Weld Neck Flanges / ASME B16.5 / MSS SP-44

[GassyChris]

6" standard ASTM A105, Gr. B, ANSI 300, weld neck flanges were used in a standard wall X52 natural gas piping system. I need to calculate the percentage of SMYS (%SMYS) when the system operates at its MAOP. The weld neck flange has a minimum neck length of 1/4" per Fig. II-7 of ASME B16.5 (see attached). Do I calculate %SMYS (Hoop stress at MAOP / SMYS) for this short Gr. B section of the flange or state that it has the same %SMYS as the pipe it is connected to? Is this Gr. B flange the weak point in my system? Why isn't high yield strength carbon steel (ASTM A694)listed in Table 1A of ASME B16.5?

 

[LittleInch]

Is it the weak spot - depends whether the flange was bored out to the same ID as the pipe. If it was then yes it is.

To be honest I've never been entirely sure if having, as I think you're getting at , a thin strip of metal with a lower SMYS than the pipe is a real problem or a theoretical one. I've normally just made sure we specify and order A694 FXX flanges.

Why it isn't in 16.5 - don't know, but it is a listed referenced standard in MSS SP44.

It all depends which code this is designed to - B31.3 and you would get away with it as the allowable stress of X52 is actually the same as A105 at "normal" temperature, but if its 31.8 then you have issues.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[racookpe1978]

No file attached.

How long is the reduced wall thickness section? (Not 1/4 inch, right?)

 

[LittleInch]

I dug the figure out and it is the flat section of the flange in section (min 1/4" in length) just before it starts to increase thickness up to the main flange body.

IMHO in practice it probably doesn't make big difference, but it is just not right.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[XL83NL]

Assuming this is for a B31.3 design, cant we handle this short piece of 'straight pipe' at the weld neck as being just a piece of straight pipe as per para 304.1.2, as we do with all pieces of straight pipe?
And, whenever someone thinks this is not allowed, one can always resort to para. 302.2.2.
This is how I do it. Never had a problem with it.
The rest of the flange (starting from where the hub tapers away) obviously is goverend by the p,T-rating of the flange itself.

 

[GassyChris]

The pipe in question carries natural gas and is a regulated line under 49CFR192. The pipe is API 5L-PLS2 grade X52. I am not worried about the pressure rating of the mated flange face; this rating does not change for a flange of a given pressure class even though the SMYS of the steel can vary from 30kpsi to 70kpsi. I have to report the %SMYS (Hoop Stress at MAOP / Pipe or Fitting SMYS = PD/2t/SMYS*100) of transmission pipe segments to PHMSA annually. It is my opinion that the 1/4" neck of the Grade B flange should be calculated independednt of the pipe. In the past, I have ignored this piece of the weld neck and just looked at the pressure rating of the mated face. I believe I have been wrong.

 

[LittleInch]

Assuming this is to B 31.8, I agree with you. It might be a ridiculously small section of "pipe", but whoever installed it goofed up IMHO.

Hopefully you wont be above 72% of SMYS....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[GassyChris]

It is B31.8 pipe, but barely over 20% SMYS at the flange.

If the pressure rating of a flange is based on the mated face properties (gasket, number of bolts, etc), why would anyone use a high yield strength slip-on flange? Is is due to welding specs? I don't see the advantage of using high yield strength slip-on flanges.

 

[LittleInch]

Then that's ok.

I don't see any advantages either for a slip on flange, only for a WN

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[weld9]

The 6" GR.B, standard, (.280 wt.) Class 300 weld neck flange will hoop stress @ 100% SMYS at 2950 psi. The MAOP of the system that this flange is in should not exceed 740 psi., as this is the maximum working pressure for Class 300 flanges. The fact that you have a GR.B flange welded onto X52 pipe won't hurt anything as long as your WPS is qualified and your MAOP doesn't exceed the weakest link in the system.

 

[GassyChris]

The problem is that the designer was supposed to keep the line below 20% SMYS at its MAOP of 650psi to keep the line out of the Transmission Category. He looked at the pressure rating of the flange and ignored the physical properties of the flange. My job is to review all as-builts. This thread was created so I could get confirmation that I was calculating the hoop stress on the weld neck flange correctly. Thanks to all.