Fabricating Low Pressure Separator vessels

October 8, 2014

While the label reads as self-explanatory, it's worth taking the time to properly delineate the function of these vessels. Separators divide substances into different states. Using the oil and gas industry as an example, the low-pressure variant involves fabricating vessels to separate liquid and gas at a stated low pressure. Pulling back to see the entire process, onshore piping takes fluids and gases through several other stages, including primary phase separation and high-pressure separation, but the model of operation is designed to act as a stepping process, reducing the energy of a well stream through intelligently positioned separator apparatus.

The fabrication criteria assigned to sizing a separator vessel is contingent on pressure carrying capacity. This same guideline affects the selection of the graded steel or other metal used in the manufacturing process. In this case, the low pressure characteristics of the design indicate a large volume, a deduction taken from the lack of compressed liquid or gas in the current stage of the process. More pressure would typically equal less volume and greater energy, leading to a smaller tank and a higher working pressure specification in the design construction blueprints. The separator designation also indicates a multi-stage dynamic. The standard stages fall almost wholly within the two-stage area, but three stage options exist. A two-stage tank would separate liquid from gas while the third stage acts to isolate free water from the setup.

The actual shape of the vessel can be seen in many forms. Horizontal and vertical designs are both common. The form descends more from manufacturer preference than any engineering requirement. As long as the separator vessel has an internal spherical shape, the physical composition of the gas and liquid will dictate the separation process. The separation action itself is performed by gravity, but many separation aids are added to the design of the vessel to cope with the properties of liquids and gases. For example, a hydrocarbon cycle positions low pressure vessels close to the wellhead to isolate different densities, the liquid flow of oil and the free-floating natural gas for further compression and piping to a storage facility. The aforementioned separation methodology is a science unto itself. Gravity is the first force to be employed, and this aligns with coalescing, centrifugal action, heat, and many other separation techniques that must be incorporated into the design for the vessel.

The general pressure capacity of this class of separation vessel falls within the 10 to 225 psi range, but other stages can handle compressed fluids and gases within an operational spectrum of 4000 to 5000 psi. Other specifications critical in the design of the vessel include a volumetric overhead to allow droplets to settle without flooding mist extractors, the provision of pneumatic, electrical, or mechanical control systems to monitor pressure and temperature, and coalescing assemblies to finitely isolate one state of the substance from the other. These factors will depend on the position of the vessel in the production line, with a final stage requiring absolute separation before sale.

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