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Pressure Classes (numbering systems)

Whilst the ANSI pressure class numbering system forms the basis for all internationally recognised design codes, specifications and recommended practices, there are other equally well-known pressure class numbering systems two of which are listed below along with CalQlata’s understanding of the associated conversion factors.

The following table uses the basic ISO pressure numbering system (PN - pressure number), which provides the most complete values across the widest pressure range from which to establish conversion factors for both ANSI and API pressure classes.


PN (ISO) Multiply PN by: ANSI B 16-5 Multiply PN by: API 6A
6 7.5 45 20 120
10 7.5 75 20 200
16 7.5 120 20 320
20 7.5 150 20 400
25 7.5 187.5 20 500
40 6 240 20 800
50 6 300 20 1000 (960)
68 6 408 (400) 20 1360
100 6 600 20 2000
150 6 900 20 3000
250 6 1500 20 5000
420 6 2520 (2500) 20 8400

The above figures in bold type are those regularly used and referred to in the associated standard. The pressure classes not in bold type are those that would occur if the same conversion factors were applied to all PN values. The PN values refer to the recommended internal pressure carrying capacity in 'bar'

BS and MSS standards use the ANSI pressure class system to define their own flange and gasket designs.

API (Spec 6DSS 2009) and ISO (ISO 14723:2009) also now recognise ANSI pressure class system in their own design specifications

Further Reading

You will find further reading on this subject in reference publications(48 & 49)

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