Introduction

Ore slurry tanks usually have agitators to homogenize the slurry before pumping it to the next stage of the material processing process. These agitators introduce cyclical forces into the side which, depending on the tank geometry, agitator rotation speed and power, can cause excessive vibrations in the equipment. These vibration levels can drastically reduce the useful life of the asset and generate a high risk of collapse due to crack propagation. In cases like these, vibration analysis by imaging the tank can provide crucial information for analyzing the situation, helping to propose reactive measures to mitigate the problem.

Case

The client found that his tanks, in a specific condition of slurry level and agitator rotation, had large vibration amplitudes visible to the naked eye. This vibration, in a short time of tank operation, was capable of causing cracks in the concrete base. Kot was therefore hired to assess the possible damage to the metal structure and propose solutions to mitigate the problem.

To measure equipment vibration in the field, Kot used motion amplification technology provided by RDI Technologies' IRIS MTM camera. The use of the IRIS camera in this case is very advantageous, as it allows vibration to be measured at various points on the tank in just a few minutes, determining the tank's vibration frequencies and displacements at various specific points on the structure. Find out more about vibration analysis by image by clicking here.

Figure 1: Vibration measurement with the IRIS camera.

In addition, it is possible to determine the complex mode in which the structure is vibrating with just a few recordings around the tank. To obtain this vibration mode with traditional measurement methods, such as accelerometry, it would be necessary to install several accelerometers at the same time around the entire side. This would be unfeasible in this case, due to the time required to install the sensors on the tank and the time required to process the signals.

Figure 2: Tank vibration - IRIS camera.

The figure below (Figure 3) shows an amplified image of the tank's vibration captured by the IRIS camera.

 

Figure 3: Amplified vibration of the tank - IRIS camera.

It was also observed that the vibration caused the bottom of the tank to lift in relation to the civil base, as shown in figure 4 below:

Figure 4: Survey of the tank bottom – IRIS camera.

The vibration data collected in the field was used to calibrate a forced vibration analysis using the finite element method (FEM). The FEM analysis provided displacement, stress, and deformation data for all points on the tank, making it possible to measure the effects of the vibration observed in the field on the structure.

 

Figure 5: Forced vibration analysis.

A high-cycle fatigue analysis of the tank structure showed that the vibration condition measured in the field is severe enough to reduce the tank's service life. If the equipment operates in this condition for about 200 hours, there is a risk of cracks appearing in the side welds, compromising the Structural Integrity asset.

Solution

The solution presented was a change in the tank's operating procedure. For the critical slurry levels for vibration, a reduction in the agitator's rotation speed was proposed. This measure moves the frequency of the exciter source away from the natural frequency of the structure, reducing vibration on the side of the tank by around 40 times. With this change, no cracks are expected to appear in the structure during the useful life of the asset.

Vale that this simple modification does not impact the productivity of the customer's operation, since the new speed selected for the agitator does not compromise the homogenization quality of the pulp. In addition, it avoids costs related to the installation of reinforcements, which include both their construction and the loss of profit due to the asset's downtime.

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