Abstract
Release valves are commonly used to protect hydraulic legs against overload caused by rock bursts or bumps. Due to an essential role in ensuring safety in the working, an application of a release valve is conditioned by a positive yield test results of a leg equipped with such a valve. A method of leg yield testing, used in Poland, enables a complex determination of an impact of not only a release valve but also of the parameters of the hydraulic leg, determining its stiffness such as for example a volume of the under-the-piston space, which has an impact on an observed pressure increase. The subject of this publication covers cognitive tests oriented onto a determination of an impact of a release valve exclusively on the pressure changes observed in the leg. The results of the efficiency tests of spring valves (Stoiński, 2018) on a rammer are discussed. The difference between the maximum pressure in the under-piston area of the leg with the release valve and the maximum pressure generated by the same dynamic load in the leg without this valve was the measure of the valve operation's effectiveness. Dynamic load, realized on a rammer, is characterized by a longer increase time than in the case of dynamic load acting on a powered roof support unit from the floor. The time process of the force in the leg is then characterized by a short load rising time – tn, large load increment factor – Kd and the average load growth rate –wp,n. Referring to that aspect, the features of a release valve were analyzed in relation to the parameters characterizing dynamic load acting on a powered roof support unit from the floor. Parameters characterizing the effectiveness of the release valve, i.e. change in the leg load increment index –Kd and change in the rate of load increase –wpn were defined. The test stand for generating the load of such parameters using the explosive method is described. Comparison of effects of the dynamic load generated by firing the same mass and the same type of explosive on a hydraulic leg with a release valve and the leg without this valve was the test objective. The effectiveness of the spring valve and two gas valves are analyzed. It was found that despite a very short load rising time, the release valve reduces the load acting on the leg. The positive values of the Kd and wp,n indices are the evidence.
Highlights
Dynamic phenomena such as rock bursts and bumps are still a big safety hazard in exploitation workings
The dynamic load increase coefficient was reduced by approximately 15%, and after the use of a release valve, it was 1.51
In the case of load, with intensive pressure increase time ranging from 2.8 ms to 7.6 ms, the release valve reduces both the pressure increase coefficient and the average rate of its increase
Summary
Dynamic phenomena such as rock bursts and bumps are still a big safety hazard in exploitation workings. According to Świątek (2020), it can be assumed that in 60% of seams, under mining, hazards of high energy bursts occur. Installation of an additional release valve in the leg's hydraulic system is a commonly used method for protecting legs against overload. In this way, the leg's yielding capacity is increased. Leg's yielding capacity is assessed by tests at dynamic load (Stoiński, 2018). An analytical method of yield assessment, used in Poland most often (Stoiński, 2018), enables to determine pressure changes in the leg under-the-piston space, caused by an external load of an accepted time function dependent, in general, on mining-and-geological conditions. The elastic properties of the mechanical system, constructed of a hydraulic leg and a release valve, impact the calculated pressure changes in the leg apart from the applied external load
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