Project Synopsis

Perform a technical evaluation and process design support for safety enhancements and a SIL 3 system specification designed to prevent possible catastrophic events related to backflow in a Propylene Oxide system.

Project Summary

Process Engineering Associates, LLC (PROCESS) was contracted by a large U.S. chemical production facility to provide engineering support regarding their flare system.  In this chemical production unit, there are three main Propylene Oxide (PO) storage facilities on-site. These include a storage tank, a day tank, and the PO railcar unloading station.  The client uses PO in this production unit as a raw material and transfers the PO throughout the unit.  During such transfers, a mixture of PO/N2 is currently vented from these three sources to a caustic venturi scrubber system.  Due to the possibility of a back flow event of caustic (from the venturi scrubber) back to the PO sources, the client contracted PROCESS to review the design associated with venting the three PO vent streams to the existing flare stack.  This new design concept supports the removal of the caustic venturi scrubber in the unit.  The removal of the venturi scrubber eliminates the possibility of backflow of caustic to the PO sources, since backflow of caustic to the PO sources could cause a catastrophic event.  This design review also consisted of reviewing the proposed pipe routing of the three PO vent streams along with reviewing a flare seal pot.

In addition, PROCESS was tasked with specifying an SIL 3 (Safety Integrity Level 3) system to prevent backflow in the liquid feed line connecting the day tank to the unit reactor, which could occur if the pressure in the reactor exceeded the day tank supply pressure.  Such backflow could create a polymerization reaction in the day tank, leading to vessel overpressure and subsequent equipment failure and loss of containment.

The primary design used an SIL 3 interlock (differential pressure) with two automated on/off valves as the means to prevent reverse flow from the reactor to the day tank. In addition, though not part of the SIL 3 system or related calculations, the design included specifying thermal expansion accumulator pots on the PO feeds lines as well as a nitrogen wedge setup between the SIL 3 on/off valves.

This above work scope resulted in the following project deliverables:

  • A report documenting the review and validation of a new flare seal pot as part of the new PO vent design and routing
  • An SIL 3 system and report to prevent backflow from the reactor to the day tank, based on a nitrogen wedge and double block valve setup
  • Thermal expansion pot specified to absorb the expanded fluid (PO), thereby limiting the pressure rise in the feed pipes during ambient heat gain events.

Industry Type

Chemicals Production


Utilized Skills

  • Relief systems design
  • Safety Integrity Level (SIL) analysis
  • Hazardous materials handling


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