Project Synopsis

Design the concept of adding a new ammonia separator within an agrichemicals facility.

Project Summary


The client is an multi-discipline detail design firm having ongoing projects with the ultimate client, a manufacturer of a wide variety of industrial and agrochemicals including anhydrous ammonia, nitric acid, ammonium nitrate, and urea.  Process Engineering Associates, LLC (PROCESS) was contracted to prepare the process design documentation for a new/modified ammonia separator system that the client has proposed for the ultimate client’s plant.

At the client’s plant, an anhydrous ammonia header is protected by three existing PSVs that provide thermal relief and discharge into a common vent header.  The client has proposed to modify this vent system by extending the vent header and adding a new ammonia vapor-liquid separator and vent stack.

PROCESS’ scope of work consisted of (1) development of a simple hydraulic model of the ammonia vapor-liquid separator, vent header system, and related piping, (2) a review and re-evaluation of three PSV’s and their inlet and discharge piping, and (3) preparation of process duty specifications for the ammonia vapor-liquid separator and vent stack.

The sizing calculations for the PSVs were redone to account for the new higher set pressure (325 psig), flashing across the valve, and two phase (vapor-liquid) discharge flow.  The valves’ inlet and discharge piping were evaluated based on these recalculated required relief flows.  A hydraulic model of the separator, vent header system, and related piping was developed based on a design flow rate that was the maximum required relief flow from the three PSVs, and the client supplied isometric drawings depicting the proposed pipe routing.  The proposed ammonia vapor-liquid separator was sized for 30 minutes of liquid holdup based on the design flowrate, and the vent stack was sized based on ANSI/ASHRAE Standard 15-2004 guidelines regarding minimum height above grade and distance from the nearest occupied building.

PROCESS provided the following project deliverables:

  • Process design basis.
  • Hydraulic model of the separator, vent header system, and related piping.  Note: The system was determined to be adequately sized, however, the vent header had low spots and was not sloped toward the separator.
  • Sizing review of the pressure relief devices and inlet/outlet piping with new calculations to account for flashing across the valve and two-phase discharge flow.  The valves and related piping were found to be adequately sized.
  • Equipment duty specification sheets for the new ammonia vapor-liquid separator and the vent stack.

Industry Type

  • Agricultural Chemicals Manufacturing

Utilized Skills

  • Separator design and sizing
  • Pressure relief device (PRD) sizing.

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