In B 31.3 sustained/ occasional stress - for longitudinal pressure stress slp = pDi^2/(Do^2-Di^2) or pDo/4tn, we consider p as design pressure(g) as it is that pressure that will provide greatest required pipe thickness & so highest slp possible as per B 31.3 design pressure definition.

But, I have noted many using various operating pressures listed in line designation table in slp computation; aren't they wrong in approach ?

regards,

sam

it is obviously that design pressure is more conservative, in some condition, if the cost is wanted to be decreased, the operating pressures can be used which based on the customer's requirement.

Thanks to Dear stress river.

I wish to know what is the intent of B 31.3 in pDo/4tn regarding p from our mentors.

If by design pressure in LDT I mean the most severe condition of coincident pressure & temperature;if the same can be taken for thickness calculation of pipe, why will there be any other pressure (operating) in pDo/4tn ?

regards,

sam

Dear Sam,

I am not sure if this is the answer relates to your posting. I just try to. Internal pressure in pipe cause 1) hoop stress Sh = P*D/2t ; 2) longitudinal pressure stress Slp=P*D/4t . Hoop stress used for minimum wall thickness calculation. It is a local effect. It will generate fail in the thinnest part of pipe wall, assumed not in all metal part. That’s why not considered as code stress. If our minimum pipe wall thickness is enough = pressure thickness (due to hoop) + CA, we don’t worry to hoop stress. If our pressure thickness is not enough, Caesar will remind us during error checker in warning. You can try trial. Longitudinal pressure stress occurs in all surface area and considered as part of code stress.

It is recommended to use design pressure rather than operating pressure regarding to both stresses.

I'd like to share you bout this one, those DESIGN Pressure and temperature are set for coincidental pressure and any other 'surprising' stress which come into system while OPERATING ones are usually used to nozzle check at equipment nozzle which intact to piping system. I suggest you to stick with Design Pressure when you arrange your piping system.

second, i have no idea why you use PD/4t in order to find wall thickness, as i know there are 3 stresses according to axis coordinate (correct me if i'm wrong):
1. Longitudinal Stress f:{PD/4t;F/A;Mb/Z}
2. Circumferential/Tangential Stress f:{PD/2t}
3. Shear stress f:{Mt/Z;VQ/A) this VQ/A is negligible
4. Radial Stress is negligible
and wall thickness design come from tangential stress checking PD/2t (hoop stress)


regards

Dears,

I haven't used PD/4t; instead,PD/2t to find wall thickness. Hoop stress is not a local effect, as opined by Mr. Manik.

My question was simple - pipe thickness calc requires design pressure; sustained & occasional load longitudinal pressure stress should also have design pressure as P isn't it?

Thus, we should have only one P1(design pressure) & HP (hydrotest pressure)in Caesar-II load case instead of multiple P1, P2, P3... operating pressures, unless the operatng pressure is above design pressure due to coincident operating temperature.

regards,

sam

multiple input PI,...,Pn are required when the piping system has complexity design. for instance, if we wanna design piping system connected to three pumps and two pressure vessels, surely we need to put various pressure and temperature there cause the nature of the piping system itself.
The other use of multiple pressure are to help us doing nozzle checking (see ASME VIII) cause we don't check the nozzle by using design pressure, operating pressure instead

Dear Sam,

I am so sorry I miss the point here. If we push F1 in Caesar regarding to pressure input, yes operating pressure required. Calculating pipe wall thickness design pressure required said by Code.

But many of us maybe (i am also/ sometimes client ask too) put P operating P1, and P design P2. In many times I run it as you also see that by using P design P2 generate higher stress ratio in SUS and OCC stress.

Coade provides 9 possibilities variation of pressure just because the line possible to have 9 operating condition or more. But we fill P design sometimes in the field. Software cares not is it P design or P operating. It only takes the value.

(In my opinion); sometimes I think P design is pressure that may be reached in the line (anticipation). The pressure not guaranteed fixed in one value I think, even not like a cyclic load. What the other think? I would like to know he...sorry if I am wrong.

Sam,

Using operating pressure on the formulas slp = pDi^2/(Do^2-Di^2) or pDo/4tn will give you the minimum wall thickness wherein the pipe will burst.


Best regards,
Q361

Sam
My opinions & experience only -
Regarding sustained stresses - I agree, you must consider the design pressure.
Regarding occasional stresses, I don't think I've ever used anything other than the design pressure myself.
However, referring to code interpretation 20-49, for a randomly occurring event you must consider all coincident operating pressures and temperatures and consider the basic allowable stress at each condition. This differs from the sustained stress check which requires you to use Sh. For occasional checks CAESAR II defaults to 1.33xSh and this should provide a safe analysis if you use the design pressure in the SUS case (assuming 1.33 to be appropriate here.)

You would want to be confident before using less than the design pressure in the analysis, ie you are saying either
(i) the particular event is not random and you can predict the operating conditions at the time, or
(ii) the design pressure does not occur simultaneously with maximum temperature & that lower temperatures have increased allowables. This is effectively committing yourself to checking two or more sets of coincident conditions including the design pressure anyway, probably requiring separate ".c 2" files to report the intended occasional allowables for each condition. I suppose that the calcs which you have seen using operating pressure may have omitted this step.

For B31.3, Design Temperature & Design Pressure are coincident values that produce the thickest straight pipe wall. Their use is for wall thickness only. Use actual coincident temperature/pressure pairs in your structural analyses. See example in Appendix S.