## 4 Discussion  
The dynamic testing of the safety valve was carried out with the hydraulic prop using this valve. During the tests, the prop was dynamically loaded. Its protection against damage due to dynamic load relies on limiting the pressure increase in the hydraulic medium inside the prop's cylinder caused by this load by the valve opening quickly at the preset yield load. The test consisted of lowering the ram from a certain height, and hitting the traverse under which the prop is located (Figure 10). In this type of test, the weight of the traverse may have a decisive influence on the test results (Figure 11). The basic instantaneous load of the prop manifests as momentary forces and inertia forces. Figure 10: Opening of the safety valve during tests for different heights of impact mass drop, where: (**1**) drop height 20 cm, (**2**) drop height 30 cm, (**3**) drop height 40 cm, (**4**) drop height 50 cm, (**5**) drop height 60 cm, (**6**) drop height 70 cm. Figure 9: Characterization of valve opening and closing pressures. The prop with the safety valve yields under the influence of kinetic energy load. The slip in the prop is more prominent with a larger flow area in the safety valve under test and a smaller flow resistance. The prop's slip and the safety valve's opening time determine the liquid's average pressure in the under-piston space of the stand during the opening of the valve. Accordingly, the flow rate of the liquid through the valve, and its flow resistance, can be determined (Table 1). When the pressure of the liquid exceeds the strength of the cylinder, the prop is destroyed (Figures 2 and 3). The safety valve (Figure 5) connected to the under-piston space of the prop is intended to prevent an excessive increase in fluid pressure in this space. The kinetic energy transferred by the ram to the tested prop is converted. This is partially for the operation of the prop's slip due to the opening of the tested safety valve, and partly for the elastic energy of the cylinder. The rest of the energy is dissipated due to vibrations in the test stand and resulting shocks. At some time after the impact, the movement of the ram is inhibited, which equates to the loss of the ram's kinetic energy. The prop's slip occurs only during ram impacts, causing the safety valve to open. The energy of the ram on the test site or the roof of the mining excavation transferred to the prop may also cause its slip when the load no longer acts on this prop. The speed of movement of the prop piston forces the appropriate flow rate of liquid from the prop sub-piston space once the preset pressure has been exceeded. The pressure relief valve must ensure that the flow rate is such that the pressure of the liquid in the under-piston space of the prop at the moment of impact cannot rise too rapidly, so the speed \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{\begin{tabular}{c} **Height of Drop** \\ **(m)** \\ \end{tabular} } & \begin{tabular}{c} **Energy of** \\ **Impact Mass** \\ **E\({}_{u}\) (kJ)** \\ \end{tabular} & \begin{tabular}{c} **Maximum** \\ **Pressure (bar)** \\ \end{tabular} & \
\begin{tabular}{c} **Leg** \\ **Slide (m)** \\ \end{tabular} &
\begin{tabular}{c} **Flow Value Q(t)** \\ **(m\({}^{3}\)s\({}^{-1}\))** \\ \end{tabular} \\ \hline
0.2 & 58.8 & 280 & 0.02 & 50 \\
0.3 & 78.4 & 480 & 0.03 & 75 \\
0.4 & 98.1 & 550 & 0.04 & 100 \\
0.5 & 117.7 & 605 & 0.05 & 125 \\
0.6 & 137.3 & 610 & 0.06 & 150 \\
0.7 & 156.9 & 620 & 0.07 & 175 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Flow value occurring in the safety valve. Figure 11: Performance characteristics of the prop, including safety valve loaded with an impact mass freely falling from the following heights, where: 1—drop height 20 cm, 2—drop height 30 cm, 3—drop height 40 cm, 4—drop height 50 cm, 5—drop height 60 cm, 6—drop height 70 cm. at which the valve release mechanism opens is very important.