Abstract
The investigation was aimed at testing the performances in fire-resistance and transmission efficiency of non-metallic flexible materials in flameproof diesel engine locomotive which may replace traditional metal flame arresters with low gas transmission efficiency. On the basis of the chain reaction mechanism, the mixed gas was burnt in the experiment, and the free radical which can be absorbed by tiny pores of flexible fiber materials and quenched was released. A 9-m-long steel pipeline with an inner diameter of 110 mm was used to simulate the environment of gas transmission. The experimental results showed that flame arresters made of flexible fiber materials can improve the gas transmission efficiency about 3% to 7%, and they also have excellent performances in fire resistance.
Keywords
Introduction
With the increase in the mining depth of coal mining and production, the pressure and the emission of gas are higher than before. The probability of gas explosion accidents had grown sharply that brought many security concerns in production of coal mine. In mining industry, different types of flame arresters are currently installed at various places where a flame or even a detonation can take place or propagate. There are two main traditional flame arresters which are widely used, including mechanical grid flame arresters and ceramic bead flame arresters. But their electrical devices are very heavy with metal shells that used in coal mine to prevent the flame from propagating in pipelines and vessels. Then, the exploitation and transportation of methane gas had caused a series problems of security occasionally with traditional flame arresters.1,2 Accordingly, in consideration of big size and high cost as well as high gas flow resistance of traditional mechanical flame arresters, appropriate structures of flame arresters are widely discussed about non-metallic flexible fiber materials. In recent years, experts at home and abroad have made a great quantity of researches on flexible fiber materials. And the materials have been widely used in the fields of military and civil engineering protection, in consideration of the outstanding inhibition to the propagation of flame and pressure wave during the explosion.3–5
At present, there is only little understanding about how flame arresters make flames quench (or fail to quench) according to related researches. In general, two main theories about quenching of combustible gas have been mentioned. Thermal theory believes that the deflagration of mixed gas was caused by overheating, and the process from smoldering to flame until the explosion is known as thermal explosion or spontaneous combustion.6,7 Most of the above-mentioned mechanical arresters are based on it. However, some kinds of materials with poor thermal conductivity show apparent effects on flame arresters. There is another theory which is different from thermal theory. Chain reaction mechanism indicates that mixed gas can decompose into free radical under external excitation such as heat and light. Meanwhile, fast recombination between free radicals and premixed gas causes a series of chain reaction. The process can be divided into three stages:
Chain initiation—free radicals generating and start of chain reaction.
Chain propagation—reaction between free radicals and premixed gas and new free radicals generating.
Chain termination—end of chain reaction and disappearance of free radicals. 8
According to the process of chain reaction, the restraints on free radicals can decrease and even terminate chain reaction.
Theoretically, refractory fibers have good performances of air permeability and complex internal space, so it can provide with adsorption area. First, irregular porous structure increases the cooling area. The temperature of flame can decrease under ignition point and quench after the heat exchange. Tiny pores of the porous materials, moreover, increases the probability of absorbing free radicals during chain reaction so as to prevent the combination of free radicals and premixed gas. Then, the chain reaction will slow down and even terminate.
The aim of this is to verify properties of non-metallic flexible fiber materials in fire resistance and transmission efficiency so that we can apply it to the flameproof diesel engine locomotive. As the main power source of conveyance in coal mine, diesel engine locomotives have the advantages of strong power and long working time. We often install metal flame arresters in the exhaust outlet of the diesel engine to prevent combustible gases from burning and exploding. But it will cause the problems of rising oil consumption and worsening emissions. 9 Therefore, it is more necessary to study non-metal flexible materials to solve these problems.
Experimental setup
Three kinds of flexible refractory fiber materials were used to verify the performance of fire resistance according to explosion-proof principle and test methods of flame arrests. And then, a comparison of transmission efficiency between flexible refractory fiber arresters and general arresters was given.
The tested materials were clad with metal mesh and a support rack on unexposed surface to contend with fire blast (Figure 1).
Structure of non-metallic flame arrester.
This test is on the basis of “The Ministry of Public Security Explosion Test Standard.” A 9-m-long steel pipeline with an inner diameter of 110 mm was used in the experiment. The pipeline was separated into two sections by a flame arrester where the flame was prevented. The upstream section where the incident CJ (deflagration) was generated and accelerated part was 6 m, and the downstream section used for test of the flame arrester was 3 m. A sketch of deflagration and detonation test device is provided in Figure 2. The inner surface of the pipeline was smooth and there was no leakage at each connection of the system. And the test points were installed on the circumference in the same section of the pipeline. The two sides of the pipeline were sealed with a plate, and the electric spark machine close to the upstream section was used as a fire source. And the specification of the test pipeline is in accordance with the flame arrester. If the length of the accelerating pipeline was not enough to affect the speed of the flame, we can take off the plate and ignite. Moreover, we can increase the speed of the flame by setting up the jamming device on the detonating side of the flame.3,10
Sketch of deflagration and detonation test device.
We can see that four flame detectors were placed in the test device to detect the flame, temperature, and visible or invisible ray radiation in the experiment. When the flame intensity exceeds the upper limit, the light is lit to indicate that there is a flame; on the contrary, the light is let go out indicating the flame is extinguishing.
The combustible gases with industrial purity used for the test included methane, oxygen, and nitrogen. The mixtures were prepared in advance by partial pressure measurements. The explosive volume concentration of methane gas ranged from 4.9% to 16%. The volume concentration of typical gases is shown in Table 1.
Volume concentration of typical gases.
The device of flame detection has an immediate impact on experimental results. Therefore, the detectors must be adaptable to flame with different intensities. Four flame detectors were installed along the pipeline and each of them was connected to a photodiode with an optical fiber that can operate in the infrared range, so it can be used to record the low signal of radiation. Moreover, an explosion-damaged diaphragm and a dynamic pressure transducer were installed at the end section of the pipeline in order to provide the ability to check whether a flame or detonation could propagate in downstream of the pipeline. The pressure transducer operates in a range of up to 3 MPa and its measuring accuracy is 0.1 MPa. 3
The transmission efficiency of flame arresters was closely related to pressure loss of the mixed gas after it propagates through the arresters. As shown in Figure 3, two test points were installed on both sides of the flame arrester and connected to pressure differences instrument, and the gas supply system can provide with different velocities.
Sketch of pressure loss test device.
Inner diameter of pipeline tested in Figure 3 was 110 mm. An air flow meter was installed along the pipeline, which was used for gas flow measuring. The velocity of flow was controlled in 5 m/s and the control accuracy was below ±0.2 m/s. The data of pressure difference was collected by pressure differences instrument after ventilation of 2 min.
Before the test, the volume concentration of methane was controlled at 8% and mixed gas was ventilated for about 1 min continuously to set up a steady cycle; then, the gas supply system was turned off and the mixture was ignited.
Every kind of material was divided into two types of thickness. Each of them will be tested for three times. And any failure will be declared invalid of fire-resistance performance. In the experiment of transmission efficiency, the pressure loss of the gas should not exceed 20% when passed the flame arrester. 10
Results and discussion
Performance of fire resistance
Three kinds of flexible fiber materials with different thicknesses were tested in the experiment. As shown in Table 2, flame detectors can detect the flame of downstream pipeline which decides the result of flame quenching. And we can also judge whether there is a flame by observing the explosion-damaged diaphragm. The thermal properties can be observed through the appearance of fire-resistance materials before and after the experiment.
Comparison result of fire-resistance performance in flame arresters.
✓ in the column of “Result of flame quenching”—can extinguish the flame effectively.
× in the column of “Result of flame quenching”—cannot extinguish the flame.
✓ in the column of “Thermal endurance properties”—can withstand high temperature.
× in the column of “Thermal endurance properties”—cannot withstand high temperature.
From Figures 4 and 5 of experiment, it showed that most of the heat was absorbed by the metal mesh because of the bad performance of heat loss in the pipeline during explosion test, so the fire-resistance device can exist at high temperatures. Therefore, the rock wool was scorched and paper-based filter material was smoldered. On the other hand, two kinds of materials also showed excellent performances in fire resistance.
Experimental materials of flame quenching: (a) rock wool and flame retardant polyester, (b) paper-based filter material, and (c) refractory glass fiber.
Experimental results of flame quenching with different materials: (a) rock wool and flame retardant polyester, (b) paper-based filter material, and (c) refractory glass fiber.
After that a new structure of fire-resistance device with thicker metal mesh in support rack was built. At the same time, the paper-based filter material was replaced by refractory fiber in the same size. The experimental result indicated that refractory fiber materials not only had excellent performances in fire resistance but also possessed the function of gas permeability. The color of material surface was dark owing to the spread of flames. Furthermore, compared with paper-based filter material, the porosity of refractory glass fiber or acupuncture cotton is bigger. The thickness can be increased to guarantee the requirements of the porosity for experimental materials.
Performance of transmission efficiency
Flexible materials cause quite small pressure loss because of excellent permeability. And pressure loss of flame arresters was related to the porosity of materials. Based on the test about flame quenching, the pressure loss performances of flexible fiber flame arresters were tested and compared with traditional arresters at the same time.
The result showed that the pressure loss of the three kinds of flexible fiber materials was almost 3% to 7% lower than general metal arresters under the same condition in Table 3.
Comparison result of pressure loss in flame arresters.
Discussion
The flexible fiber materials have exquisite pore because of spatial structure and have no channel of heat transfer. It is more likely that the complex inner structure of fiber captured the free radicals of mixture and restrained the further chain reaction. Then, the flame was quenched successfully.
According to the result, even a paper-based filter material of 1 mm was effective when the spread of flame was prevented. The thermal theory cannot explain the principle of flame quenching independently. However, a great deal of materials which have excellent heat transfer performances tend to do well in quenching.
Accordingly, the process of fire resistance cannot be explained independently by thermal theory or chain reaction mechanism. It seems that two theories are particularly effective when used together.
The flexible fiber materials always have excellent performances in fire resistance but a bad performance of heat loss. Meanwhile, flexible fiber materials have a high efficiency of gas transmission and can solve the problem of overheating by increasing the thickness of the metal mesh.
Conclusion
Based on the investigation concerning the performances in fire resistance and transmission efficiency of non-metallic flexible fiber material of flame arresters, the following conclusions can be drawn:
Flame arresters in flameproof diesel engine locomotive with flexible fiber, made of non-metallic porous materials such as rock wool, paper-based filter, and refractory fiber: They all did well in fire-resistance test. But paper-based filter materials only can be used once because it can easily be burnt out.
In view of gas transmission efficiency, materials of flexible fibers own an apparent advantage than metal arresters. The pressure loss was 3%–7% lower than metal arresters under the same condition. Accordingly, the new type of flame arrester will be used gradually.
The refractory glass fiber, acupuncture cotton, and other materials with high porosity can ensure the safety of fire resistance by increasing the thickness appropriately and guarantee the gas permeability.
Footnotes
Handling Editor: Ito Kazuhisa
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This study was financially supported by the Fundamental Research Funds for the Central Universities (grant no. 2014ZDPY10) and the Priority Academic Program Development of Jiangsu Higher Education Institutions in China.
