Big encyclopedia of oil and gas. Methods for producing dry and clean steam

§ 1.5 Emission of rheons and electron decay-evaporation The electron is as inexhaustible as the atom; nature is infinite. IN AND. Lenin, “Materialism and Empirio-Criticism,” 1908. Ritz proposed his hypothesis about the emission of rheons by elementary charges only as a way to give our

More perfect is the water regime, organized according to the scheme of stepwise evaporation. The drum is divided by a partition into two compartments (Fig. 15.13). Each of the compartments is connected to its own group of circulation circuits that are not connected by water. Only a hole in the partition separating the drum connects the water volume of both compartments. Feed water is supplied to the first (large) compartment, and purging is carried out through the second (small) compartment. Boiler water from the first compartment enters the second compartment through a hole in the partition, and the water level in it is set lower than in the first. All steam from the drum is removed through the first compartment.

The compartment where the salt content of the water is low is called clean, and the second, in which the water has a high salt content, is called the salt compartment. Attitude (SQ – concentration of impurities in boiler water) is called the concentration multiplicity. Taking for example that 80% of the total amount of steam is formed from water with low salt content, and therefore the bulk of the steam is obtained more High Quality than in the single-stage evaporation scheme, and only 20% of the steam is formed from the same water as in the simple scheme. Consequently, the quality of steam obtained with a two-stage scheme is significantly higher than with a single-stage one. The flow of water from the clean compartment to the salt compartment is internal purging of the clean compartment. Unlike external blowing, internal blowing is not accompanied by loss of either heat or working medium, and therefore its value is chosen only for the consideration of the maximum possible improvement in steam quality. In turn, the value of this purge determines the performance of the salt compartment. In this regard, the question arises about choosing the optimal performance of the salt compartment, which is established by calculation.

With intra-drum stepwise evaporation, due to the limited heights of the water and steam volumes, the difference in levels is small, and this can cause reverse flows of water. An increase in the difference due to an increase in the water level in the clean compartment is associated with a decrease in the height of the vapor volume and, therefore, with an increase in droplet entrainment, and a decrease in it in the salt compartment can cause a circulation disturbance.

When using remote cyclones as a separation volume and a link closing the circulation circuit of the salt compartment, the difference in levels in the compartments can be selected sufficient to prevent the reverse flow of water. Therefore, schemes with remote cyclones are preferable, especially when the productivity of the salt compartment is low.

The efficiency of staged evaporation increases with the number of evaporation stages, but this increase fades with increasing number of stages. The most widespread are two- and three-stage schemes. In this case, the second stage of evaporation can be organized either inside the drum, as shown in Fig. 15.13, or outside it - in remote cyclones (Fig. 15.15). In a three-stage design, usually the first and second stages are performed in a drum, and the third in a remote cyclone (Fig. 15.16).

In remote cyclones it is possible to produce steam and water volumes of any height. This ensures good steam drying (due to the high height of the steam volume) and reliable operation of the circulation circuits (due to the high height of the water volume), and also prevents the removal of water from the salt compartment into the clean compartment.

Staged evaporation allows you to increase the purity of steam at a given quality of feed water and given value purging. It also makes it possible to obtain satisfactory steam purity with water of lower quality, which simplifies and reduces the cost of water treatment. Staged evaporation also makes it possible to increase the efficiency of a steam turbine plant due to reduced blowdown without a noticeable decrease in steam quality.

With purge water. The increase in boiler blowdown caused by non-return of condensate depends primarily on the amount of chemically purified water added, as well as on the pressure in the boilers, the type of water treatment, and the presence of staged evaporation.

Standardized indicator without stepwise with stepwise evaporation more than 8 to 40 bar UP TO 8 bar

Installations with drum steam generators with steam pressure bar (in the drum) when regulating the temperature of superheated steam using water from the common feed line of the steam generator's own condensate 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 3 4 5 6 7 8 9 7 8 9 7 8 9 The listed water desalting schemes are used when, taking into account the whole complex of issues related to the preparation of additional water and the water regime, the use of magnesium desiliconization and Mac-cationization (or H-Na-cationization) in combination with stepwise evaporation turns out to be unacceptable

No staggered evaporation

In some cases, the same effect of reducing the separation work can be achieved by stepwise condensation, for example, in the demethanization unit of ethylene production units) or stepwise evaporation of raw materials (for example, in primary oil distillation units) and introducing it into the column at several points.

There is also a noticeable influence of third elements, especially sodium and potassium, with their content exceeding the content of those being determined by an order of magnitude or more. elements. The most significant decrease in line intensity in the presence of Ma and K is experienced by highly volatile impurities, which evaporate simultaneously with them. For other impurities, it is sometimes possible to noticeably reduce this influence during stepwise evaporation by distilling off the influencing elements.

For boilers fed with water with a low silicic acid content, you can limit yourself to using steam flushing with feed water, without simultaneously resorting to stepwise evaporation.

It should be noted that feeding these boilers with chemically purified water causes additional difficulties associated with the presence of selective entrainment of silicic acid. However, these difficulties are currently being eliminated by partial desiliconization of the additional water and stepwise evaporation and steam washing.

At one thermal power plant, TP-230 boilers (steam pressure 110 ama) with staged evaporation were fed with the addition of chemically purified water (Table 1). This power plant experienced a progressive decline in high-pressure turbine output, while routine flow-through flushing

Low steam from TP-230 boilers with staged evaporation. Steam samples were taken from the middle of the main drum (clean compartment) and from its right and left sides at the border of the steam exit of the salted compartments from the intermediate chambers into the clean compartment. Data from Fig. 6 show that the concentration of silicic acid in a pair of salty compartments is slightly higher than in a pair of clean compartments.

At those high-pressure power plants where evaporator distillate serves as an additive to the feed water, it is advisable to equip the boilers with staged evaporation devices with salt compartments in the form of remote cyclones. It is advisable to rinse the steam of these cyclones with boiler water from the clean compartments

Boiler water sampling devices are installed on continuous blowdown lines, and if there is staged evaporation in the boiler, also in the clean compartment. If there is a low salt concentration ratio in boilers with intra-drum devices for stepwise evaporation

If during the experiments there were no surges, but the salt content of the steam turned out to be higher than permissible standards, then the purity of the steam is determined again at a lower salt content of the boiler water. In this case, it may be necessary to conduct several long-term experiments with a decrease in the salt content of boiler water in each experiment in steps of 60-.70 fl/kg in clean compartments of boilers with stepwise evaporation.

Example 3. Determine the heat loss with unreturned condensate as a percentage of steam heat consumed by heat-consuming units, for the following conditions, saturated steam comes from a heating and industrial boiler house equipped with boilers with a pressure of kgf1cm without stepwise evaporation, the heat of the boiler blowdown water is used in the separator and heat exchanger (i np=40 kcal1kg).

The subsequent successful use of steam flushing devices in single-drum high-pressure boilers of the PK-19 and PK-20 types refuted these assumptions. The PK-19 boiler is equipped with a drum with an internal diameter of 1500 mm and is equipped with devices for staged evaporation with external cyclones. The total productivity of the salt compartments is 20% (II stage 12%, III stage 8%). All generated steam is passed through steam flushing devices located in the clean compartment of the drum (see Fig. 8).

Application on the TP-230 boiler of 3-stage evaporation and foam washout (salt compartment productivity 17%) with

Staged evaporation is a very effective method for increasing steam purity. This method allows, for a given feedwater quality and the same blowdown values, to obtain cleaner steam than with single-stage evaporation. It also makes it possible to obtain satisfactory steam purity with water of lower quality, which simplifies and reduces the cost of water treatment.

The method of staged evaporation is that the volume of the drum is divided by transverse partitions into several compartments, each of which is connected to its own group of circulation circuits (evaporation stage). All feed water is supplied to the first compartment, the boiler water from which flows to the next compartment, then to the next, etc.

Staged evaporation allows increasing steam purity for a given feedwater quality and a given blowdown value. It also allows you to obtain satisfactory steam purity with water of lower quality, which simplifies and reduces the cost of water treatment

Salt balance equation

D pv C pv = D p C p + D pr S pr

(D p + D pr)S pv = D p S p + D pr S pr

S pr = ((D p + D pr)S pv + D p S p)/ D pr, if S p = 0, then

S pr = S kv = (D p + D pr) S pv / D pr

C kv = (100 + p) C pv / r, if p = 1%

S kv =(100 + 1)S pv / 1=101S pv

Salt balance equation for compartment 1

C kv1 = (100 + p) C pv / (n 2 + r), if p = 1%

S kv1 = (100 + 1) S pv / (20+1) = 4.8 S pv

Salt balance equation for compartment 2

C kv2 = (n 2 + p) C kv1 / r, if p = 1%

S kv2 =(20 + 1) S kv1 / 1 = 21 S kv1 =101S pv

38 Why is the staged evaporation scheme with a remote cyclone better than installing a partition inside the drum.

Staged evaporation consists in the fact that zones with different salt contents in the boiler water are created in the water volume of the boiler drum. This is achieved by dividing the water volume of the boiler drum with its heating surfaces into separate compartments. Continuous blowing is carried out from the compartment with the highest salt content, and steam extraction with the lowest. The upper drum is divided by a partition with a hole (overflow pipe) into two compartments - clean and salt. Feed water enters the clean compartment, and salt water is fed from the clean compartment through an overflow pipe. Approximately 80% of steam is formed in the clean compartment, 20% in the salt compartment. Consequently, 20% of the boiler water flows from the clean compartment into the salt compartment, which is a purge water for the clean compartment. Therefore, the clean compartment is purged without heat loss, ensuring a low salt content of the boiler water in it.

A significant drawback is the possibility of reverse flow of water into a clean compartment with “sluggish” circulation. To eliminate this drawback, stepwise evaporation with remote cyclones, which are salt compartments (DKVR-20), is used. When using remote cyclones as a separation volume, the difference in levels in the compartments can be selected sufficient to prevent the reverse flow of water. Therefore, schemes with remote cyclones are preferable, especially when the productivity of the salt compartment is low.

Feed water enters the drum, which serves as a clean compartment. Blowdown water from the drum enters the cyclones, for which this water is feed. The cyclone has a separate circulation circuit and releases steam into the boiler drum. The steam passes through the clean compartment separation device and is further purified. Continuous blowing is carried out only from the cyclone, if there is one. Staged evaporation reduces heat loss through blowing and improves steam quality

The efficiency of staged evaporation increases with the number of evaporation stages, but this increase fades with increasing number of stages. The most widespread are two- and three-stage schemes. In this case, the second stage of evaporation can be organized either inside the drum or outside it - in remote cyclones. In a three-stage design, usually the first and second stages are performed in a drum, and the third in a remote cyclone.

Staged evaporation allows increasing steam purity for a given feedwater quality and a given blowdown value. It also makes it possible to obtain satisfactory steam purity with water of lower quality, which simplifies and reduces the cost of water treatment. Staged evaporation also makes it possible to increase the efficiency of a steam turbine plant due to reduced blowdown without a noticeable decrease in steam quality.

The steam coming out of the boiler drums should not contain a significant amount of moisture, salts, or sludge, since part of the surface of the superheater will be the site of evaporation and precipitation of salts contained in the water, and the metal of the pipes may be damaged. Violations in the tightness of connections may occur, and when moisture surges, hydraulic shocks and even destruction of steam pipelines may occur.

The vapor may contain non-volatile and volatile substances. Non-volatile substances usually enter the steam from boiler water, in which they are dissolved or suspended. Their solubility in low pressure steam is low. Volatile substances - ammonia МН3, carbon dioxide С02, nitrogen N2 and hydrogen Н2 - are contained in the form of gases and do not form deposits. Carbon dioxide combines with calcium to form deposits. Ammonia, getting into heat exchangers with brass tubes, causes their dezincification and destruction; In addition, ammonia is toxic. Carbon dioxide can be corrosive; Iron oxides produce sludge and deposits on heated heating surfaces.

In this regard, steam is subject to certain requirements for the total salt content, converted to sodium: at pressures up to 1.4 MPa (14 kgf/cm2) - 1.0 mg/kg; up to 2.2 MPa (22 kgf/cm2) - 0.5 mg/kg and up to 4.5 MPa (45 kgf/cm2) -0.3 mg/kg. Consequently, as pressure increases, the requirements for steam quality become more stringent.

Contamination of steam with substances occurs mainly due to the removal of impurities contained in the feed and boiler water. To get steam required quality Feed water is purified in various ways and moisture is separated from steam by separation. An increase in steam humidity is facilitated by the incorrect mode of water supply to the drum - its over-drinking, sharp fluctuations in steam pressure, non-compliance with the requirements for the quality of feed water. In particular, an increase in its alkalinity, for example, leads to the formation and entrainment of foam due to a decrease in the volume of vapor space. If steam falls under the water level, then steam bubbles, emerging on the surface of the water - a mirror of evaporation, break the shell and form large and small droplets carried into the steam space.

When a steam-water mixture enters the steam space from the pipes, in addition to the formation of drops due to the rupture of the shells of steam bubbles, water jets strike the surface of the level, the walls of the drum and parts located in the volume.

An increase in the salt content of boiler water increases its surface tension, which leads to the phenomenon of swelling of the water with steam bubbles and an increase in its humidity. Increasing pressure in the drum worsens

Sedimentation of small droplets. The large diameter of the drum and the low location of the water level in it allow for a large height of steam space. Drops of moisture carried into the vapor space, having lost their initial speed and uniting along the way with other drops in a large volume, will fall out faster. The greater the actual height of the vapor space, the better, other things being equal, natural separation will occur. The best separation for normal evaporation surface loads in low- and medium-pressure boiler units is achieved at a height of 0.6-1.0 m, as a result of which the internal diameter of the drum is usually 1.2-1.6 m. In previous designs of low-pressure boilers, steam moisture was 3-6%; now it does not exceed 0.5% and decreases with increasing pressure to 0.1-0.2%.

An increase in the salt content in boiler water not only leads to swelling, but also upon reaching a certain value (critical) causes a sharp increase in moisture loss. Up to this salt content, moisture loss is approximately proportional to the salt content of the boiler water. Steam contamination volatile substances at low and medium steam pressures it is insignificant due to the low solubility of salts in steam.

To achieve natural steam separation, reduce droplet entrainment and obtain dry and pure steam It is important to uniformly distribute the steam output from the screen and boiler pipes along the length of the drum, to prevent water jets from hitting the walls and devices

A - pipe with holes; b - fender flaps; V<- отбойные щитки, жалюзийный сепаратор и дырчатый лист; г -утопленные листы, жалюзийный сепаратор с дырчатым листом; д - щитки, утопленный лист и жалюзийный сепаратор с дырчатым листом; е - внутрибарабанные циклоны, жалюзийный сепаратор и дырчатый лист (иногда циклоны размещены вие барабана - выносные

Cyclones).

In the drum, on the evaporation mirror and uniform loading of the drum. It is also necessary to ensure uniform steam selection along the length of the drum to obtain low steam velocities in the steam space of the drum, where the primary separation of moisture occurs. However, natural moisture separation is not enough to produce dry steam. Further moisture capture is carried out by mechanical separation in devices that use inertial forces, centrifugal forces, wetting and surface tension of the liquid layer. Such devices allow you to catch droplets of water removed from the steam space.

Schematic diagrams of separating devices in the drums of low- and medium-pressure catalytic units are shown in Fig. 4-6.

The simplest of them is a pipe in the steam space of the drum with holes of different diameters on the side generatrices, distributed unevenly along the length for better separation in the volume (diagram Fig. 4-6,a). The steam speed in the pipe (final) is taken to be 30-40 m/s, the speed in the holes is taken to be greater than the speed in the pipe. Instead of installing a pipe, you can separate part of the steam space with a sheet and make holes in it according to the same principle as in the pipe. With the proper height of the steam space, uniform supply of the steam-water mixture and steam extraction along the length of the drum, sometimes it is sufficient to install fender flaps (diagram 4-6,6).

To obtain better steam separation results, you can combine the installation of baffle panels with the installation of perforated sheets in front of the steam discharge pipes. Often, blinds are installed in front of this sheet, in which the steam, changing the direction of movement several times, causes water to deposit on the walls of the blinds by inertia. This diagram is shown in Fig. 4-6, c.

If the steam-water mixture enters the drum below the water level, then uniform distribution of steam can be achieved by installing a sheet with holes under the water level, and to purify the steam, supply feed water to this sheet.

At the top of the drum, as can be seen from the diagram in Fig. 4-6d, the devices can be kept the same as in the previous diagram. When introducing a steam-water mixture below and above the level, it is advisable to use the diagram in Fig. 4-6,d, add fender flaps, - diagram fig. 4-6,e) against the pipes through which the steam-water mixture enters the drum.

At high loads inside the drum, in order to obtain high-quality steam, cyclones are installed at the inlet of the steam-water mixture, in which, when the flow is twisted, the separated water flows down the walls, and the steam exits into the steam space through the blinds on the cyclone cover; a tray is placed under the cyclone to prevent steam from passing downwards. In front of the steam removal pipes from the drum, as can be seen from the diagram in Fig. 4-6, e, install a sheet with holes and blinds.

Since the quality of the steam leaving the drum depends on the salt content of the boiler water, the value of the salt content is limited by removing accumulated salts along with hot water, carrying out blowing.

If the removal of water and salts is carried out continuously, the blowing is called continuous. In the lower elements of the boiler unit - the lower screen collectors, in the lower drum - during operation and especially at low loads and during shutdown, sludge can accumulate. To remove it during kindling and reduced loads from low points
purging is carried out, which is called periodic or slurry.

Since not only water, but also heat is lost during blowing, the amount of blowing is limited.

A method that makes it possible to obtain high-quality steam with small blowing sizes, called stepwise evaporation, was proposed in the USSR by prof. E.I. Romm in 1937 and became widespread. The essence of this method is to separate the heating surfaces, collectors and drums into parts in which gradual evaporation of water occurs. Feedwater is supplied to the first part, called the clean compartment, which produces 80-85% steam; it maintains a certain and low salt content of the boiler water due to increased blowing into the secondary part - the salty compartment. The steam from the clean compartment will be of satisfactory quality, and the boiler water in the salty compartment will have a higher salt content, which will reduce the blowdown size. The steam from the salty compartment will be of low quality and will require good cleaning, but it will not be much - 15-20%; therefore, the overall quality of the heat produced by the boiler unit will be satisfactory. Typically, staged evaporation is carried out in two, less often - three stages [L. 15].

In the circuit diagram shown in Fig. 4-7, three-stage evaporation of boiler water in a boiler unit having a boiler bank (I stage of evaporation) is shown; festoon and rear screen (II stage) and side screens (III stage of evaporation), the steam from which enters the cyclone separator removed from the drum, and from the latter goes into the drum. The productivity of stage I is p-70%, stage II - p2- = 20% and stage III pz = 10% of the total productivity of the boiler unit.

Salt balance equation for a boiler unit with three-stage evaporation at a feedwater salt content of 5P. B, the water in the clean compartment 5b salty compartment 52 And the cyclone 5pr at the purge value p will have the form:

(100 R) 5P. in = (pg + p2 4- p) 51 = (p3 -|~ p) (4-20)

From this equation you can find the blowdown percentage and salt content of boiler water in each compartment.

Boiler unit purge, %, will be:

O ___ ^P. B (100 RUR)

1 pn + pg + p %

In the second stage of evaporation the same

In stage III and purge

O _____ ^n. in (100 H~ /O

The presence of three stages of evaporation with a blowdown of 5%, even with a salt content of feed water of 500 mg/kg, allows for salt content in the blowdown

Zsch, = 500 (1^° + 5) - = 10,500 mg/kg.

From the diagram in Fig. 4-7 and formula (4-20) the effectiveness of using stepwise evaporation is visible, especially with an increased salt content in the feed water.

Due to the possible formation of deposits on heating surfaces, more than three stages of evaporation are not performed. In the USSR, all boiler plants produce boiler units with staged evaporation.