This paper provides a simplified approach to sizing 2 and 3 phase separators in low GOR (<2000 scf/STB) applications. The paper will provide the facility engineer with a straight forward method for beginning the sizing of the separation system. Once the first pass sizing has been executed, the sizing can be refined and adjusted to meet any specific requirements. The paper will also describe a simplified Stokes equation to confirm that the retention time approach has a high probability of meeting process requirements. Finally, the methodology lends itself to building an excel spreadsheet for multiple vessel sizing requirements.

The oil should be ≥ 25 API crude, although the application can be applied to heavier crudes providing that additional safeguards are applied. There are no limits on the water cut, but the paper assumes that the water cut is greater than 50%. The approach uses a retention time method which sizes the liquid section of the separator. The recommended retention time will be approximately 2 times the API 12J values for 2 reasons:

  1. To account for hydraulic short circuiting in the separator

  2. To ensure that the separation process is in the Stokes or Laminar regime.

The paper will also recommend using baffle plates in the 3 phase separation to facilitate plug flow. The paper will recommend the use of either a mesh pad or vane pack in the gas section to minimize liquid aerosols carried out with the gas.

The sizing methodology is summarized as follows:

  1. Establish the actual oil, water and gas properties and rates.

  2. Select a retention time for water and oil.

  3. Select a Length to Diameter ratio or Height to Diameter ratio.

  4. Solve for the vessel diameter and length.

  5. The gas section is then sized.

  6. The liquid control volume and alarm heights are established.

  7. Inlet and outlet nozzles are sized.

  8. Stokes calculation is executed to determine separable droplet size, oil in water, water in oil and oil in gas.

The paper will discuss the merits of vertical and horizontal separators based on production environment and environmental conditions.

Keywords:
droplet, viscosity, separation and treating, separator, diameter, terminal velocity, gas space, Upstream Oil & Gas, phase separator, level shutdown
Subjects:
Processing Systems and Design, Separation and treating
Copyright 2015, Society of Petroleum Engineers
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