b31.3 min wall thickness calcs

[SnazyDragon]

I have done my wall thickness calc for straight pipe which works for the fitting in my spec. Upon doing the calc for pipe bend with the same material I find that my intrados min wall thickness is higher than than w.t. of the fitting. Is there an exception in the code to get around this issue or do I certainly need to increase my wall thickness? Any help is much appreciated.

 

[LittleInch]

what calc for pipe bend?

Can you be more specific / add a sketch.

what fitting?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Ehzin]

Is this a formed pipe bend or a B16.9 fitting? The nature of a bend is that the the intrados will become thicker and the extrados will thin out. The I factor as part Equations 304.2.1(3d) and 304.2.1(3e) acknowledges this and also provides a basis on calculating the required thickness. The calculation also somewhat relates to the increased or decreased thickness required to contain the pressure on each side of the bend although it doesn't necessarily guarantee that after the pipe is bent you will meet the tmin. It just works out that the part that thins also requires less wall thickness for pressure.

Thanks,
Ehzin

 

[Ehzin]

To provide further information, this is discussed in the following thread (

https://www.eng-tips.com/viewthread.cfm?qid=439343)

and this exact question from a system designer point of view in ASME B31.1, Section 102.4.5: Bending and in Table 102.4.5 of B31.1

To take a small excerpt from B31.1 in 102.4.5(A.1) "In General, it has been the experience that when good shop practices are employed, the minimum thickness of straight pipe shown in Table 102.4.5 should be sufficient for bending and still meet the minimum thickness requirements of para. 104.1.2(A). Table 102.4.5 provides excess thickness over tmin to ensure proper post-bend thickness, it's dependent on the radius of bend (6 pipe diameters or greater, 5 pipe diameters, etc...)

If you're performing pipe bends then these excess thicknesses to ensure a proper intrados and extrados thickness could become significant to ensure adequate thickness, if your tmin is close to the specified pipe wall thickness (tm is 0.495 and nominal thickness is 0.5) it could become important. If you're purchasing an actual pipe fitting elbow per B16.9 or some other standard then this issue is likely not a concern. I leave it to you to evaluate further as needed. The use of the word fittings makes me think this is a standard fitting per Table 326.1: Component Standards in B31.3. (Other standards have their own references)

Thanks,
Ehzin

 

[SnazyDragon]

Thanks all,

The fitting in question would be a standard fitting from 326.1 in b31.3. My conclusion resulted in taking the thickness of the straight pipe as the necessary tmin since I am not performing any pipe bends. This thread was enlightening and helps me to confirm my decision.

Thanks,
SnazyDragon