Project Synopsis

Evaluation of process pressure relief valves and relief systems (including relief valve discharge piping, sub-headers, headers, and flares)to determine relief valve capacities relative to current process throughput rates and 30 case scenarios.

Project Summary


The client contracted Process Engineering Associates, LLC (PROCESS) to execute the following major project tasks as follow-up work to a previous PROCESS project:

  • Conduct a comprehensive evaluation of the refinery’s approximately 300 process pressure relief valves (PRV) for the purpose of determining relief valve capacities relative to current process throughput rates
  • Conduct a comprehensive evaluation of the refinery’s two (2) relief systems (including relief valve discharge piping, sub-headers, headers, and flares) for the purpose of determining relief system capacity for approximately 30 credible relief scenarios, some of which were identified during the previous PROCESS project and some of which were requested by the client.

The pressure safety valve (PSV) evaluation was initiated due to the fact that an earlier relief valve study conducted by another engineering firm rendered inadequate information to reach conclusions relative to relief valve capacities as well as inadequate information relative to what was needed to conduct the flare system evaluation.  At the initiation of this task, the PROCESS project team gathered the requisite process and engineering information at the client site.  Using the process information as well as engineering information relative to the existing relief valves, PROCESS utilized its proprietary in-house relief valve capacity computer program to evaluate each process relief valve.  PROCESS used the program to evaluate the vapor and liquid relief capacities of each valve as well as relief requirements for all identified credible relief scenarios.  For relief valves determined to be undersized, the same program was used to calculate new recommended sizes.  PROCESS used its licensed commercial process simulation software’s thermodynamics package to calculate thermophysical properties for process streams, which was required as input to the relief valve program.

During the course of this task, PROCESS discovered missing mass and energy balance information for several of the refinery process units.  The licensed commercial process simulation software was used to simulate distillation towers and other portions of units with missing mass and energy balance information.

For the relief system evaluation task, PROCESS constructed detailed computer process simulations of the refinery relief system using the simulation software’s fluid flow package.  The identified relief scenarios were then modeled for the purpose of determining the presence of excessive back-pressures (i.e., inadequate capacities).  For sections of the relief system identified as bottlenecks, PROCESS modeled modifications that would increase the system capacity.  In addition, where applicable, PROCESS developed additional recommendations to clear the bottlenecks, including pressure vessel pressure re-rating, process or operational changes to reduce the required relief loadings, and potential modifications to existing relief valves (e.g., decreasing set pressures, increasing orifice sizes, or replacing conventional relief valves with balanced-bellows relief valves).

During the relief system evaluation, it was determined that the simulation software fluid flow package did not perform well under high flow, high pressure drop compressible flow situations encountered with the client relief system.  As such, PROCESS developed a comprehensive in-house adiabatic frictional flow model for compressible fluids that much more accurately modeled certain flow situations.

Also during the course of the project, the client requested that PROCESS execute several additional tasks that were not part of the original work scope. These included: siting and hydraulic modeling of flare subsystems to be installed for two new refinery units (diesel hydrotreater and hydrogen plant), evaluation of the capacity of the existing flare knockout drum, flare thermal radiation study, and the evaluation and preliminary process design modifications required for elimination of one of the two refinery flare systems.  PROCESS completed the project (including the additional scope) on time and under budget.

Industry Type

  • Petroleum Refining

Utilized Skills

  • Mass and Energy balances
  • Computer process simulation
  • Thermal radiation analysis
  • Adiabatic frictional flow modeling for compressible fluids

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