Pipeline - Pressure elongation

Hi,

I am new to pipeline modeling in Caesar II. A client of ours in insisting that we should include the bending stresses due to pressure elongation in our pipeline stress analysis. I have gone through config settings and the different options in the Caesar file and could not find a place to enable pressure elongation effects. Is it something that can be done in Caesar II?

Kind Regards,
A

Maybe firstly you need to read the User Guide.

Then check out 'Activate Bourdon Effects' in the Special Execution Options accessed fdrom every input file.

An attempt to describe how to take into account the strain that the piping undergoes when subjected to pressure is simply based on the Generalized Hooke's Law and stress expressions when the pipe is pressure loaded.

Since this theory has no connection with the works of Eugène Bourdon, particularly with "le manomètre à tube de Bourdon, breveté le 18 juin 1849 à Paris", it's hard to understand why this stuff is placed under "Bourdon Effect" designation.

The "Bourdon" switch in C2 has two components - straight pipe elongation and bend opening. While the latter is your Bourdon effect, both are strains due to pressure.
Typically, such effects are not included in pipe stress analysis as they are insignificant. But these effects may be important in systems containing long runs, FRP pipe and/or bent pipe.

Agreed with activating the Bourdon effect for pipelines. By activating this, the stresses generated by pressure elongation is combined with thermal expansion and related displacement stresses. By this way we decouple the sustained pressure effect from elongation effect. For more information please go through the L.C. Peng paper on "pressure elongation in piping system".

Regards,
R.K.

Note that Caesar II will include the "bourdon" effect in all loads cases with pressure so it will include it in the sustained case rather than as an expansion stress. This is overly conservative. I have pushed for an option in the load case options to be able to deactivate the "bourdon" effect from load cases so that it can be treated as an expansion stress rather than a sustained stress. Hopefully this will be available in future updates.

If you want pressure elongation (when activated) in your expansion stress range, why don't you change your load case list from:

L1: T+W+P
L2: W+P
L3: L1-L2

to:

L1: T+W+P
L2: W+P
L3: W
L4: L1-L3

?

Hi,

I have a calculation of pipeline in B31.8, when i activate the bourdon effect the code stress ratio for expansion cases increases to 60%, and i note that the allowable changes


Please, can someone explain why this happens?

Tanks

Hi,

The bourdon effect is the difference between the pressures inside & outside the pipe.
If the inside pressure is greater than the outside pressure the pipe will try to expand. And if it is bent in a circular form (which normally it is) then the expansion is noticeable. This type of things normally occur when we have high pressure pipes running a long distance, or for the pipeline projects, where the pipe length is in kilometres.

If you ask me, then I will always use this effect when dealing with pipeline codes.

The other effect is the "stiffening" of pipe bends and elbows as internal pressure is increased. The stiffening results from the internal pressure resisting the "ovalization" of the cross section that normally occurs when a bending moment is applied to this section. One or both of these effects can occur in large diameter pipe with large D/t ratios.

For this kindly go through the ASME B31.3, Append D, Table 300 (for Flexibility char. and refer note 7).

I hope this help

Regards,
R.K.

80211887 - B31.8 bases allowable stress as a function of material yield stress at ambient temperature - except for the allowable expansion stress range for "unrestrained" pipe. In this case, the Code has an allowed limit that is also a function of the sustained stress (like B31.3 equation (1b)).
By turning on Bourdon pressure in CAESAR II, you are changing the sustained stress and, so to, the allowable expansion stress range for "unrestrained" pipe.

RK - on your first point... Yes, pressure will provide a load on the pipe that causes extension. In most cases, this is ignored as the pipe is usually very stiff in the longitudinal direction. Codes typically include this effect not through the solution of F=KX but instead by simply adding an additional longitudinal stress to the sustained case as PD/4t.
In very long runs of pipe, stiffness falls and the elongation may cause bending on attached pipe. In this situation, including the structural deformation due to pressure may be significant. To include this in CAESAR II, activate the bourdon effect in translation only. The program includes this effect automatically for FRP pipe (with it's lower axial stiffness) and users should consider this in (long run) pipeline applications.
The rotation effect in CAESAR II's bourdon application is intended for use with bent pipe and not forged bend fittings. Bent pipe ovalizes in the bending process and internal pressure tends to return the cross section circular, thereby opening the bend.

Hi Dave,

Agreed with the option of utilizing the burdon effect in translation ( I guess this is what we use in long run pipes); But for rotation (correct me if I am going wrong), we either selcet the option of translation or rotation, we can not impliment both the options in the same model. And rotation effect, isn't it enough that we check the amount of rotation with respect to the displacement and summarise the calculation? Since this rotation issue will be mainly in bends and not at the other areas of the pipes. For this, what if we move on with the long radius bends? Still we need to check the rotation option for burdon effect?

Appriciate your reply.

Regards,
R.K.

I find it obvious from the input selection. No Bourdon effect, translation only, or, translation and rotation.
As I said earlier, we offer rotation for those bends that are fabricated by bending straight pipe. These bent pipes are no longer circular in cross section and pressure will tend to straighten them - that's the rotation. Forged bends do not suffer this effect.

Hi Dave, My mistake....you are right..Thanks.

Regards,
R.K.

Thanks for your help

Hi,

I have a calculation of pipeline in B31.4, when I activate the bourdon effect, for hydrostatic test and sustained cases there is overstress in the branch conections, I know that this case is overly conservative, but I don't know how can I activate and present a calculation with bourdon effect for expansion case only.

Thanks