CFD Simulation of Flow-Induced Forces in a Separation Tank

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In this simulation, we investigated the dynamic flow behaviour inside a separation tank where cracks were observed at weld locations. Our client suspected that the cause was pressure-induced loading from the inflow of a mixed-phase stream of acid gas and liquid. To explore this, we performed a Computational Fluid Dynamics (CFD) study using a volume-of-fluid (VOF) interface capturing approach to model the gas-liquid interface and capture transient forces acting on the tank's internal deflector plate.

The separation tank operates with a mixture of gas and dispersed liquid entering via a single inlet. A deflector plate is welded to the tank internal walls to condense the liquid, which then collects at the bottom of the tank and is periodically pumped out. Meanwhile, the gas exits through an outlet located on the top side of the tank.

The video visualizes three key stages identified in the simulation:
1. As pressure builds up on the inlet side of the tank, the liquid is forced downward. This displacement causes the liquid level on the opposite side of the deflector plate to rise. The increasing pressure in the resulting closed-off ‘chamber’ exerts a force on the deflector plate. On the back side of the deflector plate there is a lower total force as the pressure in the liquid reduces with height, while in the gas it does not.
2. The force on the deflector plate peaks when the liquid is fully displaced to the bottom of the deflector plate. Under these conditions, the load on the deflector plate experiences a local maximum.
3. As the gas finally escapes under the deflector plate, oscillations in the forces develop. The escaping gas temporarily reduces the pressure within the chamber, alleviating the force. However, the movement of the liquid on both sides of the plate caused by the escaping gas generates transient force spikes, leading to dynamic oscillations. These oscillations exceed the previously observed maximum force.

The simulation highlights a "pressure chamber" effect as the primary driver of high, fluctuating forces—more significant than previously suspected inertial effects like slugging. These insights will inform the client’s structural Finite Element Analysis (FEA) to evaluate tank integrity and mitigate further weld failures.

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