Pumping concrete

According to Concrete Pumps, it is necessary to distinguish between the operating state of the pump (start-up, pumping, normal conveying operation, emptying and cleaning the line, malfunctions etc) and the operating state of the concrete. The type of concrete pump, either piston or peristaltic (squeezed tube) and the type of valves used for the piston pump (trunk or ‘S’ pipe) do not have any influence on the quality or the character of fresh concrete inside the pump.

Concrete can only be pushed through the delivery line when it has previously been sucked from a hopper, this being achieved by increasing the volume of the conveying space, ie the volume of delivery cylinders, of the pump so the concrete fills the conveying space to its optimum level. By decreasing the volume of the conveying space, the concrete is pushed out into the delivery line and displaces the entire concrete column in the delivery line.

On closer examination, it can be seen that the suction action is also pushing, thereby increasing the volume in the conveying space which results in a pressure level lower than the atmospheric pressure. This pushes the concrete out of the hopper into the conveying space at maximum pressure of 1 bar, but only in the absence of a continuous ‘air bridge’ between the conveying space and the atmosphere.

The low pressure level for suction and filling requires the lowest possible level of resistance to flow and deformation of the concrete. The agitator in the hopper and its geometrical shape contribute considerably towards this. The agitator not only keeps the concrete free-flowing during breaks in the conveying action but also moves and pushes the concrete during suction in an even and controlled flow rate into the suction opening. The filling rate of the conveying space is essential to the efficiency of the pump.

For optimum suction conductions, the suction openings and the conveying area diameter need to be kept as constant and as large as possible. This also results in the essential differences between piston and peristaltic pumps. Piston pumps suck in the concrete through large cross-sections and reduce the cross-section when pushing out the concrete, with the size of the output being dependent on the cylinder volume, number of piston strokes per minute and the engine power. Peristaltic pumps are limited to around 30 bar with regard to their delivery pressure and delivery performance is restricted by suction performance.

When the concrete is pressed out of the delivery cylinders of a piston pump into the delivery line, it experiences a reduction in cross-section to the diameter of the delivery line during and after passing through the valve. This results in an increase in speed together with a corresponding increase in the boundary layer per volume of unit. To reduce the associated conveying resistance, the cross-section reduction is carried out continuously as far as possible over a sufficiently long section. This reduction of the cross-section inside or immediately downstream of the pump also provides a pumpability test for the concrete.

Increasing the cross section of the line has a direct influence on the flow rate, the wear behaviour, the pressure requirement and the period for which the concrete remains in the pipeline – given the same output per hour. In comparison with the 125 mm delivery line, the cross section for the 150 mm diameter increases by around 44%. This results in a pressure reduction of around 25%, and the wear also reduces accordingly.

However, as the flow rate drops, the period for which the concrete remains in the line increases. This longer flow time must be taken into account when developing the concreting concept. With an assumed structure height of 580 m, the period for which the concrete remains in the 150 mm delivery line is around 35 minutes and added to that is the significantly higher load on the shut-off valve due to the much heavier concrete columns in the line.

It is very important for piston pumps that the delivery space is emptied as thoroughly as possible with every pump stroke. This is because as a dead volume it can remain in the delivery space, primarily on the delivery piston, at least until the next time the pump is cleaned. If this dead volume hardens or sets, there is the strong possibility that this can lead to destruction of the seals, the delivery piston and the delivery cylinder inside the wall.
This article comes from WORLDPUMPS edit released