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Concrete pumps are widely used in modern construction projects, particularly in the pumping of concrete. The most common type is the double-cylinder driven pump, which often experiences a "cut-off" phenomenon during the reversal process. This issue poses two major risks: first, it can cause concrete segregation, leading to dehydration and hardening, especially when pumping low-slump concrete; second, it creates sudden pressure changes in the pipeline and distribution valve, causing hydraulic shocks that may damage components and increase energy loss. The risk becomes more pronounced when pumping vertically above 50 meters.
To understand the root causes, we need to look at how the system operates. The pump is fully hydraulically controlled, with the main pump delivering oil through valves (8-1 and 8-2) to the main cylinders (10-1 and 10-2) and the distribution valve cylinder (9). Signals from the main cylinder cartridge valve and distribution valve cylinder control the commutation of the hydraulic valve, ensuring the proper sequence of actions for the pumping cycle. However, during each commutation, the flow of concrete often stops momentarily, creating what's known as a "flow break."
There are three primary reasons for this issue. First, when the main cylinder nears the end of its stroke, the commutation signal triggers the movement, which takes at least 0.2 seconds. Second, the suction efficiency of the concrete cylinder is typically between 85% and 95%, meaning some air is occasionally drawn in, resulting in a brief empty travel. Third, the outward pressure on the concrete during suction causes slight compressibility, contributing to another short period of empty travel. These combined factors lead to the flow interruption during pumping.
To address this problem, the key is to minimize the commutation time and accelerate the main cylinder’s movement at the start of the process. This ensures that concrete continues to flow before the pressure in the piping and distribution valve is fully released, reducing the risk of segregation and minimizing hydraulic shocks.
Most systems can resolve the cut-off issue by using an electro-proportional controlled constant power piston pump. This technology allows for faster and more precise control of the pump’s displacement, enabling quicker commutation. By adjusting the current, the pump can increase its displacement rapidly, helping to maintain continuous flow. This approach not only improves performance but also reduces wear and tear on the system.
In practice, proximity switches are often installed between the cylinder and the concrete tank. These switches help determine the starting and ending points of the cylinder’s movement based on the pump’s displacement and the cylinder’s size. The distance between the switches is usually 5% to 10% of the total travel. When the switch detects the start signal, it sends a command to the PLC or relay, which then controls the current to the proportional solenoid, increasing the pump’s displacement. This allows for rapid commutation and smoother operation.
In many countries, especially in Europe and the U.S., the use of concrete pumps is more regulated, with stricter control over concrete mix design and pump displacement. As a result, the cut-off issue is less noticeable. However, in regions where pumps with lower displacement (below 80 m³/h) are commonly used and building heights are greater, the problem is more evident. By selecting and optimizing the pump’s control mode, the flow interruption can be effectively minimized, significantly improving the overall pumping efficiency and reducing mechanical stress.