This tool utilizes HSG253 ("The safe isolation of plant and equipment") methodology for selecting ‘baseline’ process isolation standards. It is guidance on the general principles of safe process isolation. The tool can complement, but cannot replace, competent technical judgment and common sense. The reference has a free-to-download, web-friendly version through HSE website.


Name for reference:
Release Factor
Working Pressure:
Line Size:
Location Factor
People at Risk:
Plant Layout:
Fire Potential:
Baseline Standard:
Baseline Description:


This is an overview of the use of the selection tool to select a final isolation method. For more information, please refer to HSG253 Appendix 6: Example of a selection tool to establish the ‘baseline standard’ for final isolation. (HSG253 reference is available below - Figure 11 page 59).


This is an overview of the final isolation methods. For more information, please refer to HSG253 Figure 4, Final isolation methods. (HSG253 reference is available below - Figure 4 page 26 and page 64 for the description of the methods).

R: Consider whether the associated risk is acceptable or whether there is a need to further reduce risk by eg risk reduction measures, extending the isolation
envelope, plant shutdown.
I: Positive isolation (Physical disconnection, eg spool removal - Double block, bleed and spade - Single block and bleed and spade).
II: Proved isolation (Double block and bleed, DBB - Double seals in a single valve body with a bleed in between - Single block and bleed, SBB).
III: Non-proved isolation (Double valve - Single valve)


This is an overview of the appropriate substance category, These are primarily (but not exclusively) based on the classifications given in the Chemicals (Hazard Information and Packaging for Supply) Regulations 2005 (CHIP), you may check this. For more information, please refer to HSG253 Table C, Substance category. (HSG253 reference is available below - Table C page 62).


Line size and pressure give a release factor (this reflects the potential rate of release). The options are high (H), medium (M), and low (L). (HSG253 reference is available below - Table D page 63).


Consideration of the location should include numbers at risk, escalation, and damage if a release occurs. Take into account the nature of the possible consequences if the isolation fails, eg vapor cloud explosion (VCE), toxic gas cloud, jet fire with potential for escalation, etc. The options are high (H), medium (M), and low (L). (HSG253 reference is available below - Table E page 63).


Release factor and location factor give an outcome factor, in the range A-C. (HSG253 reference is available below - Table F page 63).


The substance category and outcome factor give the appropriate baseline standard for final isolation The options are R, I, II, and III. (HSG253 reference is available below - Table G page 64).


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Air separation is the most common process used to extract one or all of the main constituents of atmospheric air. The initial measurements of the constituents of air, with the exception of argon, were improved and summarized in the early 1800s by John Dalton. Earth's atmosphere is 78.09 % nitrogen, 20.94 % oxygen, 0.93 % argon, and 0.04 % carbon dioxide with very small percentages of other elements and normally not recovered (neon, helium, krypton, methane, hydrogen, xenon, and radon).  In very large air separation units (ASU) Neon, Xenon and Krypton are recovered in small amounts.

Cryogenic air separation utilizes the differing condensing/boiling points of the components of air to enable separation by distillation at cryogenic temperatures. At atmospheric pressure the main components of the air have the following condensing/boiling points:

  • Nitrogen -196° C
  • Oxygen -183° C
  • Argon -186° C


Main Air Compressor (MAC)

The MAC compresses atmospheric air, generally to 4-7 BARG, and delivers it to the system. These compressors are normally driven by electric motors. Interstage coolers are provided to remove the heat of compression between each stage of the compressor, of which there are normally 2-3.

Front End Clean Up

Modern ASUs utilize a Prepurifier Unit (PPU), which removes moisture, CO2, and most hydrocarbons from the air. Moisture and CO2 must be removed to prevent ice and dry ice from forming later in the process. A PPU is typically made up of a chiller to cool the air to 4-13 C, a condensate separator to remove free water, and 2 vessels filled with desiccant and mole sieve material, which adsorbs the contaminants while allowing the air to pass through. One bed is always online to the process, while the other bed is regenerated with heated waste Nitrogen to remove accumulated contaminants. Beds automatically switch every 5-8 hours. The air from the PPU is very close to moisture and CO2-free.


The ColdBox contains the cryogenic heat exchangers, distillation columns, and associated valves and piping. Because parts of this system are very cold, all components are mounted inside the ColdBox and then encased in insulation. Cold boxes can be rectangular or cylindrical and are usually tall, some over 200′ depending on capacity and type of Argon system. Modern cold boxes are filled with perlite insulation, which is light and easy to install and remove, when necessary. Older cold boxes may be tightly packed with cryogenic Rockwool, which is very time-consuming to install and remove.


All ASUs except some very small units have expanders. Expanders provide the required refrigeration to produce liquids in the distillation column system. Air, Nitrogen, or Waste Nitrogen is fed to the expander, causing the wheel to turn and transfer energy to a compressor, generator, or oil brake. This transfer of energy causes the gas to cool. As the process continues, the outlet temperature of the expander eventually reaches the design temperature while cooling the column system.




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