REFINERY RELIEF VALVES AND FLARE SYSTEM CAPACITY EVALUATIONS
The client, an independent petroleum refinery, contracted Process Engineering Associates, LLC (PROCESS) to execute a refinery relief valve and flare system capacity evaluation project. The project included the following major tasks:
- Create computer process simulation mass balances for six refinery unit operations (Crude, Naphtha Hydrotreater, Reformer, Isomerization, Resin Oil Supercritical Extraction (ROSE), and Methyl Ethyl Ketone Dewaxing and Deoiling)
- Conduct a comprehensive evaluation of the refinery’s approximately 300 process pressure relief valves for the purpose of determining relief valve capacities relative to current process throughput rates
- Conduct a comprehensive evaluation of the refinery’s relief (flare) system (including relief valve discharge piping, sub-headers, headers, and flare) for the purpose of determining relief system capacity for approximately 12 credible relief scenarios, which were identified conjunctively by PROCESS and the client.
PROCESS utilized the refinery’s product specifications, operating data, and equipment engineering information to generate mass balances using PROCESS' licensed commercial computer process simulation software, CHEMCAD.
The pressure relief valve (PRV) evaluation was initiated due to the fact that the client did not possess the appropriate engineering documentation for the sizing of the refinery’s relief devices. 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 pressure safety valve (PSV). 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 process simulation software’s thermodynamics package to calculate thermophysical properties for process streams, which were required as input to the relief valve program. PROCESS also recorded the protecting relief device for every piece of equipment in the refinery for the client’s equipment database.
For the relief system evaluation task, PROCESS constructed detailed computer process simulations of the refinery relief system using the simulation software’s fluid flow (hydraulics) 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 eliminate 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 course of the project, PROCESS developed a unique program, using its steady-state process simulation software, to simulate an unsteady-state fire scenario for the purposes of determining more accurate fire case relieving rates See Details. This program was used to demonstrate in one instance that the existing system could actually handle the fire relieving rates, whereas conventional fire calculations provided results that would have required a considerable change to the relief system to accommodate a flow rate that would, in reality, never be attained.
A project notebook was provided to the client that contained detailed calculations, a working copy of the simulations, and a tabulated summary of each PRV (original information and recommended changes). PROCESS followed up this project with an additional project to size new relief valves for unprotected equipment.
- Petroleum Refining
- Relief valve sizing
- Flare header design
- Advanced process simulation
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