Boiler feedwater

As feed water water is referred to, which is held in a feedwater tank and is continuously fed to a steam generator. Large amounts of feed water are required in steam power plants. The boiler produces steam, which is used to heat, for industrial processes or to drive a steam turbine or steam engine. The feed water is prepared so that harmful components of water have been removed or converted into materials for the operation of the boiler, which have no adverse effect on the operation of the boiler.

Harmful ingredients of the water used are salts of the alkaline earths, which precipitate at higher temperatures on the heating surfaces and form an insulating layer and hinder the transfer of heat. This leads to overheating of the series of thermal stress cracking. In addition, set security-related equipment by deposition except a function of scale. The dissolved gases O2 and CO2 in the water cause corrosion. When through boilers the entire feed water is evaporated. In this type of boiler, therefore, all of the dissolved constituents of the raw water must be removed from the feed water. It may therefore only deionised water ( = pure water without ingredients ) are used.

Depending on the use of the steam can more or less be used as steam condensate back as feed water. In a steam power plant, the losses must be compensated by desalination and thermal degassing by adding water. In process plants, the steam is partially used for direct heating, so that no condensate for re-use is available.

The feed water consists of the recycled condensate and treated makeup water.

• 4.1 Internal feed water treatment
• 4.2 External feed water treatment
• 4.3 Austauschenthärtung
• 4.4 Partial desalination 4.4.1 Reverse Osmosis
• 4.4.2 Ion exchange

Condensate

The condensate is collected in the condensate container and fed to the feed water container by means of the condensate pump. The quality of the recycled condensate may need to be reviewed if an ingress of foreign substances - for example, on a leaking heat exchanger - is possible.

When driven by steam engines, the condensate is contaminated with oil. If large quantities of oil from entering the boiler water, there is a danger of overheating of the heating surfaces, as the film boiling happen at lower heat fluxes. The oil must be mechanically separated by floating and residues are adsorbed by activated charcoal filters.

In dependence on the possibly invading substances, an automatic monitoring turbidity or conductivity measurement can be used which controls a 3/2-way valve, and the condensate fed to the expander. This device is compulsory when operating without constant supervision (TRD 604). In high pressure boilers (operating pressure > 64 bar ), especially in once-through boiler, a processing of the condensates is required. Unresolved components such as iron oxides, which are detached from the material of the pipes due to corrosion or erosion, and salt traces that have come from the cooling water in the condenser or in various heat exchangers in the Dampf-/Kondensatsystem must be removed. Typical equipment for condensate treatment are candle filter ( for removal of undissolved particles ), and mixed-bed filters ( for removal of dissolved salts ).

Promotion

Between the feedwater tank and inlet of the feedwater to the steam generator are located one or more feedwater pumps which deliver the feed water.

Treatment of the feed water

Thermal degassing

In general, the feed water is in a degasser disposed in the feed water tank, degassed. Result, the harmful gases oxygen and carbon dioxide are largely removed. The degassing of the physical circumstance is used, that with increasing temperature decreases, the solubility of gases in liquids. Further information on the physical process of degassing are listed under degassing ( steam and hot water technology).

The deaeration of the feedwater is predominantly at elevated pressure ( p = 0.1 .. 0.4 bar g ) and corresponding saturated steam temperatures 102-108 ° C applied. The feed water tank is heated by a steam lance in the water chamber for mixing the liquid and by the addition of steam in the gas space. On the crest of a deaerator feedwater tank is flanged. The cold make-up water and condensate will be directed to the upper distributor plate and run for scrubbing plates in the feedwater tank. In countercurrent steam flows to the top of the degasser and heats the water introduced. The bound gases are released and rise with the steam upward. The ascending vapors escape to the top of the degasser and are referred to as Fegedampf. To use the enthalpy of the vapor, a vapor condenser is used for larger systems. By the condensation of water vapor in the vapor, the treated make-up water is preheated.

The allowable pressure of the feed water container can be secured by a safety valve or a siphon. A siphon can be used only with degasifiers bar at operating pressures up to about 1.5, otherwise the pipe loop (necessary for the required static water column as a counter pressure) is too high. However, the use of a siphon has the advantage that in case of failure of the ramp metering overfilling of the feedwater tank can not occur.

In addition to the above-described degassing at a pressure of > 1.0 bar, there are vacuum degasser. These are largely comparable in structure and mode of action with Druckentgasern. The operating pressure is a function of the operating temperature of less than 1.0 bar. The vacuum is created by vacuum pumps ( predominantly water ring pumps and / or steam ). Condensates to be degassed in a Rohkondensattemperatur of about 35 -100 ° C, so-called Entspannungsentgaser can be used, which are widely used in the construction comparable with normal vacuum degassers.

Chemical degassing

In addition to the above-mentioned physical degassing is possible for the addition of water and a catalytic degassing. Here, a resin that is loaded with a heavy metal from the platinum group, is used as catalyst. The reducing agent is hydrogen or hydrazine is used. The oxygen removal is performed in a filter, which is similar in construction with an ion exchange filter. The operating temperature for this process is usually at 10 - 30 ° C.

Low oxygen levels in condensates of boilers in industry and power plants are also chemically bonded. The addition of the reducing agent is carried out according to the condenser and to remove residual oxygen after degasser. This treatment system parts made of iron from corrosion are protected by oxygen. By dosing based degasser low residual levels of approximately 5 to - 10 ppb oxygen are chemically reduced even further.

For this type of Sauerstoffentgasung following chemicals are suitable and commonly used:

• Sodium sulfite
• Diethyl ( DEHA )
• Hydrazine
• Morpholine
• Referred to as 2-butanone oxime methyl ethyl
• Ascorbic acid as ammonium ascorbate
• Diaminourea as Carbohydrazit. referred

Except for sodium sulfite ( Na2SO3 ) these chemicals are volatile in steam, and can be used in systems with forced circulation boiler. The use of sodium sulfite, however, is limited to systems with circulation boilers and hot water circuits.

Until the 1980s, predominantly hydrazine was used. Since the reaction of hydrazine with oxygen only nitrogen and water is formed, it is optimal for the chemical removal of oxygen. But as part of hydrazine according to EEC to the cancer-causing chemicals, increasingly less dangerous other agents. A disadvantage of these substitutes is that this organic degradation and form fission products such as organic acids, carbon dioxide, aldehydes or ketones. The COD content in the water - steam cycle does increase and degrades the quality.

Further treatment of the feed water

A feed water treatment with a Austauschenthärtung must be supplemented by removal or binding of oxygen and water conditioning agents which set the Resthärtebildner. Most of the gases (oxygen, carbon dioxide ) are removed by thermal degassing the feed water tank. By the addition of sodium sulfite, the residual oxygen is chemically converted into sodium sulfate. Hydrazine also can be used for the chemical ligation of oxygen. By adding trisodium phosphate the hardness are tied off and removed as sludge. A pH increase is by the addition of caustic soda or ammonia. The dose of salts such as trisodium phosphate and caustic soda is however only allowed for drum boilers. For through boiler only the use of steam-volatile chemicals is possible because salts would crystallize in the area of ​​residual water evaporation on the heating surfaces.

To simplify the water treatment, water conditioning these substances are usually supplied as a ready mix. These funds will be bottled in plastic containers and pumped by diaphragm pumps in the feedwater tank or in the makeup water. To obtain a uniform dose, the pump is driven simultaneously with the feed pump. The delivery rate of the metering pump is adjusted such that in the boiler water, an excess of the components of the processing agent is detected.

Additional water

The additional water must be treated to prevent one adverse effect of ingredients on the boiler.

Depending on the application and in particular depending on the operating pressure of the boiler is made ​​between the following processing stages:

• Salty feed water,

• Low salinity feed water,

• Salt-free feed water.

From the first auxiliary water insoluble components must be removed. Coarse components can be removed by raking or settling basins. The finer constituents are removed by filtration or flocculation.

In well water and surface water often occur harmful amounts of iron and manganese compounds which are converted by oxidation into sparingly soluble oxides and form deposits. Dissolved iron and manganese is removed when the water is aerated and the water-insoluble hydroxides developed thereby be deposited in a gravel filter. In raw water dissolved oxygen and free carbon dioxide cause particularly with plant components made ​​of steel corrosion and must be removed.

Inside feed water treatment

→ Main article: Internal feed water treatment

Inside feed water treatment means a treatment of the water in the boiler. To previously deposited on the solids, all components of the water used to enter the boiler. The oxygen must be tied by sodium or other suitable means and carbon dioxide by caustic soda. The feed water supplied to phosphates, which react with the hardness minerals to form calcium or magnesium phosphate, which accumulates as sludge on the boiler base. The boiler must therefore often be flushed away. This form of water treatment is only for shell boilers permitted as the sole means without flue pipe ( flame tube boilers) and steam locomotive boilers and thus has hardly important.

In today's boilers hardness salts and dissolved oxygen from the feed water before feeding to the boilers are largely removed. There is a conversion of the remaining damaging water constituents by water conditioning agent containing, in particular sodium, phosphates and basic ingredients for pH increase.

External feed water treatment

External feed water treatment means that the undesirable components of the water used can be removed physically or chemically prior to feeding into the boiler or converted into compounds that do not cause damage to the boiler and the steam consumers.

In modern water treatment equipment ion exchangers are used. These consist of containers with spherical resins having a particle diameter of 0.3 - 1,5 mm is filled. The regenerative exchangers operate; This means that the ion exchanger after the regeneration is reset to its original unloaded state. Regeneration is effected by treatment with acids, bases or salts. Often double exchangers are used so that in the regeneration of an exchanger of the other can be used. Prior to treatment with chemical solutions often is backflushing of the exchanger. With this backwash filtered impurities and resin fines are removed. Still takes place a loosening of the resin bed, whereby a channel is avoided during regeneration.

Austauschenthärtung

For small and medium sized boiler plants to medium vapor pressures (about 32 bar ) is carried out only a Austauschenthärtung with a downstream chemical conditioning and degassing. This softening takes place in a cation exchanger, since the positive alkaline earth metal ions are exchanged. The exchanger is regenerated with inexpensive saline. The calcium and magnesium hardness constituents, which have been deposited in the loading of the ion exchange resin are replaced by the sodium salt of the regenerating agent. 10 - - % saline solution used for regeneration is 8. This must be in excess ( 180-250 % of the equilibrium amount ) done to set a sodium ion surplus, which deviates from the equilibrium between calcium and sodium ions in the exchanger mass. It is a chemical compulsion to binding of calcium ions in the resin and release the sodium ions is thus exercised. In the exchanger is of the chemical forced to reduce the free energy by the ion exchange.

Is achieved at alkaline earths a residual content of < 0.01 mmol / l. When Austauschenthärtung the hardness to be replaced and thus the salinity of this treated water is not reduced. The surface subjected to the water boiler must therefore be drained from appropriately so that an alarming foaming of boiler water is avoided. The problem is the application of Austauschenthärtung the use of water with very high total hardness and high levels of sodium salts. By softening by ion exchange are the anions - bicarbonates, chlorides and sulfates - not diminished. The feed water then contains high levels of these sodium salts, and is corrosive to the ferrous materials of the boiler, the piping and the heat exchangers.

Partial desalination

Reverse osmosis

A method of partial desalination often used recently is reverse osmosis, since, compared to ion exchange resins can be avoided and to a regeneration system, a continuous operation is possible. Requirement for use is an upstream softening the water. The water should be clear and free of insoluble impurities, in particular, be free from organic impurities, to prevent blocking of the membranes. The principle of reverse osmosis is based on the diffusion resistance of the pores of the membranes used for the smaller water molecules is substantially less than the resistance of the larger ions dissolved in the water. The water is passed at a higher pressure in the module equipped with a membrane. The amount of pressure required is dependent on the salinity of the raw water and the type of membrane used. Usually, the pressure is fresh water at about 20 / - 5 bar. In seawater, the pressure 50 is / - 20 bar. Water and a particular share of the smaller salt ions diffuse through the membrane and form the permeate, which is available as demineralised water. The permeate of the water used is 70 - 90%. The rest is the salt-rich concentrate, which is not diffused through the membrane. This proportion is discarded.

Ion exchange

To achieve the desalination of the feed water, the added water is passed through a strongly acidic cation exchanger in hydrogen form and then passed through a strongly basic anion exchanger. In the cation exchanger, the cations are tied off and replaced by the accumulated resin in hydrogen ions ( H ). Lie behind the exchanger thus the anions as the free acid before. In the basic ion exchanger in the hydroxyl anions to be bound and replaced with a hydroxyl ion (OH - ). The strongly basic anion exchanger is also capable of ligate, carbon and silica.

Natural water, colloidal silicon dioxide ( SiO2) contain that can cause problems to boiler feed water during treatment. Particularly in the demineralization with ion exchangers this is taken into account. Non-ionic compounds are not or only slightly removed from water by ion exchange resins. When treating such waters, therefore, an additional flocculation for the removal of these colloids must be connected upstream of the ion exchange.

The cation exchanger is regenerated at the exhaustion caused by hydrochloric or sulfuric acid; for the anion exchanger is used caustic soda.

With this arrangement, all dissociated substances (salts, acids, alkalis) can be extracted from the water. For water samples with a high content of bicarbonates a CO2 Rieseler is included to relieve the anion exchanger between the two exchangers. The achievable residual conductivity of a cation-anion Austauscheranordnung is <10 ĩS / cm.

Demineralization

A demineralization of the water with a conductivity below 0.2 ĩS / cm is obtained by the ion exchange device described in CO2 Rieseler if after the addition, a mixed-bed anion exchanger, the filter is installed. In the filter are combined cation and anion exchangers. To optimize the demineralization and reduction of the amounts of regeneration funds in the processing steps in addition weakly acidic and weakly basic anion exchanger upstream of the strongly acidic or basic exchangers. The desalinated water is also referred to as deionized water. Modern demineralization plants mainly work according to the countercurrent principle. Here, the flow directions at loading and regeneration are opposite in the resin bed. When using the multi -chamber containers weak and strong acid cation resins and anion resins can be combined. An already largely ion-free deionized water without mixed bed filter with a conductivity of <1.0 ĩS / cm generated - this way, with only a slight excess chemicals - often less than 105% of the theoretical value.

Requirements on the feed water quality

A trouble-free long-term operation of steam boilers and steam power plants is to reach for the water and steam flow-through system components ( piping, boiler tubes, heat exchangers, tanks, turbines, pumps and capacitors) only when corrosion on all parts of the plant can be prevented as far as possible. In addition to proper selection and combination of suitable materials, optimum feed and boiler water quality is a prerequisite for this. More generally, the feed water to note the following:

• Materials of iron and copper are resistant only to water and steam, if dense and crack-free Oxiddeckschichten at the boundary layer medium / metal are present
• Particularly erosion-resistant protective layers of magnetite are formed only at water temperatures above 200 ° C; increases the solubility of the magnetite at a pH < 9.4

• The corrosion rate of ferrous metals has a minimum at a pH of> 9.1, the minimum of the rate of corrosion of copper -based materials in the presence of ammonia is at a pH of 8.5
• The distribution coefficient of the alkalizing agents, such as ammonia, is different for the liquid and the vapor phase. In the vapor phase of circulation boilers, the content of ammonia is significantly higher than in the water phase. In the condenser (condensate) therefore locally significantly higher ammonia levels will occur. This can at messingberohrten capacitors lead to strong corrosion.

• For the lowest possible corrosion rates must the level of electrolyte ( salts) in water and steam as low as possible, corresponding to a conductivity of ≤ 0.2 ĩS / cm. The conductivity caused by alkalizing agents is not considered.
• The content of dissolved free oxygen in the system should normally be < 0.005 mg / l; in water having a pH of 7.0 to 9.0, the corrosion rate in salt-free water samples (<0.2 ĩS / cm conductivity) but for oxygen concentration from 0.15 to 0.30 mg / l less
• Particularly important in practice, the selected pH range during operation of the water and vapor phases in the system.

The above mentioned conditions are partly contradictory. Therefore, compromises are necessary for optimal conditions, taking into account the whole system with the different materials and temperature conditions. The following qualities of the feed water are common for high-pressure systems, and operating phase in Germany:

• Neutral - driving
• Combined mode of operation
• Alkaline - driving
• High - AVT operation. (AVT = All Volatile Treatment )

The following table of baseline data for the operating modes:

• O2: > 0,050 mg / l

• O2 as a gas or
• H2O2
• Largely without ammonium hydroxide ( NH4OH ) < 0.01 mg / l

• 3-10 ug / l total Fe

Only possible with electrolyte-free conditions with ≤ 0.20 ĩS / cm ° 2 in the overall system

• Generally O2: < 0.020 mg / l
• Mostly O2: < 0.005 mg / l

• Ammonium hydroxide 0.02 to 0.25 mg / l
• Hydrazine ( N2H5OH ) < 0.02 mg / l

• 3-7 ug / l total Fe

• O2: generally < 0.020 mg / l
• O2: mostly <0,005 mg / l

• Ammonium hydroxide from 0.5 to 1.0 mg / l
• Hydrazine < 0.02 mg / l

• 3-5 ug / l total Fe

Mainly used in systems that are free of copper-containing materials,

• O2: < 0.005 mg / l

• Ammonium hydroxide > 5 mg / l
• Hydrazine > 0.02 mg / l

• < 10 ug / l
• Normal ~ 1 ug / l

Total Fe

Applied to the secondary systems of nuclear power plants with pressurized water reactors

° 1Note: In a study of the early 1990s in Germany are given for the different modes of operation following figures for the application:

• Neutral - driving: 17 plants
• Combined mode of operation: 65 plants
• Alkaline - driving: 150 plants

° 2 Note: directly measured conductivity; in the operating modes with ammonium hydroxide Dosage: according to a strongly acidic cation exchange to remove the ammonium hydroxide prior to the measurement

Guidelines for boiler and feedwater

The required quality of these waters is defined in policies. Especially the following guidelines must be observed when operating the various types of boilers and pressure ratings:

• VGB guidelines for feed water, boiler water, etc. No R450L (VGB = VGB Power Tech )
• Technical rules for steam boilers TRD from the German Boiler Committee (DDA ) and the Association of Technical Inspection Agencies ( VdTÜV ). In the TRD 611 for example, the data for steam generators of the group IV are indicated.

The data for the Greater water boiler are given in the following list:

Requirements for feed water for large water tube boiler according to EN 12953 part 10 (except injection water )