Piping Fatigue Analysis

[PVDean]

This is a Process Safety Management (PSM) issue with hydrogen gas lines. I've been tasked to look at fatigue of a long gas line. I know the designers took fatigue into account in their initial design, but I have no documentation which is ultimately needed.

I plan on applying a smooth bar fatigue analysis based on K304.8.2 (B31.3 2006) which leads me to Division II methods and the amplitude of my stress is the question I come to you with. My gas is fed by an ambient source, but the design temperature goes up to 150 F. I have recreated the piping segment in AutoPIPE and have maximum hoop stress and sustained maximum stresses, but the expansion stress is what I question. The only heating method I can come up with is radiant energy from the sun, and when I leave T1 set at ambient there is no appreciable expansion stress. If I use maximum design temperature, that is a completely different story (expansion from Amb to T1).

Now, my proposed method is to take the maximum hoop stress associated with the internal pressure and apply a factor of safety of 4 to it. Divide that number by 2 to achieve an alternating stress and feed that into the Div II fatigue curves to get an allowable number of cycles. With that I'll calculate a remaining life based on previous usage, which again will have an applied factor of safety.

Is there a method to account for radiant heating, or is my factor of safety enough to account for the added stress?

Again this is not for design purposes, but more a safety PSM issue to show we've looked at fatigue for transport of this commodity.

 

[BigInch]

I'd say that 150F is a reasonable maximum heating temperature from solar radiation in most parts of the world. The typical method of calculation is to use either a standard solar insolation of 1000 W/m2, or more (1400 W/m2 or so if you are in a low humidity, clear atmosphere hot sun desert area) * sin of the solar angle applied to exposed surfaces of the pipe/coating/insulation (hopefully painted white). That coupled with an initial morning pipe temperature, the pipe material heat capactiy, while considering natural convective cooling plus a minimal slight breeze from the surrounding atmospheric air, will give a temperature rise of the pipe per hour of radiation received, from which you will find the maximum temperature during the day.

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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

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

 

[BigInch]

BTW its usually not the simple ambient temperature that should be considered as the base temperature, unless the anchors are also moving in unison with that ambient temperature. Otherwise, it is the installation temperature that should be used as the base temperature.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

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

 

[PVDean]

Thanks.

I don't have insight into how the stress model ramps the temperature from Ambient to T1. It seems the rate of heating is assuming the commodity of the pipe to be instantaneously changing over that range therefore the temp of the pipe to be doing the same.

If I went through the exercise you outlined, and I knew the rate of temperature change in the model from Ambient to T1, I could make the program do it's own stepping function by adding load cases T2, T3..and so on until I hit 150F.

The number of these cycles will be much harder to reproduce for my remaining life analysis than pressure cycles. How many days a year does the pipe see this worst case temp change is going to be a tough bit of data. That's why I was hoping the stress due to radiant expansion would be accounted for in my factor of safety applied to the hoop.

It's a 2" A312-304 pipe running due east, sitting on Micarta pads with 3" U bolts acting as guides.

 

[BigInch]

I don't know how it works either, or even if it does transient thermal analysis (heat capacity assumed 0). Heating of the contents might be the same, insignificant, or it may be neglecting contents entirely. The heat capacity of the gas as opposed to solid surroundings may not matter much to the overall heat calc. Besides, if the gas if flowing, it may have a cooling effect, so it may be better to neglect it entirely.

If you assume its every day, 40,000 cycles is apx 27 years and 20,000 cycles is some 54 years, both which may be beyond your plant lifetime anyway. Just start with that and see if you can live with the results???

B31.3 requires approved guides and supports. Does a U bolt qualify?



**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

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

 

[PVDean]

I don't know.

I think I'll just pick a worst case temperature swing, Amb to 105F, and add that expansion stress to my hoop stress with a factor of safety. Divide that number by two for the applied stress amplitude. So, [Exp(amb-T1) + 4*Hoop(0-P1)]/2 = Sa

Probably should do another case with cold temps, maybe down to 20F, to see what stresses I'm looking at there.

I have to do something, I can fix my remaining life spreadsheet calculation or rerun these later if I can justify a more conservative approach. With this, and the factor of safety of 2 or 3 on the remaining cycles, I should have enveloped the problem sufficiently.

 

[LSThill]

PVDean (Mechanical)

http://www.eng-tips.com/viewthread.cfm?qid=259717&page=6

ATTACH

ASME B31.3 APPENDIX W HIGH CYCLE FATIGUE ASSESMENT OF PIPING SYSTEMS

Google search for pdf file:

RECOMMENDED PRACTICE DET NORSKE VERITAS DNV-RP-D101 STRUCTURAL ANALYSIS OF PIPING SYSTEMS OCTOBER 2008

Chris PVP Paper CHICAGO 2008 ASK FOR CODE CASE ASAP TO:APPENDIX W HIGH CYCLE FATIGUE ASSESMENT OF PIPING SYSTEMS (Draft 3/25/09) CONTACT ASME B31 COMMITTEE Code for Process Piping Tony PaulinWilliam J. Koves, PhD P.E. ASME FELLOW Glynn E. WoodsRonald W. HauptPlease Read Additional informationRECOMMENDED PRACTICE DET NORSKE VERITAS DNV-RP-D101 STRUCTURAL ANALYSIS OF PIPING SYSTEMSRecommended Practice DNV-RP-D101, October 2008Page 41APPENDIX JFATIGUE CALCULATION EXAMPLEThe next two pages show an example on how fatigue calculations can be performed according to PD5500.The purpose is only to show a format based on PD5500, Annex C, and PD5500 working example W.6.2.3.Refer section 3.12.4 in this RP for a description of the piping being analysed.

 

[PVDean]

Thanks for the information. I am less than the 100,000 cycle threshold for high cycle fatigue for my piping systems. I see now that I should treat the displacement stress range differently than the pressure cycling Div II analysis.

My modeling program gives me allowable amplitude and max computed amplitude for flexibility. I assumed it was a cycling issue for flexibility as well, but it appears to be a threshold type of analysis (for a low cycle situation).

Thanks again.