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