My case is this:
Piping is fabricated at 21^C, After installation, the piping experienced a temperature of 85^C due to hot climate,during operation the piping is subjected to -90^C.
My argument is this:
The piping will experience both expansion & contraction. Is it correct to calculate the expansion stress range from 85^C to -90^C and compare it to the code allowable ?
What I believe right now is that it should not be, because the piping will go back to the undeflected position;i.e at 21^C, before it contracts. Pls correct me if I am wrong.

Regards,
delpilar

I would advise you go go throughly through B31.3 ( para 319.3.1)for displacement stress range and the corresponding temperature consideration.You will get your answer (i.e. what the code wants you to do.)

Also to physically answer your problem the system sees a range of displacement corresponding to -90 to +85. So this is the "range".

Regards

When the piping is fabricated is not an issue but when the piping is installed it becomes "fixed" in place.

In other words when the pipe is laying on the shop floor at 21C it is loose and not subject to any thermal strain.

If it is installed on a hot day and has a metal wall temperature above 21 C then that temperature sets the beginning of the thermal displacement ranges.

Lets say its installed at 60C... then one range would be from 60C to 85C another range would be from 60C to -90C and then finally you need to take the total displacement stress range from 85C to -90C into account.

Fixity comes through attachment to nozzles as well as installation of restraints and other pipe supporting elements.


Lets say T1 is 85C and T2 is -90C
Assuming you have only one pressure the load cases to set up would look like....

Go to the Kaux Menu and set the ambient Temp @60C... then

1)W+T1+P1 (OPE)
2)W+T2+P1 (OPE)
3)W+P1 (SUS)
4)LC1 - LC3 (EXP)= T1
5)LC2-LC3 (EXP) = T2
6)LC2 - LC1 (EXP)= Range from T1 to T2

See the on-line user manual for further help... and I believe you can find some COADE newsletter articles on this as well.

I have read the code and it looks like that I really have to consider the expansion stress from 85^C (max) to -90^C (min).Thanks Mr. Anindja.
And I will also look for those user-manual and COADE newsletter articles for more information on these topic, thanks Mr. Luf.
There is one more thing that bothers my mind:
Why do the code consider LC6 (from the above sample)? I think it does not represent the additional stress when the pipe expands/contracts from the zero-strain condition.
I believe, by doing so, is like having the pipe subjected to a metal temperature of T3=-175^C (Range from T1 to T2 is -175 C^), which is not the case. Pls correct me if I am wrong.

Regards,
delpilar

Bon Dia, (as they say in Aruba)

It is important to remember that we are evaluating primary stresses (sustained P + W) differently from our evaluation of secondary stress ranges (thermal expansion/contraction or "displacement"). It is also important to remember how we developed the equation for the allowable stress range limit. It may be useful to look at this post:

http://www.coade.com/ubb/Forum1/HTML/000057.html .

In the B31 Codes the secondary stresses are addressed in terms of the stress range from the coldest temperature expected to the hottest temperature expected. This stress range is due to the CYCLING through thermal excursions (and other causes), and so the Code rules are focused upon fatigue. The "full" stress range combines (ADDS) the thermal excursion from the erected (as opposed to "fabricated", this is North American terminology) temperature to the hottest temperature, with the thermal excursion from the erected temperature to the coldest temperature. So in terms of temperature from your example we will have the "delta T" from the range 85 degrees C minus 21 degrees C (64 degrees C), added to the "delta T" from the range 21 degrees C minus negative 90 degrees C (111 degrees C). So the total "delta T" for the "full" cycle is 175 degrees C. Of course, there will be many "partial cycles" in the life of the piping system and these are addressed by adjustment to the stress range reduction factor, f, as described in B31.3 paragraph 302.3.5 So if you allow Caesar II to calculate the stress range from the "as installed" (i.e., "erected") 21 degrees C to -175 degrees C (if I got my sums right) you will have the calculated stress range for the "full thermal excursion stress range".

So if I understand correctly, your "erected but out-of-service" condition will result in deflections in the piping system that will strain the piping materials "in the opposite direction" from the material strains of the "in-service" condition. The thermal excursion from the "erected but out-of-service" condition to the "in-service" condition will be your "full thermal cycle range".

Now, it is also important (e.g., for hanger design and loadings on pumps and vessels) to evaluate the steady state operating condition (at -90 degrees C) to determine the actual "in-service" or "operating" deflections at that temperature (using the "delta T" of 111 degrees C to in your Caesar II model). Also, for primary stresses (P + W), it is necessary to check the calculated maximum stress against the allowable stress at both the minimum temperature and the maximum temperature - sometimes (not likely in your case) the higher temperature allowable stress will rule.

Pasa Bon Dia (can't wait to get back to Aruba)
Regards, John.

If I get it right, the effects of fatigue gets into the picture once a cycle exceeds 7000.
But for cycles less than 7000 as in my case, the code takes fatigue into account by considering the hottest and the coldest temperature.
"...Code rules are focused upon fatigue."
Thanks Mr. Breen, I really appreciate your reply.

Regards,
delpilar

Hello delpilar,

Welllllll....... not exactly.

Actually the B31 Codes address fatigue by way of the basic concept of "stress range" beginning with the first cycle. Picture an S/N (stress versus cycles) chart of the polished bar carbon steel specimen of the material at issue. The S (stress) axis is vertical and the N (number of cycles) is horizontal. The failure line is basically a 5:7 ratio, sloping down from left (cycle number one) to right (as cycles increase). The chart shows grapically that the material will fail sooner (fewer number of cycles) at higher stress magnitudes and when the stress magnitudes are lower, the material can endure more loading cycles. The first through the 7000th cycles use a stress range reduction factor, "f", of 1.0. At about (theoretically) the 7001th cycle the "f" factor is reduced ("stepped down") to keep the calculated allowable stress range within the envelope (factor of safety) of the sloping S/N curve. When the sloping failure line again approaches the limit of the envelope, the "f" factor is again reduced. This is what you see in B31.3-2002, Table 302.3.5. If you look at the latest issue, B31.3-2004 introduced an alternate way of looking at the "f" factor and it presents a fatigue failure curve ("f"/N chart - Figure 302.3.5) that shows that for cycles less than 7000, it is appropriate to use values for "f" that are (sometimes) GREATER than 1.0.

If you have read my (somewhat wordy) description of the concept of the allowable stress range you know that for secondary stress evaluation we are using (with some margin subtracted for vagaries) the range of stresses from cold material yield all the way to hot material yield. Because the allowable stress range limit is calculated to cover the "full" thermal range from coldest to warmest, your stress range calculations must also must also represent the "full" thermal range from coldest to warmest. The "f" factor is added to the calculation of allowable stress range to be sure that the rate of decline of the allowable stress range keeps pace with the declining S/N curve as cycles accumulate.

We know that in hot systems, there will be some beneficial plastic deformation in the first few cycles (typically at bends) so we have tailored the rules to allow that "relaxation" to occur in a controlled manner. The idea is to provide rules that will allow some permanent deformation in those first few full cycles but also prevent ratcheting (elastic/plastic deformation behavior) from occurring over many cycles. If the allowable stress range is respected in concert with the Code prescribed calculation of stress range, the piping system will after a few cycles "shake down" to complete elastic behavior and then from that point we will control the effect of fatigue with the application of the "f" factor.

Regards, John.

Thanks for sharing your knowledge and your time.
Hope to read more of your replies soon.

Regards,
Homer