Evaluate and size several pressure relief devices (PRDs) in ammonia service at a fertilizer plant.
AMMONIA SERVICE THERMAL PRESSURE RELIEF DEVICES REVIEW AND EVALUATION
The client is multi-discipline detail engineering firm with ongoing projects at the ultimate client facility, 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 sizing calculations for four (4) pressure relief devices (one existing, three new) that protect various sections of ammonia process piping at the ultimate client’s plant.
The scope of work consisted of preparing the sizing calculations for the PSVs including determining the recommended maximum equivalent lengths of inlet and discharge piping for the new relief valves, and a modified routing of the discharge piping for the existing relief valve that provided for free draining of the discharged liquid ammonia.
The existing PSV protects a section of a liquid ammonia railcar unloading line from overpressurization due to liquid thermal expansion. The ultimate client wished to increase this valve’s set pressure, but the valve was obsolete and new higher pressure springs were not available. Therefore, a replacement PSV was sized.
Two of the new PSVs are intended to protect lines from overpressurization due to liquid thermal expansion: one is a section of ammonia vapor line that feeds a neutralizer, and the other is a section of liquid ammonia line that feeds a water chiller. Both process lines could become liquid full and blocked in during startup and/or shutdown.
The third new PSV is intended to protect a section of liquid ammonia line that connects a urea reactor with a urea surge tank from overpressurization due to the potential failure of an upstream valve that has leaked in the past. This is a unique blocked liquid flow scenario in that failure of the upstream valve would expose the line to supercritical ammonia at 3,130 psia and 310°F. This liquid anhydrous ammonia would partially flash across the upstream valve, and the two phase vapor/liquid inlet flow would then flash across the relief valve to a higher vapor fraction as it exits to atmosphere.
PROCESS provided the following project deliverables:
- Process evaluation basis
- Sizing calculations for the four pressure relief devices and inlet/outlet piping that also account for flashing across the valve and two-phase discharge flow.
- Agrochemical / Fertilizer Manufacturing
- PSV evaluation and sizing