I'm a little confused here and this isn't my area of speciality so perhaps I'm totally wrong.

For B31.3, required thickness of the piping is calculated using the formulas and material strengths within B31.3.  However, for the flanged components, you are directed to B16.5 and for the vast majority of cases, these are the limiting components and therefore set the pressure/temperature limits for the system.

I 'thought' B31.8 had similar formulas for the strength of the piping but again, for the flanged components, you are constrained by B16.5.

Thanks for that, that's mean I still consider my flange rating in selecting the valve using ANSI B16.5?

That's my understanding.  B31.8 simply allows you to get away with thinner wall pipe versus B31.3 because of the less conservative formulas it uses (that's a lousy way of describing it but it's the best this ChemEng can do).  

However, it doesn't directly set the flange pressure/temperature ratings, those are handled by B16.5 so from your point of view, you wind up with the same flange rating but thinner pipe.

If you are designing a natural gas pipeline you must comply with CFR 49 Part 192, "Transportation of Natural and Other Gas By Pipeline: Minimum Federal Safety Standards".  I mention this because you must be careful that the pipe you choose meets the requirements of this standard, which has it's own formula to calculate at which MAOP you can operate the pipeline.  It's pretty simple to follow, but then there are a lot of other materials requirements that must be met.  

Maybe I'm misunderstanding your question.  B16.5 is for ANSI flanges.  The B31 codes govern pipe.  You need to look in B31.8 for your gas pipeline requirements, not B16.5.  

The design formulae in B31.8 describe the procedure to
determine the required wall thinckness.  YOur first step is knowing your design P & T.  Once those are known it becomes a trial-and-error process to select the wall thickness based on your material requirement.  Start with, say, X-52 as your first selection.  You get the allowable stress for X-52 from the stress tables located in the back of B31.8 based on your operating pressure.  Note that the allowable stress goes down as the operating pressure increases.  I assume you already have a diameter selected based on your pressure and delivery requirements.  So you calculate a t for X-52, then you check cost and availability for that pipe size and wall t, if too expensive, or bad dleivery, try a different material like X-60.  Trial and error.  Hope this helps!

Thanks!
Pete
P. J. (Pete) Chandler, PE
Mechanical, Piping, Thermal, Hydraulics
Processes Unlimited International, Inc.
Bakersfield, California USA
pjchandl@prou.com

Thanks for the reply and tips, but just wanna confirm again, do we need to refer to MSS SP-44 for the flange rating to this pipeline? No doubt about the thickness of the pipeline is already calculated based on the maximum pressure containment within the pipeline and using the formulae as stipulated in ANSI B31.8. My queries and wonder is about the flange specification of this pipe, where is have my valve at this pipeline.

Thanks

Rusman
Senior Process Engineer

Doesn't MSS SP-44 reference ANSI B16.5?

B31.8 is where you need to be looking for your flange requirement.  There will be something in there about fittings requirements.  That is the place to start.

Also - I made a boo-boo in my earlier post when I said that "You get the allowable stress for X-52 from the stress tables located in the back of B31.8 based on your operating pressure"; I should have said that the allowables are a function of temperature.

Thanks!
Pete
P. J. (Pete) Chandler, PE
Mechanical, Piping, Thermal, Hydraulics
Processes Unlimited International, Inc.
Bakersfield, California USA
pjchandl@prou.com