Boiler purging purpose. Boiler blowdown
Continuous blowing:
Continuous blowdown serves to maintain a constant salinity and alkalinity of the boiler water.
Previously, water was taken from the upper drum when the steam-water mixture exited the boiling pipes.
Currently, thermotechnical science has proven that the quality of boiler water is the same throughout the entire volume of the boiler and that purging can also be carried out from the lower drum.
The continuous blowing scheme is as follows:
Upper (lower) drum needle valves near the drum purge pipe continuous blowing separator: steam from the separator to the deaerator, and water to bubbler from there it cools into the sewer.
The amount of continuous blowdown of steam boilers should be:
When replenishing losses with distiller or demineralized water - 0.3-0.5%;
When replenishing losses with chemically purified water - 0.5-3%;
If consumers do not return condensate above 30% and the salt content of chemically purified water that is added is above 300 mg/kg an addition of up to 5% is allowed.
While the boiler is operating, the HVO laboratory technician regularly checks the salinity and alkalinity of the boiler water and saturated steam. If they deviate from the norm, on the recommendation of the laboratory assistant, change the amount of continuous blowing.
Periodic boiler purging:
Periodic purging of the boiler is carried out after a certain period of time and serves to remove sludge and dirt from the lower points: drum, collectors.
It is carried out briefly, but with a large release of boiler water, which during its movement captures the sludge located in the drum or collectors and carries it into the so-called expander (bubbler), designed to cool the boiler water. Cooling is carried out by mixing it with cold tap water to a temperature of 60-70°C, at which it can be released into the sewer.
Periodic purging is carried out at least once per shift. If the quality of the feed water is poor, on the recommendation of the water treatment laboratory, repeated purging is done. The duration and order of this operation is indicated in the production instructions for each boiler. Boiler room personnel, as well as everyone involved in repairing neighboring boilers, are warned about the purge. When the purge fittings are located near the front of the boiler, the purge can be performed by one operator, and if it is located on the sides and rear of the boiler, then it can be performed by two operators.
Periodic purging is performed in the following sequence:
1. Check the serviceability of the purge lines by touch. Before the first valve the pipe should be hot, and after the second valve it should be cold. The valves are checked for ease of rotation of the valve flywheels.
2. Check the serviceability of the feed pumps and the presence of a sufficient supply of feed water.
3. Blow out water indicators.
4. Fill the boiler to the upper operating level or 3/4 according to the water indicator.
5. Reduce combustion in the firebox.
6. On the line that, according to the instructions, should be blown first, carefully open first the second blow-off valve in the blow-down direction from the boiler, and then slightly loosen the blow-off valve closest to the boiler in order to warm up the blow-off line. After warming up, carefully open it. At this time, the second operator must monitor the water level in the boiler and the steam pressure in the drum. In the event of water hammer, vibration of pipelines or other problems in the purge lines; purging must be stopped.
7. When the water level decreases to the lower operating level (at the signal of the second operator), gradually close the purge valve closest to the boiler (the first), and then the second.
8. Blow out the remaining lines in the same way, observing the water level.
9. After finishing the boiler purge, you need to make sure that the purge valves are securely closed and turn the boiler back into normal operation.
10. Make an entry in the shift log indicating the start and end time of the purge.
11. After 30 min you need to check how tightly the purge valve is closed. If the fittings leak water, you should inform the boiler room manager about this and continue to monitor the water level in the boiler.
To prevent sludge, silt, sand and oil from accumulating in the boiler, the boiler is periodically purged. Bottom blowing is used to remove feedwater impurities that accumulate in the lower part of the boiler, and top blowing is used to remove oil and dirt floating in the upper layers of water.
Bottom blowing, like. As already mentioned, it is done through the bottom blowing tap, and the top blowing is done through the top blowing tap.
Top blowing is done as follows.
1) Water is pumped into the boiler above the operating level by the amount that should be removed from the boiler during purging, i.e. 3-5 cm according to the water indicator.
2) Open the kingston (onboard valve) completely.
3) Open the top blowing tap by slowly turning the handle (when this tap is opened quickly, water rushing through it into the drain pipe can cause strong shocks). At the same time, the upper layers of water go into the funnel of the faucet’s receiving pipe, dragging the foam with it
4) Observe on the water indicator glass when the water level in the boiler drops to the previous level (but not lower than the working level); and at this moment, by quickly turning the handle, close the top blow-out tap.
5) Close the Kingston.
The procedure for producing bottom blowing is the same as top blowing, but with the significant difference that the top blowing was carried out at full boiler pressure, and for bottom blowing this can only be done if a disk valve is installed as a bottom blowing valve or, when A throttle washer is installed in the bottom blowing pipeline. Otherwise, in order to reduce the risk of large amounts of water being blown out of the boiler and the possibility of exposing the ceiling of the fire chamber, before bottom blowing, the pressure in the boiler must be reduced to 2-3 at.
After bottom blowing, anti-scale agent must be introduced into the boiler.
The sequence of blowing and the amount of water that should be removed from the boiler during blowing depend on the type of boiler, the amount of water in it, its quality, the presence of feed water filters and mud traps, and are established 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 blowing is set from four to six times a day. The amount of water blown out of the boiler along the water indicator glass varies within:
for top blowing - from 2 to 4 cm;
for bottom blowing - from 2 to 5 cm.
It was noted above that in the absence of a throttle washer or disk valve, before bottom blowing, 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, due to the operating conditions of the steamship, this could not be done, then bottom blowing had to be done once every 2-6 days and more water was blown out of the boiler.
From what has been said, it is clear what importance throttle washers and disc valves have.
It should be borne in mind that blowing, especially the bottom one, is a very responsible operation, since by doing it, through oversight or inability, you can let water in and thereby cause a serious boiler failure. Therefore, a fireman can carry out bottom blowing only with the permission of his watch commander and together with him. When opening blow-off taps, it is forbidden to put a pipe on their handles or use a crowbar, since this will easily break the tap handle, and then it will not be possible to close it.
The feedwater in the drum is mixed with boiler water and is supplied through unheated downcomers to the lower collectors, from where it is distributed over heated screen pipes. The steam formation process begins in the screen pipes, and the steam-water mixture from the screen system through the steam supply pipes again enters the drum, where steam and water are separated. The latter is mixed with feed water and again enters the downpipes, and the steam flows through the superheater to the turbines. Thus, the water moves in a vicious circle consisting of heated and unheated pipes. As a result of repeated circulation of water with the formation of steam, boiler water is evaporated, i.e. concentration of impurities contained in it. An uncontrolled increase in impurities can lead to deterioration in steam quality (due to dropwise 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 provided:
- Staged evaporation and intra-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 vapors in accordance with PTE standards.
- The use of continuous and periodic blowdowns to remove excess salts and sludge.
- Preservation of boilers during summer shutdowns.
Staged evaporation
The essence of this method is to divide the heating surface, collectors and drums into several compartments, each of which has an independent circulation system.
Feed water is supplied to the upper drum of the boiler, which is part of the clean compartment. The clean compartment usually produces up to 75-80% of the total steam volume. It maintains a certain and low salt content of boiler water due to increased blowing into the salt compartments. The steam from the clean compartment is of satisfactory quality. Boiler water from salt compartments has a high salt content. The steam from the salt compartments will be of low quality and will require good cleaning, but it will not be much: 20-25%, so the overall quality of the steam will be satisfactory. Staged evaporation is carried out using remote cyclones, which are salt compartments. The clean compartment is the boiler drum. Blowdown water from the boiler drum enters a cyclone installed next to the drum, for which this water serves as feed water. The cyclone has a separate circulation circuit and releases steam into the boiler drum. Blowing is carried out only from the cyclone.
To reduce droplet entrainment, i.e. steam humidity, in the drums and cyclones of low and medium pressure boilers, various separating devices are provided in the form of steam eliminators, slotted partitions, louvres, steam tanks installed in front of the steam outlet pipe. Their action is based on mechanical separation of steam due to inertial forces, centrifugal forces, wetting and surface tension. All this allows you to separate water droplets captured by steam from the steam space.
Corrective treatment of boiler water
In steam boilers, with a high evaporation rate and relatively small water volumes, the concentration of salts in the boiler water increases so much that even with insignificant hardness of the feed water, there is a danger of scale formation on the heating surface. Therefore, in boilers, “additional softening” is usually carried out through phosphating, i.e. corrective treatment of boiler water with phosphates: trisodium phosphate, sodium tripolyphosphate, diammonium phosphate, ammonium phosphate, triammonium phosphate.
PhosphatingWhen trisodium phosphate or sodium tripolyphosphate is dissolved in a correction solution, Na+ and PO43 ions are formed. The latter form an insoluble complex with the calcium cation of the boiler water, which precipitates in the form of hydroxyapatite 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. Excess phosphates in boiler water must be constantly maintained in an amount sufficient to form sludge hardness salts. However, exceeding the phosphate content compared to PTE standards 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 scales can form.
AminationAmination 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 steam pH values normalized by PTE, i.e. 7.5 or more. The unit for dosing ammonia into additional water is located at the chemical treatment plant and is serviced by chemical shop personnel. The dosage value of ammonia, expressed as a percentage of the amount of additional water supplied to the boiler shop, is set on an automatic dosing pump by HVO personnel depending on the pH of the superheated vapors as directed by the chemical control laboratory assistant.
Simultaneous amination and phosphatingFor simultaneous amination and phosphating (when the amination unit at the chemical treatment plant is turned off), corrective treatment of the boiler water is carried out 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, NH3+ and PO43 ions are formed in the correction solution.
A phosphate or phosphate-ammonium solution is introduced into the boiler drum of the first stage of evaporation. The phosphate-ammonia solution is prepared in the phosphate preparation room on the 2nd floor of the boiler-turbine shop in a special propellant tank by dissolving salts on a grid to retain coarse impurities with hot feed water and is pumped into three phosphate tanks in the turbine department and one phosphate tank in the boiler room department, from where it is supplied to the boilers by dosing pumps. For reliable and continuous adjustment of boiler water, 2 pumps are connected to the boilers, operating either together or in single mode. Three main and one reserve phosphate pump for boilers.
The phosphate solution is prepared by the personnel of the chemical workshop and is monitored for the concentration of PO43 and, if necessary, NH4+ by laboratory assistants in the shift laboratory, recording the results in the work log. The phosphate solution is introduced and the operation of the dosing pumps is monitored by boiler shop personnel. The concentration of phosphates in the boiler water is monitored by the personnel of the chemical workshop (chemical analysis laboratory assistants of the shift laboratory). To check the correctness of the water chemistry regime in boiler water, it is necessary to control not only the concentration of phosphates, but also the pH, since the condition for compliance with 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 PTE standards (9.3 pH units for a clean compartment), there is an alkali solution tank. The alkali solution is prepared by chemical shop personnel in a propellant tank and pumped using a pump. At the direction of the chemical control laboratory technician, KTC personnel assemble a circuit for introducing alkali into the feed water.
Schot = 100% * 40 (2Shchff-Shoch) / Sc.v.,
where Shchob is the total alkalinity of boiler water; alkalinity – phenolphthalein alkalinity; 40 – equivalent weight of NaOH; Sk.v. – salt content of boiler water.
One of the main requirements for the water regime of boilers is to ensure minimal contamination of the internal surfaces of the superheater and the flow path of turbines, where salt deposits are deposited in the form of silicon compounds and sodium salts. Therefore, steam quality is usually characterized by sodium content.
The average quality of saturated steam from boilers with natural circulation over all sampling points, as well as the quality of superheated steam after all devices for regulating its temperature, must satisfy the following standards:
- sodium content – no more than 60 µg/dm3;
- pH value for boilers of all pressures is not less than 7.5.
Boiler blowing
Residual impurities contained in the feed water, entering the boiler, become concentrated as the water evaporates, as a result of which the salt content of the boiler water continuously increases. In this regard, there is a need to remove these salts from the water circulation cycle at power plants. For drum boilers, this withdrawal is carried out by continuously removing some of the boiler water from the brine compartment, i.e. by continuous blowing.
Blowdown is associated with significant heat losses; according to boiler water chemistry maps, it should be 2–4%. The percentage of blowdown is calculated based on analyzes of boiler and feed water:
- Р= 100% * (Sp.v. - Sp.) / (Sk.v - Sp.v),
where Sp.v is the salt content of feed water;
Sp. - salinity of steam;
Sk.v. – salinity content of boiler water (salty compartment).
Continuous boiler blowdown carried out by boiler shop personnel at the direction of chemical control on duty based on the results of boiler water analysis. The laboratory assistant on duty at the shift laboratory calculates the currently required salt content of the salt compartments 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 manager of the CTC.
Boiler water quality standards, continuous and periodic blowdown modes must be established based on the instructions of the boiler manufacturer, standard instructions for maintaining a water-chemical regime or the results of thermo-chemical tests carried out by the power plant, JSC Energy services or specialized organizations.
Continuous blowing is carried out to the continuous blowing separator through regulators (RNP). If necessary, continuous blowing can be carried out on a periodic blowing 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 with high salt content, goes to the heating network make-up tank or is drained.
Intermittent or slurry blowdown produced from the lower boiler manifold. The purpose of blowing is to remove coarsely suspended sludge, iron oxides, and mechanical impurities from the boiler in order to prevent drift into the screen pipes and their subsequent sticking to the pipes, and the accumulation of sludge in collectors and risers.
Periodic purging of operating boilers is carried out by boiler shop personnel as directed by the chemical control officer on duty. 1-2 times a day depending on the color of the boiler water (yellow or dark). To avoid disruption of circulation, it is not allowed to open the lower point of the boiler for a long time (more than 1 minute).
Preservation of boilers
The main element that produces deposits on the heating surface, in particular when there is an excess of phosphate ions (ferrophosphate deposits), is iron, which comes with the feed water, and is formed in the boiler as a result of standstill corrosion in the presence of carbon dioxide.
To combat parking corrosion, which occurs as a result of the absorption of oxygen and the presence of a moisture film, various methods of preserving equipment 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) suction.
Each case of boiler conservation must be reflected in the operational log of the boiler department. Chemical control involves checking excess pressure and determination of oxygen in feed water (no more than 30 μg/l), with an entry in the chemical control sheet and the boiler conservation log.
When preserving for a long time, conservation is more reliable using corrosion inhibitors, which promote the formation of protective films on the metal surface that prevent further corrosion processes.
Boiler firing
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 level, and feed water is added to the boiler. The chemical control laboratory assistant on duty takes water samples to monitor the content of total hardness, transparency and iron concentration. When the hardness is more than 100 and the transparency is less than 30, the boiler is intensively purged.
When taking a load, it is necessary to monitor the salt and sodium content in the vapor. If these indicators increase, the load rise must be delayed and continuous blowing increased.
The article provides information on continuous and periodic boiler purging, provides a real purge diagram and design drawings associated with RNP and RPP
Problems due to salts in boiler water
The boiler water must maintain a constant salt composition, i.e. the introduction of salts and contaminants with the feed water must correspond to their removal from the boiler. This is achieved by carrying out continuous and periodic blowdowns.
If there is insufficient removal of salts from the boiler, they accumulate in the boiler water and intensive scale formation occurs on the heat-stressed sections of the screen pipes, which reduces the thermal conductivity of the pipes, leads to holes, ruptures, emergency shutdowns, and, accordingly, to a decrease in the reliability and efficiency of the boiler. Therefore, 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 dissolution of salts in the feed water. The fewer dissolved salts in the boiler water and the drier the resulting steam, the cleaner it is considered. The removal of moisture with steam is considered unacceptable, since it contains salts, and during evaporation they will settle on the internal surfaces of the pipes in the form of sediment.
Inside the boiler drum there are special devices (separators) that separate moisture from steam. Very often, cyclone separators are installed inside the boiler drums, which separate water particles from steam. Louvre separators are also used; such a separator is shown in the diagram of a medium pressure drum.
To prevent scale formation on the heat exchange surfaces of the boiler, phosphates are introduced into the drum, and in the boiler water, sparingly soluble compounds are formed in the form of sludge. The removal of salts from the boiler drum is achieved through blowing.
Usually the drum is divided into a clean compartment and a dirty one. Water is blown from the clean compartment into the dirty one.
This is done in order to lose as little water as possible with purging. Blowing will be carried out from the dirty (salt compartment), where the concentration of salts is much higher than in the clean compartment, therefore, the carryover of water with blowing from the dirty compartment will be lower.
Dirty compartments are smaller than clean ones, so the main part of the steam is generated in the clean compartment and therefore the total salt content in the steam decreases. This is called stage evaporation. Staged evaporation in the boiler drum (or outside it in the case of using remote cyclones) reduces the cost of water treatment and fuel costs, since we lose heat with blowing.
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How is continuous boiler purging carried out?
Boiler water must be of such quality as to exclude:
- Scale and sludge on heating surfaces.
- Deposits of various substances in the boiler superheater and steam turbine.
- Corrosion of steam and water pipelines.
Calculation of the boiler blowdown amount:
Blowdown is determined as a percentage of the boiler's nominal steam output:
Р=Gpr/Gpar * 100%
According to paragraph 4.8.27 of the rules for the technical operation of power plants and networks of the Russian Federation, the value of continuous boiler output is accepted:
- No more than 1% for IES
- No more than 2% for CPPs and heating CHP plants, where losses are replenished with chemically purified water
- No more than 5% at heating CHP plants, with 0% steam return from consumers
That is, if you, for example, have a condensing station with a K-330-240 turbine with a fresh steam flow rate of 1050 t/h, then the blowdown amount will be 10.5 t/h.
Accordingly, the steam flow from the boiler is determined as the difference between the drinking water flow and the blowdown flow.
The size of the continuous blowdown under various operating modes must be remotely maintained using a continuous blowdown flow meter or adjusted by the boiler operator at the request of the chemical shop personnel.
Periodic purge
Periodic purge is produced to remove sludge from the lowest points of all collectors and is sent to the periodic blowdown expander and then through the bubbler to the industrial storm sewer.
Periodic purging, as the name implies, is not permanent and is carried out from time to time. Periodic purge is limited in time and lasts no more than 30 seconds. It is believed that almost all the sludge is removed immediately in the first seconds of purging.
Operating example: Periodic purging of boiler No. 3 is carried out on Wednesday and Saturday by KTC personnel under the control of the operating personnel of the chemical workshop. Each screen panel is purged by fully opening the periodic purge valve for 30 seconds. If the regimes are violated, at the request of the chemical shop personnel, extraordinary periodic blowdowns are carried out. When firing up the boiler, periodic blowdowns are carried out at 20, 60 atm in the boiler drum and when the nominal parameters are reached.
The size of continuous blowing and the time of periodic blowing are recorded in the daily reports of the express laboratory by the laboratory assistant on duty or the shift supervisor of the chemical workshop.
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Schemes and drawings of boiler purge
Boiler blowdown diagram
This is part of a real detailed diagram of a 450 MW combined cycle plant. The diagram shows how continuous and periodic purging is carried out.
The continuous purge from the high pressure drum enters the continuous purge separator/expander. The following are installed on the line along the flow of the medium: manual shut-off valves, a flow meter, an electrified regulator, a set of throttle washers, electrified valves and a set of throttle washers.
At the end of the article, an example of calculating a continuous blowdown expander is given.
The RNP is equipped with a safety valve.
In this scheme, saturated steam from the continuous blowdown separator is sent to the low pressure drum. Manual shut-off valves and a check valve are installed on the steam line. Drainage from the RNP will be sent to the clean waste tank.
The blowdown from the RNP is sent to the periodic blowdown expander; an electric control valve and manual shut-off valves are installed on the line. Next, the drainage from the RPP is discharged into the boiler drain tank.
Drawing of a steam pipeline from a continuous blowing separator to a deaerator
The design assembly drawing shows the layout of the low pressure steam line from the continuous blowdown expander to the atmospheric deaerator. There are two fittings installed on the steam line, one is a shut-off valve (position 2) and the other is a check valve (position 1) so that steam cannot flow back into the expander.
Exhaust drawing from RNP safety valve
Another drawing shows the exhaust pipe from the RNP safety valve. The pipeline from the safety valve is directed to the edge of the main building and, at the point of the columns, is led to the roof, to a height of more than 2 meters, to ensure the safety of the station personnel. A water seal is provided on the exhaust pipeline to remove drainage into the drainage collector. Based on operating experience, it is recommended to make the diameter of the water seal pipe larger than that of a conventional drain to prevent it from clogging, since leaves and other dirt can enter the exhaust pipe from the atmosphere.
Drawing of vapor from a periodic blowdown expander
thermal calculation of RNP
Let's look at expander balances using an example. We will consider the blowdown of the EP-670-13.8-545 GM boiler operating with a T-180/210-130 turbine.
Initial data: feed water consumption: Gpw = 187.91 kg/s
We accept the flow rate of purge water: Gpr = 0.3% * Gpv = 0.03*187.91 = 5.64 kg/s
We assume the pressure in the continuous blowing expander: Prnp = 0.7 MPa
We will have two equations and two unknowns, namely:
- Gpr1 - water flow at the outlet of the RNP
- Gpr2 – steam flow at the outlet of the RNP (this steam is discharged into a high-pressure deaerator 0.6 MPa)
Equations:
- Gpr = Gpr1 + Gpr2
- Gpr*hpr = Gpr1* hpr’ + Gpr2* hpr’’
Known quantities: 1.20 GB (1,300,147,052 bytes)
- Blowdown flow rate coming from the boiler drum: Gpr = 5.64 kg/s
- Enthalpy of blowing water from the drum: hpr is defined as the enthalpy of water at saturation pressure in the drum, hpr = f(Pb)=f(13.8 MPa) = 1563 kJ/kg
- Enthalpy of water at the exit from the RNP: hpr’, is defined as the enthalpy of water at saturation in the RNP: hpr’=f(Prnp) = f(0.7 MPa) =697.1 kJ/kg
- Enthalpy of steam at the exit from the RNP: hpr’’, is defined as the enthalpy of saturated steam in the RNP: hpr’=f(Prnp) = f(0.7 MPa) =2763.0 kJ/kg
All enthalpies were determined in the water steam pro program, we talked about it in the article Material balance equation and selection of a deaerator, and there are also links where you can download it.
Final equations:
- 5.64 = Gpr1 + Gpr2
- Gpr*1563 = Gpr1* 697.1 + Gpr2* 2763.0
Finding unknowns:
- Gpr1 = 3.27 kg/s
- Gpr2 = 2.36 kg/s
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In drum steam boilers, to eliminate the possibility of scale formation, it is necessary that the concentration of salts in the water is below the critical level 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, along with it, the salts supplied with the feed water are removed. As a result of purging, the amount of salts contained in the boiler water is stabilized at an acceptable level, which prevents their loss from solution. Continuous and periodic boiler blowdowns are used. Continuous blowing ensures uniform removal of dissolved salts 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 carried out from the lower drums and boiler collectors every 12-16 hours. The continuous blowdown diagram is shown in Fig. 5.19. Continuous blowdown water from the boiler enters the expander, where a pressure lower than in the boiler is maintained. In the expander, part of the purge water evaporates, and the resulting steam enters the deaerator. The remaining water in the expander is removed through a heat exchanger and, after cooling, is drained into the drainage system.
Rice. 5.19.
evaporation system
/ - a pipe with holes along its length to drain blow-off water; 2 - pipe for
feed water supply
Amount of continuous blowing R,%, is most often determined by the total salt content in the feed water and is expressed as a percentage of the boiler’s steam output
p=O pr/j- 100, (5.5)
where /) pr and /) are the flow rates of purge water and the nominal pa-
Boiler productivity, kg/h.
Feed water consumption /) p in the presence of continuous blowdown is
0„.в = IN+ About pr. (5.6)
The amount of water removed by continuous blowdown is determined from the boiler salt balance equation
AmA.v = + Af^pr’ (5-7)
where /) pw - feed water consumption, kg/h;
5 PV, 5 I and 5 - salt content of feed water, steam and
purge water, kg/kg.
In low- and medium-pressure boilers, the amount of salts carried away by steam is insignificant, and the term Lb/ can be neglected. Then the amount of water removed with blowing is equal to
Af = AyA.v / Af-
Substituting the value About pp from expression (5.6) and taking into account formula (5.5), the amount of blowing is determined
Р=5 p. in 100/(5 p. -5 p. in). (5.8)
In order to reduce heat losses due to blowing, one should strive to reduce the amount of water removed from the boiler. An effective method of reducing the amount of blowdown water is stepwise evaporation of water, the essence of which is that the boiler evaporation system 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. Purge water from the second compartment enters the third compartment, etc. 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 blowdown systems are usually made of two or three compartments. The increase in the salt content of water during several stages of evaporation occurs in steps and within each compartment is set constant, equal to the output from a given compartment. With two-stage evaporation, the system is divided into two unequal parts - a clean compartment, where all the feed water is supplied and 75-80% of the steam is produced, and a salt compartment, where 25-20% of the steam is produced. In Fig. Figure 5.20 shows a diagram of an evaporation system with two-stage evaporation with salt compartments located inside the boiler drum, at its ends. With two-stage evaporation, the relative total steam production of the salt compartments, necessary to ensure a given salt-holding of water in a clean compartment in the absence of water transfer into it from the salt compartments, is determined from the expression
Pc > (100 + /?)? pv / ? in 1 -R, (5.9)
Where p and - steam production of salt compartments, %;
Pv and 5 in1 - salt content of feed water and water in the clean compartment, kg/kg; R - purging from the salt compartment, %.
The optimal steam production of salt compartments during two-stage evaporation is determined by the permissible salt retention in the steam and with a blowdown value of 1% it is 10-20%, and with a blowdown of 5% it is 10-30%.
![](https://i2.wp.com/studref.com/im/39/5582/754434-95.jpg)
Rice. 5.20.
1 - feed water supply; 2 - steam removal; 3 - purging the clean compartment; 4 - purging 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 feed water and purging the boiler. Therefore, a correction method of water treatment is additionally used, in which Ca 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.
Trisodium phosphate No. 3 P0 4 12H 2 0 is used as corrective reagents. When this reagent is introduced, a reaction occurs with calcium and magnesium compounds
6Ш 3 ?0 4 + YuSa80 4 + 2NiO =
ZCa 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 fall out in the form of sludge, which is removed by periodic blowing from the lower drum and boiler manifold.
In some cases, complexons are used as corrective substances.