Stress Analysis of Carbon Steel Piping 3

[Shooze]

All Stress Engineers,

I am a pretty junior engineer in the field of stress engineering in piping.

I have been getting the round around with different stress engineers about when to worry about stress due to thermal expansion.

Just wondering, what range of temperatures should I be worried about stress due to thermal expansion? For example: if I have sweet gas operating for 5 degC to let say 40 degC should I be worried about carbon steel lines expanding.

Just trying to get a good understanding so I can determine if I need to add expandsion loops, guides etc.

Some people seem to think that every piping design need to be stressed.

Any comment or opinions would be helpful

Thanks,

 

[rneill]

If you look at the PIP specs (

http://www.pip.org),

you may find that their document PNC00004 "Piping Flexibility Analysis Criteria for ASME B31.3 Metallic Piping" has what you need. It is $50 to buy from them.

In particular, I think that Section 7 of this document addresses your question as to what piping systems require formal flexibility analysis.

 

[BigInch]

Although it might be borderline situation as far as temperatures are concerned, there is a possibility that 5 to 40 C can overstress particular piping configurations. For example, a straight configuration is probably not overstressed, by either axial tension or compression, but could buckle, depending on length and fixity of ends. Once buckled, secondary bending stress would add and overstress. It is also possible that somewhat flexible configurations exchange axial stress for bending stress in an inproportional manner, increasing the bending stress above axial stresses and throwing you over the combined allowable stress, where a longer loop might solve both problems.

The answer to your dilema is quite simple. You should do stress analysis on al of them until you know which configurations do not get overstressed. You'll quickly develope a feel for when you need to do calculations and where you don't.

**********************
"The problem isn't finding the solution, its trying to get to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[pennpiper]

"The answer to your dilemma is quite simple. You should do stress analysis on all of them until you know which configurations do not get overstressed. You'll quickly develop a feel for when you need to do calculations and where you don't."

"BigInch" has given you the very best advice (above) for any novice Piping Designer or Pipe Stress Engineer.

I will offer the following:

Pipe Stress Criteria
The question:
What are the criteria for which lines need formal stress calculation?”

My answer:
You will get lots of opinions on this one. Here is what I use and teach.

1. “The 1500 rule”
(from David Diehl, COADE)
a. If the line size (nominal pipe size) times (x) the temperature (degrees F) are below 1500 then the line "may" not need formal stress analysis.
Example 3”(x) 400 degrees (F) = 1200

b. If the line size (nominal pipe size) times (x) the temperature (degrees F) are above1500 then the line "may" need formal stress analysis.
Example 4" (x) 400 degrees (F) = 1600

2. If the line (regardless of temperature) connects to a pump, a compressor or other alignment sensitive equipment then it should be submitted for formal stress analysis.

3. The "May" noted above relates to the application of good judgment when considering other factors such as metallurgy and the wall thicknesses above schedule 80.


Pipe Stress Criteria #2 (Revision #20)
The question:
What is the minimum recommended temperature or temperature range for which flexibility analysis should be done per ASME B31.3?

My answer:
I do not have a copy ASME B31.3 (Old or current 2009) so I can't cite any specifics therein. However I would be surprised if B31.3 would give a minimum temperature for the requirement of flexibility analysis. There are just too many variables.

Flexibility Analysis might come in a number of forms and should consider many situations.
Forms:
- Visual Review
- Informal Calculations
- Formal Analysis for excessive stress caused by common factors (Temperature induced expansion, Dead/Live Loads)
- Special Analysis (i.e.: Analog studies for reciprocating compressors, seismic studies, Sea-Force studies for Offshore Platforms)

Many engineering companies may set some minimum analysis criteria on a project by project basis but there is always the exception for even "That" project.
Exceptions may include:
- Very large diameter pipe
- Very heavy wall pipe
- Very thin wall pipe
- Exotic Alloy Pipe
- Attachment to strain sensitive equipment (Pumps, Glass lined vessels, etc.)
- Extremely low temperature (Cryogenic) systems
- Any pipe installed in Arctic locations
- All other lines below the stated minimum level on a job installed in a place like Oman or Kuwait

Even if the Code makes a recommendation on this subject each Piping Lead and the assigned Pipe Stress Engineer are responsible for the proper engineering and design of the project.

 

[BigInch]

Nice list.

Yes as for question 2. Its best not to ask questions about minimum and maximum temperatures, but start thinking in terms of temperature ranges from min to max and where the installation temperature falls within that range. Not all systems are installed when the temperature is 70F. A summer installation temperature of 95F could give trouble when the temperature drops to -5F and even require heat tracing, if the fluid's minimum operating temperature itself is not warm enough to keep the pipe at a higher temperature.

Pennpiper's rule for equipment. Keep that one in mind! I've fixed a lot of piping configurations that had misaligned their compressors. The original designers thought a big 90deg turn into the station before the anchor would give enough flexibility, but those kind of things often act as levers against the equipment, if the pipe is buried, or there is a badly placed guide or anchor. Fully fixed anchors are difficult to achieve without a 10' x 12' x 30' anchor block behind them.

**********************
"The problem isn't finding the solution, its trying to get to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[LSThill]

Shooze (Mechanical)

Pipe Stress Engineering
Order #: 802854
ISBN #: 9780791802854
Published: 2009
Product Type: Print-Book
No. of pages: 500
This item is currently unavailable.
The scheduled delivery date is: 12/2009

Do detail search in Eng-Tips Piping Stress

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L S THILL

 

[SNORGY]

I recently wrote a project-specific guideline at work on this very topic, after researching numerous valuable previous posts in past threads on this site and a few published references, combining them with my own experience and judgement. If you are interested I'll strip the title block stuff out and post the raw text of it in a file you can link to.


Regards,

SNORGY.

 

[unclesyd]

There is older very good book "Process Piping Design' by Rip Weaver published by Gulf Publishing Co. that you should get a copy of. He tackles the thermal analysis of piping using the back of the napkin method. It is easier to calculate than think about, especially for old men like me.

 

[LSThill]

Expansion Loop Design thread378-235691

http://www.eng-tips.com/viewthread.cfm?qid=235691