Flange/Pipe derating factor

[MTPipeliner]

Does a forged steel flange have a derating factor associated with it for construction type?

Example:
In a pipeline used for natural gas transmission service there is a derating factor for piping used in areas with higher populations of people .6 for areas that are remote and .5 for areas like a compressor.

This derating factor only seems to apply to piping.

Is there a derating factor already built into flanges and valves?

I've never seen any consideration given to location or construction type when dealing with valves and flanges. An ANSI 600 flange seems to be good for 1480 (at room temp)regardless of where it is located. It seems illogical to derate only the pipe but not the flanges or valves unless there is already a derating factor included.

Also why are flanges available in different grades of material if they are already rated to a certain pressure? Is this only because of the weldability concerns between the pipe and the flange. It would seem that the only differnce between an ANSI 600 flange with a yield stength of 35ksi and one with a yield stength of 42 ksi is the weldability of the pipe since the ANSI 600 rating sets the maximum pressure.

Sorry if this is all jumbled, but I'm a little confused about it all.

Pat

 

[bvi]

The factor applies to wall thickness calculations, not flanges in accordance with listed standards.

The rating of the flange depends upon the geometry, and the material strength (specifically yield strength, so the maximum allowable pressure depends upon the class (which establishes the DIMENSIONS of the flange) and the material.

 

[MTPipeliner]

So if the the flange was the weakest part of the setting and the design factor was changed from .6 to .5 and the same flange was used. What was gained?

Pat

 

[bvi]

The "official" position is that the location factor increases the wall thickness of pipe in locations with greater population density because there is a greater probability of third party damage due to excavation and similar activities. So, in this context, increasing the wall thickness of the pipe increases its ability to withstand such damage.

Note that the pipe (regardless of location class) and flanges are fully capable of carrying the pressure. Also note that the flanges are used at their pressure rating in other piping systems, with a more conservative stress basis than ASME B31.8 (e.g. B31.1).

Also, in some instances, people use lower design factors near installations where people congregate, so that if there is a corrosion failure, it is more likely to occur elsewhere in the piping system. This is relevant for pipe, but not flanges.

So, based on any of the above considerations, there is no need to reduce flange pressure ratings when the design factor changes with location class.