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Explosion proof valve in cement factory

Date:2019-06-20 16:50     writer:cement editor     Views:

1 Introduction


In the cement factory, the pulverized coal preparation workshop is the most vulnerable workshop for safety accidents. Due to the characteristics of pulverized coal, the pulverized coal workshop is prone to accidents of pulverized coal combustion and explosion. In order to safely produce pulverized coal workshop, various measures need to be taken in design and operation, for example, in system configuration: in each closed device An explosion-proof valve is installed on the upper part; the temperature and CO concentration in each closed device are monitored, and the alarm is timely; the system is equipped with an automatic fire extinguishing system for carbon dioxide or nitrogen.

This paper mainly discusses the selection of explosion-proof valves in the pulverized coal preparation workshop of cement factory, the calculation and arrangement of pressure relief area, and the precautions for the design and use of explosion-proof valves.


2 Explosion-proof valve selection


Explosion-proof valve, also known as pressure relief valve and safety valve, can be automatically opened according to the working pressure of the system. It is usually installed on airtight equipment or pipelines. When an abnormal situation such as explosion occurs inside the equipment, the internal gas pressure acts on the explosion-proof valve. Explosion-proof valve discs bounce or break, draining the internal pressure of the equipment due to explosions, and protecting equipment and piping.

Explosion-proof valves commonly used in cement plants can be divided into two categories, one is a diaphragm type explosion-proof valve, and the other is an automatic opening and closing type explosion-proof valve.

When the diaphragm type explosion-proof valve explodes inside the device, the diaphragm ruptures and releases internal pressure. The explosion-proof valve of this type has the advantages of simple structure and low cost; however, the disadvantage is that after the diaphragm is broken, the outside air easily enters the container to cause a secondary explosion. The diaphragm of this type of explosion-proof valve is mostly aluminum plate or stainless steel plate, and the thickness is more than 1mm. The one side of the film plate is engraved with a cross-shaped indentation in the middle of the film to facilitate the explosion inside the device. When installed, the notched side should face outward.

When the automatic opening and closing type explosion-proof valve explodes inside the equipment, the diaphragm bounces and releases the internal pressure. After the explosion is over, the diaphragm is closed by gravity or spring force. The advantage of this explosion-proof valve is that it can minimize the damage to the equipment and the damage to the workers, and can quickly resume normal production. The disadvantage is that this structure is prone to air leakage and requires reasonable design and careful maintenance at a later stage.

When conditions permit, it is recommended to select an automatic opening and closing type explosion-proof valve as much as possible. In the explosion accident of the coal grinding system, many of them are due to the selection of the diaphragm type explosion-proof valve, which causes the explosion accident to be more serious.

3 Calculation of pressure relief area of ​​explosion-proof valve


(1) Effective area of ​​explosion-proof valve


When arranging the explosion-proof valve, the hole is usually opened in the closed equipment, and then the pipe is led from the opening to the appropriate position, and then the explosion-proof valve is installed at the end of the pipe. When an explosion occurs, the high-pressure gas is discharged through the opening of the device, the guiding short pipe, and the explosion-proof valve. Therefore, the effective area of ​​the explosion-proof valve is the minimum of the opening area, the cross-sectional area of ​​the guiding short pipe, and the area of ​​the explosion-proof valve plate.


(2) Determination of the effective area of ​​the explosion-proof valve


1 The cross-sectional area of ​​the explosion-proof valve on the inlet and outlet pipes of the mill shall not be less than 70% of the cross-sectional area of ​​the pipe.

2 On the top cover of the classifier, cyclone separator and coarse powder separator, the total cross-sectional area of ​​the explosion-proof valve can be calculated according to the volume per cubic meter of the separator not less than 0.04m2.

3 The total cross-sectional area of ​​the explosion-proof valve on the pulverized coal bin can be calculated according to the volume of 0.01 m2 per cubic meter of the pulverized coal bin, but the minimum should not be less than 0.5 m2. (1)

4 The quantity and area of ​​the explosion-proof valve of the coal dust remover are determined by the design of the dust collector manufacturer. In the calculation of the area of ​​the explosion-proof valve, the method of the hole ratio is simple, that is, the effective area of ​​the explosion-proof valve / the effective volume of the equipment to be protected, which is recommended to be 1/40 of the domestic gas industry. However, this method is more empirical, and the results are often quite different. Therefore, the VDI3673 nomogram of Germany is widely used, and the process designers can refer to this nomogram for design (2).

(3) Requirements for guiding the short pipe of the explosion-proof valve

1 The length should not be greater than 10 times the short tube equivalent diameter.

2 Short tubes should be short and straight, and arranged as vertically as possible. When they are not vertically arranged, the angle between the pipe and the horizontal plane should be not less than 70°.


4 Explosion-proof valve arrangement


Four equipments such as coal mill, dust collector, classifier and coal powder bin must be equipped with explosion-proof valves. The original coal bunker can be flexibly controlled according to the original coal quality. It is recommended that the factory using bituminous coal or high-volatility raw coal should be equipped with an explosion-proof valve on the original coal bunker.

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(1) Coal mill explosion-proof valve


If the coal mill is a ball mill, an explosion-proof valve (see Figure 1 and Figure 3) should be installed at the exit of the grinding head, the grinding tail, and the classifier. Grinding heads and grinding tails conveying raw coal and pulverized coal should be designed to ensure that their angle with the horizontal direction is not less than 70° (3) to prevent spontaneous combustion explosion caused by coal powder accumulation (4). When using a ball mill as a coal mill, it is also necessary to provide an explosion-proof valve (position as shown in Figure-2) at the rising pipe that enters the dust collector to protect the pipe and equipment.

If the coal mill uses a vertical mill, the coal mill explosion-proof valve is placed on the rising pipe of the vertical mill to the coal dust remover. And the explosion-proof valve should be arranged vertically (as shown in Figure-4). In order to make the explosion-proof valve effectively protect the pipeline and the vertical grinding at the same time, the angle between the inclined pipe and the horizontal line should be greater than 70° when the pipe is designed.

(2) Coal dust collector explosion-proof valve


The explosion-proof valve of the pulverized coal dust collector generally adopts a door type structure, which is called an explosion-proof door or a pressure relief door, and is arranged on two sides of the dust remover, and is arranged in an array. The dust collector explosion-proof door is part of the dust collector equipment. The dust collector manufacturer shall determine the number and area of ​​the explosion-proof door according to the comprehensive factors such as the dust collector volume and the predetermined pressure relief pressure.

The explosion-proof valve of the dust collector should be equipped with a matching maintenance platform, and the maintenance platform should be provided with handrails. When designing the railing, the rail should not block the trajectory of the explosion-proof valve disc when it is blown up, so as not to close the valve panel after it bounces. A cement plant in Beijing has caused an accident due to this reason (5).

(3) Pulverized coal warehouse explosion-proof valve


The explosion-proof valve of the pulverized coal bin is generally led out from the top of the pulverized coal bunker, and an explosion-proof valve is installed at the end of the pipe (see Figure-5). The opening direction of the explosion-proof valve plate should be arranged away from the feeding tube of the pulverized coal bin to prevent the airflow from the explosion from damaging the facilities of the pulverized coal warehouse top such as the feeding pipe.

5 Process Design Considerations


(1) Explosion-proof valves should avoid the pedestrian passages in the workshop. The opening of the explosion is prohibited from facing the inspection and inspection passages, electrical cabinets, main equipment and facilities, and if necessary, adding steel plate protection to avoid sudden explosion pressure and leakage to the person and equipment. Facilities cause harm.

(2) Explosion-proof valves cannot be placed in the closed construction workshop. Some cement production lines that were built earlier, the coal grinding workshop is a closed reinforced concrete structure with four walls and walls. When the explosion-proof valve is arranged, the pipe should be led to the outside of the building structure to facilitate the release of pressure during the explosion.

(3) When arranging the explosion-proof valve, it is generally to open a hole in the closed equipment, and then lead the pipe from the opening to the appropriate position, and then install an explosion-proof valve at the end of the pipe. Pipes should be properly designed during installation to avoid the formation of coal at pipe joints and corners.

(4) Explosion-proof valves should be installed at locations that are convenient for maintenance personnel to repair. If they cannot be satisfied for any reason, special inspection and maintenance platforms should be set up.

6 Conclusion


The explosion-proof valve is an indispensable equipment for the pulverized coal preparation system. Appropriate selection of the explosion-proof valve and reasonable process design can avoid or reduce the loss caused by the explosion of the pulverized coal preparation system. In addition, due to the simplicity of the equipment, the cement factory is very arbitrarily installed when installing the explosion-proof valve, resulting in the explosion-proof valve not achieving the design effect, which is worthy of attention by the installation unit and the supervision unit.

references:

(1) "Design specifications for cement plants" GB50295-2008

(2) "Exploration of explosion-proof valves for coal dust collectors", "Cement Technology", No. 1, 2005, Xu Lixin, etc.;

(3) "On the safety design of the new dry process cement production coal powder preparation system", "Production Technology", Wang Zhijie.

(4) "Causes and Countermeasures for Fire and Explosion of Pulverized Coal Preparation and Storage System", "New Century Cement Herald", No. 3, 2010, Yang Dongsheng.

(5) "Causes and Preventive Measures for Coal Mill System Explosion", "Cement", No. 4, 2003, Yang Hongbin.

五、Smart factories need to create real value for cement companies


At the end of last year, the Ministry of Industry and Information Technology issued the "Intelligent Manufacturing Development Plan (2016-2020)", proposing a "two-step" strategy for smart manufacturing. It is required that by 2020, the foundation and support capacity of intelligent manufacturing will be significantly enhanced. The key areas of traditional manufacturing will basically achieve digital manufacturing. The conditional and basic key industries will achieve significant progress in intelligent transformation. By 2025, the intelligent manufacturing support system will be basically established. Key industries have initially achieved intelligent transformation.

As a key basic industry of the national economy, it is also an important area of ​​supply-side structural reform. The intelligent transformation of the cement industry is crucial for the upgrading of its own industry and better service to the social economy. However, even in recent years, the widespread application of DCS control systems and new technology equipment has greatly improved the automation level of the domestic cement industry, but there is still a big gap from the real smart factory.

During the talks, Ren Yong also talked about his experience in the German cement factory in 1991. In the 1990s when the computer was far from popular, the automation of the German cement factory developed well, such as unattended hardware systems, expert collaboration software, etc. With the rise of computer big data applications, foreign cement factories have begun to move from automation to intelligence. The business intelligence system based on a large number of automation equipment and advanced control systems and based on big data applications provides more scientific guidance for the production and operation of cement enterprises, and cement companies are increasingly becoming a combination of modern intelligent technology.

In China, cement smart factories have been widely mentioned in recent years, especially in the field of equipment manufacturing, "smart" is becoming the subject of the company's process of promoting its own products. However, the definition of what practical assistance the smart factory should bring to the production and operation of domestic cement companies is relatively vague.

Ren Yong believes that the intelligence of cement plants can not be just a gimmick. Specifically, the cement intelligent factory system needs to optimize the entire cement production process based on the actual plant operation data collected, thereby reducing the labor intensity of workers, improving the operating efficiency of the entire cement production system, and eliminating cement plants. Safety hazards, fundamentally reduce the operating costs of the factory, only to achieve these goals, intelligence is really practical

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