Operating Case Stresses 1

[chakaad]

I'm analysing a steam line based on B31.3. After analysing the case with CAESAR II, the code stresses (Sustained, Expansion and Sustained+Occasional) were all OK, but there are some high stresses in the Hot Operating Case. Also it happens for the Hot Operating+Occasional Load. Should I ignore the high stresses in these operating cases and go on with the code compliance?

Regards
Chakaad

 

[DSB123]

chakaad,
Ignore them at your peril. Suggest you determine the cause of the high operating stresses. Is it poor supporting of the system? Do some of the supports which were active in the sustained case become inactive when temperature is included. You cannot simply ignore high operating stresses because the Code compliance aspect has been met. It's up to you as the "Designer" to ensure the system is "fit for service". Meeting the Code compliance does not clear you of this responsibility. You must ensure you have considered everything and that the system will be operating in a safe manner.

 

[NozzleTwister]

chakaad,

DSB123 has given you some good advice, Code compliance per CAESAR is not all there is.

High operating stresses usually mean very high thermal loads on your stops, guides and equipment connections. Your attached restraints and structure also need to be designed for these loads. Also, you need to verify your equipment connections aren't overloaded if applicable.

It's quite possible that you have too many restraints or your guides are too close to the elbows on your loops or other offsets. Also make sure your CAESAR configuration is set up so your not considering corrosion allowance for the thermal code cases.

Good luck,

NozzleTwister
Houston, Texas

 

[xhpipe]

In theory if all is correct as far as the basis of the analysis is concerned an interaction of less than 100% of the code allowed stresses is all that is required from the world of the beam element models point of view.

However a couple of things come to my mind....

The code is concerned with localized stresses at attachments have they been adressesd?

Have the all the possible temperature profiles been accounted for versus the installation temperature and has the TOTAL DISPLACEMENT STRESS RANGE been adressed???

Regards,
XHPIPE

 

[chakaad]

Thanks for valuable comments.
For this case there is no equipment. No disengagement from the supports happens and the guides are well located. In the Expansion load case the stresses rise up to 83% of the allowable code stress. Combined with stresses from sustained loads, the total goes high. But the question is that against which reference the stresses in Operating Case should be compared? For the Sustained load, Expnasion and Sustained+Occasional load the B31.3 provides the reference stresses, but for the operating case what's the reference? Are these high values really high or not?

Regards,
Chakaad

 

[xhpipe]

The current edition of of B31.3 does not and has never adressed operating loads. Neither does B31.1.

The new edition of B31.3 (2004) which may be available in March will offer a non-mandatory approach towards dealing with operational loads/stresses.

The codes are based upon a paper authored by Messr Markl et. al. entitled "piping flexibilty analysis". His work and the work of that group of people from the 1950s has stood the test of time.

If you have met the displacement stresses pf SE, SA, as well as SUS and OCC your system complies assuming that all is done correctly.

Regards,
XHPIPE

 

[CRG]

Are these high values or not?

The code allowable is just that, the maximum allowed by code. Some factors that should be considered are how accurate you model is and how close to the design will the field installation be. I see no problem designing up to 99 percent of the allowable providing:

There is a constraint in pipe routing that does not allow you reduce the loading. Sometimes there is insufficient room to for large expansion loops.

Careful consideration has been made for the number of thermal cycles that the piping system will go through. If it is a high cycle system, then you should be very careful as you come close to the allowable thermal stress. In contrast if you are designing for a 2000 full cycles and the reduction factor is 1 for 7000 cycles, then there is less concern.

How close are you to the mechanical properties of the pipe? If you are designing a steam system that operates at 350 degF and the allowable stress given in the code is listed at between -20 and 650 degF before a reduction in allowable is given, there is a slight comfort factor here.

There will be no adverse affect on flange loading.

How accurate are the design temperatures & pressures. Perhaps it is time to discuss this with the person that wrote the specs.

There are other limiting details that may give you comfort or concern as you approach 99% of the allowable

So the bottom line is if every allowable factor of the design is pushed to the limit and there is no practical way to reduce the stress, proceed with extreme caution. In contrast, if there are large margins of safety in other limiting aspects, caution still needs to be exercised. If you are uncomfortable with the stress, and I assume you are because you started the thread, you may consider hiring a consultant with more experience to review your work.

 

[xhpipe]

Well said CRG!!!!!!

I personally feel comfortable with my own work at 99.9% somebody else’s work that I have not reviewed is an entirely different matter!!!!

Interestingly if you ask some members of the committee when is a line overstressed (this question arose in recent meeting I was in) various answers were thrown out 75% tops, 99.9% is ok, and even some at + 100%.

But all these answers were dependent upon the issues you have addressed.


Regards,
XHPIPE