ASME B31.3-2018 Section 319.4

[Mr Leaf]

Hello,

I just have a question for my understanding on ASME B31.3-2018 Section 319.4(c) Equation 16. I see the limiting factors and the foot note warning, but within the codes itself so this is a more of "the math doesn't seem to add up when I read it - what am I missing?" question?

If we look at the terms L and U, for a straight run of pipe these cancel out and the denominator becomes 0. Lets assume its anchored in a straight run of pipe between two pieces of equipment that do not induce thermal expansion, vibrations, etc (say a tank or so). Assume the pipe is straight with no bends or slopes, such that L=U. I can absolutely see applications like this. Obviously at some point the sag and bend will be too much, but the equation does not account for that when L=U.

What am I missing here? Am I misunderstanding a terminology?

 

[csk62]

The flexibility and stress in a straight run of pipe between two anchor points is easily determined; why would you use this formula?

In your case for a straight run with no displacement, if L and U are equal, y should be 0 and the equation becomes indeterminate. If you find you have displacement in the system, then y is not 0, and you should not run a straight length of pipe, but change the routing to add in flexibility.

----------------------------------
Not making a decision is a decision in itself

 

[1503-44]

Yes. L includes any length of dogleg and loops (added to get better flexibility).

 

[LittleInch]

What you're missing is section 319.1
Basic Requirements. Piping systems shall have
sufficient flexibility to prevent thermal expansion or
contraction or movements of piping supports and terminals from causing
(a) failure of piping or supports from overstress or
fatigue
(b) leakage at joints
(c) detrimental stresses or distortion in piping and
valves or in connected equipment (e.g., pumps and
turbines), resulting from excessive thrusts and
moments in the piping

A straight pipe between two fixed items will only be able to comply with 319.1 if there is no temperature difference between the time of installation and operation.

This is rarely the case.

Also tanks have a notoriously low acceptance for axial force on nozzles so that is a bad example.

As soon as y is >0, the equation becomes infinite as division by 0 is infinity. This is most certainly larger than K1.

Pipe subject to positive temperature change between two fixed anchors will develop compressive stress until the pipe buckles or the anchors break.
Similarly negative temp change will develop axial stress until the pipe or the anchors break.

319.1 requires you to find out if this is going to happen and if it is then you need to introduce flexibility into the system.



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

 

[XL83NL]

Also, dont forget that when you’re doing a pipe stress analysis on a straight section with intermediate anchors, (thermal displacement) stresses will most probably be zero, even when dT > 0. This is due to the fact that an (in)finite straight section pipe between two hard, rigid anchors doesn’t see any bending moments, thus stresses will be 0.

Huub
- You never get what you expect, you only get what you inspect.

 

[1503-44]

Bending stresses will be zero. Axial stresses will be maximum, may buckle the pipe and introduce secondary bending stresses..

 

[XL83NL]

You're correct 1503-44, bending stress is 0 due to 0 bending moment (319.4.4 eq 18). Axial forces (S[sub]a[/sub]) are indeed > 0, so S[sub]E[/sub] > 0.


Huub
- You never get what you expect, you only get what you inspect.

 

[Mr Leaf]

All,

Sorry for the delay. This is the point I was missing in considering this:

Per csk62: "In your case for a straight run with no displacement, if L and U are equal, y should be 0 and the equation becomes indeterminate." (note divide by zero is indeterminate. A limit that approaches 0 can be infinite).

The displacement would change the developed length and L > U. Further reading indicates that U and L are the trigonometric relations (e.g., straight line in 3D space between points for U).

Ultimately this is an empirical equation that I know must be understood within its limits and applications. Interestingly, I found it has also been debated by the code committee to the merits of including it in the code.