Project Synopsis

Perform a relief device sizing study for an Isomerization Unit and associated Propane Refrigeration Skid at an independent U.S. oil refinery.

Project Summary


The client, an independent oil refinery in the U.S., contracted Process Engineering Associates, LLC (PROCESS) to provide a pressure relief device study for their Isomerization Unit and associated Propane Refrigeration Skid.  The purpose of this project was to determine required relieving capacities for approximately thirty one (31) existing relief devices protecting equipment within the Isomerization Unit and compare results with actual flow capacities for the installed relief device.

PROCESS completed the following tasks in support of this effort:

  • Provided on-site support to assemble relevant equipment information, develop isometric sketches of pressure safety valve (PSV) inlet/outlet piping, and collect operating data from the unit.
  • Reviewed existing documentation including previous PSV studies, process flow diagrams (PFDs), piping and instrumentation diagrams (P&IDs), etc.
  • Developed a computer process simulation model of the Isomerization Unit in HYSYS to provide accurate mass balance and physical property information needed for the study.
  • Determined equipment protected by each relief device and identified any equipment shown in the P&IDs that are not credibly protected.
  • Identified all potential relief scenarios for each relief device.  Both the required capacity and available capacity were evaluated using sizing equations and general guidelines as set forth in American Petroleum Institute (API) guidelines 520, 521, 526, and 2000.
  • Rate dependent overpressure scenarios were evaluated at both the current feed rate and a proposed future increased feed rate for the unit.  The current maximum Isomerization Unit capacity was calculated, and recommendations made in order to increase the maximum unit capacity.
  • Calculation results and supporting documentation were provided for each relief device as well as a capacity summary table showing evaluation findings in tabular form.  The table provided relevant device information; the controlling relief scenario; required size and capacity for the controlling scenario; actual capacity for the controlling scenario; inlet/outlet piping pressure drop (as percent of set pressure) at the device’s actual capacity for the controlling scenario; calculation results for non-controlling scenarios; estimated unit capacity for the controlling non-fire related scenario; and recommendations related to any deficiencies identified.
  • Rigorous hydraulic calculations for each relief device’s inlet and outlet lines were completed.  Where existing lines do not meet API 520/521 guidelines, piping modifications were recommended.
  • For the purposes of the relief device calculations, the relief devices’ discharge piping sections to the flare were evaluated using a flare header back pressure as determined by others in previous flare header calculations.  Where no flare header back pressure calculations are available, best engineering judgment was used to establish the flare header back pressure.

Industry Type

  • Petroleum Refining

Utilized Skills

  • PRD evaluation and sizing

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