Thermal Neutral points and bends 1

[Orefunto]

Hello Guys, its me again.

Once more I apologize for any inconveniences, I've checked the FAQ and I don't think this question has been asked before.
Please what would be "Thermal Neutral Points" in a pipeline (laid directly on the ground but to be to be anchored at intervals . Please, I would like to seek clarification about what this is???

2. If I wanted to calculate the stress at bends or elbows in the above ground line,
[Anc1]-----bend/elbow------[Anc2]

a) would it be o.k to just multiply the stress in the straight section by the stress intensification factor of the bend or elbow;

b) I was also wondering what the best way to obtain the moments at the bends or elbows would be. Would it be best to obtain the stress and back calculate the moment by using S = iMY/I or iM/Z or is it possible to use static formulas such as or WL^2/8 or WL^2/10 and multiply with the stress intensification factor??
I don't think I need to worry about out of plane moment as there shouldn't be any since this is merely resting on the ground

Once again apologies for any inconveniences but any help would be appreciated!

Once again, sorry for the bother....

 

[BigInch]

Points where thermal expansion or contraction in any direction are 0 when there are no anchors in the line. These are places where, if you put an anchor there, it would have no effect.

It's not OK. a) No. You must calculate the forces and moments at the joint (bend), then the stresses and multiply those by the intensification factor.

The whole point of calculating forces and moments in any structure, building, or pipe, is so you can obtain the stresses and determine if it will fail based on that level of stress. If you already know the stresses, wouldn't it be a big waste of time back-calculating the forces and moments, or is that just a fun thing that you like to do??? We don't make those determinations by examining the level of moment, or axial force; its the resulting maximum shear stress compared to yield stress that is (usually) the criteria to predict ductile material failures.



We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer

 

[Orefunto]

Thank you very much BigInch
On the Thermal Neutral Points: .
If we have a straight line, I suppose it would be reasonable to assume the mid point.But if we have a line of say 5 miles with bends and corners, these may be at any point.
Or would it be reasonable to assume they are at the mid point between bends?
Is there a way to obtain these points by hand calculation without using a software like CEASEAR

b)On the second point about stresses at the elbow, I already know the stresses in the straight section due to thermal using equation in the code for SL and Sh and checked against 0.9SMYS.
I thought it may be possible to just multiply the stress in the straight section by the stress intensification factor in order to get the stress at the elbow?

But it appears I will have to calculate the forces at the elbow seperately? Is this because we could consider the elbow to be unrestrained and the Se equation different at the elbow?

Sorry for the long questions...

 

[BigInch]

Its hard to make assumptions in pipe stress. For example to assume you have no movement at the center would require symetric ... everything.

A symetric pipe expansion loop (up-over & down) for example would me more likely to have neutral points 2/3 up the "columns". If you can draw a moment diagram, they will more than likely be at the points of zero moment, but that only says there is no member curvature there. You could easily have axial deflection, lateral deflection and growth there due to thermal expansion.

You can do these things by hand, if they are simple. Any number of quite a few structural analysis procedures will do. Trouble is that frame analysis gets complicated fast. Pipe is not anchored as many times as buildings have foundation supports, so its more mobile, and the pipe can take a number of twists and turns between anchors, making for complex calculations to get the joint movements and ultimate displacements.

I don't like to do anything more than the most simple configurations, one plane, an L shaped, or a one bent expansion loop by hand.

We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer

 

[Orefunto]

Again , thanks very much BigInch.

I downloaded an evaluation version of CEASAR and I'll try it to see if I can figure out how to use it to obtain obtaining the thermal neutral points. I've used used other software to analyze mainline pipe, I'm only recently slowly getting involved with pipe design within the facility itself.

Please on the second part of my question regarding obtaining the stresses at the elbow/bend; assuming I already calculated my elongation stresses on the straight section as SS, the following is is the procedure I want to obtain the stresses at the elbow/bends and would like to run it by you.

a). I'll use the guided moment method to obtain the shear forces f at the ends supports adjacent to the bends, then the moments at the bends will be sum of the moment due to the shear force in each leg at the elbow/bend, i.e M = sum (f1*Leg1 + f2*leg 2)

b)The stress will then be due to i*(SS + M/Z) where i is my stress intensification factor and also I think my M should only be in-plane moments since my pipe is supported flat on the ground?

Pls does this approach make sense?

 

[BigInch]

There may be a moment at the joint center (the net sum of all fixed end moments acting at the intersection of the two pipe axes) in addition to the shear forces. That moment should be distributed to each pipe and then each "fixed end" moment translated to the beginning or end of the elbow (adjusting shears as you do).

Shear forces at joint centers can only balance vertical or horizontal forces and loads at the joint itself, as theoretically there is no leg, or any length, in a joint free body diagram to multiply those shear forces by and develop a moment on the joint free body diagram. Moments entering joints arrive as distributed mmoments "fixed end" moments from the adjoining members and must be balanced by the net sum of moments, from the other pipe entering the joint and any moment applied to the joint (usually 0 for pipes).

You may have out of plane moments, because of torsion loads on a member in a one plane structure, or because your loads are in two planes on a 1-plane structure.

You can have out of plane moments in a 1-plane structure. If you consider an L-shaped pipe configuration laid in the flat horizontal plane. Weight of pipe and contents would be a distributed load applied in the vertical plane, thus bending in the vertical plane in each leg will create an out of plane moment, but on the other pipe. If both pipes are continuously supported, as you say, then there would be no out of plane moments.

We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer