Analytical evaluation of explosion-proof safety based on the international standard for Ex ‘d’ type LED working-lights

2.1 Types of Explosion-proof

Explosion-proof structures are classified according to the technique that prevents the device from becoming an ignition source of flammable vapors, combustible gases, and dust environments. Each explosion-proof structure has different requirements for the design structure and test. The types and specifications of explosion-proof structures are shown in Table 1.

Table 1:

Explosion-proof structure type and IEC international standards

2.2 Flameproof Enclosures

The LED work light developed in this study has a flameproof enclosure. The enclosure or container has a mechanical strength that withstands the explosion pressure so that gas does not affect the external gas in the surrounding area even if an electric spark or heat causes an explosion due to gas penetration inside the flameproof lamp. Furthermore, internal flames should not be transferred to the surrounding area through the joint surface of the structure. They should be designed to prevent the surface temperature of the enclosure from reaching the ignition temperature of the external gases. Flameproof equipment is tested according to the methods required by the explosion-proof general requirements [5] and the flameproof structure specification [6].

2.3 Test Requirements for Flameproof LED Working Lights

The type, method, condition, and sequence of explosion-proof type tests change according to the device's construction materials, joint structure, intended use, environment, and rating. The mobile work lights developed in this study had an explosion-proof rating of Ex db IIC T6 Gb, and the enclosure consisted of aluminum luminaires and glass floodlights. The joint structures used were cemented joints, threaded joints, and flanged joints, and the operating temperature was –20 to 40 °C. The required tests included temperature measurement, thermal endurance to heat, thermal endurance to cold, resistance to impact, drop test, thermal shock test, determination of explosion pressure (reference pressure), overpressure test, and the test for non-transmission of an internal ignition. Each test method and conditions are as follows.

2.3.1 Temperature measurement

In the temperature measurement test, the operating temperature (T_s) was evaluated by measuring the maximum temperature of the external surface with the measurement configuration, as shown in Figure 1, at the rated voltage and driving conditions of explosion-proof equipment. Moreover, the maximum temperature of the external surface of the appliance was measured at 90%, 100%, and 110% of the rated voltage, and the temperature rating was determined according to Table 2.

Figure 1:

Temperature measurement

Table 2:

Explosion-proof device temperature rating according to the highest surface temperature

2.3.3 Resistance to impact

In the impact resistance test, a 1-kg mass is dropped from the height specified in Table 3 to the point where it is easy to be exposed to the outside of explosion-proof equipment enclosure to evaluate any damage or deformation due to impact. The shape of the impact part of the falling object is a hemisphere having a diameter of 25 mm.

Table 3:

Tests for resistance to impact [5]

2.3.7 Determination of explosion pressure (Reference pressure)

The standard pressure test is designed to inject an explosive gas mixture into the sample and forcibly ignite it to ensure that no damage or deformation of the equipment enclosure occurs during the internal explosion, and to measure the explosion pressure during the explosion. The concentration conditions of the explosive mixture used in the test are shown in Table 4. The test diagram is shown in Figure 2.

Table 4:

Explosive mixture for reference pressure

Figure 2:

Test diagram for determination of explosion pressure

2.3.8 Overpressure test

The overpressure test is a test to prove that no damage or change of the equipment occurs when a selected pressure from 1.5 times, 3 times, and 4 times of the pressure determined in the reference pressure test is induced by using water for 10 s. The test diagram is shown in Figure 3, and the routine test frequency changes depending on the pressure level.

Figure 3:

Test diagram for overpressure test

2.3.9 Test for non-transmission of an internal ignition

The explosive ignition test is a test to inject the same explosive mixture into the inside and outside of the explosion-proof equipment test sample and to prove that the explosive flame caused by the forced ignition inside the sample does not affect the external gas environment. The test diagram is shown in Figure 4.

Figure 4:

Test diagram for non-transmission of an internal ignition

3.1 Product and analysis modeling overview

The target product of this study was an explosion-proof LED work light that can be applied to explosive areas (explosion-proof areas) and powered by a battery without a power cable. This eliminates the possibility of electric spark (explosion ignition source) caused by cable damage of a metal halide explosion-proof work light that uses wires and can save energy by applying LED as a light source. The specifications of the developed product are shown in Table 5.

Table 5:

Specification of Ex d LED working lights

As shown in Figure 5, the 3D model of the product that was developed and used in the analysis of analytical evaluation based on the IEC explosion-proof standards to ensure the explosion safety consists of a light enclosure made of AL 6061, a 10 mm-thick sight glass window, an LED, a metal PCB, and a thermal PAD, which is a silicon series material to transfer heat between the luminaire and PCB interface. The light enclosure consists of three assembled parts. In the analysis, the 6 degrees of freedom displacement constraint was used considering the bolt connection. Similarly, the 6 degrees of freedom displacement constraint was used for the light source part, which is composed of an LED and a metal PCB, considering that the light source is fixed by bolts. Individual LEDs and metal PCBs were modeled as equivalent solids.

Figure 5:

3D model of explosion type LED for analytic evaluation

3.2 Thermal analysis

Thermal analysis can be used to estimate the best surface temperature of explosion-proof equipment to assess their suitability for temperature rating. For thermal analysis, ANSYS Discovery AIM 2019 R1, which is a Multiphysics analysis program, was used. Generally, about 30% of the power input to the luminaire is converted to light energy and the rest to thermal energy. Therefore, for 77 W power input, which is 110% of the rated power consumption, the power consumption through heat was set to 53.9 W, and it was assumed that heat is transferred through the LED and the metal PCB (see Figure 5). The analysis showed that the maximum temperature outside the product was 54.3 °C, and the result is shown in Figure 6. The maximum operating temperature based on the ambient temperature, and the correction factor was 74.3 °C, which is suitable for the T6 temperature rating, which requires an operating temperature to be 85 °C or lower.

Figure 6:

Cross-sectional view of the thermal analysis result of the explosion-type LED with a source heat flux 53.9 W

3.3 Impact resistance analysis

During the impact resistance test, a 1 kg hemispherical collider with a diameter of 25 mm was dropped freely at the height of 0.7 m on an explosion-proof LED work light and collided into the center of a light-transmitting glass. The impact resistance was analyzed using ANSYS Discovery AIM 2019 R1. The gap between the colliding body and the colliding surface, which is the glass surface, was defined as 200 μm. When the collider falls freely at a height of 0.7 m without considering the air resistance, it will reach a speed of 3.7 m/s just before the collision. Therefore, this speed was used as the collision speed. Only the first collision was analyzed, and it was found that the maximum displacement was 1.3 mm, as shown in Figure 7 (a). The maximum stress was found to be 140.1 MPa, as shown in Figure 7 (b). Considering that the yield stress of the aluminum enclosure is 275 MPa and the breaking strength of 10 T of tempered glass is 150 MPa, it was found that the impact resistance of the explosion-proof LED work light meets the explosion-proof standard.

Figure 7:

Resistance to impact analysis result of the explosiontype LED with impact force 0.7 N

3.4 Drop analysis

The suitability for the drop test can be evaluated by analyzing the impact that occurs when the explosion-proof equipment is dropped at the height of 1 m onto the horizontal concrete surface. ANSYS AUTODYNE 2019 R1 was used for the drop test. The distance between the explosion-proof LED work light and the ground was defined as 200 μm. When the collider falls freely at the height of 1 m without considering the air resistance, it will reach 4.4 m/s just before the collision. Therefore, this speed was used as the collision speed. Only the first-order collision was analyzed, and the results showed that the maximum displacement was 3.1 mm, and the maximum stress was 310.8 MPa at the end of the explosion-proof LED work light enclosure. The results confirmed that the permanent deformation that occurred during the collision did not impair the operation of the product. In most areas of the enclosure of the explosion-proof LED work light, the stress was found to be less than 200 MPa, which confirmed that the explosion-proof performance was not inhibited or invalidated (See Figure 8).

Figure 8:

Drop analysis result of explosion type LED with drop height 1 m

3.5 Explosion Pressure Analysis

Analysis of the explosion that occurs inside the explosion-proof equipment can be performed to evaluate the safety of the explosion-proof enclosure and to estimate the explosion pressure. ANSYS AUTODYNE 2019 R1 was used to conduct the explosion analysis because it is suitable for solving a significant deformation problem caused by explosions. The TNT equivalency method was used to simulate an explosion event inside the equipment. The TNT equivalent method was proposed in the US Army Technical Manual TM 5-1300. The TNT equivalence method can be used to calculate the equivalent TNT mass that can generate the same amount of energy from an explosion situation [7] [re].

The equation for calculating the TNT equivalent mass by the TNT equivalent method is shown in Equation (1).

where W is the TNT equivalent mass (kg), μ is the explosion yield, M is the amount of explosive gas leak (kg), and E_c is the amount of thermal energy released per unit mass (kcal/kg) due to the explosion. An explosion of 9.8% hydrogen was converted into TNT equivalents using an explosion yield of 0.5, and it was found that the TNT equivalent mass was 23.9 kg, which was then applied to the explosion analysis using AUTODYN. The analysis results showed that the explosion pressure was 899 kPa and did not impair the safety of the explosion-proof enclosure (see Figure 9).

Figure 9:

Explosion pressure analysis with TNT equivalency method

3.6 Overpressure Analysis

Explosion-proof safety for the overpressure test can be evaluated based on the pressure analysis in which high-pressure is applied inside the explosion-proof equipment to destroy the product. In the actual test, one of the conditions of 1.5 times, 3 times, and 4 times the explosion pressure determined in the determination of explosion pressure is selected to apply the pressure using water and to verify no damage and deformation occurs for more than 10 seconds. In the analysis, 1,350 kPa, which corresponds to 1.5 times the reference pressure, was statically applied to the inner surface and the light-transmitting part of the enclosure. As a result of the analysis, the maximum stress of 205.9 MPa between the light-transmitting part and the enclosure of the product was found, which is less than the aluminum yield strength. However, when 3,560 kPa, which corresponds to 4 times the standard pressure, was applied, a maximum stress of 992.1 MPa was observed, which is higher than the yield strength of the aluminum enclosure.

Figure 10:

Overpressure analysis result

Author Contributions

Conceptualization, M. Yoo; Methodology, I. Lee; Software, M. Shin; Validation, M. Yoo and W. Lee; Formal Analysis, I. Lee; Investigation, S. Paek and Y. Seo; Writing—Original Draft Preparation, I. Lee; Writing—Review & Editing, M. Yoo; Visualization, I. Lee; Supervision, M. Yoo; Project Administration, M. Yoo; Funding Acquisition, M. Yoo.