Un-de-rating flanges

Flanges have pressure ratings defined by a standard, most commonly ANSI/ASME B16.5 for which the pressure ratings decrease as temperature rises above 38ºC.  However the basis for the rating is conservative and AS 2885 allows some exemption from the de-rating requirements:  pressure de-rating of B16.5 flanges is not necessary until the temperature exceeds 120ºC (see Clause 3.4.3, including the useful notes that follow it).

The effects of de-rating can be very significant. For example, the NT Gas pipeline (1400 km to Darwin from fields west of Alice Springs) is nominally a Class 600 (10.2 MPa) pipeline but has an MAOP of 9.65 MPa.  As far as I know this MAOP was selected to meet the flange de-rating requirement at some mildly elevated design temperature. (It was designed in 1985, before the de-rating exemption was introduced.)  This looks to me like a severe case of the tail wagging the dog. MAOP of 1400 km of pipeline reduced by 550 kPa because of a few flanges?  That 5% pressure reduction would have been worth about 10% of pipeline capacity.  (No criticism of the designers implied here – there may have been other factors I’m not aware of.  And as it turns out the gas fields are declining so maximum capacity is unlikely to become an issue anyway.)

These days the usual practice in response to flange de-rating is not to reduce MAOP but to require specification of flanges from the next higher pressure class. So a 10.2 MPa (Class 600) pipeline would be fitted with Class 900 flanges (15.3 MPa).  This reaches its extreme on things like filter vessels. I’ve seen filters on a Class 900 (15.3 MPa) system fitted with Class 1500 flanges that were considerably larger than the vessel itself.  In the overall scheme of things that might not matter, but it offends my sense of proportion.  Unfortunately the AS 2885 de-rating exemption doesn’t help in this case because he vessel design is governed by AS 1200, although a design check of the lower-rated flange might allow it to pass the AS 1210 requirements.

Standards other than B16.5 are not so restrictive – MSS SP44 and ISO 15590.2 don’t require de-rating until above 120ºC.

In summary:  For piping flanges (not vessel flanges) at ordinary pipeline temperatures there is no need to de-rate the allowable pressure, or to use a higher pressure class in order to maintain the desired rating.  Vessel flanges may be more complicated – do the design check, use non-B16.5 flanges, or just grit your teeth and accept flanges of the next higher pressure class.

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9 Responses to Un-de-rating flanges

  1. Mark Coates says:

    As mentioned, if you take a step back and design the flanges to the required pressure, you may find that compliance will occur at elevated temperatures. Flanges rated to ASME B16.5 for Classes 150 and 300 are traditionally very close to the design limit, whereas Classes 600 and 900 are more conservative.

    By following flange design in accordance with either AS1210 or ASME BPV Section VIII, the Class 600 and Class 900 flanges may be suitable to pressures higher than those mandated by ASMe B16.5.

    The main proviso is that flanges in services outsdie of ASME B16.5 are not supposed to have the ASME B16.5 or Class stamp on them. If the stamping was left on (eg off the shelf purchases), then some type of formal audit trail would need to exist to show compliance at the extended limits.

    Thanks

    • Chris Hughes says:

      If your pipeline design process does not have a formal audit trail then it probably doesn’t comply with AS2885 anyway.

  2. Chris Hughes says:

    Another part of the problem is designers specifying design temperatures which are unrealistically high. The SEAGas pipeline, for example, had a maximum design temperature of 55degC because that was the maximum expected output temperature at the compressor stations – unfortunately no allowance was made for the gas temperature drop between the Coomandook compressor and the Adelaide meter stations and the 55decC design criterion was applied to these stations – resulting in Class 1500 flanges on the filter vessels as described by Peter. 38degC would have been a perfectly justifiable design temperature for these stations.

    • petertuft says:

      Very true about the design temperatures. It’s foolish to specify a single design temperature across a pipeline that passes through a range of locations with a range of different environments and operating conditions, but that practice is surprisingly common. And you are exactly right that I was thinking of the SEA Gas filters.

  3. Chris Hughes says:

    Another problem is that designers, when considering B16.5 flanges, take the pressure-temperature ratings from Group 1.1 materials only (ASTM A105, etc). Instead they could use Group 1.2 materials ASTM A350 Gr LF6 Cl. 2) which for Class 600 allow working pressures in excess of 10.3 MPa up to 100degC. They will cost a bit more and are more difficult to obtain, but in the overall project this is irrelevant.

    Alternatively specify your flanges to MSS-SP44 which allows you to go to 250degF (121degC) before derating. Could this be where AS2885 got its idea from?

  4. Gary says:

    I wonder how ME001 is going considering the derating exemption for B16.5 flanges?

    • petertuft says:

      Not well. There is apparently one committee member who will not consider any change so it is unlikely to happen in the foreseeable future. The alternative is to procure flanges stamped for compliance with ISO 15590.3 instead of B16.5 or do a specific calculation for each flange, both of which have drawbacks.

  5. Pingback: A350 lf6 | Selectdesigns

  6. Pingback: Temperature de-rating | Pipelines OZ

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