This paper introduces the working principle, category and application precautions of the pilot operated check valve, and gives an example of the correct use of the pilot operated check valve to avoid the accident. At the same time, also lists the hydraulic check valve often appear the fault, causes and troubleshooting. Hydraulic control check valve is a kind of one-way valve which can be opened and controlled reversely. Such valves are widely used in metallurgical equipment. Due to the particularity of its structure and principle, the on-site control valve is used improperly Faults often occur. 1, hydraulic check valve structure and working principle of liquid-controlled check valve structure shown in Figure 1. When the control port pressure hydraulic oil, the oil can only from PA â†’ PB; when the control port through the pressure oil, the positive and negative oil can be freely passed. When the oil flow in the opposite direction, the force balance of the valve core expression is: pKAK-pAAK-FKM = pBA-pAA = Fs + Fs + W (1) Where pK --- control oil pressure in N; pA --- Reverse oil pressure in oil chamber, in N; pB --- Reverse oil pressure in oil chamber, in N; FKM --- Control piston frictional resistance in N; FM-- - cone poppet total frictional resistance in N; Fs --- spring force in N; W --- spool weight in N; AK --- control piston area in m2; A- - Seat area, in m2. When the frictional resistance between the control piston and the poppet is omitted, the control oil pressure is: pK = [(pB-pA) A + Fs + W] / AK + pA (2) The control oil pressure. If the valve port is closed, the reverse flow of oil stops, then the pressure of the oil chamber pA = 0. Piston according to the control of the upper chamber of the drain is divided into different types of internal leakage and leakage type. The internal bleed is also the simple pilot check valve shown in Figure 1. Leakage-type liquid-controlled check valve shown in Figure 2, this leak-type liquid-controlled check valve reverse the pressure pA out of the cavity only in the control piston at the top, compared with the structure of Figure 1, the role of the area should be small Much, at the same time, reverse the oil pressure of the oil chamber and control the pressure oil to leak into the control piston between the upper and lower segments, can be led directly through the external leakage to the valve body to avoid the accumulation of leakage of oil control piston Of the upward movement, it is called leak-type pilot-operated check valve. Duplex structure of liquid-controlled check valve shown in Figure 3, unidirectional valve spool with a small unloading spool. When the control piston goes up, first open the small spool to relieve the pressure of the main oil line, and then open the check valve spool, the control pressure is only 4.5% of the working pressure, there is no unloading spool control of the pilot operated check valve Pressure is 40% ~ 50% of working pressure. Fig.1 Simplified pilot operated check valveFig.2 Leakage pilot operated check valveFig.3 Two-way pilot operated check valve 2. Application of pilot operated check valve The pilot operated check valve is characterized by low leakage and good locking performance, Reliable and widely used in metallurgical hydraulic system, such as iron and steel plant mud gun rotary hydraulic device, electric furnace electrode lifting circuit, electric basket hoist hydraulic circuit suspension, rolling mill and coiler hydraulic system is the extensive use of hydraulic control one-way Valve, shown in Figure 4 for the hydraulic check valve in a mill looper hydraulic circuit applications. Figure 4 hydraulic looper system pilot check valve specific applications are as follows: (1) to maintain pressure. Sliding valve valve has a gap leakage phenomenon, only a short time holding pressure. When the packing requirements, you can add a pilot on the oil-way valve, the use of cone valve closure of the tightness, the oil pressure for a long time holding. (2) Hydraulic cylinder "support." In a vertical hydraulic cylinder, the piston and piston rod may slide downward under the weight of the piston and piston rod due to leakage of the spool valve and the pipe. The hydraulic check valve connected to the lower chamber of the hydraulic cylinder, you can prevent the hydraulic cylinder piston and sliding part of the sliding parts. (3) to achieve hydraulic cylinder lock. When the reversing valve is in the neutral position, the two pilot-operated check valves are closed and the oil in the two chambers of the hydraulic cylinder can be tightly closed. At this time, the piston can not move due to the external force. (4) large flow discharge. The effective working area of â€‹â€‹the two chambers of the hydraulic cylinder varies greatly. In the return of the piston, the hydraulic cylinder right chamber of the sudden increase in oil displacement, this time if the use of small flow spool valve, will have throttling effect, limiting the piston back speed; if the addition of pilot-operated check valve in the hydraulic cylinder When the piston is retracted, the control pressure oil will open the pilot check valve so that the right cavity fluid can be discharged smoothly. (5) for oil filling valve. Vertical hydraulic cylinder piston in the high-speed descent process, due to the role of high-pressure oil and dead weight, resulting in rapid decline, resulting in suction and negative pressure, you must add a charge device. Hydraulic check valve as a filling valve to complete the filling function. (6) combined into a valve. In the design of hydraulic circuit, sometimes can be combined into a pilot valve to use the valve. For example: With two pilot operated check valves and a check valve in parallel (check valve centered), it is equivalent to a 3/3-way valve reversing circuit. It should be pointed out that when controlling the pressure oil port does not work, it should be back to the tank, otherwise difficult to reset the control piston, one-way valve can not stop the reverse flow. 3, the use of liquid control check valve site practice has proved that the use of liquid-controlled check valve prone to problems in the course of the maintenance, the following is a precaution. (1) must ensure that the pilot check valve has enough control pressure, absolutely not allowed to control pressure loss. Should pay attention to control the pressure to meet the reverse opening requirements. If the control of pilot-operated check valve is taken from the main system, then the influence of the pressure change of the main system on the pressure of the pilot line should be analyzed to avoid the misoperation of the pilot-operated check valve. (2) Depending on the position of the pilot operated check valve in the hydraulic system or the flow resistance (back pressure) after the reverse oil outlet chamber, the structure (simple or double) of the pilot operated check valve and the drain Way (internal leakage or leakage). For the internal leakage type check valve, when the reverse oil outlet pressure exceeds a certain value, the hydraulic control part will lose control, so the internal liquid discharge check valve is generally used for reverse the oil chamber back Pressure or back pressure smaller occasions; and leak-type liquid-controlled check valve can be used to reverse the oil chamber back pressure higher occasions to reduce the minimum control pressure, saving control power. As shown in Figure 5, if the system uses the internal discharge type, the plunger cylinder will intermittently drop to make vibration and noise. Figure 5 pilot-operated check valve for the reverse back to the oil chamber back pressure higher occasions Figure reverse the oil into the chamber pressure is high, then use unloading spool of the hydraulic check valve, then control the oil Pressure is reduced to a fraction of the original one to one-twentieth. If you choose a leak-type pilot-operated check valve, pay attention to the external drain connected to the tank alone as shown in Figure 6. In addition, the hydraulic cylinder rod chamber and rod chamber ratio can not be too large, otherwise it will result in hydraulic check valve can not open. As shown in FIG. 7, the ratio of the acting area of â€‹â€‹the upper and lower chambers of the hydraulic cylinder is larger than the ratio of the acting area of â€‹â€‹the control chamber and the one-way valve core of the pilot-operated check valve. If the acting area ratio of the upper and lower chambers of the hydraulic cylinder is 4: 1, Hydraulic control check valve control piston area and the role of the area of â€‹â€‹the upper one-way valve is generally the ratio of (2.6 ~ 3.5): 1, then the pilot-operated valve will never open. At this time, the hydraulic cylinder will be the same as a supercharger, the hydraulic cylinder under the serious pressurization, the pressure of several times the working pressure, resulting in accidents. Therefore, the ratio of the area of â€‹â€‹the working chamber of the general industrial hydraulic cylinder with the working area of â€‹â€‹the rodless chamber usually takes 3: 7-9: 10. Figure 6 Leakage pilot operated liquid control valve Figure 7 Single-sided hydraulic cylinder locking circuit (3) with two pilot-operated one-way valve or a double check valve hydraulic system to achieve hydraulic cylinder lock , Should pay attention to the selection of Y-type or H-type performance of the reversing valve to ensure that the pilot-controlled pilot pressure in the center can immediately release the one-way valve immediately closed, the piston stopped. If the use of O-type or M-type function, the reversing valve to the center, due to pilot-operated check valve control chamber pressure oil is closed, the pilot check valve control oil pressure is still there, so that the hydraulic control Check valve is still open. But can not make it immediately shut down, the piston can not stop immediately, resulting in movement phenomenon. Until the control valve is relieved by the internal leakage of the directional control valve, the pilot check valve can be closed, affecting the locking accuracy. However, the choice of H-type medium-performance should be very careful, because when the hydraulic pump flow through the drain pipe, if the exhaust pipe slender or partial obstruction or other causes of local friction caused by friction (such as with low-pressure filter Or pipe joints, etc.), may make the control piston subjected to higher control pressure, resulting in hydraulic check valve can not be closed and make the hydraulic cylinder malfunction. Y-type median will not form this result. (4) The working flow should match the rated flow of the valve. (5) When installing, do not mix the main port, the control port and the drain port and recognize the main port of the positive and negative directions, so as not to affect the hydraulic system's normal work. (6) The pilot operated check valve with unloader spool is only suitable if the reverse flow is a closed chamber, such as a chamber or accumulator of a hydraulic cylinder. The pressure in this closed chamber can be relieved by simply releasing a small amount of flow. The reverse flow is generally not connected to a continuous supply of hydraulic power. This is because the unloading valve spool opening flow area is very small, high oil pressure, pressure loss, coupled with the continuous supply of oil when the hydraulic pressure will cause the reverse pressure drop down, you need a lot of liquid Control pressure to make the pilot check valve main spool open. If the control of the pipeline when the oil pressure is small, there will be no open liquid-controlled check valve failure. (7) As shown in Figure 8, the system check valve can not be used alone to balance the circuit. Otherwise, when the piston descends, the descending speed of the piston exceeds the speed set by the oil inlet due to the dead weight of the moving part, so that the cylinder 6 Vacuum appears in the upper chamber. The control oil pressure of the pilot operated check valve 4 is too low, the check valve is closed and the piston motion is stopped. After the pressure in the upper chamber of the hydraulic cylinder is re-established, the check valve is opened again and the piston begins to descend again. Such duplication is the phenomenon of crawling or jitter, vibration and noise. In the rodless chamber and hydraulic pilot check valve 4 in series between the one-way throttle valve 5, the system constitutes a return to throttle throttling circuit. This will neither be due to the weight of the piston and reduce over-speed, but also ensure that there is sufficient pressure oil, so that the pilot-operated check valve 4 remains open, the piston steady decline. Valve 3 should be H or Y type function, if using M-type function (or O-type function), then due to hydraulic control check valve oil can not be timely relief, the circuit lock, the work of a stop Not allowed, resulting in movement phenomenon. Figure 8 balance circuit 1 - hydraulic pump 2 - relief valve 3 - valve 4 - hydraulic check valve 5- one-way throttle valve 6- hydraulic cylinder 4, check valve and fault diagnosis and troubleshooting Control one-way valve of the common fault and troubleshooting shown in Table 1. The common faults and diagnosis of liquid-controlled check valve troubleshooting Table 1 5, Conclusion Understand the structure and working principle of liquid-controlled check valve, fault law and maintenance knowledge, to master the correct use of the hydraulic system to improve and reduce accidents, It is important to improve the efficiency of hydraulic system maintenance.
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