Correction factor for i (SIF) & k (Flex.Factor)

Dear All,

Observations Regarding SIF calculation for BENDS

1.As per the CODE,
For THIN cylinders -- CAESAR-II using correction factor for I & k.
For THICK cylinders --- CAESAR-II not using correction factor for I & k.
2.Pressure:
As I increase the pressure value – SIF value reduces—on what basis/formulae --CAESAR-II reducing this SIF value. (I checked CAESAR is not using correction factor for this calculation)

Please correct me if I am wrong.

I'm guessing you're using B31.3 (although it really doens't matter)?

The correction factor is always computed, but it is limited to a minimum value of 1.0 c2 divides the SIF by this correction factor.

The concept of THIN versus THICK is too vague. This was discussed in "Interpretation 12-11 dated 9/17/93", which stated in the reply:

A large diameter thin wall elbow is one in which the pressure significantly affects the calulated values of k and i when corrected by the equation in Note 7.

Dear Richard,
Thanks for ur reply. Still I am not clear.
I considered one example for SIF Calculations

INPUT

OD mm = 88.900
Thk mm = sch40 = 5.4864
Pressure kgf/cm2 =14.2
Bend radius mm =114.30
Elastic Modulus "EC" Kgf/cm2 = 2.0739E+4

CALCULATIONS

Mean radius r2= (OD/2)-(THK/2) =41.7068 mm
Flexibility Characteristics "h" =thk*(od/2)/(r2^2)= 0.3605
FLEXIBILITY FACTOR "K" =1.65/h=4.5768

Inplane SIF = 0.9/(h^2/3)= 1.7768
outplane SIF= 0.75/(h^2/3)= 1.4806

correction factor for i=1.694 (based on note 7 formulae in Asme B31.3)
correction factor for k=1.653 (based on note 7 formulae in Asme B31.3)

corrected inplane SIF= 1.7768/1.694=1.0486
corrected outplane SIF= 1.4806/1.694=0.8739(SIF can't be less than 1)
corrected k=4.5768/1.653=2.7684

CAESAR –II RESULTS:

INPLANE SIF=1.765
OUTPLANE SIF=1.470
K=4.547

CAESAR-II results and my calculations are not matching.
In this case CAESAR-II Using correction factor or not?

Please Clarify/correct me If I am wrong.

My CAESAR II results using your data produces your expected results.
What version of CAESAR II are you using?
Do you have more than one pressure defined in your model?
Are you certain of the P & E used in your calculations? They can be changed; see the Load Case Options.

Thanks for reply,

1. These values are coming?

CALCULATED outplane corrected SIF= 0.8739 But As per the CODE SIF cant be less than 1.

MY EXPECTED RESULTS

corrected inplane SIF= 1.7768/1.694=1.0486
corrected outplane SIF= 1.4806/1.694=0.8739
corrected k=4.5768/1.653=2.7684
2. I am using CAESAR-II 5.3.1 VERSION

3. Even if two pressures are there, CAESAR-II is calculated with EC and Pmax?

Please Correct me. If I am Wrong.

This stuff is verified at least once a week. Open a Support Request on eCustomer and send in your job file so we can see "exactly" what's going on in your model.

Thanks for your reply Mr. Richard & Dave.

I will open support request.

For final confirmation. According to this data. Can you please provide me the SIF’S (INPLANE & OUTPLANE)

OUTER DIAMETER "OD" mm 88.9000
Thickness mm 5.4864
Pressure kgf/cm2 14.2000
Bend radius mm 114.3000
Elastic Modulus "EC" Kgf/cm2 20680.0000

My mistake in unit conversion of "Ec"

CAESAR-II values are Exactly matching with the calculated values..

Thanks Dave and Richard. For ur valuable information.

Good Day.

Glad you figured it out.

Dear Richard & Dave.

As you said, CAESAR-II takes Default means Correction factor K and i is used for calculation.

As we believe that, CAESAR-II is performing calculations by Default means It is safe and conservative approach.

Can you please explain, On what basis CAESAR-II is considering automatically as DEFAULT correction factors.

why I am asking this Doubt because

If my d/T ratio is Large then, Corrected SIF Values are reduced drastically. there by Bending stress is also reduced by large amount.

I am missing any thing. Pl correct me if I am wrong.

We are simply applying the terms defined in Appendix D of the Code.
The adjustment you mention is most important in large diameter, thin wall pipe bends. We apply to all bends. You have control over this adjustment in the Load Case Options screen.

Thanks Dave Diehl for your reply,

1. Can you please check this statement -- "According to the CODE For Large Diameter Thin wall Pipe( We can use APPENDIX D up to d/T ratio less than 100, so for this correction factor the upper limit is d/T>=100) Bends User has to consider correction factor for i and K." -- Please give me any reference for more details on this topic.

2. In my case one more problem,, we have two pressure cases DESIGN and OPERATING. for every time we are making two files with DESIGN PRESSURE and OPERATING PRESSURE. As you know that if i select default for OPE pressures Stresses are more compared with Design. That is why we are making into two files.
Please give us a solution for making one CAESAR-II file whether with DESIGN OR OPERATING.

Please correct me if I am wrong.

Note 1 of Appx. D states (all) values are useful for D/t<100. The bend adjustment in another note does not change this.
CAESAR II collects P1, P2 etc. with no regard for "design" or "operating".

Dear Dave,

Thanks for your Reply..

As you said: CAESAR II collects P1, P2 etc. with no regard for "design" or "operating". -- sorry I am not getting what you are trying say.

I will explain in Detail about my Doubt

CONDITION: DEFAULT for correction factor of i and K.


----------------DESIGN pressure--------OPE pressure

Pressure value-----Large---------------Small

SIF----------------Small---------------large

K------------------Small---------------large

Stresses-----------Small---------------large

After seeing this comparisions, we are in confusion whether we have to use OPE pressure or design pressure.. that is why we are calculating two times one with DESIGN PRESSURE and one with OPE pressure and cheking stresses..

Is there any alternative for this by doing this in with any one pressure

As d/t ratio approaches 100, stresses in OPE case are higher and higher compared with DESIGN pressures
.

A few years ago B31.3 changed the terms used in the bend adjustments. The Code used to reference design pressure and reference modulus of elasticity. Now it uses P & E for the case under consideration.
So does CAESAR II, but you can choose others in the Load Case Options.

Dear Dave,

Thanks for your reply,

1.Yes, Code uses P & E for the case under consideration. But what about Sustained case which pressure I have to consider. Generally we are considering Pmax, but the problem with Pmax is: If default the correction factor is applied and stresses are reduced.

2.I have checked with another piping stress analysis software “AUTO PIPE”. There Default is: not considering Correction factors. If my d/T ratio near to 100 huge difference in stress results for the same file analysis in CAESAR-II & Auto pipe.
CAESAR-II gives less stresses compared with Auto pipe. Pl help me up to which maximum d/T ratio we can apply this correction factors.

Can you please help us, what we can explain if any client asks about this difference in stresses?

Those Code adjustments for bend stiffness and strength are specifically for large diameter, thin wall bends.
If you wish to not use them, choose a different pressure to set them in the Load Case Options.

Dear Dave Diehl,

Thanks for your reply,

Yes, I agree. Code doesnt give any explicit statement on large dia and thin wall bends.

Can you please provide any path regarding this topic. because with out clarity how can we apply default always ...

Check this previous post, especially the reference to the "Interpretation". I think this will help.

Dear Richard,

Thanks for your prompt reply.

In that post only I asked this doubt.

Can You please provide me any path so that I will get some information regarding this topic. I searched in this forum also but the final conclusion up to which D/T ratio is safe and conservative is not there.

please help me to close this issue..

I can't answer this question. Recent work in the PRG lab has shown issues with D/t ratios (on fittings) as low as 61. Much more testing is needed here.

I don't believe a single D/t value can be used as a decision point. Rather I think that each fitting will have a recommended limit, but even that may be based on the type of loading (sustained or cyclic).