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With the development of concrete pumping technology to date, concrete pumps have evolved towards higher pressure and greater flow rates. In the construction of high-rise buildings, it is necessary to consider the backflow of concrete when the pump stops operating, as well as the measures to prevent such backflow. 1 Backflow of concrete and its effects The so-called “stopping the pump” refers to the cessation of concrete pumping by the concrete pump (truck) during intervals in pumping (with the engine still running) or before cleaning work is carried out after completion. When the main cylinder is operating normally and the pump is stopped (not at the starting or stopping point), the concrete with a high slump value (18–22 mm) in the vertical piping flows back under the effect of its own heavy weight, through the horizontal piping and the concrete distribution valve. The backflow pressure is transmitted to the end face of the concrete piston, and from there, via the piston rod, it is transmitted to the piston of the main cylinder that drives the intake cylinder, as shown in Figure 1. In the absence of a structure to prevent concrete from flowing back, this backflow pressure will push the concrete back toward the stop pin (or tube) in the washing chamber. At the same time, the piston of the main cylinder is also subjected to backflow pressure as a result, causing it to return to its starting point A. Concrete pumping state of the main pumping system: 1. Y-shaped tube; 2. Swing tube; 3. Concrete delivery cylinder; 4. Washing chamber; 5. Stop pin; 6. Main cylinder; 7. Closed loop. Figure 1 shows the concrete pumping state of the main pumping system. When pumping stops, the oil pump in the main system is in a unloaded state. The high backflow pressure generated in cylinder A due to the backflow of concrete exerts significant stress on the oil pump. Since the rod cavity is in a closed configuration, when the piston of the left cylinder returns to point A at the position where pumping stops (indicated by the dashed line in the diagram), the volume of the closed loop increases. This leads to two possible scenarios: ① Under the high negative pressure resulting from the increased volume, the oil inside the closed loop becomes less dense, meaning the distance between oil molecules increases, and the air present in the oil separates out and forms bubbles. While emulsification occurs, external fluid will leak into the closed loop until the negative pressure in that loop becomes less than the sealing contact stress of all space agencies. As a result, internal leakage is the primary issue, with oil emulsification being secondary; this leads to an increase in the amount of oil within the closed circuit (in extreme cases, both main cylinders retract to their starting points A and B). During re-pumping, in addition to overcoming the resistance associated with concrete pumping, the main cylinder must also deal with the higher back pressure generated within the rod chamber as the increased volume of oil is discharged through the safety valve or check valve in the closed circuit. If the sum of the above two values reaches the overflow pressure of the main system, the main system will stop pumping. ②When the sealing performance of the closed loop is good, the main change in the oil within the loop is the emulsification of the hydraulic oil, with internal leakage being a secondary phenomenon. That is, the negative pressure in the closed circuit quickly reaches a level where it is less than the minimum contact stress of all sealing elements (in the extreme case, the oil in the original circuit is simply emulsified and its consistency doubles). During the period when the main cylinder moves toward rapid direction change during re-pumping, the emulsified oil is compressed and returns to its original state, so the pumping pressure does not increase. As it continues to move toward the end of its journey ； Since the extremely small amount of excess oil that has seeped in must be completely removed, the pumping pressure increases suddenly; at this point, the sum of the pumping pressure and the back pressure can reach the safe overflow pressure of the main system, resulting in failure of the pump in that system. 2 Design principles and applications of protection structures (1) On the main inlet and outlet oil passages of the master cylinder, the surface energy of a three-way four-position valve is utilized to isolate the main system’s oil circuit so that it remains in a unloaded state when the pump is stopped, as shown in Figure 2. This structure is used in the series of concrete pump (truck) products manufactured by Schwing Company. The three-way four-position valve controlling the master cylinder and counter-pump features a fluid-controlled spring-centered reset mechanism, with the spool having an M-shaped profile. Its working principle is as follows: when the pump stops, the hydraulic pressure pk controlled by fluid disappears. The three-way valve spool is in the middle position under the action of the springs on both sides; at this time, the oil outlet of the valve is closed, and the main oil pressure p is in a state of oil return and unloading. Blocked by the check valve. In some closed primary pumping systems, the three-way four-way valve used to control the direction of the main cylinder also adopts the three-position four-way valve structure shown in Figure 2, but its spool mechanism is of the “H” type. Although it meets the requirements for oil inlet and outlet in closed-loop systems, it cannot prevent concrete from flowing back. The author believes it is better to adopt the M-type function. It can meet the requirements of closed systems while preventing concrete from flowing back. 1. Three-way four-position valve 2. Electro-hydraulic directional control valve. Figure 2 shows one of the structures for preventing concrete backflow (2), which uses a shuttle valve together with a check valve, as shown in Figure 3. 1. Solenoid valve 2. Shuttle valve 3. Check valve. Figure 3: The second structure for preventing concrete backflow – the check valve can be locked to prevent the backpressure generated by concrete from affecting the cylinder. This structure is used in the pumps (vehicles) of IHI Corporation and similar domestic products. Its working principle is as follows: when pumping stops, the solenoid valve is in the position shown in the diagram. The backpressure pk generated by the backflow of concrete on the cylinder passes through the four-way valve of the main cylinder (two-position type), which pushes the spool valve core to the right position; this pressure then flows through the spool valve and the solenoid valve before acting on the spring chamber of the check valve. Since the check valve is of the DF type right-angled design, it allows the return pressure of the master cylinder to be separated from the main system, which is in a unloaded state. (3) Use the main spool of the sequence valve to open and close the main oil circuit. In the main system where the sequence valve operates in sequence, the closure of the main spool of the sequence valve when the pump is stopped is used to isolate the oil inlet path of the main cylinder from the unloading system, as shown in Figure 4. This structure is used in the pumps (vehicles) of IHI Corporation and similar domestic products. The principle is as follows: in the current structure, the check valve 2 and its control oil circuit f do not exist. When pumping stops, solenoid valve 3 is in the upper operating position shown in the diagram; the external control oil circuit of the pilot valve connects to the return oil circuit, causing the pilot valve to close immediately. The main spool also closes as a result. The back pressure generated by the backflow of concrete on the cylinder is blocked by the main spool. However, since the main spool of the sequence valve uses a conical valve seat seal, its oil inlets and outlets are also at right angles. Therefore, there is always an effective compressed annular area on the sealing cone surface that enables the main core to open. When the backflow pressure reaches a certain value, the force exerted on this compressed annular area becomes greater than the elastic force of the main core spring, causing the main core to open and the backflow pressure to flow back into the oil tank. Therefore, the height to which this structure can prevent concrete backflow is limited, and it needs to be improved. The principle of the improvement is as follows: an externally controlled sequence valve is used to direct the excess sealing oil from the solenoid valve to control the check valve installed on the main oil circuit, namely the box formed by the check valve and channel f as shown in Figure 4. Its working principle is as follows: when pumping stops, the solenoid valve is in the operating position shown in the diagram. The pressure from the pressure-holding circuit of the distribution valve, which comes from the accumulator, passes through this solenoid valve and the oil passage to reach the spring chamber of check valve 2, causing the check valve to close. This is because the compressed area of the spring chamber is much larger than the effective annular compressed area of the sealing cone surface. Therefore, the additional check valve 2 can effectively prevent the back pressure generated by the backflow of concrete from affecting the cylinder. 1. External control pilot-operated sequence valve 2. Check valve 3. Solenoid valve. Figure 4: The third structure for preventing backflow in concrete pumping (4): In the main pumping system using cartridge valves (as shown in Figure 5), backflow prevention is achieved by means of the cartridge valve 2 itself. Through the pilot control valve 1 of the cartridge valve, the backflow back pressure is regulated and controlled; this backflow back pressure acts on the spring chambers of the cartridge elements, thereby isolating the master cylinder back pressure from the unloaded main system. Its working principle is essentially the same as that of the second and third types of structures; in all cases, backflow pressure or externally applied pressure is used to close the check valve in the opposite direction (toward the spring chamber). Prevent backflow from the master cylinder. Its principle model is shown in Figure 6, where p_back represents the backpressure exerted on the main cylinder when concrete flows back, S is the effective working area of the spring chamber, and S_ring is the effective annular area at the sealing location subjected to p_back. 1. Pilot control valve 2. Plug-in valve 3. Check valve 4. Plug-in valve 5. Pilot relief valve 6. Solenoid valve Figure 5: Structure for preventing concrete backflow – Part 4 1. Sealing check element 2. Spring 3. Valve body Figure 6: Backpressure sealing for check valves or plug-in elements 3. Prevention and handling of concrete backflow during actual pumping operations In concrete pumps that are already in use but lack mechanisms to prevent backflow, special operations can be carried out on the main system to avoid concrete backflow; that is, pumping should be stopped immediately after the concrete distribution cylinder completes its direction change. At this point, the upward pressure of the concrete forces the concrete piston tightly against the washing stoppin (or tube). If concrete backflow has occurred and it is confirmed that (excess) oil has entered the closed circuit of the main cylinder, after pumping for a period of time, open the shut-off valve or adjustable flow valve controlling the oil volume in the return circuit to remove the excess oil from it; however, it should be closed immediately afterward. (end)