One-Dimensional Dynamic Stability Analysis of Liquid Relief Case Study

The Challenge

An energy company contacted ioMosaic to evaluate dynamically the stability of eight separate Seal Oil Pressure Relief Devices (PRDs) during a blocked outlet scenario because of pocketed outlet piping. The main challenge encountered during this project was to accurately estimate the acoustic velocities of the oil. The velocities were estimated based on a first order regression relating acoustic velocity, temperature, pressure and API gravity, established in the Ph.D. thesis titled “Wave Velocities in Hydrocarbons and Hydrocarbon Saturated Rocks – With Applications to EOR Monitoring” (Z. Wang, Stanford University, 1988).

Our Approach

The project was conducted using Process Safety Office^® SuperChems^TM. The 1D dynamic analysis for each system was based on the following steps:

• Develop the PRD Single Degree of Freedom (SDOF) model and confirm the SDOF prediction of the blowdown
• Re-create the piping isometrics with elevation and orientation changes in Process Safety Office^® SuperChems^TM
• Establish the pipe/fluid system speed of sound and speed of sound reduction (if applicable)
• Run the one-dimensional (1D) dynamics
• Collect and animate the data produced

The Results

The deliverable for each system analyzed was a report illustrating the following results:

• SDOF model, which uses the PRD geometries, mass in motion, spring constant, and backpressure reduced fit data to simulate the relief device performance; i.e., calculation of the implied blowdown.
• Steady state evaluation based on the engineering force balance methodology as stated in API Standard 520 Part II to determine the potential for valve instability.
• One-dimensional (1D) dynamic evaluation to accurately predict the PRD behavior dynamic profiles, including:

• PRD lift at 10% overpressure Inlet, outlet mass flow profile at 10% overpressure
• Reaction force components on relief device inlet and outlet pipings
• Reaction force components on specific fittings in the relief layout
• Dynamic pressure and dynamic impulse propagation animations through the relief discharge piping

The analysis showed that the systems operate under stable manner and therefore, contingent upon the presence of appropriate drainage at the lowest point of the relief device discharge piping, these pressure relief systems can be considered to be consistent with Recognized and Generally Accepted Good Engineering Practices (RAGAGEPs), such as API Standard 520 Part II and ASME Section VIII.

Learn More

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