Boiler purge. The order of its execution. Appointment. Boiler blowdown Periodic boiler blowdown
general characteristics
Minimizing the amount of boiler blowdown can significantly reduce energy losses because the temperature of the blowdown water is directly related to the temperature of the steam produced in the boiler.
When the water evaporates, dissolved solids remain in the boiler, which leads to an increase in the total content of dissolved solids inside the boiler. These substances can precipitate out of solution to form deposits that hinder heat transfer. In addition, the increased content of dissolved substances contributes to the formation of foam 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 blowdown 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 on an intermittent basis (a few seconds every few hours);
- top blowdown is designed to remove dissolved impurities that accumulate near the surface of the water, and, as a rule, is a continuous process performed automatically.
Discharge of boiler blowdown water results in energy losses amounting to 1-3% of the generated steam energy. In addition, additional costs may be associated with cooling the discharged water to the temperature set by the regulatory authorities.
There are several ways to reduce the amount of purge water:
- condensate return. The condensate does not contain solid suspended or soluble impurities that could accumulate inside the boiler. The return of half of the condensate reduces the amount of blowdown by 50%;
- Depending on the quality of the feed water, softening, decarbonization and demineralization of the water may be necessary. In addition, deaeration of the water and its conditioning with the use of special additives may be necessary. The required amount of blowdown is determined by the total content of impurities in the feed water entering the boiler. If the boiler is fed with raw water, the blowdown ratio can reach 7-8%; water treatment can reduce this value to 3% or less;
- an option to install an automated blowdown control system may also be considered. As a rule, such systems are based on the measurement of electrical conductivity; 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 impurities with the highest concentration and the corresponding limit value for the given boiler (e.g. silicon - 130 mg/l; chloride ion<600 мг/л). Дополнительная информация по данному вопросу приведена в документе EN 12953 -10;
- the descent of the blowdown water at medium or low pressure, accompanied by evaporation, is another way of utilizing 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. In terms of exergy, this solution can be more efficient than simply recovering the heat from the blowdown 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 around 103 °C. Corresponding losses can be minimized by optimizing the deaerator steam flow.
Environmental benefits
The energy content of the blowdown water depends on the boiler pressure. The corresponding dependence is presented in table. The blowdown value 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 into steam. Obviously, reducing the amount of blowdown can provide energy savings.
In addition, reducing the amount of blowdown will reduce the volume of wastewater, as well as the cost of energy or cold for any cooling of these waters.
Impact on various components of the environment
Discharges of chemicals used for water treatment, regeneration of ion exchange resins, etc.
Production information
The optimum amount of blowdown is determined by various factors, including feedwater quality and associated water treatment processes, condensate return rate, boiler type and operating conditions (water flow, operating pressure, fuel type, etc.). As a rule, the blowdown ratio is 4-8% of the fresh water fed to the boiler, but can be as high as 10% in case of high dissolved solids content in the make-up water. For optimized boiler rooms, the blowdown value should not exceed 4%. In this case, the blowdown value should be determined by the content of additives (antifoaming agent, oxygen absorber) in the treated water, and not by the concentration of dissolved salts.
Applicability
Decreasing the blowdown value below a critical level can lead to foaming and scaling problems. Other measures described above (condensate return, water treatment) can be used to reduce this critical level.
Insufficient purge volumes can lead to wear and damage to equipment, while excessive purge volumes can waste energy.
Economic aspects
Significant savings in energy, reagents, make-up water and cold are possible, making this approach applicable in almost any situation.
Implementation Motives
- economic considerations
- reliability of the production process.
Adapted from the "Background Document on Energy Efficiency Best Available Techniques"
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Steam boiler blowdown systemsThe feed water of boiler installations is not completely demineralised. Salts are supplied both with make-up water and with salts of complexones used in the chemical treatment of feed water, and can also be formed during steam condensation and come along with condensate return.
When water boils in the boiler, the concentration of salts increases, since the dissolved salts remain in the boiler water and are not carried away with the steam. Foam is formed at the phase interface, which entails a number of negative consequences.
Foam can affect the accuracy of measuring the water level in the boiler and, consequently, the safe operation of the equipment.
Rising with steam into the steam lines, the foam leads to a decrease in the dryness of the steam, sticks to the surfaces of the steam lines and heat exchangers and, consequently, leads to a decrease in the efficiency of heat transfer.
Maintaining high steam quality, characterized by cleanliness and dryness, as measured by the feed water salinity (TDS), is a function of the blowdown. Sometimes this purge is called continuous or top purge (Fig. 1). It can be either manual or automatic, but in both cases, the design of the shutter does not have a fundamental feature.
Continuous blowdown is most efficiently achieved by automating this process - continuously measuring the TDS level and controlling the blowdown intensity with a control valve, as well as recovering the secondary steam and using the heat of the drained boiler water to heat, for example, the same make-up.
APPEC | APPEC |
Picture 1.
Solid salt residues sink under their own weight to the bottom of the boilers, forming an accumulating layer of insoluble salts. To remove this sediment, periodic bottom blowing is used (Fig. 2). The abrupt opening of the bottom purge valve with a large flow area creates a large pressure drop on the valve seat, under the action of which a vacuum is formed on the seat, sucking in most of the salts.
Figure 2.
The need to ensure high speed and the presence of solid sediments in the drained water impose certain requirements on the design of the bottom purge valves. Blowdown valves can be manually operated and pneumatically operated in automatic systems.
Blowdown butterfly valves are an extremely successful solution for this task.
Water quality and parameters (pressure and temperature) also determine the design of blowdown expanders or vapor separators used to recover heat and cool water before it is discharged to the drain.
The bottom blowdown does not replace the blowdown that reduces the salinity of the feed water. Salinity control (TDS) by using only bottom blowdown leads to large losses of boiler water and heat carried away with the water.
Compared to manual control, the accuracy of maintaining the desired TDS level by automatic means is much better, especially when the TDS level of the feed water varies. High accuracy limits the rate of foam formation, while reducing heat loss in the top blowdown and reducing the rate of solid residue deposition or loss of boiler water in the bottom blowdown.
In drum steam boilers, in order to exclude the possibility of scale formation, it is necessary that the concentration of salts in water be below the critical one, at which they begin to fall out of solution. To maintain the required concentration, a certain part of the water is removed from the boiler by blowing, and together with it, the salts coming with the feed water are removed. As a result of blowing, the amount of salts contained in the boiler water stabilizes at an acceptable level, excluding their precipitation from the solution. Both continuous and periodic blowdowns of the boiler are used. Continuous blowing provides uniform removal of dissolved salts from the place of their greatest concentration in the upper drum. Periodic blowdown is used to remove sludge that has settled in the boiler elements, and is carried out from the lower drums and collectors of the boiler every 12-16 hours. The scheme of continuous blowdown is shown in fig. 5.19. The continuous blowdown water from the boiler enters the expander, where the pressure is maintained, which is less than in the boiler. In the expander, part of the blowdown water evaporates, and the resulting steam enters the deaerator. The water remaining in the expander is removed through the heat exchanger and after cooling it is drained into the drainage system.
Rice. 5.19.
evaporation system
/ - a pipe with holes along the length for the removal of purge water; 2 - pipe for
feed water supply
The amount of continuous purge R,%, most often set by the total salt content in the feed water and expressed as a percentage of the steam output of the boiler
p \u003d O pr / d- 100, (5.5)
where /) pr and /) - purge water flow rates and nominal pa-
boiler productivity, kg/h.
Feed water consumption /) n in in the presence of continuous blowing is
0„.v = AT+ O ex. (5.6)
The amount of water removed by continuous blowing is determined from the boiler salt balance equation
AmA.v \u003d + Af ^ pr '(5-7)
where /) pv - feed water consumption, kg / h;
5 PV, 5 I and 5 - salinity of feed water, steam and
purge water, kg/kg.
In low-pressure and medium-pressure boilers, the amount of salts carried away by steam is insignificant, and the term Ab/ can be neglected. Then the amount of water removed with blowing is equal to
Af \u003d AyA.v / Af-
Substituting value About pp from expression (5.6) and taking into account formula (5.5), the amount of purge is determined
P \u003d 5 p. In 100 / (5 inc -5 p. In). (5.8)
In order to reduce heat losses with blowdown, one should strive to reduce the amount of water removed from the boiler. An effective method for reducing the amount of blowdown water is the staged evaporation of water, the essence of which is that the evaporative system of the boiler is divided into a number of compartments connected by steam and separated by water. Feed water is supplied only to the first compartment. For the second compartment, the feed water is the purge water from the first compartment. The purge water from the second compartment enters the third compartment, and so on. The boiler is purged from the last compartment. Since the concentration of salts in the water of the second or third compartment is much higher than in the water during single-stage evaporation, a smaller percentage of blowdown is required to remove salts from the boiler. Staged evaporation and purge systems are usually made of two or three compartments. The increase in the salinity of water at several stages of evaporation occurs in stages 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 75-80% of steam is produced, and a salt compartment, where 25-20% of steam is generated. On fig. 5.20 shows a diagram of an evaporative system with two-stage evaporation with salt compartments located inside the boiler drum at its ends. With two-stage evaporation, the relative total steam capacity of the salt compartments, which is necessary to ensure a given salt content of water in a clean compartment in the absence of water transfer into it from the salt compartments, is determined from the expression
Pts > (100 + /?)? pv / ? in 1 -R, (5.9)
where p and - steam capacity of salt compartments, %;
Pv and 5 v1 - salinity of feed water and water in the clean compartment, kg/kg; R - purge from the salt compartment, %.
The optimal steam capacity of the salt compartments with two-stage evaporation is determined by the permissible salt content in the steam and is 10-20% with a blowdown of 1%, and 10-30% with a blowdown of 5%.
Rice. 5.20.
1 - feed water supply; 2 - steam outlet; 3 - purge clean compartment; 4 - purge of the salt compartment; 5 - evaporative heating surfaces included in the salt compartment; 6 - evaporative heating surfaces included
into a clean compartment
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 the feed water and blowing the boiler. Therefore, a corrective method of water treatment is additionally used, in which Ca and salts are converted into compounds that are insoluble in water. To do this, reagents are introduced into the water, the anions of which bind and precipitate calcium and magnesium cations in the form of sludge. The resulting sludge is removed by periodic blowing.
As corrective reagents, trisodium triphosphate No. 3 P0 4 12H 2 0 is used. When this reagent is introduced, a reaction occurs with calcium and magnesium compounds
6Sh 3 ?0 4 + YuCa80 4 + 2NOH =
3Ca 3 (P0 4) 2 + Ca(OH) 2 + YuNo 2 80 4 . (5.10)
The resulting substances are Ca 3 (P0 4) 2; Ca(OH) 2 ; 1(Sha 2 80 4 - have low solubility and precipitate in the form of sludge, removed by periodic blowing from the lower drum and the boiler collector.
In some cases, complexones are used as corrective agents.
To prevent the accumulation of sludge, silt, sand and oil in the boiler, the boiler is periodically blown out. To remove impurities from the feed water that accumulate in the lower part of the boiler, the bottom blow is used, and to remove oil and dirt floating in the upper layers of the water, the top blow is used.
Bottom blowing, how. already mentioned, it is produced through the lower blowdown valve, and the upper one through the top blowdown valve.
Top blowing produced as follows.
1) Water is pumped into the boiler above the working level by the amount that must be removed from the boiler during blowing, i.e. by 3-5 cm according to the water indicator.
2) Fully open the kingston (side crane).
3) Slowly turn the handle to open the top blowdown valve (when this valve is quickly opened, water rushing through it into the drain pipe can cause strong blows). At the same time, the upper layers of water go into the funnel of the intake pipe of the tap, dragging the foam with it
4) They observe on the water-indicating glass when the water level in the boiler drops to the previous one (but not lower than the working one); and at this moment, with a quick turn of the handle, the top blowing valve is closed.
5) Close the kingston.
The procedure for producing the bottom blowdown is the same as the top blowdown, but with the essential difference that the top blowdown was carried out at full boiler pressure, and for the bottom blowdown, this can only be done if a disk valve is installed as a bottom blowdown valve or when a throttle washer is installed in the lower blowdown pipeline. Otherwise, in order to reduce the risk of large water blowing out of the boiler and the possibility of exposing the ceiling of the fire chamber, the pressure in the boiler must be reduced to 2-3 at.
After the bottom blowing into the boiler, it is necessary to introduce antiscale.
The sequence of blowdowns and the amount of water to be removed from the boiler during blowdowns depend on the type of boiler, the amount of water in it, its quality, the presence of feed water filters and mud collectors, and are set by the ship's mechanic in agreement with the shipping company's mechanical and ship service.
Taking into account all these circumstances, the sequence of purges is set from four to six times a day. The amount of water blown out of the boiler on the water-indicating glass varies within:
for top blowing - from 2 to 4 cm;
for lower blowing - from 2 to 5 cm.
It was noted above that in the absence of a throttle or a disk valve, the steam pressure in the boiler had to be reduced to 2-3 at. This means that in order to comply with the specified sequence of purges, the pressure must be reduced up to six times a day. If, according to the operating conditions of the steamer, this could not be done, then the bottom blowing had to be done once every 2-6 days and a larger amount of water had to be blown out of the boiler.
From what has been said, it is clear how important throttle washers and butterfly valves are.
It should be borne in mind that blowing, especially the lower one, is a very important operation, since, by performing it, due to negligence or inability, water can be lost and thereby cause a serious boiler failure. Therefore, the stoker can carry out the bottom blowing only with the permission of his watch officer and together with him. When opening the cocks for blowing the crpoj-go, it is forbidden to put a pipe on their handles or use a crowbar, since it is easy to break the handle of the cock., and then it will not be possible to close it.
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. Boiler purging 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 adversely affect the condition of the surfaces of the unit, not to mention the quality of the liquid coolant it serves. Equipment with natural circulation of water and steam must be purged in order to remove harmful deposits into special separator tanks. There are different ways to implement cleaning, but the complete failure of this preventive measure can lead to the wear of the unit to the point of unusability. So, in relation to hot water and steam boilers, purge 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 equipment in the form of a pipe located in the boiler drum. To regulate the intensity of the process, valves and shut-off valves are additionally connected.
Boiler Purging Purpose
Each boiler model has its own schedule for the purge operation, taking into account the mode of operation of the equipment and the quality of the serviced water. Usually, a special line is provided for this operation, connected to the purge line. The procedure is performed sequentially at each contour point of the withdrawal of foreign particles. Due to the small volumes of accumulated water, care must be taken when purging the salt chambers of the cyclones.
What effect should the boiler blowdown provide? Again, a lot depends on the current state of the hardware. With complex purge, elements such as sludge, ash, salt, soot and scale are removed from the circuits and functional containers. If they are not removed in time, then over time the risk of burnouts will increase, which will lead to a decrease in boiler performance, an increase in fuel consumption and even pipe rupture.
Types of purge
There are two types of purge - continuous and intermittent. In the first case, respectively, the cleaning process is carried out without stopping, and in the second - in a short-term mode after certain periods of operation. The technique of continuous removal of undesirable substances focuses rather on washing out the salts in the boiler water. In turn, periodic blowing is activated in cases of removing more solid settled substances such as scale and sludge.
Continuous blowdown of the steam boiler is more commonly used as it ensures better maintenance of equipment surfaces. It is another matter that such a method cannot be applied during major comprehensive cleaning. Blowing at longer intervals is rather considered as an additional maintenance operation, the purpose of which is to remove local dry sludge accumulations.
Continuous purge technique
The procedure can be performed from any part or circuit of the boiler equipment with piping. In particular, you can start with the lower or upper drum capacity, as well as with remote 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 with the help of a boiler installed in the drum. Further, valves are connected to the regulation circuits, which adjust the intensity of the water supply. Sometimes a continuous blowdown of the boiler is organized through the lower outlets of the salt chambers with two active small-format valves. It is also recommended to additionally install restrictive washers with 3-8 mm diameter valves on the lower purge line.
Permanent purge shutdown
Salt-containing water is cleaned already outside the boiler using a separator. If at a certain operational interval the planned alkali indicator is normal, then the boiler blowdown can be set to a minimum degree of work or completely turned off. After the contaminated liquid is removed, the valve of the connected pipeline closes, cutting off the line of separated water. The filtered salts and sludge are sent to the drainage circuit.
How to Perform Intermittent Purge
This method involves connecting the output circuits only through the lower points of the collectors or drums in order to remove the sludge to the separators. Technically, the process of periodic purging of boilers is performed in the following sequence:
- The adequacy of the liquid supply in the nutrient deaerator is checked.
- Water-indicating measuring equipment is blown.
- The tightness of the purge fittings, 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.
- The purge process keeps the water at or above the normal operating level (medium range).
- The procedure is performed in turn on each node of the collector or boiler drum.
- First, the second valve in the purge line opens completely, and then the first one. Next, purge begins with a duration of no more than 30 seconds.
- The valves close in reverse order.
- Simultaneous cleaning from two lower points is not allowed.
- When a water hammer occurs, the purge stops. It is possible to eliminate the risk of such phenomena with the help of buffer hydraulic tanks.
Conclusion
The regulation of saline water in the boiler is an important operation, but energy-intensive and demanding on the technical and structural design of the piping. That is, not in every unit it is even theoretically possible. In modern boilers, for example, means are used for the biochemical decomposition of alkalis with the removal of processed products through regular waste disposal channels. By itself, blowing down 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 constantly creates conditions for contact between the piping circuits of the equipment and alkaline products. The optimal solution to the problem of clogging 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.
