treatment is necessary, and therefore it may be advantageous to maintain pressures below this figure. High steam and temperatures can also influence selection of boiler materials such as alloy steels. The upper limit for industrial applications is around 60 bara and 540°C. 

Other notaries are the Consensus on Operating Practices for the Control of Feedwater and Boiler Water Chemistry in Modem Industrial Boilers (1994 edition), published by the American Society of Mechanical Engineers, and BS 2486 1997 Recommendations for Treatment of Water for Steam Boilers and Water Heaters from the British Standards Institution. The 1994 Consensus (with its engineering background) and the 1997 version of BS 2486 (with its strength in operational chemistry) complement each other well. I consider that the tables and propositions contained in these two booklets jointly represent a true standard for boiler water treatment operational control. Consequently, I am pleased to be able to reproduce in this book all the tables from both publications, having received permission from the respective organizations to do so. 

In ammonium phosphate production and mixed and blend fertilizer manufacturing, one possibility is the integration of an ammonia process condensate steam stripping column into the condensate-boiler feedwater systems of an ammonia plant, with or without further stripper bottoms treatment depending on the boiler quality makeup needed. 

In most cases, condensate does not require treatment prior to reuse. Makeup water is added directly to the condensate to form boiler feedwater. In some cases, however, especially where steam is used in industrial processes, the steam condensate is contaminated by corrosion products or by the inleakage of cooling water or substances used in the process. Hence steps must be taken to reduce corrosion or to remove the undesirable substances before the condensate is recycled to the boiler as feedwater. 

Power Production. Steam cycles for generation of electric power use various types of boilers, steam generators, and nuclear reactors operate at subcritical or supercritical pressures and use makeup and often also condensate water purification systems as well as chemical additives for feedwater and boiler-water treatment. These cycles are designed to maximize cycle efficiency and reliability. The fuel distribution of sources installed in the United States from 1990—1995 are as follow coal, 45% combined cycle, 27% miscellaneous, 14% nuclear, 11% solar, oil, and geothermal, 1% each and natural gas, 0.3%. The 1995 summer peak generation in the United States was 620 GW (26). The combined cycle plants are predominantly fired by natural gas. The miscellaneous sources include bagasse, black liquor from paper mills, landfill gas, and refuse (see Fuels frombiomass Fuels fromwaste). 

PHYSICAL WATER TREATMENT - Refers to the treatment of removing dissolved gases from the boiler feedwater, using steam. 

Membranes are a candidate technology for each of these applications. However, they are only used regularly in the treatment of boiler feedwater for steam turbine makeup application. The selection criterion is related more to the economics rather than performance. Each of these applications is examined in more detail to understand where membranes can be utilized and what developments are required for broader adoption. 

G. Navitsky and F. Gabrielli, Boiler water treatment, feedwater treatment, and chemical cleaning of drum-type utility steam generators. Combustion 52(2), 19 (1980). 

Utilities include electricity, steam, process fuel, process water, boiler feedwater, cooDng water, deionized water, compressed air, instrument air, refrigeration, inert gas, and effluent treatment. The unit use is dependent on die process technology, and the cost is site dependent. For each utility, unit use and unit cost should be recorded. The estimated use of utilities for different fertiDzer processes is shown in Table 21.6. Some offsite utilities such as electricity are often priced with a fixed cost component and a variable cost component. The fixed and variable portions need to be calculated separately to provide an average variable cost per unit of production for a given annual production. 

The hot-process softener, sometimes used for treating boiler feedwater, uses steam to heat the water to more than 200°F (93°C). Hot-process hme treatment has been used to remove silica from boiler feedwater where extremely low concentrations requiring ion exchange are not necessary. The sUica is removed by adsorption on freshly precipitated magnesium hydroxide. The removal is therefore dependent on the amount of Mg(OH)2 precipitated. The effective-... 

Internal Feedwater Treatment. Internal feedwater treatment is generally nothing more than the addition of the proper chemicals to prevent the deposition of scaling materials on the hot surfaces of the boiler. A sludge formed by the chemicals with calcium and/or magnesium carbonates drops to the bottom of the boiler or remains in suspension. In steam boilers, this sludge can be removed by proper blowdown procedures. The chemicals that are to be added to the boiler water, the blowdown procedure, and the analysis and maintenance of the feed-water conditioning should be handled by a water-treatment consultant. 

The wastewater from a hydrogen plant typically consists of only the blowdovm from the boiler system. The boiler feedwater that feeds the steam generation system has small amounts of impurities such as sodium, chlorides, silica, and organic carbons. These impurities will accumulate within the boiler system and create sludge, scaling of the boiler tubes, and possible carryover of solids into the process steam. Blowdown of the boiler water is performed to prevent these issues from affecting the operation of the steam system. The blowdown is typically sent to the sewer or the on-site waste treatment plant for treatment and disposal. 

The steam system is an area that requires constant attention to ensure proper operation of the hydrogen plant. If the steam quality decreases, it can lead to solids carryover from the steam drum. These solids will plate out in the feed preheat coil of the convection section and subsequently lead to an equipment failure and plant shutdown. To reduce the probability of an upset, the steam drum should be manually blowndown on a regular basis to reduce the amount of dissolved solids and other impurities in the steam drum. Next, the boiler feedwater should be analyzed on a regular basis to ensure that the treatment it is receiving is adequate for the desired steam generation conditions. 

On the other hand, 30 percent condensate recovery as a percentage of the steam generation rate is rather bad and represents sloppy operations and bad condensate recovery design practices. Very approximately, 10 percent of the cost of steam generation could be saved by recovering the condensate. The condensate contains few silicates, thus no blowdown (explained below) is needed, no chemical treatment or deaeration is required, and no energy that would otherwise be used to preheat the boiler feedwater will be wasted. 

The steam drum is the entry point for boiler feedwater and internal chemical treatment. It is also the withdrawal point for continuous blowdown, purging from the solute of suspended solids to maintain the minimum level of dissolved and suspended solids. If blowdown is not done deposits of solids would form continuously on the boiler tubes and/or contaminate the system. The solid contents of steam should, in principle, be kept to zero. 

On the other hand, in a once-through type boiler, boiler feedwater is fed once-through and preheated at the economizer, evaporated water wall and evaporator, superheated at superheater and led to the steam turbine. Therefore, impurities contained within boiler feedwater will deposit inside the evaporator or be carried into the steam turbine. Consequently, once-through type boilers require more severe water quality control than natural circulation type boilers. AVT has been applied as feedwater treatment for all once-through type boiler plants for many years, but Combined Water Treatment (CWT) Oxygen Treatment has been used with good results since about 10 years ago. Since then, water treatment in once-through type boilers has been switched from AVT to CWT in sequence. 

In addition to the requirement to conform to steam purity needs, there are concerns that the boiler water not corrode the boiler tubes nor produce deposits, known as scale, on these tubes. Three important components of boiler tube scale are iron oxides, copper oxides, and calcium salts, particularly calcium carbonate [471-34-1]. Calcium carbonate in the feedwater tends to produce a hard, tenacious deposit. Sodium phosphate is often added to the water of recirculating boilers to change the precipitate from calcium carbonate to calcium phosphate (see also Water, industrial water treatment). 

In lower pressure boilers a variety of additional treatments may be appropriate, particularly if the steam is used in chemical process or other nonturbine appHcation. Chelants and sludge conditioners are employed to condition scale and enable the use of less pure feedwater. When the dmm pressure is less than 7 MPa (1015 psia), sodium sulfite may be added direcdy to the boiler water as an oxygen scavenger. It has minimal effect on the oxygen concentration in the system before the boiler. 

The treatment of raw water, condensate, feedwater and boiler water, and the subjects of carry-over and steam purity are considered in detail in the sections which follow. 

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