Calculating Displacement Stress in Flexibility Calculation acc. B31.3

[soletto]

Hallo,

i have a understanding problem and i hope you guys can help me.

While calculating the maximum occuring displacement Stress (analytical).

according code the Thermal Force is calculated in uncorroded condition using nominal wallthicknesses.

In Detail:

The Area for stresscalculation is the nominal wallthickness.
The Area for calculating the force is also the nominal wallthickness

But:

Calculating the thermal force (nominal less fabrication and less corrosion allowance which is really high)
and the Area (nominal less fabrication and less corrosion) i get much higher displacement stresses Which then exceeds my conservative allowable Stress SA.

Or Short: Why is the Expansional Case not considered (Displacement Stresses @ final lifetime of plant when pipe is fully corroded with allowable corrosion allowance) ?

Thank you,
BR Stefan

 

[KevinNZ]

The most conservative approach is to calculate stress by;

1. Expansion force from nominal wall thickness / area from corroded thickness

This would be the case if there was ring of corrosion within a section of uncorroded pipe.

The other options are

2. Expansion force from nominal wall thickness / area of nominal thickness pipe

3. Expansion force from corroded wall thickness / area from corroded thickness

If corrosion was pitting and local then case 2 would be OK.

Case 3 assumes pipe has corroded evenly.

Our designs use nominal wall thickness to calculate the forces on supports and attached equipment and also case 3 for the stress checks. Some clients require case 1.

There is a B31.1 code case where the committee says case 2 applies (for sustained loads), (

https://cstools.asme.org/Interpretation/InterpretationDetail.cfm?TrackingNumber=13520)

But the designer's can more be conservative than the code if they want.

B31.1 also says
102.4.1 Corrosion or Erosion. When corrosion or erosion is expected, an increase in wall
thickness of the piping shall be provided over that required by other design requirements. This
allowance in the judgment of the designer shall be consistent with the expected life of the piping.

 

[soletto]

Ok thank you.

But using corroded thickness is a real szenario because when plant starts up at their final lifetime when inner diameter is bigger due to corrosion I also need to check if stresses are ok.

I use the following:

Thermal force using nominal (without fabric and without corrosion(biggest force)
compares to corroded wall less fabrication

Whe do always need to check if corrosion allowance is within design limits (sustained loads check for inner design pressure)

Yes this is exactly the case i mean what you describe for case 1. And this is the case what realy exists at the final lifetime of a plant when the pipe inner diameter is fully corroded and the stresses rises because of the smaller area

Case 3 is the case when a plant with a corroded jacket starts up at the final lifetime. ITs a real szenario so B31.3 is not considering this case

I need to considere corrosion allowance for example of 3mm at a wall of like 11mm so like 25% smaller wallthickness to consider. So corroded inner diameter is Dicorroded=DI uncorroded+6mm

Thank you for the input

 

[soletto]

I do not understand why corrosion allowance should not be considered since this has an influence on the loads.

(Bigger inner diameter due to corrosion allowance leads to bigger required wallthickness due to bigger loaded pressure)

Normaly force should be calculated with nominal and area for calculating stress should be smaller due to corrosion. So that the worst case beeing considered (also its no real szenario its a combination of corroded -stressed area and uncorroded - thermal force properties)