A client approached Dynaflow with concerns about potential dynamic loads on their piping system during the activation of a Pressure Safety Valve (PSV) protecting a degasser. During PSV opening, both gas and liquid (methane and water) are discharged together, creating complex flow conditions that could lead to transient forces and unwanted vibrations in the connected piping.
To address these concerns, Dynaflow carried out a high-fidelity multiphase CFD analysis aimed at:
- Understanding the flow behavior across different gas–liquid configurations
- Quantifying the forces exerted by the transient multiphase flow on the piping system
- Later used these forces in a dynamic stress analysis in CAESAR II to verify that the stresses remain below the allowable stresses.
CFD Model Overview
The PSV geometry was modeled in detail, based on client specifications:
- Inlet diameter: 92 mm
- Outlet diameter: 102 mm
- Inner opening height: 41.5 mm
The overall setup included:
- A DN200 inlet pipe (1 meter length)
- A DN150 outlet pipe (765 mm length) with a 90° bend at the end
The CFD analysis produced a complete picture of the flow dynamics, including:
- Pressure and velocity field animations
- Detailed force reports in multiple directions
- Time histories of flow behavior and force fluctuations
One key result was the force acting in the X-direction on the outlet bent pipe. While the raw signal showed high-frequency oscillations likely due to numerical noise, a filtered trend revealed a more realistic force profile. The system reached steady state around 0.3 seconds, once all initialized water/gas regions had moved through the domain.
From CFD to Vibration Analysis
Using the transient force data extracted from the CFD model, Dynaflow conducted a detailed vibration analysis in CAESAR II. These force inputs allowed for an accurate evaluation of the dynamic response of the piping system, helping to identify potential risks and ensure compliance with relevant design codes. In addition, by introducing the necessary restraints to mitigate overstress conditions, the analysis also provided the support reaction forces required for the final support design.
Conclusion
The investigation into the dynamic loads and vibrations associated with the Pressure Safety Valve (PSV) discharge system has yielded critical insights into the operational challenges faced by the client. Through multiphase CFD analysis, Dynaflow successfully characterized the complex flow dynamics during PSV activation, where a mixture of methane gas and water is discharged. The analysis revealed significant transient forces acting on the piping system, particularly at the outlet bend, where localized accelerations and phase interactions can occur.
Subsequent vibration analysis confirmed that while most flange loads, specifically those at the PSVs in Trias 1 and on the degasser, were within acceptable limits, the PSV in Trias 2 presented a concerning scenario. The thermal loads imposed on the flange at this location were found to exceed acceptable thresholds, posing a potential risk for leakage under full design pressure and temperature conditions.
To mitigate these risks, it is imperative to implement the recommended adjustments, specifically by extending the blow-off of the PSV in Trias 2. This measure aims to reduce the thermal loads on the flange, ensuring the integrity and safety of the piping system.
In summary, the comprehensive analysis not only addressed the client’s concerns but also provided actionable recommendations that enhance the safety and reliability of the PSV discharge system, reinforcing the importance of ongoing assessments and adjustments in response to dynamic operational conditions.
