Producing clean steam and regulating steam temperature. Great encyclopedia of oil and gas

§ 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

One of the most effective methods Reducing heat loss from boiler water with continuous blowdown and obtaining cleaner steam is step evaporation.

Figure 2.21 – Staged evaporation

It lies 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. With staged evaporation, heat loss from blowing is reduced and the quality of steam is improved.

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.

Figure 2.22 – Scheme of three-stage evaporation with an external third stage:

1 – boiler drum; 2 – lower collector; 3 – downpipe; 4 – riser pipe; 5 – feed water supply; 6 – removal (purging) of part of the water from the circulation circuit; 7 – saturated steam removal; 8 – remote cyclone; 9, 10 – lowering and steam-generating pipes of the salt compartment circuit; 11 – discharge of steam-water mixture into the cyclone; 12, 13 – water and steam transfer pipes; 14 – periodic purging.

2.2.9 The influence of internal deposits on the efficiency and reliability of the boiler system

Reliable and economical operation of the boiler depends on the quality of the water used to feed the boilers.

Sources of water supply for boilers can be ponds, rivers, lakes, groundwater or city water supply. Natural waters contain impurities in the form of dissolved salts and mechanical impurities, therefore they are unsuitable for feeding steam boilers without preliminary cleaning.

Water quality is characterized by the following indicators:

1. Dry residue contains the total amount of substances dissolved in water (calcium, magnesium, sodium, iron, aluminum, etc.) that remain after evaporating the water and drying the residue at 110 o C. The dry residue is expressed in milligrams (impurities) per kilogram (of water) .

2. Hardness of water characterized by the total content of calcium and magnesium salts in water, which are scale formers. There are general, temporary (carbonate) and permanent (non-carbonate). Water hardness is expressed in milligram equivalent per 1 kg of solution (mg-eq/kg).

3. Water alkalinity characterized by the content of alkaline compounds in it (hydrates, for example sodium hydroxide NaOH, carbonates Na 2 CO 3 - soda ash, bicarbonates). There are hydrate, carbonate and bicarbonate alkalinity.

4. Oxidability characterized by the presence of oxygen and carbon dioxide in water, expressed in milligrams per kilogram.

Some impurities always remain in the feed water entering the boiler.

In the process of generating steam and removing it from the boiler, as well as the entry of new portions of feed water into the boiler, the amount of salts in the boiler water increases, since dry steam does not dissolve them.

When the salt content increases above the norm, they begin to precipitate and form scale on the heating surface and sludge in the water column, foaming will appear and the entrainment of boiler water with dissolved salts by steam will increase. This will lead to salt contamination of the steam line.

To obtain steam of the required quality, boiler water is treated with special reagents that cause scale-forming salts to precipitate in the boiler in the form of sludge, which is easily removed by blowing.

In addition, the reliability of the boiler installation is affected by metal corrosion. Corrosion of the boiler walls can occur from exposure to oxygen, carbon dioxide, hydrogen, etc. dissolved in the feed water.

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 stage 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

The method of stepwise evaporation, the essence of which boils down to the creation in a steam generator of evaporation circuits with different concentrations of impurities when they are sequentially fed with water, was proposed in 1936 by E.I. Romm. To do this, the circulation circuit is artificially divided into two or three parts, called compartments or evaporation stages.

More than three stages of evaporation are not used, since this significantly complicates the design of the steam generator, and the resulting gain in steam purity decreases as the number of stages increases. For energy boilers, the number of evaporation stages is usually two.

The first compartment contains the main group of circulation circuits, which is connected to part or all of the drum. A small group of circulation circuits is allocated to the second compartment, which is connected either to part of the drum or to external cyclones. The vapor-generating surfaces of the individual compartments do not communicate with each other. Feed water enters the first (clean) compartment. The second (salt) compartment is fed with boiler water from the first compartment. The steam generator is always purged from the last compartment. Water flows from one compartment to another through special water transfer pipes. This mode is called the internal purging mode. The flow of water through water bypass pipes in the desired direction is ensured by the difference in water levels on both sides of the partition in the drum.

The total steam output of a steam generator with staged evaporation is the sum of the steam output of all compartments.

The figure below shows a diagram of two-stage evaporation without remote cyclones.

The water volume of the drum is divided into two compartments by a partition slightly protruding above the water level. At the bottom of the partition there is a short water overflow pipe. The circulation circuits connected to the clean and salt compartments are separated from each other by a blind partition in the lower manifold.

The performance of the salt compartment depends on the design features of the boiler and also to a greater extent on the quality of feed water. For medium-pressure drum boilers, make-up water is prepared according to softening schemes and the clean compartment includes about 65% of the drum volume and all side screens. If the steam-water circuit is fed with desalted water and the feed water has a very low salt content, the productivity of the salt compartment does not exceed 3-5%.

Due to the increased concentration of salts in the boiler water of the salt compartments, screen surfaces with relatively low thermal stress are allocated to the circulation circuit of these compartments (as a rule, these are pipes of the side screens).

The multiplicity of salt concentration between compartments (a value characterizing the efficiency of stepwise evaporation) should be no more than 10. At higher multiplicity values, conditions are created for scale formation in the screen pipes of the salt compartment.

The resistance of the bypass pipe from the clean compartment to the salt compartment should be small so that the levels in them differ little. But the low resistance of this pipe contributes to periodic pushes of water in the opposite direction. This so-called transfer of water from the salt compartment to the clean one increases the salt content of the water in the clean compartment, which in turn leads to a deterioration in the quality of steam, with all the ensuing negative consequences.

For high and ultra-high pressure boilers, make-up water is prepared using desalting schemes, which has made it possible to improve the quality of boiler water and switch to organizing salt compartments in remote cyclones.

Modern heating and hot water supply systems for enterprises require considerable investment. Therefore, in our time, you need to choose an equipment supplier whose prices for boiler rooms will be lower than others.

When the last stage of evaporation is performed with external cyclones, the overflow is eliminated almost completely. The ability to have a large difference in levels in the drum and the cyclone allows you to make a water transfer pipe with greater resistance.

The use of scheme (b) makes it possible to obtain steam of higher quality by washing the steam of the salt compartment with water from a clean compartment.

The idea of ​​staged evaporation, which played a positive role for medium-pressure drum boilers, exhausted itself in the 60s of the last century during the development of high- and then ultra-high-pressure boilers.

The fact is that at first it was used only in cases where it was not possible to ensure the salinity content of the boiler water below the critical value due to a reasonable amount of blowdown. Staged evaporation was considered as a way to reduce the blowdown (internal blowdown). However, it soon began to fulfill its main function - improving the quality of steam, but this was effective only for medium and high pressure boilers.