Flare System Computer Process Simulation and Capacity Evaluation

Project Synopsis

Project Summary

REFINERY FLARE SYSTEM PROCESS SIMULATION AND CAPACITY EVALUATION

Process Engineering Associates, LLC (PROCESS) was contracted by the client to perform a capacity evaluation of a flare system at one of its refineries.  Six flare system relief scenarios, including loss of cooling water, power failure, and four multi-relief valve fire scenarios were evaluated to determine the adequacy of the capacity of the relief system for these scenarios. Commercial process simulation software was used as the primary tool for the evaluation; the results and conclusions of the evaluation were summarized in a report prepared for the client.

Flare capacity evaluation project tasks included:

• Refinery process and flare system engineering documents review
• Heat and mass balance and vapor-liquid equilibrium calculations preparation
• Flare system computer process simulation model development
• Pressure relief scenarios modeling
• Flare system modeling results evaluation and modification alternatives development
• Flare system recommendations development.

PROCESS reviewed the results of a 1997 refinery-wide relief valve evaluation (previous PROCESS project) to identify significant relief scenarios to the north side flare system and reviewed plant layout drawings and relief valve information to identify congested areas in the refinery that could discharge significant fire relief rates to the north side flare system.  PROCESS then presented potential scenarios to the client who requested that detailed flare system hydraulic studies be conducted for the six relief scenarios.  PROCESS re-characterized all relieving streams taking advantage of the detailed thermodynamics package associated with the process simulation software.  Reviewing client-generated isometric piping drawings, PROCESS then constructed a detailed hydraulic computer model of the flare system, which included mass and energy balance and two-phase flow prediction capabilities.  The pressure relief scenarios were simulated using the process model, as were flare system modifications that could be made to increase system capacity as necessary.  Resulting process modification alternatives and recommendations for relief valves and vessels exposed to high back pressure and for increasing flare system capacity included: (1) increasing pressure vessel ratings, (2) decreasing relief valve set pressures, (3) adding relief valves and/or replacing standard relief valves with balanced-bellows or pilot-operated relief valves, and (4) modifying flare system design to decrease system back pressure and increase capacity.

PROCESS’ use of commercial process simulation software generated less conservative results than the more traditional approaches that have been typically used for flare system analysis.  The capability of the software to determine and factor in the thermophysical properties of the relieving fluids under high pressures led to reductions of more than 10 percent for some flare relief vapor flow rates and reduced back pressures of up to 33 percent for some scenarios as compared to values generated by traditional calculation methods.  This prevents unnecessary system modifications undertaken based on overly-conservative results.

Industry Type

• Petroleum Refining

Utilized Skills

• Flare system hydraulics analysis
• Heat and mass balances
• Computer process simulation
• Advanced flare system modeling
• Two-phase flow modeling
• Pressure relief valve (PRV) or pressure safety valve (PSV) sizing