CGA P-8.7, Safe Location of Oxygen, Nitrogen, and Argon Gas Vents

The second edition of CGA P-8.7 defines the criteria for determining the safe venting locations for oxygen (O₂), nitrogen (N₂), and argon (Ar) gases at air separation and cryogenic liquefaction plants – across normal operating, upset, and emergency conditions.

Air Separation

Air separation is the process of extracting key gases from the atmosphere—primarily oxygen (O₂), nitrogen (N₂), and argon (Ar) and other trace gases. These gases are vital to industries such as healthcare, electronics, food processing, and energy production. Air separation plants make it possible to deliver these gases in both gaseous and liquid forms, tailored to specific industrial needs.
There are two typical ASU configurations for producing pressurized oxygen. In the gas plant configuration (also called gaseous oxygen process or classic gas process), oxygen is taken as a vapor from the bottom of the low pressure column and warmed by incoming air in the main heat exchanger. If a high pressure oxygen product is needed, it is compressed to the required pressure. A liquid oxygen purge stream is taken from the sump of the low pressure column to prevent the trace contaminants from concentrating above allowable safety limits. In the pumped liquid oxygen process (also known as the internal compression process), oxygen is taken as a liquid from the low pressure column sump, pumped to the required pressure, and vaporized in the main exchanger against high pressure air from the booster air compressor. The pumped oxygen stream removes trace contaminants from the low pressure column sump, so a separate liquid oxygen purge stream from the low pressure column sump may be eliminated.\

Oxygen has the highest boiling point of the three main components and is taken from the bottom of the low pressure column. Nitrogen is taken from the top of the low pressure or high pressure columns. An argon-rich stream can be withdrawn from the middle of the low pressure column and refined to a pure product in other distillation columns. The product streams are warmed to ambient temperature against incoming air in the main heat exchanger to recover the refrigeration. It is also possible to remove the products from the distillation system as liquid, if sufficient refrigeration is available. Producing large quantities of liquid products requires extra refrigeration, often supplied by a nitrogen liquefier unit.

What’s New in This Edition

A number of notable changes to the 2nd edition of this harmonized publication include:

  • This edition uses the Ventjet dispersion model developed and evaluated by a CGA and EIGA joint task force;
  • New definitions for concentration boundary and concentration limit were added;
  • Factors for consideration in the location of vent discharges were updated;
  • Calculations for the determination of safe vent locations were updated to include information about dispersion models including typical inputs as well as information about concentration limits, inputs for generating separation distances, vent elevation, vent discharge velocity,and vent velocity ranges;\
  • Figures 1, and 2 were updated and Figures 3a, 3b, 3c, 4a, 4b, 4c, 5a, 5b, and 5c were added to illustrate separation and elevation parameters for different vent orientations. Figures 6 illustrates separation distances and elevations with a range of weather conditions for a vertical release of oxygen; Figure 7 illustrates composite separation curves for a vertical release of oxygen, and Figure 8 shows an example of determining velocity ranges and horizontal separation distances;
  • Information was updated and added to address guidelines, limitations, and recommendations regarding the calculation of separation distances for vent locations;
  • The most extensive changes were made to Appendices A, B, and C. These appendices now provide tables of dispersion modeling results for both the warm and cold gas at specific concentrations, at specific vent elevations, and for three different vent orientations (horizontal, angled, and vertical). Appendix A provides oxygen dispersion modeling results, Appendix B provides nitrogen dispersion modeling results, and Appendix C provides argon dispersion modeling results;
  • Revisions were also made to the oxygen adjustment mathematics information found in Appendix D;
  • Appendix E was added to provide three examples of the application of the tables found in Appendices A, B, and C.

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