- P = pressure
- S = allowable stress
- t = wall thickness
- D = outside diameter
The design of a complex pressure containment system involves much more than the application of Barlow's formula. For almost all pressure vessels, the ASME code stipulates the requirements for design and testing.
The formula is also common in the pipeline industry to verify that pipe used for gathering, transmission, and distribution lines can safely withstand operating pressures. The safety factor is multiplied by the resulting pressure which gives the maximum operating pressure for the pipeline. This safety factor is dependent on class locations which are defined in DOT Part 192. There are four class locations corresponding to four safety factors:
- Class 1: An offshore area or any class location unit that has 10 or fewer buildings intended for human occupancy. The Class 1 safety factor is 0.72.
- Class 2: any class location that has more than 10 but fewer than 46 buildings intended for human occupancy; safety factor 0.60.
- Class 3: any location that has more than 46 or more buildings intended for human occupancy or any area where the pipeline lies within 100 yards (91 meters) of a building or a small, well-defined outdoor area (such as a playground, recreation area, outdoor theater, or place of public assembly) that is occupied by 20 or more persons at least five days a week for 10 weeks in any 12 month period—weeks need not be consecutive. Safety factor 0.50.
- Class 4: any class location unit where buildings with four or more stories above the ground are prevalent; safety factor 0.40.
- Or pressure vessel, or other cylindrical pressure containment structure.
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