Project Synopsis

Evaluate an existing but non-commissioned vapor recovery unit for both operability and safety issues at a crude oil terminal facility.

Project Summary

TRUCK LOADING STATION / OIL STORAGE FACILITY VAPOR RECOVERY UNIT (VRU) EVALUATION

The client, an engineering company, was asked by the ultimate client, a pipeline operating company, to evaluate an existing vapor recovery unit (VRU) for a truck loading station and sweet crude oil storage facility in U.S. Great Plains region.  The VRU is a skid-mounted unit that consists of an oil-flooded screw compressor along with its associated suction and discharge knockout drums, oil cooler, and oil filter.  The VRU was connected to the process piping in the facility after installation, but was never commissioned.  The client asked Process Engineering Associates, LLC (PROCESS) to perform the evaluation of the VRU to determine if the equipment is adequately designed for the current operation of the facility given a proposed expansion of the facility and changes to the feedstock composition.  Specific tasks for this analysis included:

  • Establishing the current design criteria for the VRU, including the design flow, the suction pressure and temperature of the VRU compressor, the specific gravity of the material flowing through the VRU compressor, and the required discharge pressure from the VRU compressor
  • Performing a hydraulic analysis of the vent header from the storage tanks to the VRU to help ensure that the new VRU design conditions associated with the current facility operation did not cause the pressure in the storage tanks to exceed their maximum allowable working pressure (MAWP) while venting during the design case
  • Submitting the current design criteria for the VRU to the original equipment manufacturer to determine what changes, if any, may need to be made to the equipment to help ensure that it will operate properly
  • Performing a qualitative risk evaluation of the VRU to help ensure that there are no obvious safety hazards associated with commissioning the VRU given the current facility operation.

PROCESS assumed that the suction temperature, the discharge pressure, the site elevation, and the ambient temperature listed in the specification sheets for the VRU did not change.  PROCESS then established both the flow and specific gravity of the vent streams from the various storage tanks connected to the VRU vent header for two design cases: the “working losses” case and the “standing losses” case.  The “working losses” case accounts for (1) vapors that flash when a liquid stream enters a tank from a higher pressure source and (2) vapors that are displaced as the vapor head space in a tank decreases due to an incoming liquid stream.  The “standing losses” case accounts for vapors that evaporate from liquid in a tank when that tank is isolated from incoming sources and exposed to ambient heating.  PROCESS utilized the guidance set forth in the American Petroleum Institute’s (API’s) Venting Atmospheric and Low-Pressure Storage Tanks, API Standard 2000, as well as licensed commercial computer process simulation software to help determine the flow and specific gravity of the vent stream for the controlling case.

PROCESS also used simulation software to perform a hydraulic analysis of the vent header from the storage tanks to the VRU to help ensure that the backpressure at the storage tanks did not exceed the MAWP of the tanks when venting during the controlling case.  PROCESS concluded that the existing vent header was adequately sized to accommodate the vent stream for the controlling case without producing excessive backpressure at the tanks.

PROCESS submitted the current design criteria to the original VRU manufacturer to allow them to independently evaluate the existing VRU to help determine if the VRU is still adequate given the differences between the original design conditions and the current design conditions.  Their evaluation determined that the motor sheave and belt for the VRU screw compressor will need to be changed to accommodate the current design flow due to the higher specific gravity of the gas in the current design conditions versus the original design conditions.

PROCESS concluded its analysis of the VRU by performing a qualitative risk evaluation to help identify any potential safety hazards associated with commissioning the VRU given the current facility operation.  As a result of the evaluation, PROCESS concluded that that air may be introduced into the vent header from the truck loading stations at the facility during the truck loading process, which could lead to an explosion in the VRU when the oxygen in the air along with the hydrocarbons in the vent header reached the VRU compressor.  Therefore, PROCESS recommended that the ultimate client (1) vent any gas from the tank trucks to the new proposed truck loading flare at the site during the truck loading process, (2) remove any connections from the truck loading stations to the tank vent header, and (3) provide a natural gas blanket for the storage tanks that connect to the VRU vent header to inert the head space in the tanks and prevent an ingress of oxygen into the system.

PROCESS provided a letter report to the client presenting the results of the VRU evaluation.

Industry Type

  • Oil Terminal Facilities

Utilized Skills

  • Process systems evaluation

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