GOSSTROY USSR Glavproystroyproekt

S0UZSANTEHPRONKT State Design Institute SANTEHPRONKT

APPROVE:

/ DI^EK^R GPI SANTEKHPR01ZhT --N.KOHANESHSO

Moscow - 1974

SODESHENIE

1. Purpose and tasks of boiler blowdown .............................................. 3

2. Calculation of the amount of blowdown of boilers ... b

3. Boiler water quality holes...... 9

4. Schemes of continuous blowdown of boilers .. 13

5. Calculation of continuous blowdown separators..............................I e

6. Discharge of purge water

boilers....................... 21

7. Literature ..........................26

State Design Institute San tekhp roekt Glz vp proio troyproekt Gosstroy USSR

(GPI Sa ktehiroe kt), 1974

d kL ~ K s - dry residue of boiler water, respectively, in clean and salty compartments, mg/l;

H c - ool multiplicity, determined by the formula

h - steam input of the salt compartment, jC from the total steam output of the boiler (accepted according to the passport data of the boiler);

Rpr - the estimated value of the boiler purge,

The absolute value of alkalinity (in mg-eq/l) of boiler (blow-down) water is not standardized. According to the Baisky Boiler Plant, during testing, the alkalinity of the boiler water was about 180 mg-eq/l, and the purity of the steam did not deteriorate. The minimum alkalinity of boiler water in a clean compartment, as well as in a boiler without staged evaporation, when boilers are fed with softened water, is taken to be at least 1 mg-eq / l.

Relative alkalinity of boiler water (in %) for the protection of boiler metal from intercryotallite corrosion is taken into account in accordance with paragraph 6.2-3 of the Rules of the State Mining and Technical Supervision. To protect the boiler metal from intergranular corrosion ("alkaline corrosion") during the installation and operation of boilers, it is necessary to prevent and eliminate high mechanical and thermal overstresses in the metal. In addition, as a protective measure, we can recommend the dosage of sodium nitrate into the boiler water, which passivates the boiler metal (protects against corrosion).

for boilers that allow intra-boiler (reagent or magnetic) treatment, the design norms for the quality of boiler water can be taken from Table 2.

Water pipes without pipes

Formative in the form of sludge, the minimum alkalinity of boiler water should be taken for all boilers not below 7-10 mg-eq.

2. The standards do not apply to boilers operating on gas and fuel oil, since in this case the use of intra-boiler treatment is not allowed.

Boiler water quality standards for overproduction boilers with a clarity of more than 30 t / h are given in Table 3.

Table 3

iolesoderaanie,> 11 rime-mg / l _! chaoya

boiler-; nourishing] howl; body! water I water

Paropro-! Working! exhaust pressure) tal- | kgf/sn*-(noot, ;

					Belgorod boiler plant

4.Schemes of continuous blowdown of boilers

Continuous blowdown of boilers is carried out according to various schemes. Saratov and Taganrog boiler plants produce expanders (separators with a diameter of 450, 600 and 800 mm) with two tangential supply of blowdown water. In low-pressure boiler houses, the scheme shown in Fig. 3 is used for these separators.

Depending on the amount of blowdown and, consequently, the required steam volume, the separator was placed on one or two boilers. Expansion and vaporization took place directly at the inlet of the purge water into the expander (operator).

As shown by the work of TsKTI, in order to reduce the volume of expanders, improve the quality of the resulting steam, and ensure their more reliable and uniform operation, one should use a connection scheme for continuous blowing of boilers into a collector in which the boiler water expands and the initial formation of a water-steam mixture.

czb

frogenetic/ t (Sha

From the paired pores

original cold soda

G/ -lls 1 -

Descent / Heated i

Prodtsobochnaya 6a I sewerage 1 -trf~wc

Heated initial Soda for post-treatment Rio.3.Principal scheme of continuous blowdown of boilers

I - boiler; 2 - continuous purge separator (expander); 3 - heat exchanger; 4 - safety lever valve

Figure 4 shows a diagram of connecting the blowdown water from the boilers to the collector, which supplies steam and water to the separator.

This improvement made it possible for the Biysk boiler plant to produce a new separator Du 300 oo with a flattened nozzle at the steam-water inlet (the diameter of the collector in which the expansion occurs is taken according to the diameter oo:ma); the highest steam capacity of the separator is 1.2 t/h. Such a separator in the boiler house is installed one for several boilers in accordance with the amount of blowdown allowed by the Biy-skin boiler plant.

Technical characteristics of separator DN 300

Oval body diameter Du, mm........ 300

Working overpressure in oepara-

tore, kg / ohm2 .......... 0.2-4), 6

The highest steam capacity, t / h .. 1.2

Blow water consumption at pressure in the boiler drum, t/h;

Rio.4. Scheme of connecting the separator to the continuous blowdown of boilers

R I 14 KGO / OY 2 ............... 7

P i 20 kg/cm^ ....................... 6

I 30 kg/cm2 ............... 5

Drawings of a general view of the separator Du 300 are given in rio.5.

Figure 6 shows a diagram of a continuous blowdown installation recommended for low and medium pressure boiler houses, in which a Du 300 separator is installed at the Biysk Boiler Plant. The separator in this scheme is not calculated, but is taken according to the characteristic given by the manufacturer.

5. Calculation of continuous purge separators

Having calculated the amount of boiler blowdown according to equation (5) and having solved the issue of economic feasibility of installing continuous blowdown equipment, the amount of water to be removed from the boiler is determined using the refined formula *

t "_* A * / in \

where is the value of continuous purge or co

quantity of water removed from the boiler, t/h; 2) p - steam capacity of the boiler house (boiler), g / h;

J_ x - the share of chemically treated water in the feed water - or, which is the same, the loss of steam and condensate as a fraction of the steam capacity of the boiler house;

Oukhoi residue of chemically treated water, mg/l; $k6 ~ °Y X °Y the rest of the boiler water is taken according to the passport data of the boiler manufacturer, mg / l (om.section 3);

I - the proportion of steam that is evaporated in the oepara-torus (expander) of continuous purge

“ T kgo / "si ^;

U dbl-! D9l-

New 1 unit ■! volume!mass!steam, ."steam.

Taplosoderm-1 nie. kcal/kg

Water I couple I !

1 Latent!heat “! transform! zation, !kcal/kg


		1 1,725 ! 0,5797

UDC 621.187.2 I. Purpose and tasks of blowing boilers

The normal operation of the boiler unit is determined primarily by the quality of the boiler water.

The quality of the boiler water depends on:

a) steam purity;

b) cleanliness of the heating surface of the boiler;

c) corrosion safety of the boiler metal and steam-condensate path.

The main means for maintaining the boiler water quality required by the standards, in addition to the appropriate treatment of waste water and, if necessary, condensate, is blowing the boiler. With the help of blowing, it is possible to regulate the concentration of salts and alkalis in boiler water in a wide range, remove suspended solids and sludge-like substances from the boiler. precipitation.

Compliance with the rational regime of boiler blowdowns, which varies depending on the quality of the boiler water and steam, is one of the radical measures to organize the water regime that ensures the normal operation of the boilers. The greater the loss of condensate in the total steam-water balance of the boiler house, replenished with chemically treated water, the greater the value of the boiler blowdown. There are two ways to blow down boilers: periodic blowdown and continuous blowdown.

Periodic blowing is carried out to remove coarse dioper sludge deposited in the lower collectors (drums) of the boiler or other low-active areas of the boiler circulation system (in places of "sluggish" circulation). Periodic blowdown is carried out according to the schedule set during commissioning, but at least once per shift, the six of which periodic blowdown is performed depend on the boiler design: lower drums, collectors, lower points of screens, in case of staged evaporation - lower points of remote cyclones.

the volume of vapor separated in the expander-body, s 3;

the specific volume of steam at pressure l oeparators, is taken according to table k for saturated steam, m 3 /kg;

the degree of steam dryness is assumed to be 0.97;

the vapor pressure of the steam volume of the oe-parator is assumed to be 800-1000 m 3 / m 3 when working according to the scheme of Fig. 3, when working according to the scheme of Fig. 4 - see the technical characteristics of the separator Du 300, which is not calculated, but is taken according to factory data.

According to the obtained volume of separated steam, a separator manufactured by the Saratov heavy engineering plant and the Taganrog plant "Krasny Kotelshchik" is selected, based on the volume of the vapor space of the separator.

6. Discharge of boiler blowdown water

The calculation of the amount of water discharged from the boiler blowdown into the barbater is given in Section 2. Various easily soluble sodium salts enter the steam boiler with feed water, since hardness cations are practically removed during the two-stage pre-boiler treatment, and an insignificant amount of iron in the feed water is allowed

The opening of the purge valves is usually carried out alternately for no more than 30 seconds. (including opening and closing times) with increased monitoring of the water level in the boiler. Particular care is required when blowing through saline compartments (cyclones) due to the small water volumes of the latter. Simultaneous purge of several points is not allowed. To ensure a more complete removal of sludge, periodic blowing should be carried out with the greatest possible intensity, with the obligatory condition to significantly but disrupt the circulation in this section of the boiler and not drop the water level in the boiler below the permissible limits. The intensity of blowing down the lower points should be limited to a flow rate of blowing water of 400-500 kg/min.

Periodic blowing of 2-3 points does not provide complete removal of evil from the boiler, complete removal of sludge is achieved by equipping the lower drums (or sumps) with special collectors (Fig. I), which ensure the intake of sludge along the length of the drum.

Fig. I. Purge manifold for removing sludge from the lower drums and sumps

It is not recommended to blow out the lower points in order to remove sludge at a low intensity, an exception can only be in the process of sludge removal during rubbing (reagent or stepping) treatment of water, when prolonged blowing is required, while

It is necessary to limit the flow of purge water, which is achieved by installing a restrictive washer dia.

meter 12-15 mm on the bypass line at the valves of the next purge (Fig. 2).

Removal of sludge from the lower points of the boiler during intra-boiler processing can be carried out both periodically and continuously.

1 - lower drum, collector or sump;

2 - shut-off valve; 3 - purge control valve; 4 - restrictive washer;

5 - purge water into the intermittent purge expander or purge well

Continuous blowdown is carried out to maintain acceptable salinity in the boiler water, which ensures the production of clean steam.

For a long time, there was an opinion that continuous blowing should be carried out by removing the most dangerous layer of water (in the zone of the evaporation mirror), which contains the maximum concentration of salts. Special studies have established that the concentration of ools in boiler water (with single-stage evaporation) is the same in any

point of the boiler circulation circuit, the only exception is the place where the feed water is introduced; with stupefied evaporation, this is confirmed for clean and salty sections of the boiler.

Water removal during "" intermittent blowing of boilers with single-stage evaporation should be carried out through a water intake manifold (with staged evaporation from salt compartments, cyclones), located in the zone of the most "calm" water in order to exclude the possible capture of steam bubbles.

The collector must be located at a depth of at least 300 mm from the normal water level in the drum and be as far away from the feed water inlet as possible. The previously common devices for removing water from the evaporation mirror cannot be used and must be dismantled.

Continuous blowing of boilers is safe periodic, since it does not sharply reduce the water level in the boilers and is more economical, since it allows the use of separated steam and the heat of blowdown water. However, the use of continuous blowing by no means eliminates the need for periodic blowing.

2. Calculation of the amount of blowdown of boilers

As mentioned above, maintaining a certain acceptable boiler water quality is achieved by blowing the boiler. The main factor determining the required amount of boiler blowdown is the total salt content of the boiler water, which ensures the production of clean steam.

Let us imagine the balance of salts in the cycle of the boiler house when the maximum salinity of the boiler water is reached: the amount of salts that enters the boiler with feed water must be continuously removed from the boiler by the blowdown water, which can be expressed by the equation

Sh.f> (3)n *■ Yupr)- Sh.6. "'Dn F y (i)

P R~ S*.t-Sn.6 *

where Sn6 - salt content of feed water, g/t;

Dn - the amount of steam evaporated in the boiler. t / h;

Sh.6 - salt content of boiler water, g/t;

L)lr - the amount of blown boiler water, t / h.

From the above simplified salt balance equation in the boiler, we obtain the value of the boiler blowdown D - Sn * R ",

^ n R Sh.6 - Sn6 (£)

or, when expressing the blowdown value as a percentage of the boiler capacity, equation (2) will take the form p-JM-JOSL , (h)

R P r - the value of the boiler blowdown, $ - from overproductive ti.

The salt content of feed water can always be determined analytically, based on the quality of the individual components included in the feed water and the ratios in which they are mixed, for example, the salt content of boiler feed water can be determined from the equation

n Sx 'bya * SiUk g (*)

where - about oles ode neighing of feed water, mg/l;

5 K - condensate coolant content, yg/l;

A*, - proportion of chemically purified

water and condensate in the feed water, the amount of which is taken as a unit:

For preliminary (approximate) calculations, the values of boiler blowdown in equation (4) neglect the value of the salinity of the condensate, since it is insignificant compared to the salt content of chemically treated water (100 or more times less). Equation (4) will then take the form

By substituting this approximate value of the feedwater oleocontent into expression (3), an equation is obtained that is usually used to determine the amount of boiler blowdown in boiler rooms

where оСх is the proportion of chemically purified water in the feed

noah, or what is the same, the loss of steam and condensate, replenished by chemically purified water;

For industrial and heating boiler houses, according to SN 350-€b "Guidelines for the design of boiler plants", the calculated value of the purge of low-pressure boilers should not exceed Yu £ of the steam capacity of the boiler room. For boilers of similar pressure P \u003d D0 kgf / s ^, the value of the boiler purge is allowed up to (si. "Rules of technical

operation of power stations and networks").

In low-pressure boiler rooms, continuous blowing is carried out if it is necessary to remove boiler water with a salt of 2/£ from the steam generator, introducing the boiler room capacity, but not less than 0.5 t / h. With a blowdown value of less than 0.5 t/h, the expediency of continuous blowdown should be confirmed by calculation. When blowing from 0.5 to I t/h, only a continuous blowing separator is installed, when blowing more than I t/h, a separator is installed to the heat exchanger to use the heat from the discharge of the separated water. With a blowdown value of less than 2> of the steam capacity of the boiler house and less than 0.5 t / h, in order to maintain an acceptable salt content of boiler water, periodic blowdown of the boilers is sufficient, which is usually carried out not in the river

I time in Oman.

The foregoing does not apply to DKER-20 boilers with an operating pressure of 13 and 23 kgf / cm ^, which, due to their design features, require continuous blowing of at least 5% of the boiler steam output.

Features of the operation of boilers DKVR-20 are detailed in the supplement to the instruction "Gift boilers: DKZR" of the Biysk boiler plant.

When the blowdown water flow rate is less than 1-10.5 t/h and less than 2 of the steam capacity in the low-pressure boiler house, the feasibility of installing continuous blowdown equipment can be checked from the following expression:

_, // Pnp "Dn It p.6 - Lc.S) A" B760

where<* - ежегодные амортизационные отчисления для

economically acceptable payback period. capital costs, unit share;

U, - the total cost of the installation for using the heat of the blowdown water, rub.;

Pgr - the size of the boiler blowdown, fractions of a unit;

% - boiler steam output, t/h;

L - cost of I t of standard fuel, rub.

3. Boiler water quality standards

These recommendations do not deal with the choice of water treatment systems for steam boilers and the quality requirements for feed water, but give boiler water quality standards that provide clean steam, and recommendations for ensuring these requirements. The quality of the boiler (purge) water is normalized according to the

The dry residue of the boiler (purge) water for the DKV and DKVR boilers (at P = 14.24 and Pa40 kgf / ohm 2), according to the Biysk boiler plant, is shown in Table 1.

Notes.I. In the event of improper operation of gas-oil burners, limit values of boiler water oozing in the second stage should not be allowed.

H. Features of the application of boiler water quality standards for the DKVR-20 boiler are set out in work f 4 3.

The amount of dry residue of boiler water in the first stage of evaporation is determined by the formula

Requirements for the quality of boiler water of steam boilers in terms of total salt content (dry residue) should be adopted according to the data of boiler manufacturers.

Alkalinity of boiler water in terms of phenolphthalein in the clean section of boilers with staged evaporation and in boilers without staged evaporation should be taken as ^ 0.05 mg-eq / kg for condensate-distillate feeding and ^ 0.5 mg-eq / kg for feeding boilers with the addition of softened water.

The highest value of alkalinity of boiler water is not standardized.

The indicated boiler water quality standards are supported by pre-boiler treatment of feed water, intra-boiler (correctional) treatment of boiler water and regulation of boiler blowdown. When the boiler is purged, both dissolved salts and suspended sludge are removed from it.

In the conditions of concentration of boilers, continuous and periodic blowing is used.

Continuous blowdown is carried out to maintain a certain salinity (dry residue) or alkalinity of the boiler water at the level necessary to obtain clean steam.

Continuous blowdown equipment includes a blowdown separator with a separated water heat exchanger (if any) and a cooler for sampling the boiler water from the pipeline.

The separator is started up in the following order: the second valve from the boiler on the continuous blowdown line is fully opened, then the first valve from the drum is 7s - "/b of a turn, after which the valve is opened at the water outlet from the separator to discharge into the sewer.

It is necessary to slowly open the valve on the pipe for blowdown water from the boiler to the separator, then the valve on the pipe for the outlet of separated water through the level controller to the heat exchanger or to the drain. During normal operation, the pressure in the separator should not exceed 0.02-0.03 MPa (0.2-0.3 kgf/cm2).

Disabling the continuous blowdown separator must be carried out as follows: first, close the valve on the blowdown water supply line to the separator and, lastly, the valve on the blowdown water line from the boiler, then close the gate valve at the outlet of the separated water from the separator to the heat exchanger or to the drain.

From time to time, the sludge accumulated in the lower part of the separator must be discharged through the drain pipe.

Maintaining a given alkalinity in the boiler (salt content) is carried out by adjusting the degree of opening of the first valve along the flow of water from the boiler.

The size of the continuous blowdown is determined during the thermochemical tests of the boiler, based on the quality standards of feed and boiler water.

Periodic blowdown is carried out to remove sludge deposited at the lowest points of the boiler. Purging is carried out from the lower drum and collectors of the steam boiler screens, and in case of staged evaporation - from the lower points of remote cyclones.

Periodic purging should be carried out when the boiler is fired up with a pressure of 0.3-0.4 MPa (3-4 kgf / cm2) during hours of reduced loads or short-term stops. In order to avoid leakage of water, the boiler must be purged by two drivers (stokers): one monitors the water level in the boiler using water-indicating instruments, the other performs the purging. Before starting it, it is necessary to inform the shift maintenance personnel about this, and also make sure that other boilers that are being repaired or cleaned are disconnected from the common blowdown line with plugs.

Before starting the purge, it is necessary to check whether there is water in the deaerator (the stock is at least half the level in the tank). In addition, the water level in the boiler drum should be slightly higher than normal for automatic feeding, and higher for manual feeding.

After making sure that there is a sufficient amount of water in the deaerator tank and the boiler drum and that the nutritional, water-indicating devices are in good condition, the boiler can be purged. First, the valve second in the direction of the blowdown water from the boiler is completely opened, then slowly and carefully the first valve from the boiler drum.

The duration of the purge from one point should not exceed 30 s, including the time of opening and closing the valve.

It is necessary to carefully purge the salt compartments (cyclones) due to their small water volume.

At the end of the purge, the valves are closed in the reverse order - first the first, and then the second from the boiler.

Make sure that the shut-off valve on the vent line is closed securely and does not allow water to pass through.

If it is not possible to tightly close the purge valves after purge, then increase the power supply and immediately stop the boiler. The use of levers when opening and closing the purge valves is prohibited.

In the event of hydraulic shocks in the purge pipeline or a sharp drop in the water level in the boiler, the purge must be stopped, and after a secondary check of the boiler condition, continue again.

The number of blowdowns is set depending on the sludge content of the boiler water within the time limits set by local schedules, at least once a day, if the boiler water is clear. In the event of a malfunction of the feed pumps or water-indicating glasses, it is forbidden to purge.

Reading time: 3 min

Even with the use of high-quality water and constant maintenance, from time to time there is a need to clean the equipment from slag. Blowing out the boiler can help with this.

Purpose of boiler blowdown

Make-up water, as a rule, contains impurities, which, when they enter the drum boiler, accumulate, which leads to an increased salt content in the water.

This entails the need to remove these substances from the water cycle system. Drum boilers use a continuous removal process called blowdown.

The purpose of this process is to get rid of sludge, oxidized iron, mechanical debris, in order to prevent these substances from entering the screens of the thermal installation and concentrating in collectors. Water must meet the standards specified in the equipment documentation, water chemistry instructions.

Flushing is carried out on the working equipment by the employees of the boiler house by order of the chemical control specialist up to two times a day, depending on the color of the water.

Steam boiler blowdown

When water evaporates in steam equipment, salts settle on the heating surface, which, upon further heating, come in the form of sludge in the lower units of the boiler. This leads to a deterioration in heat transfer and an increase in fuel consumption, and ultimately to damage to the pipes and drum.

Therefore, it is important to ensure the operation of equipment without scale and timely ventilation of the furnace. To maintain the correct mode of operation, steam installations are cleaned, that is, foreign impurities are removed along with water. Blowing can be of two types, which we will discuss in detail below, namely, periodic - it is performed periodically to remove sludge from the screens, drum, collectors of the lower part of the boiler.

This process is fast, but with a large flow of water. Water is discharged into a cooling expander before entering the sewer. Continuous washing of steam boilers removes impurities constantly from the upper part of the boiler. The water enters the separator where steam and water are separated.

Hot water

To increase the stability of operation, hot water boilers are connected to the heating system through water-to-water heat exchangers.

Make-up water in the boiler must be of high quality; for this, continuous and periodic purging of a closed circuit is carried out. This procedure eliminates the content of iron compounds.

Types of purge

As mentioned above, there are two types of cleaning: periodic and continuous. The first is designed to get rid of sludge impurities, and the second to ensure the required minimum level of salt content in the boiler water. The frequency of intermittent purges and the volume of continuous purges are determined by the maintenance personnel.

Periodic purge

This type is performed by two employees with the water level in the drum above average. In this case, one directly carries out the flushing, and the second controls the water level.

Periodic washing is carried out in several stages:

Make sure that the line is working. The section of pipe between the boiler and the flush valve must be hot, and behind the valve - cold.
Make sure that the shut-off valves and measuring instruments are working.
The second valve slowly opens.
After that, the first valve from the boiler is carefully opened to avoid water hammer.
The opening time period of the first valve must not be longer than 30 sec. Next, the second valve behind it closes.

After the process is completed, make sure that all valves are closed and the line is tight. The start and end times of the procedure are displayed in the shift log.
During periodic flushing, water with impurities is removed from the lower part of the boiler. In addition, flushing helps to reduce the salt content in the boiler water. The quality of the procedure is determined by the device for measuring the pressure in the line.

Continuous blowdown of the boiler

Continuous blowdown is carried out through a permanently open valve, which is located in the flush line, from the top of the boiler. A pipe with holes is laid around the drum for a uniform flow of water.

This is necessary to get rid of saline water, which is replaced by the same volume of cleaner make-up water. As a rule, the volume of washing is up to 3% of water. This is enough to maintain the required level of salt content in the water.

A specialist in chemical analysis determines the amount of salts in the water, and depending on this, the percentage of water withdrawal is set. Water from the boiler enters the separator, where the separation of steam and water takes place, the steam enters the deaerator, and the contaminated water after the cooling expander goes to the sewer.

Boiler purge schemes

The figure shows a diagram of continuous and periodic blowdowns of a combined cycle gas plant with a capacity of 450 kW. Saturated steam from the continuous blowdown expander is directed to a reduced pressure separator. The steam pipeline is equipped with shut-off valves and a check valve.

Drainage from the RNP enters the clean wastewater tank. After RNP, the liquid enters the intermittent blowdown expander, and after that, the contaminated water is discharged into the drain tank from the boiler.

Drawing of the steam pipeline from the continuous blowdown separator to the deaerator

This design drawing shows the design of the low pressure steam pipeline from RNP to the atmospheric deaerator. The steam pipeline is equipped with shut-off valves and a check valve to prevent steam from entering the expander.

Exhaust drawing from RNP safety valve

This drawing shows the exhaust piping from the blowdown expander relief valve. It passes to the main building and then goes to the roof (no higher than 2 m) to ensure the safety of employees. A hydraulic seal is installed on the exhaust pipeline to divert drainage.

Drawing of flash steam from the intermittent blowdown expander

The figure shows the vapor from the RPP. He is taken outside the premises. Unlike exhaust, vapor is constantly discharged. The evaporator must be cooled, for which a device for supplying cold water to the pipeline (vapour cooler) is used.

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

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

When the water in the steam boiler evaporates, any impurities contained in the feed water are not carried away with the steam, but remain in the boiler water. As a result, 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 surface of the boiler. Foam from the surface is carried away from the boiler into the steam pipeline. Foaming is also the reason for turning off the boiler on the "Level in the drum" protection.

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

Dnp \u003d Dk * Spv / (Smax - Spv)

Dnp - consumption of continuous purge;
Dto - consumption of feed water for the boiler (t/h);
FROMpv - salinity of feed water (µg/kg);
FROMmax - maximum salt content in the boiler (µg/kg)

Heat loss with continuous blowdown will be:

Qpot \u003d Dnps * inp - Dnpb * isb

Qsweat - heat lost with continuous blowing (kcal/h);
Dnpc - existing consumption of continuous blowing (t/h);
Dnpb - consumption of continuous blowdown, after installation of the heat recovery unit for continuous blowdown (t/h);
inp - enthalpy of continuous blowing at pressure in the boiler (kcal/kg);
iSat - continuous blowdown enthalpy after installation continuous purge 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 the salt content in the boilers are carried out once a day and in order to prevent the salt content in the boilers from exceeding the limit, it is necessary to maintain the salt content in the boiler at the minimum allowable level.

Exceeding the discharge of the continuous blowdown of the boiler leads to thermal energy losses 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 salinity 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 to use the heat of continuous blowdown to generate flash steam and heat up 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 plant operation:

Continuous blowdown of steam boilers

Continuous blowdown of steam boilers Continuation of the heading “Conflict of interest. How not to harm the system by improving the operation of individual installations”, today we will talk about how they affect the overall

What is a steam boiler blowdown and why is it needed

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 from slag and impurities. Even frequent blowing of the steam boiler will not save you from this.

There are two types of rinsing - cold and warm rinsing. Cold - steam comes out and the boiler itself cools to a temperature of thirty, thirty-five degrees. And after that, the water is drained, and the boiler naturally cools down 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 way that does not require special equipment.

To ensure the specified water regime, it is necessary to regularly remove (purge) salts coming 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 blowdowns: periodic and continuous.

Periodic is carried out 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, takes out sludge and other sediments into the so-called expander (bubbler), designed to cool the boiler water.

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

The composition of the water must be maintained in the boiler water, i.e. the input of salts and contaminants with feed water must correspond to their removal from the boiler. This is achieved by continuous and intermittent blowing.

In case of insufficient removal of salts from the boiler, they accumulate in water and scale formation on pipe sections, which reduces their thermal conductivity, leads to bulges, ruptures, emergency shutdowns, 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.

Steam separators in the drum

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 during evaporation they will settle on the inner surfaces of the pipes in the form of a precipitate.

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.

Brief description and description of the operation of boilers

Feed water in the drum is mixed with boiler water and is fed through unheated downpipes to the lower collectors, from where it is distributed through heated screen pipes. The process of steam formation begins in the screen pipes, and the steam-water mixture from the screen system through the steam supply pipes again enters the drum, where the steam and water are separated. The latter is mixed with feed water and re-enters the downcomers, and the steam through the superheater enters the turbines. Thus, the water in the boiler moves in a vicious circle, consisting of heated and unheated pipes. As a result of 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 the quality of the steam (due to droplet entrainment of boiler water and its foaming) and to the formation of deposits on the 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 resulting steam.
Corrective treatment of boiler water (phosphating and amination) to reduce the amount of deposits and maintain the pH of the vapors according to PTE standards.
The use of continuous and periodic purges in order to remove excess salts and sludge.
Preservation of boilers during summer downtime.

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.

Feed water is fed into the upper drum of the boiler, which is part of the clean compartment. A clean compartment 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. Boiler water of salt compartments has a high salinity. The steam from the brine compartments will not be of high 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 with the help of remote cyclones, which are salt compartments. The boiler drum serves as a clean compartment. The blowdown water from the boiler drum enters a 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. Purge 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 baffles, slotted partitions, shutters, dry steamers installed in front of the steam outlet pipe. Their action is based on the mechanical separation of steam due to the forces of inertia, centrifugal forces, wetting and surface tension. All this makes it possible 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 in the boiler water, the salt concentration increases to such an extent that even with a slight feed water hardness, there is a risk of scale formation on the heating surface. Therefore, in boilers, “re-softening” is usually carried out by means of phosphating, i.e. corrective treatment of boiler water with phosphates: trisodium phosphate, sodium tripolyphosphate, diammonium phosphate, ammonium phosphate, triammonium phosphate.

When dissolved in a correction solution of trisodium phosphate or sodium tripolyphosphate, Na +, PO43 ions are formed. The latter form an insoluble complex with the calcium cation of boiler water, which precipitates in the form of hydroxyapatite sludge, which does not stick to the heating surface and is easily removed from the boiler with blowdown water. At the same time, a certain alkalinity and pH of the boiler water can be maintained by phosphating, which ensures the protection of the metal from corrosion. The excess of phosphates in the boiler water must be maintained constantly in an amount sufficient to form sludge hardness salts. However, exceeding the content of phosphates in comparison with the norms of PTE is also not allowed, since in the presence of a large amount of iron and copper in the boiler water, ferrophosphate deposits and scales of magnesium phosphate 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, normalized by the PTE, i.e. 7.5 or more. The unit for dosing ammonia into make-up water is located at the HVO and is serviced by the personnel of the chemical workshop. The amount of ammonia dosage, expressed as a percentage of the amount of additional water supplied to the boiler shop, is set on the automatic dosing pump by the HVO personnel, depending on the pH of the superheated vapors, as directed by the chemical control laboratory assistant.

Simultaneous amination and phosphating

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

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 displacing tank by dissolving salts on a grate to retain coarse impurities with hot feed water and pumped into three phosphate tanks in the turbine room and one phosphate tank in the boiler room section, from where dosing pumps are supplied to the boilers. For reliable and continuous correction of the boiler water, 2 pumps are connected to the boilers, working either together or in single mode. Three main and one standby phosphate pump boilers.

A solution of phosphates is prepared by the personnel of the chemical workshop and controlled by the concentration of PO43 and, if necessary, Np + by laboratory assistants of the shift laboratory, recording the results in a work log. The input of the phosphate solution and the monitoring of the operation of the dosing pumps is carried out by the personnel of the boiler shop. The control over the concentration of phosphates in the boiler water is carried out by the personnel of the chemical workshop (laboratory assistants of the chemical analysis of the shift laboratory). To check the correctness of the water chemistry regime in the boiler water, it is necessary to control not only the concentration of phosphates, but also pH, since the condition for compliance with this regime is the correspondence between the concentration of phosphates and pH.

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

Shield = 100% * 40 (2Schff-Schob) / Sk.v.,

where Schob is the total alkalinity of the boiler water; Aff - alkalinity in terms of phenolphthalein; 40 is the equivalent weight of NaOH; Sk.v. – salinity of boiler water.

One of the main requirements for the water regime of boilers is to obtain steam of acceptable quality, which ensures minimal contamination of the internal 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 the sodium content.

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

sodium content - no more than 60 µg/dm3;
pH value for boilers of all pressures is at least 7.5.

Boiler blowdown

Residual impurities contained in the feed water, entering the drum boiler, are 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 withdraw these salts from the water circulation cycle at power plants. For drum boilers, such a withdrawal is carried out by continuously removing some part of the boiler water from the salt compartment, i.e. by continuous blowing.

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

Continuous blowdown of the boiler carried out by the personnel of the boiler shop at the direction of the on-duty chemical control based on the results of the analysis of boiler water. The laboratory assistant on duty at the shift laboratory calculates the salinity of the salt compartments required at the moment to maintain the blowdown value of 2-4%, depending on the salinity 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 blowdowns should be set on the basis of the instructions of the boiler manufacturer, standard instructions for maintaining the water-chemical regime or the results of thermal-chemical tests conducted by the power plant, services of AO Energos or specialized organizations.

Continuous purge is conducted to the separator of continuous purges through regulators (RNP). If necessary, continuous purge can be carried out on the separator of periodic purges in addition to 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 a high salinity goes to the heating system make-up tank or is drained.

Intermittent or sludge blowdown produced from the lower collector of the boiler. The purpose of the blowdown is to remove coarsely weighed sludge, iron oxides, mechanical impurities from the boiler in order to prevent drift into the screen pipes and their subsequent sticking to the pipes, accumulation of sludge in collectors and risers.

Periodic purging of operating boilers is carried out by the personnel of the boiler shop at the direction of the duty officer for chemical control 1-2 times a day depending on the color of the boiler water (yellow or dark). In order to avoid disturbance of circulation, it is not allowed to open the lower points 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 the iron that 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, which occurs as a result of oxygen absorption and the presence of a moisture film, various methods of equipment conservation are envisaged. The simplest method of conservation 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 being sucked in.

Each case of conservation of boilers must be reflected in the operational log of the boiler room. Chemical control provides for checking the overpressure and determining oxygen in the feed water (not more than 30 µg/l), with an entry in the chemical control list and the boiler conservation journal.

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

Boiler kindling

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

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

Brief description 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

Boiler water regime

In drum boilers with natural and repeated forced circulation, in order to exclude the possibility of scale formation, it is necessary that the concentration of salts in the water be below the critical one, at which they begin to fall out of solution. In order to maintain the required concentration of salts, a certain part of the water is removed from the boiler by blowing, and together with it, salts are removed in the same amount as they come with the feed water. As a result of blowing, the amount of salts contained in the water stabilizes at an acceptable level, which excludes their precipitation from the solution. Apply continuous and periodic blowdown of the boiler. 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 blowing is used to remove sludge that has settled in the boiler elements, and is carried out from the lower drums and boiler headers every 12-16 hours.

The scheme of continuous blowdown of boilers is shown in fig. 12.5. The continuous blowdown water is supplied to the expander, where the pressure is kept lower than in the boiler. As a result, 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.

Continuous purge p, %, is set according to the allowable concentration of soluble impurities in the boiler water, most often by 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 blowdown water and the nominal steam output of the boiler, kg/h. Feed water consumption D n.v. In the presence of continuous purge is

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

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 the heating surfaces, related 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 equal to zero. The normal water regime of the boiler does not allow the deposition of salts on heating surfaces, and the D S0 term in this equation should also be equal to zero. Then the amount of water removed with blowing,

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

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

The use of continuous blowing, which is the main means of maintaining the required water quality of the drum boiler, 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 purge and feed water, kJ/kg; % - boiler efficiency.

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

With the specified blowdown rates and partial use of the heat of the blowdown water, the heat loss with the blowdown is 0.1-0.5% of the heat of the fuel. In order to reduce heat losses with blowdown, one should strive to reduce the amount of water removed from the boiler. An effective method to reduce blowdown is the staged evaporation of water. The essence of staged evaporation or staged blowdown 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 - 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 the water with one-stage evaporation, a lower percentage purge. The use of staged evaporation is also effective as a means of reducing carryover of silicic acid due to the high hydrated alkalinity that occurs in the salt compartments. Staged evaporation and purge systems are usually made of two or three compartments. Currently, most medium and high pressure drum boilers use staged evaporation. 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 the feed water is supplied and 75-85% of the steam is produced, and a salt compartment, where 25-15% of the steam is generated.

On 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 capacity of the salt compartments, %, required to provide a given salinity of water in a clean compartment, in the absence of water transfer into it from the salt compartments, is determined from the expression

where n and – steam capacity of salt compartments, %; S n.v and S vl - salinity of feed water and water in the clean compartment, kg/kg; р – purge from the salt compartment, %. The optimal steam capacity of the salt compartments with two-stage evaporation and blowdown, determined by the allowable total salt content in the steam, is 10-20% with a blowdown of 1%, and 10-30% with a blowdown of 5%.

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„ \u003d C / Cc-b mg / kg; here

K l and K ll are the 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 is the multiplicity of concentrations in the clean compartment and feed water. Salt concentration in clean compartment water, mg/kg,

Salt concentration in the purge water, mg/kg,

Multiplicity of concentrations between the salt and clean compartments in the absence of water transfer from the salt compartment during two-stage evaporation.

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

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

Permissible limit values of salinity, silicon 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. Additionally, a corrective method of water treatment in a boiler is used, in which Ca and Mg salts are converted into compounds that are insoluble in water. To do 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 precipitate in the form of sludge removed by periodic blowing. When feeding boilers with condensate with the addition of chemically purified water, a phosphate-alkaline water regime of the boiler is created, in which free alkalinity is maintained. When distillate and chemically desalted water are added to the condensate, the pure phosphate water mode of the boiler is maintained in the absence of free alkalinity. The following excess of RO in water is recommended: for boilers without staged evaporation 5-15; for boilers with staged evaporation in the clean compartment 2-6 and in the salt compartment - no more than 50 mg/kg.

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

The hydrazine-ammonia water mode of the boiler, the oxygen remaining after thermal deaeration is bound by hydrazine. The remaining carbon dioxide is bound by 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 complexing water mode of the boiler, in addition to ammonia and hydrazine, introduces a complexing agent into the feed water - usually ethylene aminetetraacetic acid (EDTA). This leads to an increase in the thermal conductivity of deposits and their movement 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 - iron hemoxide), forms iron complexonates that are well soluble in water, which, under the action of a higher temperature along the medium decompose with the formation of a dense layer of magnetite falling out on the inside of the pipes, which protects the metal from corrosion.

In once-through boilers that do not have a purge, 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 a once-through boiler has the form

Hardness salts and metal corrosion products are partially deposited on the walls of the heating surface 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 allowable concentration of this compound in the feed water is determined by the allowable intensity of deposits in the boiler per unit mass of incoming water:

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

i.e., the intensity of deposit growth is proportional to the derivative of solubility with respect to enthalpy and the average heat flux density 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 pressure, the solubility of a number of compounds, including silicic acid and sodium chloride, is quite high, and their concentration does not reach saturation in the boiler. These impurities are taken out together 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 for the reliable operation of turbines, in the flow part of which, with a decrease in steam pressure, deposits can form.

The salts settled 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.

Updated on 03/06/2012 15:54

Conditions for the formation of scale. Steam boiler blowdown

When water evaporates, the concentration of salts in it continuously increases. If salts are not removed from the boiler, then at a certain concentration in 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 (Ca (HCO3) g, Mg (HC03) 2) dissolved in water decompose, forming sludge, and precipitate at the lower points of the boiler (lower drums and collectors).

Scale is concentrated on the most heat-stressed surfaces of screen and boiler 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 fuel consumption on scale thickness

Scale thickness, mm

Average value of excessive fuel consumption, %

Due to the low thermal conductivity of the scale, the metal of the boiler and screen pipes is poorly cooled and subjected to severe overheating, as a result of which its strength decreases. This leads to the appearance of bulges, cracks, rupture of pipes, and even to the explosion of drums and boilers.
On modern water-tube boilers, the operation of the boiler under the condition of scale formation is unacceptable. Boilers must operate in a scale-free mode.
Steam boiler blowdown
Boilers are purged to maintain the permissible salinity of the boiler water.
Purging is the removal of foreign impurities from the boiler together with the boiler water (salts, sludge, alkalis, suspended solids, etc.) while replacing the blown water with feed water. Purge is periodic and continuous.
Periodic blowdown is carried out at certain intervals and is intended to remove sludge from the lower points of the boiler: the drum, screen collectors, etc. Purging is carried out into an expander designed to cool water before it is discharged into the sewer.
Continuous purging provides a constant removal of dissolved salts of constant hardness to maintain their acceptable concentration. Continuous blowing is usually from the top drum and controlled by a needle valve. The water is diverted to the expander (separator), where the steam is separated from the 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 instructions or the head of the boiler room (according to the instructions of the laboratory).

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Scale formation conditions

Feed water in the drum is mixed with boiler water and is fed through unheated downpipes to the lower collectors, from where it is distributed through heated screen pipes. The process of steam formation begins in the screen pipes, and the steam-water mixture from the screen system through the steam supply pipes again enters the drum, where the steam and water are separated. The latter is mixed with feed water and re-enters the downcomers, and the steam through the superheater enters the turbines. Thus, 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, the boiler water is evaporated, i.e. concentration of impurities in it. An uncontrolled increase in impurities can lead to a deterioration in the quality of the steam (due to droplet entrainment of boiler water and its foaming) and to the formation of deposits on the 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 resulting steam.
Corrective treatment of boiler water (phosphating and amination) to reduce the amount of deposits and maintain the pH of the vapors according to PTE standards.
The use of continuous and periodic purges in order to remove excess salts and sludge.
Preservation of boilers during summer downtime.

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.

Corrective treatment of boiler water

Phosphating

Amination

Simultaneous amination and phosphating

A solution of phosphates is prepared by the personnel of the chemical workshop and controlled by the concentration of PO43 and, if necessary, NH4+ by laboratory assistants of the shift laboratory, recording the results in a work log. The input of the phosphate solution and the monitoring of the operation of the dosing pumps is carried out by the personnel of the boiler shop. The control over the concentration of phosphates in the boiler water is carried out by the personnel of the chemical workshop (laboratory assistants of the chemical analysis of the shift laboratory). To check the correctness of the water chemistry regime in the boiler water, it is necessary to control not only the concentration of phosphates, but also pH, since the condition for compliance with this regime is the correspondence between the concentration of phosphates and pH.

Shield = 100% * 40 (2Schff-Schob) / Sk.v.,

where Schob is the total alkalinity of the boiler water; Aff - alkalinity in terms of phenolphthalein; 40 is the equivalent weight of NaOH; Sk.v. – salinity 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 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 the sodium content.

sodium content - no more than 60 µg/dm3;
pH value for boilers of all pressures is at least 7.5.

Boiler blowdown

Residual impurities contained in the feed water, falling into, are 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 withdraw these salts from the water circulation cycle at power plants. For drum boilers, such a withdrawal is carried out by continuously removing some part of the boiler water from the salt compartment, i.e. by continuous blowing.

czb

Purpose of boiler blowdown

Steam boiler blowdown

Hot water

Types of purge

Periodic purge

Continuous blowdown of the boiler

Boiler purge schemes

Drawing of the steam pipeline from the continuous blowdown separator to the deaerator

Exhaust drawing from RNP safety valve

Drawing of flash steam from the intermittent blowdown expander

What is a steam boiler blowdown and why is it needed

Brief description and description of the operation of boilers

Staged evaporation

Corrective treatment of boiler water

Boiler blowdown

Boiler conservation

Boiler kindling

Boiler water regime

Staged evaporation

Corrective treatment of boiler water

Boiler blowdown

Boiler conservation

Boiler kindling

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