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Reasons for Cracks in Boiler Casting Light Burning Powder

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Reasons for Cracks in Boiler Casting Light Burning Powder

Date of release:2018-12-27 Author: Click:

It is a dangerous phenomenon that cracks occur after pouring material of light powder fired boiler is built. This situation must be dealt with in time to avoid accidents. The causes of cracks are as follows:

First, the construction technology is unreasonable

(1) The "ash-water ratio" of castables is not well controlled during mixing. Excessive water addition, high porosity, low strength, long natural solidification time and long curing time of castable after moulding. Too little water is added, material fluidity is not good, vibration is not compact, easy to leave pores, caves, etc., strength will be greatly reduced.

(2) The mixing time and vibration time are not well controlled. The mixing time is too short, the material mixing is not uniform, the density is not compact, and the strength decreases. The castable should be poured by layered vibrating rammer. If the vibration time is too long, the material is easy to be delaminated, the fine powder floats on the surface, and the aggregate sinks at the bottom, which leads to the decrease of material strength and easy peeling. Castables should be used up within 30 minutes after mixing. It is advisable to pour the castables once to the specified thickness and height. Castable construction should be carried out under the environment above 5 C, because the temperature is too low, the material is not easy to solidify, even solidification, it is also a pseudo-solidification phenomenon, castable should generally be continuously poured, before the first layer of castable initial solidification, the next layer of castable should be poured. If the construction gap exceeds its initial setting, it should be treated according to the requirements of construction joints.

(3) The demoulding time is not well controlled. The castable is demoulded before hardening. Only when the strength of castable can ensure that its edges and corners are not damaged by demoulding can the castable be demolished. The load-bearing formwork shall not be dismantled until the castable reaches 70% of its strength. In order to facilitate demoulding, a layer of oil should be coated on the surface of all moulds before pouring.

(4) The curing time of castable is not well controlled. After the boiler is built, there must be enough natural drying period, so that most of the water in the refractory layer can be precipitated, so as to prevent a large amount of water from being discharged in time and make the refractory layer burst and fall off.

(5) Baking quality control is not good. Drying oven is to remove free water and crystalline water which can not be excluded by natural drying of lining materials, and curing castables at high temperature to achieve a certain strength.

Before drying, the lining material must have a certain natural strength (enough curing period) in order to carry out. The principle of oven drying is "long, not short, slow, not fast". The heating rate should be uniform and stable, the constant temperature time and temperature should be controlled well, and the fluctuation of temperature should not be more than (+20 C). To prevent excessive heating, water can not be discharged in time, breaking through the wear-resistant and refractory layer; secondly, to prevent the temperature gradient in the fire-resistant and wear-resistant layer from being too large, resulting in huge thermal stress, so that the wear-resistant and refractory layer cracks, bulges and falls off.

It is also an important factor that the number of exhaust holes in the steel shell outside the wear-resistant refractory layer (such as the return leg and the cone of cyclone separator) is too small.

2. unreasonable structure design and poor castable quality

2.1 unreasonable design of fireproof and anti-wear structure of light burning powder

(1) The design of expansion joint of refractory and wear-resistant material is unreasonable. The design quantity of circumferential and longitudinal expansion joints is insufficient. After the operation of the boiler, wear-resistant refractories are heated, expanded and extruded, resulting in cracks. The expansion joints of wear-resistant refractory castables and refractory bricks in the inlet flue of cyclone separator were changed from 5 mm to 10 mm, and sufficient clearance was reserved to meet the expansion requirements. Expansion joint is changed to Z shape to prevent boiler ash from penetrating and scouring insulation layer. Expansion joint filling material requires calf skin layer fireproof fiber felt on both sides to prevent fireproof slurry from being immersed in filling material. The design interval of circumferential expansion joint was changed from 1500 mm to 1000 mm. In large areas such as the outlet flue of separator and the cone of separator, the pouring surface is changed from (2.5 x 2.5 m) to small area (1.5 x 1.5 m) by one-time forming and masonry, and expansion joints are set at the same time. The width of expansion joints is less than 3 mm. The filling material requires rigid plywood to prevent deformation during vibration. After boiler operation, plywood burns down at high temperature and space becomes expansion joint.

(2) The cyclone separator cylinder is equipped with brick-lined bracket along the height direction to achieve the purpose of layered unloading of brick wall. By calculating the actual weight of refractory bricks in the cylinder, the design of one-layer bracket can not achieve the purpose of unloading, and the bracket is replaced by two-layer arrangement according to the actual situation in the field. The bracket of wear-resistant and refractory castable for the riser of the reclaimer was changed from three to four layers, and the spacing was reduced to 2.5m, so that the castable could be unloaded in layers.

(3) The shape of Y-type screw in the boiler needs to be improved, and the material should be heat-resistant. The optimum angle is 60-80 degrees. Only by ensuring the top shape of the screw can the screw grasp the castable more effectively.

Grab nails, air distributor, slag discharge pipe and air duct are metal materials. The expansion coefficient after heating is much larger than that of refractory. If not pretreated, the interface between metal and refractory will form net cracks, leading to cracking and falling off. Refractories contact metal surfaces to remove oil and ash stains and evenly coat with asphalt more than 1 mm. To ensure the asphalt concentration and coating thickness, we must resolutely stop using asphalt paint instead of asphalt oil.

2.2 The quality of refractory and wear-resistant materials is not up to standard

(1) Matching and bonding between aggregate and matrix of wear-resistant materials have important influence on wear resistance of materials. The mismatch between aggregate and matrix will lead to cracks in the material body after forming, and the poor bonding between aggregate and matrix will lead to the matrix being eroded first, then the aggregate will be isolated, and then fall off, even if the aggregate has poor bonding with the matrix.

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