Discussion on the key points of risk control and s

2022-08-17
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Talking about the grasp of the key points of risk control and safety evaluation of cement plant

in the building materials industry, the cement industry is not a high-risk industry, and its production process is relatively safe. Especially after the emergence of the new dry method (preheating and pre decomposition) cement clinker calcination process in the early 1960s, due to its advantages of large production scale, low heat consumption and high clinker quality, it has achieved rapid development all over the world and has a history of nearly 50 years. At present, China has more than 1000 new dry process cement clinker production lines of various types/scales, which have become the mainstream production process in the cement industry, and their production safety has been proved by practice. 2. The position and direction of sample cutting, but how to accurately grasp its risk control points and key points of safety evaluation in the work of hazard prediction and safety evaluation really needs to be paid great attention by colleagues in the industry

I. key points of safety evaluation of cement plant grasp the key points of safety evaluation of cement plant lie in the following three unique production links and facilities of cement plant (including personal safety and equipment safety)

⑴ clinker burning system: it belongs to high-temperature thermal system, and the calcination temperature is up to 1450 ℃. It mainly includes three parts, namely, preheating and pre decomposition system (kiln tail), rotary kiln and grate cooler. The working characteristics of the firing system are: ① in a high temperature state, ② continuous production line. High temperature increases the failure rate of equipment (besides mechanical failure, there is also thermal failure). After the equipment fails due to assembly line production, its upstream and downstream equipment, and even the whole process production line, have to stop, causing huge shutdown losses

⑵ pulverized coal preparation system: pulverized coal is inflammable and explosive and must be treated seriously. Including raw coal homogenization and storage, coal mill, pulverized coal storage and pneumatic conveying

⑶ waste heat power generation system: it mainly includes waste heat boiler, steam turbine, generator and other equipment, similar to a small thermal power station. The waste heat boiler and steam turbine are special equipment with high temperature (310 ~ 400 ℃) and high pressure (generally 1.35 MPa)

other production processes and electromechanical facilities, such as crushing, transportation and storage of raw materials, minerals and mixtures, grinding of raw materials and cement, cement packaging, as well as auxiliary production facilities such as water supply, power supply, machine maintenance and material transportation, are similar to those of general industrial enterprises, and their requirements for the analysis of dangerous and harmful factors and prevention and control measures are similar

II. Grasp the key points of risk control in cement plant

(I) main accidents and preventive measures in the firing system of cement plant

the firing system of cement plant mainly includes three parts, namely, preheating and pre decomposition system (kiln tail), rotary kiln and grate cooler

1, that is, the main process accident of preheating and precalcining system (kiln tail) is the scaling and blockage of preheater. Once the blockage occurs, it is very dangerous to deal with it, because it is very dangerous to dredge the blockage manually from the stabbing hole (hit with a high-pressure water gun). Once it is poked, the blocking material on it collapses. This high-temperature material flow has strong fluidity, and it will gush out of the stabbing hole, and even spray out of the kiln head through the rotary kiln, causing casualties

the main cause of scaling and blockage is the "alkali cycle" of volatile harmful elements such as potassium, sodium, chlorine and sulfur in raw materials and fuels between the kiln and preheaters at all levels. In the production process, these harmful elements volatilize in the high-temperature area of the kiln and condense in a low-temperature part of the preheater system, or when the combustibility of materials is poor, the calcination temperature is too high, or there is incomplete combustion in the kiln and a reducing atmosphere occurs, these elements will be recycled and enriched, adhere to a relatively stable lining wall of the preheater system, and form thicker and thicker crusts, If these crusts are not treated in time, it will lead to the blockage of the preheater system. The direct consequence of blockage is to reduce the effective section at the preheater outlet, increase the system resistance, and reduce the total air volume, thus affecting the output of the kiln system and even interrupting production. Serious blockage will even cause the raw material to block the feeding mouth continuously, forming a "sausage filling" phenomenon. This dozens of tons of additional accident load will seriously threaten the structural safety of the kiln tail tower

The main preventive measures for the accident of preheating and precalcining system include:

① strictly control the content of harmful components in raw materials. Among them, limestone ore requirements: R2O (k2o+na2 as for rough fracture zone o) ≤ 0.6 ~ 0.8%, SO3 ≤ 1%, cl- ≤ 0.03%; MgO ≤ 3.0%, R2O ≤ 2%, SO3 ≤ 2% in other correction raw materials; R2O in raw meal ≤ 1.0%; After homogenization, the total sulfur content of coal is ≤ 3.0%, and the molar ratio of sulfur to alkali in the kiln is ≤ 1.0 (generally 0.6 ~ 0.8). This requires the homogenization/matching mining of limestone mines, and is also one of the constraints on the comprehensive utilization of the interlayer and stripping of the mine

② the firing system should maintain a stable thermal system to reduce the amount of alkali circulation. If necessary, technical measures such as by-pass wind protection and kiln ash discharge (that is, the kiln dust discharged by the dust collector at the end of the kiln will no longer enter the kiln) can be taken

③ strengthen sealing measures to prevent cold air from leaking in. Since the hot materials in the kiln tail and low-level preheater are particularly easy to condense into crusts in cold places, especially near the air leakage point, every effort should be made to prevent the existence of air leakage points at the seal of the kiln feeding end, the preheater discharge valve and the low-level preheater. In addition, the leakage of cold air will also cause heat loss and increase the energy consumption of the system

④ set up an air cannon at the unloading cone that is easy to be skinned, and blow and clean it in turn at a certain frequency to prevent the further development of skinning

2. Main process accidents of rotary kiln:

⑴ power failure accident. The accident example of Hunan Xinhua cement plant: lightning caused power failure and kiln shutdown, and the subsequent heavy rain caused serious warping and deformation of the kiln body, cracking of the kiln lining, scrapping of the kiln, and top damage to the kiln seal, forcing the whole plant to stop production. The lesson is that the lightning protection design of the general step-down station in the plant must have high standards; After the accident power failure, the emergency power supply (self provided diesel power station) must be started in time within 5 ~ 10 minutes to provide security power, so that the auxiliary transmission of the rotary kiln can carry out the slow pan kiln and prevent the kiln body from deformation. At present, the auxiliary drive of the kiln, high-temperature fan at the kiln tail, grate cooler fan, central control room (CCR), fire pump, etc. have been generally listed as the first-class load in the design of the cement plant, and the dual circuit power supply has been implemented to effectively ensure the stability of the power supply

⑵ "red kiln" accident. During normal production, the surface temperature of the kiln carcass shall not exceed 250 ~ 320 ℃ (firing section), which is the normal working range that the heat-resistant steel can withstand (referring to strength and stiffness). However, if the refractory bricks in the kiln fall off locally or become thinner due to serious wear, the high-temperature clinker (1300 ~ 1450 ℃) will approach or even directly contact the steel plate of the kiln shell, resulting in a "red kiln" accident (dark red, about 400 ~ 600 ℃), which will soften the steel plate and, in serious cases, deform the kiln body and fail to operate normally. Due to the extremely poor working conditions of the refractory bricks of the rotary kiln, they have been subjected to high-temperature thermal corrosion for a long time, chemical corrosion caused by the reaction of ingredients, and fatigue damage caused by alternating mechanical stress and thermal stress when China is still in the deep development stage of industrialization, informatization, urbanization, marketization and internationalization when the kiln rotates. The superposition of these three effects, together with the possible quality problems of the refractory bricks themselves or the quality problems of masonry and installation, The phenomenon of wear and tear and brick falling is difficult to avoid in the production process. The preventive measures are:

① use an infrared scanner to continuously monitor the surface temperature change of the kiln carcass in real time, automatically record the temperature curve of the kiln carcass, and deal with it immediately when an abnormal temperature alarm is found, stop feeding in time, and prevent further large-area collapse of refractory bricks

② strictly implement the pre inspection and repair system, and timely and regularly replace the kiln lining (refractory bricks, castables) with serious corrosion to prevent trouble

once the kiln shutdown accident of large-scale brick collapse occurs, it will cause huge shutdown losses of the enterprise. It will take 10 to 15 days to deal with the accident, and its output value loss is as high as tens of millions of yuan, while the cost of planned pre inspection and repair is much lower

(II) main accidents and preventive measures of pulverized coal preparation system

as pulverized coal is flammable and explosive, long-term accumulation will cause spontaneous combustion, so the preparation, transportation and storage system of pulverized coal is another hazard source of cement plant. Necessary safety facilities must be provided to prevent fire and explosion accidents. Generally, the safety facilities of pulverized coal system shall be:

⑴ CO2 automatic fire extinguishing system (nitrogen can also be used): CO automatic analyzer and temperature measurement device shall be set in the dust collector and pulverized coal bunker. The liquid CO2 gas is stored in the steel cylinder and automatically sprayed under the following conditions:

① alarm when the CO concentration at the outlet of the dust collector reaches 1.5%, and automatic spraying when it exceeds 2%

② alarm when the temperature of dust collector ash hopper and pulverized coal bunker reaches 70 ~ 90 ℃, and spray automatically when it exceeds 90 ℃. This system requires qualified units to design, manufacture and install. CO2 gas storage cylinders and their automatic sprinkler devices belong to pressure vessel equipment

⑵ the coal mill dust collector shall adopt a special explosion-proof bag dust collector, and the pulverized coal bunker, powder concentrator and dust collector shall be equipped with explosion-proof valves to ensure timely pressure relief in case of accidents

⑶ the fluidity of pulverized coal shall be maintained in the pulverized coal bunker, and it is not allowed to overstock and store pulverized coal for a long time (because there is the possibility of spontaneous combustion). During equipment maintenance, the pulverized coal in the silo shall be emptied. Accident example: an explosion accident occurred during the welding repair of the pulverized coal bunker in a cement plant. The lesson is that the pulverized coal in the bunker must be emptied and fully ventilated before the welding repair to reduce the concentration of coal dust

(III) main accidents and preventive measures of waste heat power generation system

waste heat power generation devices in cement plants include waste heat boilers and their piping systems at the kiln head and kiln tail, steam turbines and their steam and cooling water circulation systems, chemical water treatment systems, generators and their electrical control systems, which are similar to a small thermal power station. The waste heat boiler and steam turbine are special equipment with high temperature (320 ~ 400 ℃) and high pressure (1.25 ~ 1.5MPa)

its characteristic is that the cement plant takes cement production as the main task, and the waste heat power generation system is its auxiliary device. The purpose is to comprehensively utilize the waste heat generated in the clinker calcination process as secondary energy for energy conservation and emission reduction, which is different from the production purpose of thermal power stations in nature. Therefore, the waste heat power generation of cement plant is bound to be subject to the cement production line, which must operate on the premise of ensuring the quality, output and comprehensive energy consumption of cement clinker. When the waste heat of the kiln system fluctuates, it is natural to fluctuate the capacity of the power generation system (which is not allowed for thermal power stations). In addition, the electric energy produced by the waste heat power generation system is not used by the plant, so the requirements for its power supply quality (referring to the stability of electrical parameters such as voltage, current, frequency and phase) are not as strict as those of thermal power stations. However, waste heat power generation needs to be connected with external power supply and supply power to the plant, and there are many large-scale high-voltage variable-frequency speed motor devices in the cement plant, so the power supply quality of self generated power should be controlled

from the perspective of safety, the waste heat power generation system should meet the following requirements:

⑴ its installation accuracy, thermal expansion gap reservation, pipeline welding, pressure test, mandatory inspection and inspection items, system commissioning and other aspects, in addition to the implementation of power

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