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Concrete pumps are widely used in construction projects, especially in the process of pumping concrete. Most commonly, double-cylinder driven pumps are used, and during the pumping reversal, a phenomenon known as "cut-off" often occurs. This issue poses two main risks: first, it can cause concrete segregation, with some parts drying out and hardening, leading to blockages—particularly when pumping low-slump concrete. Second, it causes sudden changes in pressure within the pipeline and distribution valve, which can damage hydraulic components and increase energy loss. The risk becomes even more severe when pumping vertically above 50 meters.
To understand the root cause, let's look at how the pump operates. A full-hydraulic control system uses a constant power main pump that supplies oil to the main cylinders and the distribution valve cylinder via hydraulic valves. When a commutation signal is sent, the hydraulic valves switch, allowing the main cylinder to change direction. This process typically takes at least 0.2 seconds. During this time, the suction efficiency of the concrete cylinder is only about 85% to 95%, which may result in air being drawn in, causing a short period of empty travel. Additionally, the outward pressure on the concrete during suction can compress it slightly, further contributing to this empty travel. These factors together lead to the flow interruption problem.
To address this issue, the key is to reduce the commutation time and accelerate the main piston movement at the start of the cycle. By doing so, the concrete can be continuously pumped before the pressure in the pipeline and distribution valve fully releases, minimizing segregation and reducing the impact caused by pressure changes.
One effective solution is to use an electro-proportional controlled constant power piston pump. This type of pump allows for precise control over displacement, enabling faster commutation and smoother operation. By adjusting the current to the proportional solenoid, the pump can increase its displacement quickly, ensuring that the concrete is pushed forward before the pressure drops too much. This helps maintain a continuous flow and improves overall performance.
In many systems, proximity switches are installed between the cylinder and the concrete tank. These switches help determine the starting and ending positions of the main cylinder based on its stroke and bore size. Typically, the distance between the switches is 5% to 10% of the total travel. When the switch detects the start signal, it sends a message to the PLC or relay, which then controls the current to the pump’s proportional valve. This allows the pump to reach maximum displacement quickly, ensuring smooth commutation and efficient pumping.
In countries like the U.S. and Europe, where pump usage is common and concrete specifications are more strictly controlled, the cut-off issue is less noticeable due to smaller building heights and better material quality. However, in China, where pumps with lower displacement (below 80m³/h) are widely used and buildings are taller, the cut-off problem is more pronounced. By optimizing the pump control mode, it's possible to significantly reduce flow interruptions, minimize mechanical stress, and enhance the overall performance of the concrete pump.