The constant increase in the extraction depth and concentration of output in hard coal deposits is one of the primary reasons for the rising absolute methane emission rates and coal seam methane pressure in mining plants (Cybulski et al., 2018). It has a direct influence on the increase in methane explosion hazards present in deep mines. Coal dust and methane explosions are some of the most common causes of mining disasters in hard coal mines all over the world, and research continues to be conducted with the purpose of understanding the mechanisms of explosion as well as improving explosion prevention, monitoring and risk reduction (Shepherd et al., 1981; Takla & Vavrusak, 1999; Cioca & Moraru, 2012; Hudeček et al., 2012; Shao & Ma, 2012; Brune, 2013; Krause & Smoliński, 2013; Li et al., 2013; Hao et al., 2014; Krause & Skiba, 2014; Yuan, 2016; Trenczek, 2015; Burtan et al., 2017; Song et al., 2021). Broad-scale research has been conducted in Poland and worldwide for many years to reduce the risk of methane and coal dust explosion, with the intent of drafting protection standards and developing methods for explosion mechanism identification and prevention and ignition source detection (Pytlik et al., 2021). The introduction of electrical equipment into mining plants in 1870 necessitated the commencement of research on explosion hazards (Górny, 2013). It should be noted that the first studies on the parameters that determine methane ignition were conducted in Germany as early as 1884–1885, whereas the first fireproof shield tests were carried out at the University of Sheffield. This work resulted in regulations and standards being issued in countries such as Germany and the UK. The first standard organisation with an international reach was the International Electrotechnical Commission (IEC), established in 1906. In Poland, the first standard concerning explosion-proof equipment was issued in 1929 by the Association of Polish Electrical Engineers (Stowarzyszenie Elektryków Polskich [SEP]), in cooperation with the Czechoslovakian Electrotechnical Association (Elektrotechnický Svaz Československý [ESČ]). The research institution named the Experimental Mine ‘Barbara’, the Central Office of Mining Rescue and the Magnetic Observatory in Mikołów was established nearly 100 years ago (in 1925), and some of its activities included the testing of devices, equipment and materials used in mines, as well as research on gas and coal dust explosion phenomena. Currently, the Experimental Mine ‘Barbara’ is a part of the Central Mining Institute in Poland. The testing grounds of the Experimental Mine ‘Barbara’ constitute the only site in Europe that is capable of accommodating full-scale gas and dust explosion tests.
In order to standardise the regulations concerning basic requirements for equipment intended for use in explosive atmospheres, the European Union member states adopted the ATEX Directive 2014/34/EU (Eckhoff, 2006; Petitfrere & Proust, 2006; Jespen, 2016). Before a device is permitted for use in the field, it is important to carry out its conformity assessment process, part of which involves tests for conformity with standards harmonised with the ATEX directive. Should no harmonised standards exist, the required tests and extent of testing are determined by a body notified within the scope of the directive, and some of the basic certification tests include maximum temperature determination and potential methane and coal dust ignition source identification (Kałuża, 2017; Jurca et al., 2020). The ATEX directive encompasses requirements for both electrical and non-electrical (Rogers, 2003; Gakhar et al., 2006; Thurnherr et al., 2007; Ghicioi et al., 2010a and 2010b; Jurca et al., 2020) devices. Unlike electrical device standardisation, the standardisation of non-electrical devices is a relatively recent endeavour, and its greatest development began with the adoption of the ATEX directive in 1994 (Górny, 2017).
One technical source of ignition of methane can be mechanical sparks, resulting from friction of rocks between each other as well as during operation of mechanical equipment, and the high surface temperature of equipment components during their operation in an explosive atmosphere. (Cioca & Moraru, 2012; Trenczek, 2015; Prostański, 2018; Pytlik et al., 2021; Song et al., 2021; Zhang et al., 2021). The issue of where the mechanical sparks are formed on the steel support and the formation of high-temperature areas, which can pose a risk of ignition of an explosive gas mixture, is poorly recognised, as is also indicated by the small literature in this area.
Initial tests of straight sliding joints constructed from V29 sections under static and dynamic loading were presented in a publication concerning loads exerted on the support by a suspended monorail system (Pytlik, 2019a). The mechanical sparking phenomenon observed during the tests motivated the author to continue the research on full-scale support frames (Pytlik, 2019b; Pytlik, 2020) to better understand the surface temperature generated as a result of arching sliding joint yield and to determine the locations of sparking.
This paper presents the methodology and results of thermal imaging and strength testing of straight and arching roadway support sliding joints constructed from V32 sections under dynamic loading. Tests of friction joints were carried out in 2020 and are a continuation of the tests that were the subject of Pytlik's (2020) article, which consists of the load capacity of steel arch support under static loading and friction straight joints under dynamic loading with a maximum impact energy of 28 kJ (according to PN-G-15533:1997) including thermal imaging tests. In this article, the testing of friction joints has been extended, compared to the requirements of the Polish standard (PN-G-15533:1997), to include dynamic testing on curved elements, and the maximum impact energy of the drop mass during testing of friction prop has been increased to approx. 50 kJ at an impact velocity of approx. 2.8 m/s. The research was carried out to identify areas of mechanical spark formation (using a thermal imaging camera) and to determine at what energy and impact velocity the permissible temperature of 150ºC of the section surface is exceeded during sliding under conditions simulating rock burst. The main objective of the study of friction joints under dynamic (impact) loading simulating rock burst, presented in this article, is to identify possible sources of ignition of explosive atmospheres during impulsive loading of a yielding steel arch support under rock burst conditions. This research is new to the extensive studies of friction joints presented by Brodny (2012a and 2012b) and other researchers (Ciałkowski, 1996; Pacześniowski and Pytlik, 2008; Horyl et al., 2014, 2017 and 2019), which were mainly focused on determining the resistance of friction joints under static and dynamic loading.
Compared to the previous dynamic sliding joint testing, the presented work was expanded with arching element tests to better understand the phenomena that occur during yielding. The primary purpose of the tests under dynamic loading was to identify the potential explosive atmosphere ignition sources during the impact loading of the yielding steel arch support under the conditions of rock bursts. The scope of testing encompassed temperature measurements of support sliding joint elements at yield. Thermal imaging measurements were carried out by means of a high-speed camera with a refresh rate of 128 Hz. The obtained information regarding the temperature distribution in the joint at yield is necessary to identify the locations that influence the increase in frictional resistance or indicate the risk of support element failure. Determining the maximum surface temperature generated as a result of the sliding joint yield makes it possible to inspect whether the maximum temperature defined in standard PN-EN ISO 80079-36:2016 has not been exceeded. The standard in question is harmonised with the ATEX directive concerning group I non-electrical equipment used in explosive atmospheres. Some of the aspects defined in the standard include the maximum temperature
Yielding support frames and friction props are assembled using elements constructed from open V32 sections (or other sections, e.g. with V, TH or U profiles) coupled via overlaps by means of screw pin shackles. Yielding frames and props are commonly utilised at great mining depths under difficult geological and mining conditions resulting from increased rock mass stress and dynamic phenomena in the form of rock mass tremors and rock bursts. Straight and arching sliding joints constructed from V32 sections (PN-H-93441-3:2004) with increased mechanical properties of steel (
The methodology for testing ŁP frame and SV friction prop sliding joints under dynamic impact loading was developed based on the following assumptions:
The dynamic loading of the support frame and friction prop joints is exerted by the impact of masses of rock, resulting from a rock burst (Dubiński & Konopko, 2000). The collision of the ram against the crosshead with a velocity
The dynamic loading of the support frame and friction prop joints is exerted by the impact of masses of rock, resulting from a rock burst (Dubiński & Konopko, 2000).
The collision of the ram against the crosshead with a velocity
The impact velocity
Following the collision, the combined masses move jointly with a velocity
The total mechanical energy
The determined emissivities were as follows:
ε = 0.842 – for a slightly corroded V29 section, ε = 0.863 – for a slightly corroded V32 section, ε = 0.873 – for a strongly corroded V32 section (arching element) with visible tarnish and flaking corrosion products.
ε = 0.842 – for a slightly corroded V29 section,
ε = 0.863 – for a slightly corroded V32 section,
ε = 0.873 – for a strongly corroded V32 section (arching element) with visible tarnish and flaking corrosion products.
The axial force exerted on the joint during the free fall of the ram was measured during the tests by means of a C6A strain gauge force sensor (class 0.5) connected to a measuring amplifier of the MGCplus type from Hottinger Brüel & Kjær The joint yield
The dynamic sliding joint load capacity tests were performed based on a methodology for dynamic sliding joint strength testing (PN-G-15533:1997; Pacześniowski & Pytlik, 2008). The straight and arching sliding joints were constructed from V32 sections with enhanced mechanical properties. The test methodology was based on the free fall of a drop mass (ram)
The load capacity of SV32 friction props (two shackles in the joint), with clevises tightened with a torque
Result compilation of SV32t straight sliding joint tests under dynamic loading.
The SV32t sliding joint test results indicate that the increase in the ram impact velocity
In the case of SV32t prop testing according to standard PN-G-15533:1997 at a ram height of fall of 0.7 m (
Thermal traces can be observed on the section flanges in the thermal images, which are generated by the movement of the shackles along the sections. The maximum section surface temperature
Tests of strongly corroded arching sliding joints obtained from ŁP10/V32/4/A frames were carried out as well, and example test results are presented in Figs 8 and 9. The strongly corroded state of the section resulted in the total yield of an arching joint with two shackles coupled with screws with a torque of 450 N m at an impact velocity
Very strong sparking accompanied by an increase in temperature to over 200°C (Fig. 9c) was also observed in the case of the same type of joint (Fig. 9) with an additional third (middle) shackle (
The straight and arching joints exhibit different characters of operation primarily because the tips of the arching segments ‘tear’ through the sections in the joint during the yield. Their key contact points are in the areas of the upper and lower shackles. Intensified sparking and increased surface temperatures are observed during a dynamic yield (Pytlik, 2020), and these result not only from the nature of the dry friction itself, but also from the tearing of the surfaces.
One solution to the problem of the excessive joint yield may be to add a fourth restraining shackle under the joint. This shackle is installed directly under the joint on a short piece of a V32 section with a length of about 150–200 mm (Fig. 10).
All the shackles were tightened with a lowered torque
It can be clearly observed that the temperature falls significantly below the standard value of
The calculated values of the kinetic coefficient of friction m
Another example of a design solution for a frictional joint, compared to a joint with a braking shackle, which significantly reduces the sliding value, is the use of a shackle with a resistance wedge (Brodny, 2012b). Such shackle causes a significant increase in the resistance force of the joint during the sliding of the sections in the friction prop. However, in the available literature, no results of surface temperature studies of the joint components have been encountered.
As the issues concerning the places of mechanical spark formation on yielding steel arch support and the formation of high-temperature places that may pose a risk of ignition of explosive gas mixtures are poorly recognised, the authors of the article had difficulty in referring to both Polish and foreign literature. The article fills a gap concerning the sparking phenomena of steel arch yielding support during dynamic loading.
Compared to the temperature
Tests of straight sliding joints at impact velocities greater than what is defined in standard PN-G-15533:1997 demonstrated that the surface temperature rises significantly above the permissible temperature
A similar situation occurred during the tests of arching sliding joints obtained from ŁP10/V32/4/A support frames. The maximum surface temperature during testing also exceeded 200°C, and strong sparking in the areas of the upper and lower shackles was observed at yield. This was primarily due to the tearing of the surfaces by the tips of the arching sections and the melting of the corrosion products as a result of the temperature increase at the contact points between the arching sections.
The temperature increase of sliding joints subjected to dynamic impact loading can be limited by preventing excessive joint yields. An additional fourth shackle restraining the movement of the joint, installed directly under the joint, is applied for this purpose in certain types of friction props (e.g. SVtw). This solution is currently not used in arching support frame joints, primarily due to economic factors. The tests demonstrated that using a fourth shackle in the joint prevents rapid joint element temperature increases as a result of excessive yielding. During the testing of straight joints with four shackles, at a torque
In the case of steel arch support systems, the arching sections should be made with the same bend radius to limit the tearing effect of the sections in the joint, which is the primary cause of mechanical sparking. Determining such mechanical parameters of the joints that would limit yielding to a minimum and reduce sparking in the joint requires the conduction of further testing – particularly arching joint testing – involving various types of the V section and the various states of its corrosion, as well as different numbers of shackles in the joint and varied shackle screw nut torques. The aim of further research should also be to test different sliding joints: both existing on the market and new ones, with a view to minimising section slide in the joint, joint friction surface temperature and sparking. The adapted methodology is applicable to all friction straight and arch joints tested, existing on the market, under static and dynamic loading under laboratory conditions.
Result compilation of SV32t straight sliding joint tests under dynamic loading.
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