Weld Cracking Investigation on T Bar Section of Cyclones

Cyclone FEA Model - Failure Analysis & Design Modifications
Cyclone FEA Model - Failure Analysis & Design Modifications

Keywords: T Bar Section; Cyclones; Modal Analysis; Design Recommendations

Weld cracks were found on the T Bar section of cyclones at a manufacturing plant. We performed a finite element analysis (FEA) to determine the mode of failure – and recommended design modifications.

Although the Tee Bar section had been weld repaired, cracking in this area had become a recurring problem. We performed modal analyses on the cyclone to determine natural frequencies which may be excited, leading to the cracking in the Tee Bar. We also investigated various design modifications to prevent future issues.

A three-dimensional finite element model was created using ANSYS. The model consisted of approximately 60,000 shell elements and 60,000 nodes. The model was completely constrained (no translation or rotation permitted) at the base of its eight support legs.

The finite element model included various assumptions and simplifications including:
1. Material properties evaluated at room temperature.
2. Welds were not explicitly modeled.
3. Minor components such as small bore piping, bolting, and lifting lugs were not modeled.

A modal analysis was performed to determine the natural frequencies of the cyclone. The resulting mode shapes were then evaluated to determine which natural frequencies would result in significant stresses in the Tee Bar.

Thirty modes were extracted using the Block Lanczos method in a frequency range of 0 to 50 Hertz. Of the thirty modes extracted, three result in significant stresses in the area of the Tee Bar: Mode 3 at 16.7 Hz, Mode 4 at 22.1 Hz, and Mode 5 at 22.9 Hz.

All three of these modes included a rocking motion of the upper cylinder with respect to the lower cylinder of the cyclone. Modes 3 and 4 are similar with the difference being the axis about where the rocking occurs.

Inspection of the modes of vibration that most likely lead to cracking in the Tee Bar suggested two possible design modifications: locally stiffen the area of the Tee Bar to reduce the level of stresses, and/or locally stiffen the upper cylinder of the cyclone inhibiting the range of motion of the mode of vibration, thereby reducing the magnitude of the stresses in the Tee Bar area.

After further investigation, it was concluded that:
• The natural frequency that is most likely being excited is a rocking mode and occurs at 22.9 Hz. This finding is in close agreement with field vibration testing on the actual unit.
• A proposed modification of increasing the thickness of the Tee Bar does not result in a significant decrease in the level of stresses in the area of cracking.
• The proposed modification of adding radial stiffeners between the upper and lower cylinders inhibiting the relative rocking motion between the two cylinders showed a decrease in stress of approximately 33%. The addition of radial stiffeners between the upper and lower cylinders should delay or eliminate the cracking in the area of the Tee Bar. The stresses and fatigue life of the stiffeners and the welds joining the stiffeners to the cyclone should be evaluated.


 

O’Donnell Consulting Performs (Modal, Vibration, Thermal & Fatigue) FEA Analysis – and Make Design Recommendations – for Clients in Industries including Manufacturing, Petrochemical and Energy.

(412) 835-5007

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