PROCEEDINGS
OF THE TOMSK POLYTECHNIC INSTITUTE
OF THE ORDER OF THE OCTOBER REVOLUTION AND THE ORDER OF THE RED BANNER OF LABOR
named after S. M. Kirov

Volume 263　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　1975

STUDY OF THE DYNAMIC CHARACTERISTICS AND DURABILITY OF EKOR-TYPE CHECK VALVES IN POWERED ROOF SUPPORTS

V. F. GORBUNOV, L. N. GRINIS, E. G. FRANK

In recent years, in the coal industry, comprehensive mechanization of work in stopes has been carried out on a broad industrial scale with the use of hydraulically powered roof supports. The distinctive features of the hydraulic drive of powered supports are: the use of a water-oil emulsion as the working fluid; high values of working pressures developed in the closed hydraulic systems of hydraulic props (up to \(800\ \text{kg}/\text{cm}^2\)); the great length of hydraulic lines (\(200 \div 250\ \text{m}\)); operation under contaminated conditions; and high requirements for the reliability and durability of the elements and of the systems as a whole. In connection with these specific features, the hydraulic drive of powered supports differs substantially, in its circuits, design solutions, and operating characteristics, from the hydraulic systems of other branches of mechanical engineering.

Taking into account that, first, the durability of individual elements of the hydraulic systems of supports is very limited [1, 2], and second, that problems of the dynamics of the hydraulic drive of powered supports have not been considered sufficiently fully in the literature [3], it is advisable to study the dynamic processes occurring in hydraulic props and the durability of parts and assemblies [1, 2].

According to data from the manufacturer (the Pneumatika plant, Leningrad) and from a number of mines in Kuzbass, the least durable element of the hydraulic systems of props is the EKOR check valve.

The present work presents the results of experimental investigations of the dynamic characteristics and of the possibility of increasing the durability of a controlled relief check valve of the EKOR type with a ball shutoff member and a plastic (P-68 resin) seat.

The experimental investigations were carried out under laboratory conditions on a special test stand that made it possible to vary the parameters and operating modes of the valve over a wide range and to record the principal processes occurring in the valve under steady-state and transient conditions. Figure 1 shows the basic diagram of the EKOR valve and of the sensors required for recording transient processes in the valve. The purpose and principle of operation of the valve are described in [1].

To record the transient processes in the valve, the following sensors were used (Fig. 1):

1. Three strain-gauge pressure sensors \(D_A\), \(D_B\), and \(D_C\) for measuring the pressure \(P_A\) in cavity \(A\), connected with the closed piston-type

the high-pressure cavity of the hydraulic prop, pressure \(P_B\) in cavity \(B\) of the drain, and pressure \(P_C\) in cavity \(C\) of the prop unloading, respectively.

1. Inductive sensor 1 for the displacement \(x\) of stop 6 of the valve.

Fig. 1. Schematic diagram of an EKOR-type check valve and sensors for pressures, gate displacement, and ball rotation.

1. Sensor 2 for the angular velocity \(\omega\) of rotation of the valve ball, made in the form of a rigid thin rod soldered at one end into the ball. At the other end of the rod a light pinwheel is mounted, reflecting the signals of a photo-pulse stroboscopic tachometer, which recorded the rotational speed of the pinwheel.

The transient processes were recorded by means of an N-105 loop oscillograph.

Experimental investigations showed that the most severe operating modes of the valve, from the standpoint of dynamic loads and durability, are:

1. The “unloading” mode of the prop, which consists in opening the valve by means of pusher 3 under the action of pressure \(P_C\) and emptying cavity \(A\) of the prop.

2. The “setting” mode of the prop, consisting in filling with liquid and increasing the pressure of the closed cavity \(A\) of the prop up to the setting pressure of the overflow valve of the pumping station. At the end of the “setting” cycle the forces of static pressure acting on the gate are balanced, and under the action of spring 4 of the valve it closes.

The “unloading” mode is accompanied by intense longitudinal oscillations of the moving system (stop 6, ball 5, and pusher 3) of the valve and by oscillations of pressures \(P_A\) and \(P_C\) in the system. Figure 2 shows a typical oscillogram of the transient processes in the valve at the moment the valve opens in the “unloading” mode. It can be seen from the oscillogram that the amplitude \(x\) of oscillations of the stop is on average \(0.5\ \mathrm{mm}\), the amplitude of the change in pressure \(P_A\) in the prop cavity is \(200\ \mathrm{kg/cm^2}\), with the pressure overshoot reaching 50% of the initial pressure (\(320\ \mathrm{kg/cm^2}\)); the amplitude of the change in pressure \(P_C\) in the valve unloading cavity is \(10\ \mathrm{kg/cm^2}\); the oscillation frequency is approximately \(150\ \mathrm{Hz}\), which corresponds to the natural frequency of oscillations of the moving system.

High-frequency oscillations of the gate with significant amplitude are one of the causes of wear of the seat, stop, and valve spring, which is confirmed by the fact that valve parts that have failed (according to laboratory and production tests in this mode) show signs of fatigue failure.

Pressure surges \(P_A\) in the prop cavity, occurring at high speed and high frequency, cause significant impact loads on the moving elements and are the reason for premature failure of high-pressure flexible hoses, rapid wear of seals, and other elements of the hydraulic system [3] as a result of fatigue failures; this is also confirmed by operating practice for hydraulic props and by laboratory durability tests of the valve.

Fig. 2. Oscillogram of transient processes in the valve in the “unloading” mode

In the “setting” mode of the prop, in addition to longitudinal oscillations, intense torsional oscillations of the ball, stop, and valve spring were detected. These oscillations are caused, first, by the hydrodynamic action of the fluid jet, accompanied by strictly periodic shedding of vortices from the streamlined body of revolution and, consequently, by the occurrence of a periodic force acting in the direction transverse to the motion of the fluid flow [4]; and, second, by the presence in the shutoff element of a second degree of freedom in a direction perpendicular to the valve axis (to the motion of the flow), owing to the considerable radial clearance between the stop and the valve body.

Experimentally, a relationship was found between the rotational speed \(\omega\) of the ball and the principal parameters of the system. The greatest influence on the rotational speed of the ball is exerted by the volumetric flow rate \(Q\) of the fluid passing through the valve and by the stiffness \(c\) of the stop spring (Fig. 3). The nature of the dependence of the speed \(\omega\) on the principal parameters of the system agrees with the theory of separated flows in the boundary layer [5], which qualitatively confirms the nature of the origin of the torsional oscillations of the valve ball. It is important to note that the rotational speed of the ball is considerable (Fig. 3) and, under the most unfavorable combination of parameters (within the real range of their variation), reached 6000 rpm. At such high rotational speeds of the ball together with the stop and the valve spring, with significant radial clearances in the joints, considerable impact loads and friction forces arise, causing intense wear of the seat (Fig. 4, a), the stop (Fig. 4, b), and other valve parts.

Simple design modifications of the valve that eliminate the main causes of longitudinal and torsional oscillations made it possible to substantially reduce wear of the valve parts. Figure 4 presents photographs of the seat (Fig. 4, a) and the stop (Fig. 4, b) of a production valve

manufactured (by the Pneumatika plant) and the same parts (Fig. 4, c and d) of the valve of improved design, made after durability tests under laboratory conditions as close as possible to real ones, with identical conditions and test duration over the complete operating cycle of the valve.

Production tests at the Polysaevskaya-1 mine of Kuzbass of a series of improved-design EKOR-type valves, manufactured in the hydraulic-automation laboratory of TPI, showed that their service life, in comparison with the durability of serially manufactured valves, increased by more than 8 times.

Fig. 3. Dependence of the rotational speed of the valve ball on the flow rate of the fluid flowing through the valve in the “spacer” mode and on the stiffness of the shutter spring

Fig. 4. Characteristic wear of valve parts after durability tests: a) seat and b) valve stop of serial manufacture; c) seat and d) stop of an improved valve design; the tests were carried out under identical conditions

REFERENCES

1. V. N. Khorin, S. V. Mamontov, V. Ya. Kashtanova. Hydraulic Systems of Mechanized Supports. Moscow, Nedra, 1971.

2. E. G. Frank, L. N. Grinyas. Study of the dynamic characteristics of an EKP-type safety valve for hydraulic props of mechanized supports. In: Automation of Production Processes in Mechanical Engineering, Part 4, Tomsk, Tomsk University Publishing House, 1970.

3. I. A. Zakutskii, A. L. Koroblev. On hydraulic shocks in hydraulic systems of mine mechanized supports. In: Mining Machines and Automation, Issue 2, Moscow, Nedra, 1966.

4. N. S. Marein. Determination of the lift force for a circular cylinder oscillating in a fluid flow. Proceedings of the Moscow Power Engineering Institute, Issue 32, Moscow, 1959.

5. Ya. Ts. Fyn. Introduction to the Theory of Aeroelasticity. Moscow, Publishing House of Physico-Mathematical Literature, 1959.