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Selection of lining refractory under different construction conditions of chimney

Words:[Big][Medium][Small] Mobile Page Two-Dimensional Code 2018-10-15     

Brief introduction of lining selection for chimney construction

Cl-80 castable is adopted. Firstly, due to the technical limitations of chimney construction and investment control requirements, it cannot cooperate well with chimney sliding form construction, and the construction period is long and the investment is high.Second, due to the climate and engineering characteristics, there are difficult to solve the problem of realkali spalling.Therefore, although there are successful cases of chimney construction with cl-80 castable, it is not suitable for this project.Although acid-resistant brick is not as economical as cl-80 castable in structural design, its chimney construction technology is feasible in this project, and it can meet the requirements of construction period and save the total investment. Therefore, acid-resistant brick is finally selected as chimney lining in this project.

It can be seen that the selection of chimney lining and the influence of chimney construction conditions are quite important.

1. Project overview

Traditional chimneys of this kind of engineering are often lined with acid-resistant bricks, and a new type of cl-80 castable is proposed to be used in this project.The characteristics of cl-80 castable are small bulk density (1/3 of acid-resistant bricks), which can reduce the amount of concrete and foundation steel in chimney wall to a certain extent.Low thermal conductivity (castable 0112W/mk, acid-resistant brick 019W/mk), no heat insulation layer is needed.Generally speaking, it is more economical than acid-resistant brick in design, and the performance of castable is similar to that of concrete. Through the combination of embedded steel bar and concrete, the overall mechanical performance is better.See table 1 for details.

2. Lining selection

Traditional chimneys of this kind of engineering are often lined with acid-resistant bricks, and a new type of cl-80 castable is proposed to be used in this project.The characteristics of cl-80 castable are small bulk density (1/3 of acid-resistant bricks), which can reduce the amount of concrete and foundation steel in chimney wall to a certain extent.Low thermal conductivity (castable 0112W/mk, acid-resistant brick 019W/mk), no heat insulation layer is needed.Generally speaking, it is more economical than acid-resistant brick in design, and the performance of castable is similar to that of concrete. Through the combination of embedded steel bar and concrete, the overall mechanical performance is better.See table 1 for details.

3. Inconsistency between lining selection and construction conditions

From the perspective of design, cl-80 castable is more economical and reasonable than acid-resistant bricks. However, when discussing the specific construction scheme, it is found that there are two major problems in using cl-80 castable:

1) it is difficult to reconcile construction technology, construction period and lining design. Due to the tight construction period requirements, the construction of chimney barrel must start from the end of February to the end of April, so as to make room for safe construction work for surrounding structures.In addition, the physical and chemical properties of cl-80 castable material are similar to that of concrete, which is suitable for pouring. The construction unit initially chooses the scheme of double sliding form, which can strive for almost synchronous construction of outer cylinder and inner lining and save time.However, there are irreconcilable contradictions between the construction technology of double sliding form and the detailed design of cl-80 castable, mainly including:

A. Synchronous double slide molds require a minimum gap of about 5cm between the lining and the cylinder wall (usually separated by a heat insulation layer, as shown in figure 2) in order to install the separator material and avoid mutual infiltration of concrete and castable.However, there is no insulation layer between the liner of cl-80 castable and the chimney barrel wall. The castable is combined with concrete through the embedded steel bar in the cylinder wall, and the two materials contact directly (FIG. 1). Although the overall mechanical performance is good, it is impossible to achieve synchronous double sliding mode.

B. If synchronous double sliding is not carried out, the scheme of staggered 015m sliding mode of tube wall and lining is adopted.As the best time for concrete cylinder wall to slide is about 015MPa concrete strength, slump is about 5~6cm, according to the local spring weather conditions, the estimated speed of sliding is about 215m per day, that is, 1m/8~9h.And CL - 80 castable in 24 h rear can release, at the same time to avoid mutual permeability with concrete, must reach a certain intensity in concrete construction after the same level position of the castable, both in the construction time is difficult to cooperate, so are compared.in casting must wait after a period of CL - 80 demoulding, time limit for a project will extend the three times, are not suitable for the project construction period.

C. In the case that the double-sliding mode scheme is not feasible, whether the single-sliding mode scheme can be adopted or not, further discussion finds that there are still various problems in the single-sliding mode using cl-80 castable.

The single slide scheme is to finish the construction of the tube wall first, and then to carry out the construction of the lining after there is enough construction work surface.If the lining is constructed by supporting formwork, the internal formwork system is complex, and the support and operation are difficult.After further market investigation, it was found that many construction cases of this castable in the north of the Yangtze river adopted spraying or daubing scheme.This scheme is: the main structure construction, and then use mechanical spraying or manual daub.For tall structures like chimneys, manual daub is not only required for working platform, but also for personnel quality, quality assurance system and construction speed.In addition to the above problems, mechanical spraying will also produce a large loss of materials, which is not economically feasible.

Therefore, the above two methods are rarely used in tall structures such as chimneys.

2) the physical properties and chemical composition of castable are similar to that of concrete, and the alkali-reducing problems of concrete under specific conditions also exist in castable.

The local spring and summer are humid and hot. During the construction of the chimney in this case, there will be a period of re-damp weather in March, which is quite different from the dry climate in the north. Due to the lack of air circulation, moisture accumulates indoors, and the air humidity reaches 95% or so.

After the construction of alkaline building materials in the indoor part due to high air humidity, water can not be discharged in time, if exposed to the air, will produce a series of chemical reactions (such as cement mortar prepres in such conditions often occur alkali phenomenon).And exposed outdoor parts due to air circulation, water vapor will not accumulate, but rarely occur.Because the physical and chemical properties of castable are similar to those of cement, it is likely that the problem of alkalification will occur.

The concrete manifestations of alkalification phenomenon are: without dry cement and other pouring bodies under long-term hot and humid environment conditions, the surface will peel loose, long-term untreated, the reaction will gradually deepen, eventually leading to layer by layer peeling.Main reasons for this phenomenon is: the casting body is porous structure determines the casting body has certain moisture content, some of the alkali metals and alkaline earth metal material will gradually into hydroxyl, under the influence of water when the water in the casting body big enough, under the effect of capillary pressure, the water along the wool stoma can rise up to 10 cm, the free water in these hydroxide as the casting body also gradually migrate to the surface of the casting body, arrived at the surface, it is exposed to air react to generate expansion of carbonate, the casting body surface peeling off osteoporosis.Taking concrete as an example, the main chemical reaction equation is as follows: hydroxides migrate to the surface of the pouring body and contact with air to form water-insoluble carbonates:

Ca (OH) 2 + CO2 = CaCO3 + H2O

Cl-80 is similar to cement in physical and chemical properties. During pouring, it has high water content and great fluidity. After pouring, it covers the concrete cylinder and is directly exposed to the air inside the cylinder.In practice, the manufacturer also found that after the completion of refractory castable casting, in the hot and humid climate in the south can not be timely baking surface dealkali, if not timely treatment, will gradually deepen and produce layer by layer peeling phenomenon, and the internal gradually crisp powder.The phenomenon and reason of caustic soda of castable is consistent with that of cement pouring body. The key lies in the existence of free water. To solve this problem, it is necessary to drain out the free water in time and form stable compounds before the alkali metal and alkali earth metal substances are converted into hydroxides when they meet water, so as to prevent the occurrence of caustic soda.

If the above problems are to be solved, it is necessary to bake the pouring body in time and discharge the free water in the pouring body, so that the alkali metal and alkali earth metal substances will react with each other to form stable compounds before they are converted into hydroxides and migrate, and thus prevent the occurrence of alkalification.But because the project first boilers have requirement of ignition until the end of April, the next year, distance chimneys castable construction over a year, that is to say after the casting a year time to available boiler baking, to undergo two get damp, FanJian phenomenon inevitably occur, will cause irreparable economic loss (loss of 1.8 million yuan materials alone), but also technically difficult to repair, replace lining.If prebaking is carried out in other ways (for example, wood, coal or coke is used for lighting and baking from the bottom of the chimney after pouring as proposed by the manufacturer, small fire at the bottom of the chimney (150~200℃)&ge is adopted in sections.Medium fire at the bottom of chimney (250~350℃)≥Fire at the bottom of chimney (150~200℃)≥36h. It is difficult to implement technically, difficult to control the effect, and difficult to control the cost. Therefore, it is not feasible.

On the contrary, if acid-resistant brick is used as lining, single sliding mode scheme is adopted for the construction of the cylinder, with an average of 2~3m/d and 60d to complete the sliding mode.After the completion of the tube slide form, the lining masonry will not affect the construction of surrounding structures.Acid-resistant brick will not occur the phenomenon of material alkalinization, masonry using the movable suspension plate, suspension plate Φ48 & times;The 315 pipe is assembled, which is 350mm smaller than the liner diameter at +10.00m. Six hand-lever hoist (3t) are used as lifting system for lifting and lowering of the platform. A 1000-times is set in the center of the platform.800 holes are used as the loading port of lining material, and the board is used to block up the loading, which solves the construction technology and time limit of chimney barrel and lining.Therefore, using acid-resistant brick as lining conforms to the chimney construction conditions of this project.



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