Project Synopsis

Evaluate and size existing relief devices, a vent header, and a new anhydrous ammonia vapor-liquid separator for an engineering company at a fertilizer chemicals plant.

Project Summary


The client is an EPC firm with ongoing projects with the ultimate client, a manufacturer of a wide variety of industrial chemicals and agrochemicals including anhydrous ammonia, nitric acid, ammonium nitrate, and urea.  Process Engineering Associates, LLC (PROCESS) was contracted to prepare sizing calculations for three existing pressure relief devices (pressure safety valves, PSVs), evaluate and revise the design of the vent header, and prepare process design documentation for a new ammonia vapor/liquid separator that the client has proposed for the ultimate client plant.

At the plant, the three existing PSVs protect the following: (1) an anhydrous liquid ammonia vaporizer, (2) the downstream ammonia vapor superheater, and (3) the interconnecting piping.  The three relief valves discharge into a vent header that will be routed to a new ammonia vapor-liquid separator with vent stack.

The scope of work consisted of: (1) preparing sizing calculations for the three PSVs, (2) developing a hydraulic model of the vent header system, ammonia vapor-liquid separator, and vent stack, and (3) preparing the process duty specifications for the ammonia vapor-liquid separator and vent stack.

New sizing calculations for the three PSVs were generated because the original ones were not available for review.  The vaporizer and superheater PSVs were sized for vapor discharge based on a “control failure” scenario.  The third PSV that protects the piping was sized for liquid thermal expansion relief, and accounted for flashing across the valve, and two-phase (vapor-liquid) discharge flow.  The hydraulic model of the system identified the need for a larger diameter vent pipe for each of the two vapor discharge flows to eliminate critical flow conditions.  The proposed ammonia vapor-liquid separator was sized for 30 minutes of liquid ammonia holdup from the thermal relief PSV, and much larger vapor flowrates from the other two PSVs.  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.

Project deliverables included the following:

  • Process design basis.
  • Sizing calculations for the three pressure relief devices and inlet/outlet piping.  The thermal relief calculations accounted for flashing across the valve and two-phase discharge flow.  The three valves were adequately sized, but the vent header piping was undersized.
  • Hydraulic model of the vent header system, separator, and related piping.  An isometric drawing was also submitted that recommended changes to the vent header sizing and routing to the separator to eliminate critical flow conditions, low spots, and provide for the proper slope.
  • Equipment duty specification sheets for the new ammonia vapor-liquid separator and the vent stack.

Industry Type

  • Fertilizer / Agrichemicals Manufacturing

Utilized Skills

  • Relief device evaluation
  • Hydraulic line sizing
  • 2-Phase separator design

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