Our client reported multiple cracks at weld locations on a separation tank. These cracks are suspected to be caused by dynamic flow effects from the mixed inflow of acid gas and liquid. To address this issue, the client requested a Computational Fluid Dynamics (CFD) analysis to quantify the forces exerted on the drum internals due to the inflow.
The separation tank operates with a mixture of gas and dispersed liquid entering via a single inlet. A deflector plate is installed 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 objective of this study was to understand the key flow phenomena inside the tank and provide time-varying forces acting on the tank internals. These results will enable the client to conduct a structural Finite Element Analysis (FEA) to assess the drum’s integrity under operational conditions.
Approach to solving the Navier-Stokes equations for two phases
Computational
Fluid Dynamics (CFD) is a powerful numerical tool used to simulate fluid flow
and related physical phenomena by approximating solutions to the Navier-Stokes
equations. These equations, which govern all fluid motion, are notoriously
complex to solve directly. CFD methods attack this challenge by dividing the
flow domain into a computational mesh made up of numerous smaller cells. Within
each cell, the equations are solved iteratively, enabling the calculation of flow
properties such as velocity, pressure, and temperature within each cell.
In this
study, CFD was used to investigate the causes of high weld stresses in the
tank, both qualitatively and quantitatively. A volume-of-fluid (VOF)
phase-fraction-based interface capturing approach was employed to track the
interface between the gas and liquid phases throughout the simulation.
Additionally, to account for the expected large pressure variations in the
tank, the equations were solved for compressible flow. Ultimately, the pressure
data calculated within the cells adjacent to the tank walls were extracted as a
deliverable for a subsequent Finite Element Analysis (FEA) by the client.
Excessive liquid levels cause a “pressure chamber” effect
From the analysis of the given process conditions, it was identified that the primary cause of excessive forces on the tank internals was the high liquid level, which was not pumped out frequently enough. When the liquid level rose above the deflector plate, it created a closed chamber where pressure could build up before the gas escaped under the plate.
Simulations were conducted for varying initial liquid condensate levels. The results showed that forces on the deflector plate were low when the gas could flow freely beneath it. However, when the liquid level rose above the deflector plate, the following stages of flow development were observed:
- 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.
- 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.
- 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.
Key Insights
This study successfully identified the mechanism responsible for the high loads on the deflector plate of the separation tank. The forces acting on the plate were quantified and provided to the client as inputs for their structural Finite Element Analysis (FEA). The analysis revealed that the initial condensate level significantly influences the loading on the deflector plate, whereas the inlet flow velocity has a comparatively minor effect.
The maximum force on the deflector plate was not solely determined by the energy required to displace the liquid until a gas flow path was established beneath the plate. Once this pathway formed, a dynamic “gulping” phenomenon was observed, characterized by intermittent gas escape and blockage. This cyclical process caused fluctuating loads on the plate, with the peaks exceeding those observed during the initial liquid displacement.
Conclusions & Recommendations
This study found that high weld stresses in the separation tank are more likely caused by pressure chamber effects and transient variations in liquid levels, rather than inertial impacts like water hammer or slugging flow, as initially suspected by the client. For future work, the following recommendations are proposed:
- The pressure data from the CFD simulations should be incorporated into Finite Element Analysis (FEA) to evaluate stress distributions. While higher liquid levels lead to greater total loads, the FEA study should consider the pressure data for various initial liquid levels. This would capture the effects of shear forces and moments at weld points, which could also contribute to weld failure. For instance, higher liquid levels may reduce these moments by creating a counterweight as liquid is pushed on top of the deflector plate.
- Improved accuracy in condensate level measurements is recommended for a more precise analysis. Current assumptions are conservative due to limited data on liquid levels during operation.
