MAWP of WN Flanges 1

[zakk]

How is the MAWP of ASME B16.5 flanges determined? We have NPS6 Sch 40 and Sch 80 flanges which sources say are both rated for 1480 psig, but common sense says the Sch 40 should be rated lower. We have fittings manufactured from SA350-LF2 with Sch 40 bevels which ASME calcs says we can only rate at 950 psig based on the minimum thickness at the weld bevel. When end-users question why the fitting side of the weld is rated at 950 psig and the flange side is rated at 1480 psig, we are at a bit of a loss. Am I missing something simple here?

 

[PAN]

Refer to ANSI B16.5, Mat. Group No. 1.1 (Carbon Steel) Class 600 (Temp. 100 F or less), you have pressure rating at 1480 psig. This is pressure rating which is not depended on pipe schedule (thickness)....For more understanding, slip on or weld neck flange would be the same pressure rating.

 

[zakk]

Pan:

Thanks for the info.

I realize that the pressure/temp ratings on a specific ANSI Class does not specify a pipe schedule. My question is "Why wouldn't it?". Why would a thinner-walled Sch 40 flange have the same rating as a Sch 80? If a flange manufacture says they have the same rating, how do they justify it mathematically?

Incidently, the 950 psig MAWP on the fitting is based on a 1/8" Corrosion Allowance. If flange calculations are based on lower a C.A., this may explain everything.

 

[PAN]

Let's see the related subject. It may give you an idea.

COMPARISON BETWEEN CLASS 1500 AND SCHEDULES
thread292-16682

 

[twjag]

See paragraph 2.6 of B16.5 for basis of rating of weld neck flanges, & required thickness of hub at weld end.

 

[Raydel]

Hi zakk

If you want to be on the safe side, use ASME VIII Division 1 Appendix 2 for the design of flanges. The values given in B16.5 are 'legal', but are based on parameters whose origins have been lost in the mists of time. If you know someone with CodeCalc software from COADE, bribe them to run the calculations for you.

 

[Elvie]

Zack,
in a piping system is the ASME B16.5 Flange class rating which define the rating of the entire system ,and this depending of the material group.The piping schedule (or WT-wall thickness) is calculated as per the relevant piping design code ,say ASME B31.3.for the most demanding couple of coincident design conditions (pressure & temperature).

Flanges are standard for same rating and size no mater of material group but its WN may vary.
The piping schedule may be different for same size and design conditions as a function of the material allowable stresses at temperature listed in the piping design code.
In order to join them together (by welding) a flange is ordered with the WN (welding neck)schedule equal to that of the pipe (or fitting) to be connected to.
It results that the WN schedule is the limiting factor in your case;the maximum pressure you may sustain with your NPS 6"Sch 40 WN you may be easily calculated based on your piping code formula for the flange material you have in hand.But make sure the Sch 40 piping/fitting to be welded to and the weld itself are not even weaker.