Blowdown of boilers. Boiler blowdown

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What is boiler blowdown and what is it for

No matter how carefully you monitor the boiler, and no matter how hard you try to use only clean water, the time will come when it will be necessary to clean the boiler of slags and impurities. Even frequent blowing out of the steam boiler will not save you from this.

There are two types of flushing - cold and warm flushing. Cold - steam comes out and the boiler cools itself to a temperature of thirty, thirty-five degrees. And after that, the water is drained, and the boiler is naturally cooled to ambient temperature. After that, it is washed with cool water supplied by a special pump under pressure (usually 5-6 kg / cm2). This is the most convenient method that does not require special equipment.

What is a steam boiler passport for?

In order to avoid problems with the boiler in the future, it is better to buy it at a trusted company. These include a website company that produces high-quality and time-tested steam boilers and industrial steam generators. But remember, quality isn't everything. You need to take care of him in time. Then it will serve you for a long time.

To ensure the specified water regime, it is necessary to regularly remove (purge) the salts supplied with water, otherwise there will be a rapid increase in the alkalinity of the boiler water, its foaming, and obvious corrosion damage to the boiler tank will appear.

There are two types of boiler blowdown: intermittent and continuous.

Periodic is produced at intervals and is designed to remove sludge from the drum, collectors, etc., is carried out quickly. But with a significant discharge of water from the boiler, which, during its movement, carries out sludge and others into the so-called expander (bubbler), designed to cool the boiler water.

Continuous blowdown is carried out from the upper drum of the boiler. For a more even intake of boiler water, a pipe with holes is laid along the drum through which water enters the pipe.

Problems due to salts in the boiler water

The boiler water must maintain a constant water composition, i.e. input of salts and impurities with feed water must correspond to their output from the boiler. This is achieved by continuous and intermittent blowdown.

With insufficient removal of salts from the boiler, they accumulate in the water and form scale in the pipe sections, which reduces their thermal conductivity, leads to blowouts, ruptures, emergency stops, and a decrease in the reliability and efficiency of the boiler. Therefore, the optimal and timely removal of salts and sludge from the boiler is of decisive importance.

Drum steam separators

The higher the steam parameters, the worse the salts dissolve in the feed water. The less dissolved salts in the boiler water and the drier the resulting steam, the cleaner it is. The removal of moisture with steam is considered unacceptable, since it contains salts, and upon evaporation, they will settle on the inner surfaces of the pipes in the form of sediment.

Boiler water must be of such quality as to exclude:

Limescale and sludge on heating surfaces.
Deposits of various substances in the boiler superheater and steam turbine.
Corrosion of steam and water pipelines.

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In addition to periodic adjustments of automation and maintenance of optimal technical condition, it also needs regular maintenance. Preventive maintenance consists in cleaning the surfaces of pipelines and internal cavities of the structure from harmful salts, alkalis and scale. The boiler blowdown technology allows you to effectively cope with such tasks.

General information about the method

The process of operation of hot water and steam boilers is associated with the accumulation of salt-containing products, which negatively affect the condition of the unit's surfaces, not to mention the quality of the coolant it serves. Equipment with natural circulation of water and steam should be purged in order to remove harmful deposits into special separator tanks. There are different ways to implement cleaning, but a complete rejection of this preventive measure can lead to wear of the unit to a state of unusability. So, in relation to hot water and steam boilers, blowdown is the removal of a certain volume of water from its structure and associated pipeline circuits, which contains salts, sedimentary elements and sludge. Technically, the procedure is performed using a plug-in attachment in the form of a branch pipe located in the boiler drum. To regulate the intensity of the process, valves and shut-off valves are additionally connected.

Purpose of boiler blowdown

For each boiler model, its own schedule for performing the purging operation is prescribed, taking into account the operating mode of the equipment and the quality of the served water. Typically, a dedicated line is provided for this operation, connected to the purge line. The procedure is performed sequentially at each contour point of the removal of foreign particles. Due to the small volumes of accumulated water, care should be taken when blowing the salt chambers of the cyclones.

What effect should the boiler blowdown provide? Again, a lot depends on the current state of the equipment. With complex blowdown, elements such as sludge, ash, salt, soot and scale are removed from the circuits and functional tanks. If they are not removed in time, then over time the risk of burns will increase, which will lead to a decrease in boiler performance, an increase in fuel consumption and even pipe rupture.

Purge types

There are two options for blowing - continuous and intermittent. In the first case, respectively, the cleaning process is carried out without interruption, and in the second - in a short-term mode after certain periods of operation. The technique of continuous removal of undesirable substances is oriented rather towards washing out the salts in the boiler water. In turn, periodic blowdown is used in cases of removal of more solid settled substances, such as scale and sludge.

Continuous boiler blowdown is more commonly used because it guarantees a higher quality of equipment surface maintenance. It is another matter that such a method cannot be applied in case of capital all-round cleaning. Rather, large-gap blowing is viewed as an additional maintenance operation to remove local dry sludge build-up.

Continuous Purge Technique

The procedure can be performed from any part or circuit of the boiler equipment with piping. In particular, it is possible to start from the bottom or top of the drum, as well as from external cyclones. The setting point of the connected communications for purging does not matter, since the operation is performed with small resources with a minimum pressure load. The process is organized using a boiler installed in the drum. Further, valves are connected to the control circuits, which adjust the intensity of the water supply. Sometimes, continuous boiler blowdown is organized through the lower outlets of the brine chambers with two active small-format valves. It is also recommended to add restrictive washers with valves with a diameter of 3-8 mm on the lower purge line.

Disable continuous blowdown

Purification of saline water outside the boiler is carried out using a separator. If the planned alkali value is normal at a certain operating interval, then the boiler blowdown can be set to a minimum degree of operation or completely turned off. After the contaminated liquid is discharged, the valve of the connected pipeline closes, cutting off the separated water line. Filtered salts and sludge are sent to the drainage circuit.

Periodic Purge Procedure

This method involves connecting the outlet circuits only through the lowest points of the collectors or drums in order to remove the sludge to the separators. Technically, the process of periodic boiler blowdown is carried out in the following sequence:

The sufficiency of the liquid supply in the feed deaerator is checked.
Water-indicating measuring equipment is being blown out.
The tightness of the purge valves, the reliability of the boiler shutdown mechanisms are checked.
The water level in the boiler rises by 2/3 by the standards of the pointing device.
During the purging process, the water is kept at a level not lower than the normal operating level (medium range).
The procedure is performed in turn on each collector or boiler drum assembly.
First, the second valve on the purge line opens completely, and then the first one. Then, the blowdown begins with a duration of no more than 30 seconds.
The valves close in reverse order.
Simultaneous cleaning from the two lower points is not allowed.
When a water hammer occurs, the blowdown stops. The risk of such phenomena can be eliminated by using buffer tanks.

Conclusion

The regulation of saline water in the boiler is an important operation, but it is energy-intensive and demanding in terms of the technical and structural performance of the piping. That is, it is not even theoretically possible in every unit. In modern boilers, for example, means of biochemical decomposition of alkalis are used with the removal of processed products through standard waste disposal channels. By itself, purging the boiler is not only costly in terms of resources, but can also be harmful to pipeline circuits. This is especially true for continuous cleaning, which continuously creates conditions for contact between equipment piping circuits and alkaline products. The optimal solution to the problem of salting of boiler units is to prevent the dissolution of sediments and sludge elements. This is done in various ways - in particular, by flushing the circuits with softened water during staged evaporation.

general characteristics

Keeping the boiler blowdown to a minimum can significantly reduce energy losses, since the temperature of the blowdown water is directly related to the temperature of the steam produced in the boiler.

When water evaporates, dissolved solids remain in the boiler, which leads to an increase in the total dissolved solids inside the boiler. These substances can fall out of solution with the formation of deposits that impede heat transfer. In addition, the increased content of solutes promotes foaming and entrainment of boiler water with steam.

In order to maintain the concentration of suspended and dissolved solids within the established limits, two procedures are used, each of which can be carried out both automatically and manually:

bottom blowing is performed to remove impurities from the lower parts of the boiler in order to maintain acceptable heat transfer characteristics. Typically, this procedure is performed manually in a batch mode (a few seconds every few hours);
top blowing is designed to remove dissolved impurities accumulating at the surface of the water, and, as a rule, is a continuous process performed in an automatic mode.

The discharge of the boiler purge water results in an energy loss of 1-3% of the energy of the steam produced. In addition, additional costs can be associated with the cooling of the discharged water to the temperature set by the regulator.

There are several ways to reduce the volume of purge water:

condensate return. The condensate does not contain suspended solid or soluble impurities that could accumulate inside the boiler. The return of half of the condensate reduces the blowdown rate by 50%;
Depending on the quality of the feed water, softening, decarbonization and demineralization of the water may be necessary. In addition, water deaeration and conditioning with special additives may be necessary. The required blowdown is determined by the total impurity content of the feed water entering the boiler. If the boiler is supplied with raw water, the blowdown ratio can reach 7-8%; water treatment allows you to reduce this value to 3% or less;
the option of installing an automated purge control system can also be considered. Typically, such systems are based on conductivity measurement; their use allows for an optimal balance between reliability and energy saving considerations. The blowdown value is determined on the basis of the content of the impurity with the highest concentration and the corresponding limit value for the given boiler (for example, silicon - 130 mg / l; chloride ion<600 мг/л). Дополнительная информация по данному вопросу приведена в документе EN 12953 -10;
Evaporation of purge water at medium or low pressure is another way to recover some of the energy contained in this water. This method is applicable in those enterprises where there is a steam network with a lower pressure than that at which steam is produced. From an exergy point of view, this solution may be more efficient than simply recovering the heat of the purge water using a heat exchanger.

Thermal deaeration of feed water also leads to energy losses of 1-3%. The deaeration process removes CO 2 and oxygen from the pressurized feed water at about 103 ° C. The corresponding losses can be minimized by optimizing the deaerator vapor flow rate.

Environmental benefits

The energy content of the blowdown water depends on the boiler pressure. The corresponding dependence is presented in table. The blowdown rate is expressed as a percentage of the total feedwater consumption. Thus, a blowdown value of 5% means that 5% of the feed water entering the boiler is used for blowdown and the rest is converted to steam. Obviously, reducing the amount of blowdown can provide energy savings.

In addition, reducing the amount of blowdown will result in a reduction in the volume of waste water, as well as the energy or cold required for any cooling of this water.

Impact on various components of the environment

Discharges of chemicals used for water treatment, regeneration of ion exchange resins, etc.

Manufacturing information

The optimal blowdown rate is determined by various factors, including the quality of the feed water and the associated water treatment processes, the proportion of condensate returned, the type of boiler and the operating conditions (water flow, operating pressure, type of fuel, etc.). Typically, the blowdown ratio is 4-8% of the fresh water supplied to the boiler, but can be as high as 10% in the case of high solute content in the make-up water. For optimized boilers, the blowdown should not exceed 4%. In this case, the amount of blowdown should be determined by the content of additives (antifoam agent, oxygen scavenger) in the treated water, and not by the concentration of dissolved salts.

Applicability

Decreasing the blowdown value below a critical level can lead to problems with foaming and scale formation. Other measures described above (condensate return, water treatment) can be used to reduce this critical level.

Insufficient blowdown volumes can lead to wear and damage to equipment, and excessive blowdowns can waste energy.

Economic aspects

Significant savings in energy, reagents, make-up water and cold are possible, making this approach applicable in virtually all situations.

Motives for implementation

economic considerations
reliability of the production process.

Based on the "Reference Document on the Best Available Technologies for Energy Efficiency"

To add a description of the energy-saving technology to the Catalog, fill out the questionnaire and send it to marked "to the Catalog".

Natural circulation steam boilers shall be equipped with devices for continuous and intermittent blowdown.

Blowing- This is the removal from the boiler, constantly or periodically, of a certain amount of water with salts, sediments and sludge contained in it.

Continuous blowdown serves to reduce the salt content in the boiler water and ensure the purity of the steam. It is carried out from any part of the boiler. It can be upper, lower drums or remote cyclones.

Continuous blowdown is safer than periodic, since it does not drastically reduce the water level in the boiler, and more economical, since the heat of continuous blowdown can be used in the deaerator.

Continuous blowing is carried out through a perforated pipe located in the boiler drum. Outside on the pipe, two valves are installed (the second for safety), they regulate continuous blowdown. In the event that the salinity of the boiler water increases, the operator opens the valve, increasing the amount of water flowing out of the boiler.

Scale, sludge, ash, soot lead to burns, pipe ruptures, excessive fuel consumption and a decrease in boiler steam output. They are poor heat conductors, which leads to overheating of the boiler metal. Scale, is formed due to the accumulation of salts during the evaporation of water. Salts, having reached the solubility (saturation) limit, precipitate, forming hardly soluble scale in places of high thermal stress. Sludge is a sludge-like sediment that precipitates at the lower points of the boiler and consists of mechanical impurities, metal oxides and products of intra-boiler water treatment. The sludge is easily carried away during intermittent blowdown.

Periodic blowdown is performed from the lower points of the boiler, lower drum, lower collectors, cyclones. Periodic blowdown is understood to mean the removal in a short time of a large amount of water, with which sludge, precipitation, and salt are carried away. The number of periodic blowdowns is determined by the commissioning organization based on the analysis of the boiler water. For periodic blowing, a pipe with holes is laid in the drum through which sludge and sediments are carried away.

Each boiler for intermittent blowdown has a blowdown line, which is connected to a common blowdown line laid downstream of the boilers. The purge water enters the pressureless purge tank or well. Purging is performed sequentially at each point. Due to the small volume of water, it is especially necessary to carry out blowing out of the salt compartments - cyclones with particular care.

Continuous blowdown water is supplied to expander 1 (Figure 9.3), in which its pressure drops to atmospheric. As a result, part of the water evaporates and the resulting steam 5 enters the deaerator, where its heat is used. The remaining water flows into the drain well through the heat exchanger 12, where part of the heat of the blowdown water is also used.

To comply with the established steam quality standards, periodic or continuous blowing is carried out, i.e. part of the water from the steam boilers is released and replaced with feed water. Periodic blowdown, if continuous, serves to discharge the sludge. Continuous blowing in drum boilers is performed from the upper drums 9 (Figure 9.3), where more salts are concentrated, and periodic blowing from the lower drums or collectors. Continuous blowdown must ensure that excess salts are continuously removed from the boiler water during boiler operation. Boiler water of continuous blowing from drum 9 is discharged into an apparatus called continuous blowdown separator, in which the expansion of water and the separation of steam occurs. From the separator, steam is discharged to the feed water deaerator, and hot water containing salt is discharged into drain 11, or is used to heat raw water.

Continuing the section “Conflict of Interest. How not to harm the system by improving the operation of individual units ", today we will talk about how the measures aimed at optimizing the operation of boiler equipment, namely, the automation of continuous blowdown of the steam boiler and the use of continuous blowdown heat, affect the overall efficiency of the steam system.

Let's try to figure out why continuous blowdown of the steam boiler is necessary.

When water evaporates in a steam boiler, any impurities in the feed water are not carried away with the steam, but remain in the boiler water. In this regard, the concentration of dissolved solids in the boiler water increases more and more over time. The salt content in the boiler increases, which in turn leads to foaming on the boiler surface. Foam from the surface is carried away from the boiler into the steam line. Foaming is also the reason for the boiler shutdown for the "Level in the drum" protection.

To eliminate these problems, boiler manufacturers determine the maximum value of the salt content in the boiler. By the value of the maximum salt content in the boiler and the existing salt content in the feed water, you can find the minimum value of continuous boiler blowdown:

Dнп = Dк * Spv / (Cmax - Spv)

Dnp - continuous blowdown consumption;
DTo - consumption of feed water to the boiler (t / h);
WITHpv - salt content of feed water (μg / kg);
WITHswing - maximum salt content in the boiler (μg / kg)

Heat loss with continuous blowdown will be:

Qpot = Dnps * inp - Dnpb * isb

Qsweat - heat lost with continuous blowdown (kcal / h);
Dnpc - existing flow rate of continuous blowdown (t / h);
Dnpb - continuous blowdown flow rate after installation of the continuous blowdown heat recovery unit (t / h);
inp - enthalpy of continuous blowdown at boiler pressure (kcal / kg);
iSat - enthalpy of continuous blowdown after installation continuous blowdown heat recovery unit (kcal / kg).

In the absence of automation for continuous blowdown of the boiler, the existing flow rate of continuous blowdown significantly exceeds the minimum required flow rate of continuous blowdown. This is due to the fact that analyzes of salinity in boilers are carried out once a day and to prevent salinity on boilers above the limit, it is necessary to maintain the salinity in the boiler at the minimum permissible level.

Excessive discharge of continuous blowdown of the boiler leads to losses of thermal energy, amounting to 1–3% of the thermal energy of the produced steam.

In the presence of automatic control of continuous blowdown, it is possible to maintain the salt content in the boiler by 2-3% below the maximum allowable salt content, which leads to a decrease in the consumption of continuous blowdown.

When automating continuous blowdown, my colleagues and I propose using the heat of continuous blowdown to generate flash steam and heat some existing stream:
- make-up water to the deaerator, (Fig. 1)
- feed water in front of the steam boiler. (Fig. 2)

Let us analyze the impact of the listed energy efficiency measures in relation to their impact on other parameters of the installation:

Continuous blowdown of steam boilers

Continuous blowdown of steam boilers Continuing the section “Conflict of interest. How not to harm the system by improving the performance of individual installations ", today we will talk about how they affect the overall

What is boiler blowdown and what is it for

There are two types of boiler blowdown: intermittent and continuous.

Continuous blowdown is carried out from the upper drum of the boiler. For a more even intake of boiler water, a pipe with holes is laid along the drum through which water enters the pipe.

Drum steam separators

Boiler water must be of such quality as to exclude:

Limescale and sludge on heating surfaces.
Deposits of various substances in the boiler superheater and steam turbine.
Corrosion of steam and water pipelines.

Brief characteristics and description of the operation of boilers

The feed water in the drum is mixed with boiler water and is fed through unheated downpipes to the bottom collectors, from where it is distributed through the heated wall pipes. In the wall tubes, the vaporization process begins, and the steam-water mixture from the screen system through the steam supply tubes again enters the drum, where steam and water are separated. The latter mixes with the feed water and re-enters the downpipes, and the steam is supplied to the turbines through the superheater. Thus, the water in the boiler moves in a closed circle consisting of heated and unheated pipes. As a result of the repeated circulation of water with the formation of steam, the boiler water is evaporated, i.e. concentration of impurities in it. An uncontrolled increase in impurities can lead to a deterioration in steam quality (due to droplet entrainment of boiler water and its foaming) and to the formation of deposits on heating surfaces. To prevent these processes, a number of measures are envisaged:

Staged evaporation and in-boiler separation devices to improve the quality of the generated steam.
Corrective treatment of boiler water (phosphating and amination) to reduce the amount of deposits and maintain the pH of the vapor in accordance with the PTE standards.
The use of continuous and intermittent blowing in order to remove excess salts and sludge.
Preservation of boilers during summer shutdown periods.

Staged evaporation

The essence of this method consists in dividing the heating surface, collectors and drums into several compartments, each of which has an independent circulation system.

The feed water is fed into the upper drum of the boiler, which is part of the clean compartment. The clean section usually produces up to 75-80% of the total steam volume. It maintains a certain and low salinity of boiler water due to increased blowing into the salt compartments. The steam from the clean compartment is of satisfactory quality. The boiler water of the salt compartments has an increased salt content. The steam from the brine compartments will be of low quality and will require good cleaning, but there will be little: 20-25%, so the overall steam quality will be satisfactory. Staged evaporation is carried out using external cyclones, which are salt compartments. The boiler drum serves as a clean compartment. The blowdown water from the boiler drum enters the cyclone installed next to the drum, for which this water is feed. The cyclone has a separate circulation circuit and delivers steam to the boiler drum. The blowing is carried out only from the cyclone.

To reduce drift, i.e. humidity of steam, in drums and cyclones of low and medium pressure boilers, various separating devices are provided in the form of steam baffles, slotted partitions, louvers, dry steam tanks installed in front of the steam exhaust pipe. Their action is based on mechanical separation of steam due to inertial forces, centrifugal forces, wetting and surface tension. All this makes it possible to separate the water droplets entrained by the steam from the steam space.

Boiler water corrective treatment

In steam boilers, with a high evaporation rate and relatively small water volumes in the boiler water, the concentration of salts increases so much that even with an insignificant hardness of the feed water, there is a risk of scale formation on the heating surface. Therefore, in boilers, usually "softening" is carried out by means of phosphating, i.e. correctional treatment of boiler water with phosphates: trisodium phosphate, sodium tripolyphosphate, diammonium phosphate, ammonium phosphate, triammonium phosphate.

When trisodium phosphate or sodium tripolyphosphate is dissolved in a correction solution, ions Na +, PO43 are formed. The latter form an insoluble complex with the calcium cation of the boiler water, which precipitates in the form of hydroxylapatite sludge, which does not adhere to the heating surface and is easily removed from the boiler with blowdown water. At the same time, by phosphating, a certain alkalinity and pH of the boiler water can be maintained, which protects the metal from corrosion. The excess of phosphates in the boiler water must be kept constantly in an amount sufficient for the formation of sludge hardness salts. However, an excess of the phosphate content in comparison with the PTE norms is also not allowed, since in the presence of a large amount of iron and copper in the boiler water, ferrophosphate deposits and magnesium phosphate scale can form.

Amination is carried out to bind carbon dioxide released into steam due to thermal decomposition and hydrolysis of bicarbonate and carbonate alkalinity. In this case, it is possible to achieve the pH values of the steam, standardized by the PTE, i.e. 7.5 and more. The unit for dosing ammonia into the make-up water is located at the water treatment plant and is maintained by the personnel of the chemical department. Ammonia dosage value, expressed as a percentage of the amount of additional water supplied to the boiler shop, is set on the automatic metering pump by the HVO personnel, depending on the pH of the superheated vapors, as instructed by the chemical control laboratory assistant.

Simultaneous amination and phosphating

For simultaneous amination and phosphating (when the amination unit is switched off at the cold water treatment plant), the boiler water is treated with a mixture of ammonium salts of phosphoric acid in different ratios depending on the pH of the superheated steam. When the above salts are dissolved in water in the correction solution, NH3 + and PO43 ions are formed.

Phosphate or phosphate-ammonia solution is introduced into the boiler drum of the first evaporation stage. Phosphate-ammonia solution is prepared in the phosphate preparation room on the 2nd floor of the boiler-turbine shop in a special displacement tank by dissolving salts on a grate to retain coarse impurities with hot feed water and pumped into three phosphate tanks in the turbine compartment and one phosphate tank in the boiler room compartment, from where it is supplied to the boilers by dosing pumps. For reliable and continuous correction of the boiler water, 2 pumps are connected to the boilers, which work either together or in a single mode. Three main and one standby phosphate pump for boilers.

The phosphate solution is prepared by the staff of the chemical department and monitored by the concentration of PO43 and, if necessary, by Np + laboratory technicians of the shift laboratory with the recording of the results in the work log. The phosphate solution is injected and the dosing pumps are monitored by the personnel of the boiler department. The control over the concentration of phosphates in the boiler water is carried out by the personnel of the chemical department (laboratory assistants of the chemical analysis of the shift laboratory). To check the correctness of the water-chemical regime in the boiler water, it is necessary to control not only the concentration of phosphates, but also the pH, since the condition for observing this regime is the correspondence between the concentration of phosphates and pH.

To quickly eliminate a sudden drop in the pH of boiler water below the PTE standards (9.3 pH units for a clean compartment), there is an alkali solution tank. The alkali solution is prepared by the chemical department personnel in a propellant tank and pumped over using a pump. At the direction of the chemical control laboratory assistant, the CTC staff assembles a circuit for introducing alkali into the feed water.

Schob = 100% * 40 (2Schff-Schob) / Sc.c.,

where Schob is the total alkalinity of the boiler water; Shff - phenolphthalein alkalinity; 40 - equivalent weight of NaOH; Sk.c. - salinity of boiler water.

One of the main requirements for the water regime of boilers is to obtain steam of an acceptable quality, which ensures minimal contamination of the inner surfaces of the superheater and the flow path of the turbines, where salt deposits are deposited in the form of silicon compounds and sodium salts. Therefore, the quality of steam is usually characterized by sodium content.

The average quality of saturated steam for boilers with natural circulation, as well as the quality of superheated steam after all devices for regulating its temperature, must meet the following standards for all sampling points:

sodium content - no more than 60 μg / dm3;
pH value for boilers of all pressures is not less than 7.5.

Blowing out boilers

Residual impurities contained in the feed water, getting into the drum boiler, concentrate as the water evaporates, as a result of which the salt content of the boiler water increases continuously. In this regard, it becomes necessary to remove these salts from the water cycle at power plants. For drum boilers, such an outlet is carried out by continuously removing some of the boiler water from the salt compartment, i.e. by continuous blowdown.

The blowdown is associated with significant heat losses; according to the water chemistry cards of the boilers, it should be 2–4%. The blowdown percentage is calculated from the analyzes of the boiler and feed water:

Continuous boiler blowdown carried out by the personnel of the boiler department at the direction of the duty chemical control based on the results of the analysis of the boiler water. The duty laboratory assistant of the shift laboratory calculates the salt content of the salt compartments required at the moment to maintain the blowdown value of 2-4%, depending on the salt content of the steam and feed water, and reports the obtained value to the boiler operators and the shift supervisor of the CTC.

Boiler water quality standards, the modes of continuous and periodic blowdown should be established on the basis of the instructions of the boiler manufacturer's plant, standard instructions for maintaining the water-chemical regime or the results of thermal-chemical tests carried out by the power plant, AO Energo services or specialized organizations.

Continuous blowdown is carried out to the separator of continuous blowdowns through the regulators (RNP). If necessary, continuous blowdown can be carried out to the intermittent blowdown separator in addition to the RNP. In separators, part of the purge volume in the form of steam is returned to the cycle through the heating steam line to the deaerators. The other, in the form of water of high salinity, goes to the feed tank of the heating system or is drained.

Intermittent or slurry blowdown produced from the lower header of the boiler. Purpose of the blowdown is to remove coarse-weighted sludge, iron oxides, mechanical impurities from the boiler in order to prevent drift into the wall pipes and their subsequent adhesion to the pipes, accumulation of sludge in the collectors and risers.

Periodic blowdown of operating boilers is carried out by the personnel of the boiler department as directed by the chemical control officer 1-2 times a day depending on the color of the boiler water (yellow or dark). To avoid disturbance of circulation, it is not allowed to open the lower point of the boiler for a long time (more than 1 minute).

Boiler conservation

The main element that gives deposits on the heating surface, in particular, with an excess of phosphate ions (ferrophosphate deposits), is iron, which comes with the feed water, which is formed in the boiler as a result of parking corrosion in the presence of carbon dioxide.

To combat parking corrosion resulting from the absorption of oxygen and the presence of a film of moisture, various methods of equipment preservation are provided. The simplest method of preservation for a short period (no more than 30 days) is to fill the boilers with feed water while maintaining excess pressure to prevent air (oxygen) from sucking in.

Each case of conservation of boilers must be reflected in the operational log of the boiler department. Chemical control involves checking the overpressure and determining oxygen in the feed water (no more than 30 μg / l), with a record in the chemical control register and the boiler conservation log.

When preserving for a long period, preservation is more reliable with the use of corrosion inhibitors, which contribute to the formation of protective films on the metal surface, preventing further corrosion processes.

Heating up boilers

Before lighting the boiler, it is slowly filled with water. If the boiler was filled with a preservative solution (alkali), then the latter drops to 1/3 of the level, and feed water is added to the boiler. The chemical control technician on duty takes water samples to control the content of total hardness, transparency and iron concentration. If the hardness is more than 100 and the transparency is less than 30, the boiler is blown intensively.

When taking a load, it is necessary to monitor the salt content and sodium content in the vapors. With an increase in these indicators, the increase in the load must be delayed, and the continuous blowdown must be increased.

Brief characteristics and description of the operation of boilers

Brief characteristics and description of the operation of boilers Brief characteristics and description of the operation of boilers Feed water in the drum is mixed with boiler water and through unheated downpipes

Boiler water mode

In drum boilers with natural and multiple forced circulation, in order to exclude the possibility of scale formation, it is necessary that the concentration of salts in the water is below the critical value at which they begin to fall out of solution. In order to maintain the required concentration of salts, some part of the water is removed from the boiler by blowing, and together with it, salts are removed in such an amount in which they come with the feed water. As a result of blowing, the amount of salts contained in the water is stabilized at an acceptable level, excluding their precipitation from the solution. Apply continuous and intermittent boiler blowdown. Continuous blowing ensures uniform removal of accumulated dissolved salts from the boiler and is carried out from the place of their highest concentration in the upper drum. Periodic blowdown is used to remove sludge deposited in the boiler elements, and is produced from the lower drums and boiler headers every 12-16 hours.

The scheme of continuous blowdown of the boilers is shown in Fig. 12.5. Continuous blowdown water is supplied to the expander, where the pressure is kept lower than in the boiler. As a result, part of the purge water evaporates and the resulting steam enters the deaerator. The water remaining in the expander is removed through the heat exchanger and, after cooling, is drained into the drainage system.

Continuous blowdown p,%, is set according to the permissible concentration of soluble impurities in the boiler water, most often according to the total salt content, and is expressed as a percentage of the boiler steam output:

where D np and D are the flow rates of the purge water and the nominal steam capacity of the boiler, kg / h. The flow rate of feed water D n.v. In the presence of continuous blowdown is

The amount of water removed by continuous blowdown is set from the equation of the salt balance of the boiler

where D n.v - feed water consumption, kg / h; S n.v, S n and S np - salinity of feed water, steam and blowdown water, kg / kg; 50 T is the amount of substances deposited on heating surfaces, referred to 1 kg of the resulting steam, mg / kg.

In low and medium pressure boilers, the amount of salts carried away by steam is insignificant, and the D Sn term in equation (12.3) can be equated to zero. Normal boiler water mode does not allow salt deposition on heating surfaces, and the D S0 term in this equation should also be equal to zero. Then the amount of water removed with the blowdown is

Substituting the value of D pw from expression (12.2) taking into account formula (12.1), we determine the blowdown,%,

In high-pressure boilers, the entrainment of impurities by steam due to the solubility in the vapor of metal hydroxides and SiO 2, as well as their deposition cannot be neglected, and the blowdown value should be determined taking into account the term D S and equation (12.3) by the formula

The use of continuous blowdown, which is the main means of maintaining the required drum boiler water quality, is associated with an increase in feed water consumption and heat losses. For each kilogram of blowdown water, heat is consumed, kJ / kg,

where h np and h p.v are the enthalpies of the blowdown and feed water, kJ / kg; % - boiler efficiency.

According to the rules of technical operation, continuous blowdown when feeding the boiler with a mixture of condensate and demineralized water or distillate should be no more than 0.5; when chemically purified water is added to the condensate - no more than 3; if the loss of steam taken for production exceeds 40% - no more than 5%.

At the specified rates of blowdown and partial use of the heat of the blowdown water, the heat loss with blowdown is 0.1-0.5% of the fuel heat. In order to reduce heat losses with blowdown, one should strive to reduce the amount of water removed from the boiler. Stepwise evaporation of water is an effective method of reducing blowdown. The essence of staged evaporation or staged blowdown is that the boiler evaporation system is divided into a number of sections connected by steam and separated by water. Feed water is supplied only to the first compartment. For the second compartment, the purge water from the first compartment serves as feed water. Purging water from the second compartment enters the third compartment, and so on.

The boiler is purged from the last compartment - the second with two-stage evaporation, the third - with three-stage evaporation, etc. Since the concentration of salts in the water of the second or third compartment is much higher than in water with single-stage evaporation, a lower percentage is required to remove salts from the boiler. purge. The use of staged evaporation is also effective as a means of reducing the entrainment of silicic acid due to the high hydration alkalinity that occurs in the salt compartments. Staged evaporation and purge systems are usually made up of two or three compartments. Currently, the majority of medium and high pressure drum boilers use staged evaporation. An increase in the salinity of water at several stages of evaporation occurs in steps and within each compartment is set constant, equal to the outlet from this compartment. With two-stage evaporation, the system is divided into two unequal parts - a clean compartment, where all feed water is supplied and 75-85% of steam is produced, and a salt compartment, where 25-15% of steam is produced.

In fig. 12.6, a shows a diagram of an evaporative system with two-stage evaporation with salt compartments located inside the boiler drum, at its ends, and in Fig. 12.6, b - with remote cyclones, which, together with the screens included in them, form the salt compartments of the boiler. With two-stage evaporation, the relative total steam production of the salt compartments,%, required to ensure the specified salt content of water in the clean compartment, in the absence of water transfer to it from the salt compartments, is determined from the expression

where n and - the steam capacity of the salt compartments,%; S n.в and S вl - salinity of feed water and water in the clean compartment, kg / kg; p - blowdown from the salt compartment,%. The optimal steam production of the salt compartments with two-stage evaporation and blowing, determined by the permissible total salt content in the steam, with a blowdown of 1% is equal to 10-20%, and with a blowdown of 5% it is 10-30%.

With two-stage evaporation, the total salt content of steam, mg / kg, is determined by the formula

where S nt = C, Sn, mg / kg; Sn „= C / Cts-b mg / kg; here

K l and K ll - coefficients of salt removal from the first and second degrees of evaporation; at low and medium pressures K l = fti l = 0.01 / 0.03%; C l - multiplicity of concentrations in the clean compartment and in the feed water. The concentration of salts in the water of the clean compartment, mg / kg,

Salt concentration in blowdown water, mg / kg,

The multiplicity of concentrations between the salt and clean compartments in the absence of water transfer from the salt compartment with two-stage evaporation.

For a system with three-stage evaporation, the total salt content of the steam, the salt concentration in the compartments and the purge water, as well as the concentration ratio are determined by equations similar to those given above.

In the case of application - steam washing of the second and third stages of evaporation with water of a clean compartment, the total salt content of saturated steam is determined by the formula

The permissible limit values of salinity, silica content and alkalinity of water in drum boilers depend on their design, steam pressure, etc. It is not always possible to avoid the appearance of scale on the heating surfaces of a drum boiler only by improving the quality of feed water and blowing the boiler out. Additionally, a corrective method for treating water in a boiler is used, in which Ca and Mg salts are converted into compounds that are insoluble in water. For this, reagents are introduced into the water - correction substances, the anions of which bind and precipitate calcium and magnesium cations in the form of sludge.

In boilers at a pressure of more than 1.6 MPa, trisodium phosphate Na 3 PO 4 l 2 H 2 O is used as a corrective reagent. When this reagent is introduced, a reaction occurs with calcium and magnesium compounds:

The resulting substances: Ca 3 (PO 4) 2, Ca (OH) 2 and Na 2 SO 4 - have low solubility and fall out in the form of sludge removed by periodic blowing. When the boilers are fed with condensate with the addition of chemically purified water, a phosphate-alkaline water regime of the boiler is created, in which free alkalinity is retained. When distillate and chemically demineralized water are added to the condensate, the purely phosphate water regime of the boiler is maintained in the absence of free alkalinity. The following excess of PO in water is recommended: for boilers without staged evaporation 5-15; for boilers with staged evaporation in a clean compartment 2 - 6 and in a salt compartment - no more than 50 mg / kg.

To correct the water quality of drum boilers with a pressure above 6.0 MPa, in recent years, in some cases, either ammonia with hydrazine or a complexone are dosed into the feed water.

The hydrazine-ammonia water regime of the boiler, the oxygen remaining after thermal deaeration is bound by hydrazine. Remains of carbon dioxide are bound by the ammonia dosed into the feed water, which completely neutralizes CO 2 and increases the pH of the medium to 9.1 ± 0.1, which helps to reduce the corrosion rate. The complex water regime of the boiler, in addition to ammonia and hydrazine, introduces a complexone into the feed water, usually ethylenedaminetetraacetic acid (EDTA). This leads to an increase in the thermal conductivity of deposits and their displacement to less heat-stressed surfaces (economizer). At 80-90 ° C, aqueous solutions of EDTA and ammonia form a trisubstituted ammonium salt of EDTA, which, interacting with iron corrosion products (at 110 ° C with iron hemioxide), forms well-soluble iron complexonates, which, under the action of a higher temperature along the course of the medium decompose with the formation of a dense magnetite layer precipitating on the inner side of the pipes, which protects the metal from corrosion.

In once-through boilers without blowdown, all mineral impurities entering it with feed water crystallize on the surface, forming scale deposits, or are carried out by steam from the boiler. Accordingly, the salt balance of the direct-flow boiler has the form

On the walls of the heating surface, hardness salts and metal corrosion products are partially deposited in the area where their minimum solubility at a given pressure is less than the concentration of these compounds at the boiler inlet. In this case, the permissible concentration of this compound in the feed water is determined by the permissible intensity of deposits in the boiler per unit mass of incoming water:

where C add is the permissible concentration of this impurity in water; C min - the minimum solubility at a given pressure; From min additional - allowable deposits in the boiler. Above, the dependences of the solubilities of various mineral impurities on the water temperature were shown. Comparison of the concentrations of individual compounds in the feed water with the characteristics of their solubility makes it possible to determine whether deposits will form, and, if any, the place where deposits start and the rate of their growth.
The growth rate of deposits, kg / (m 2 * year), is determined based on the equation of changes in enthalpy and solubility of impurities along the length of the pipe according to the formula

that is, the intensity of the growth of deposits is proportional to the derivative of solubility in terms of enthalpy and the average density of the heat flux on the inner surface of the pipe. In high-pressure boilers, salt deposition begins when the moisture content of the steam decreases to 50 - 20%, and ends when the steam is overheated by 20 - 30 ° C. The greatest deposition of impurities occurs in the area where the steam humidity is less than 5 - 6%.

In once-through boilers at high and supercritical pressures, the solubility of a number of compounds, including silicic acid and sodium chloride, is quite high, and their concentration does not reach the saturation state in the boiler. These impurities are carried out along with the steam and are almost not deposited on the heating surface. Therefore, the permissible concentration of silicic acid and sodium chloride in the feed water is determined only by the conditions of reliable operation of turbines, in the flow path of which, with a decrease in steam pressure, deposits can form.

Salts deposited in the boiler pipes are removed during shutdown periods by water and acid washing. Water flushing is carried out at the next stop of the boiler with water at a temperature of 100 ° C. Acid washing is carried out every 2-3 years with a weak solution of chromic or hydrochloric acid.

Last Updated on 03.06.2012 15:54

Scale formation conditions. Blowdown of steam boilers

When water evaporates, the concentration of salts in it increases continuously. If the salts are not removed from the boiler, then at a certain concentration in the water they fall out of the solution and are deposited on the heating surface in the form of scale. When heated to 80 - 100 ° C, Ca and Mg bicarbonates dissolved in water (Ca (HCO3) g, Mg (HC03) 2) decompose, forming sludge, and precipitate at the lower points of the boiler (lower drums and collectors).

Scale concentrates on the most heat-stressed surfaces of the wall and heating pipes and boiler drums. Scale conducts heat 40 times (from 20 to 100 in different boilers) worse than iron, therefore, when working with scale, fuel consumption increases and the reliability of the boiler heating surfaces decreases. (Soot conducts heat 400 times worse.)

Dependence of excessive consumption of fuel on the thickness of the scale

Scale thickness, mm

Average value of excessive consumption of fuel,%

Due to the low thermal conductivity of the scale, the metal of the boiler and wall pipes is poorly cooled and undergoes strong overheating, as a result of which its strength decreases. This leads to the appearance of blowouts, cracks, rupture of pipes and even to the explosion of drums and boilers.
On modern water-tube boilers, the operation of the boiler, subject to the formation of scale, is unacceptable. The boilers must operate in a scale-free operation.
Blowdown of steam boilers
To maintain the permissible salinity of the boiler water, the boilers are purged.
Blowing is the removal of impurities from the boiler together with the boiler water (salts, sludge, alkalis, suspended solids, etc.) while simultaneously replacing the blown feed water. The blowing can be periodic and continuous.
Periodic blowdown is carried out at regular intervals and is intended to remove sludge from the lower points of the boiler: drum, screen collectors, etc. It is performed for a short time, but with a large discharge of boiler water, which carries away the sludge and carries it out. Blowing is carried out into an expander designed to cool water before discharging it into the sewer.
Continuous blowdown ensures constant removal of dissolved salts of constant hardness to maintain their acceptable concentration. Continuous purging is usually done from the top drum and is controlled by a needle valve. Water is discharged into an expander (separator), where steam is separated from water. Both steam and water are used to heat raw or chemically treated water (their heat is used).
The timing and duration of blowdowns are set by the instruction or by the head of the boiler room (according to the instructions of the laboratory).

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Scale conditions