ASME B31.3 VERSUS B31.1 RELATED TO BEND THINNING ALLOWANCE 1

[rhino27]

I was hoping to gain some insight regarding the following issue. In that calculation of minimum wall thickness per b31.3, I know that you can use the formula in section 304.1.1 for a straight pipe. My confusion comes in when you calculate the wall thickness for example a 6D pipe bend. I see in B31.3 that you can calculate the required wall thickness for the intrados & extrados and that through theses two calculations you will discover that the extrados can be thinner than that of the calculated thickness of the straight pipe minimum wall that was calculated. Contrary, the intrados calculation shows that it must be thicker than that of the calculated minimum wall of the straight pipe. My logic is that if you take a straight piece of pipe and bend it then the outside of that bend or extrados will experience thinning & the inside will experience thickening. So if I were to order a pipe that were to bent on a 6D radius, could I not order what I calculated for the straight length of pipe due to the fact that once that straight length of pipe is bent it will thin on the outside (which the extrados can be thinner to due more exposed area to contain the pressure force)& the inside will become thicker (which the intrados has to be thicker do to less exposed area to contain the pressure force)? I feel like this would balance each other out appropriately. Furthermore, I see in B31.1 paragraph 102.4.5 Bending that the minimum wall thickness at any point in a completed bend shall not be less than what is calculated using the straight pipe equation. Additionally, Table 102.4.5 guides the designer for ordering pipe based off Bend thinning allowances being added to the straight pipe min. wall thickness before bending the pipe. B31.3 does not have such a statement nor a bend thinning allowance. Therefore, do I not have to account for a bend thinning allowance if I am designing to B31.3 and can order a pipe wall thickness based off what I obtained for the straight pipe minimum wall thickness?

 

[LittleInch]

I'd like to see this with some example numbers.

Be careful though. What 31.3 is giving you are minimum thicknesses AFTER bending.

It's not giving you the calculation to use that shows what the thinning is from a straight pipe. Usual ROT for a 5D bend is 8% thinning. The calculation in 31.3 looks to be about 4.5%, but it needs the full calculation to compare thickness of straight pipe and at the intrados and extrados of the bend with all parameters filled in.

Of course you need to account for bend thinning when ordering straight pipe to be bent to 31.3.

All 31.3 gives you is MIN thickness AFTER bending.

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[rhino27]

Thanks for your reply.. " B31.3 is giving you the minimum thickness after bending." I agree to this statement fully. I just do not understand why you have to worry about the thinning from bending due to that as you bend the straight pipe you are exposing more area for the pressure force to act on which is proportional to the thinning that occurs. Therefore, I feel that the thinning from bending will produce exactly the correct thickness it needs to be because as the pipe is bent the extrados is thinning but you are also gaining margin by exposing more area for the pressure force to act on so the two cancel each other out as well as the intrados is getting thicker proportionally as it is losing exposed area the the pressure force is acting on. My assumptions are based off of the Lorentz factor. I will try to work out an example problem for further clarification.

 

[LittleInch]

Your hypothesis doesn't make sense to me. Pressure is pressure acting on all parts equally. The fact there is a bigger area is irrelevant.

There might be different forces because it is a bent peeve of pipe and not straight,

My point is that as toy bend the pipe it gets thinner faster than the allowed thickness measurement changes.

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

 

[rhino27]

What basis are you using to state that the pipe is getting thinner faster than the allowed thickness changes?

 

[LittleInch]

Simplease trig.

For a 6d bend the outer circumfere is approximately 8% more than the neutral axis. That can only come from thinning of the pipe. My calculations of 31.3 showed acceptable thinning of only 4%.

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

 

[rhino27]

Thanks you.. last question. Why does b31.1 give you percentages per bend thinning and b31.3 not give them as seen in table 102.4.5 of b31.1?

 

[LittleInch]

Now that I don't know. Probably lost in the mists of time

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