Pipe Stress due to Thermal Expansion

[mxms80809]

I have a stainless steel pipe system that runs about 35' along the x-axis and then 10' along the y-axis. The system is fixed at both ends. The working fluid is air, and temperature goes from 50F to 300F.

The system experiences thermal expansion due to the temperature increase. I have calculated this expansion to be about 0.83 inches in x-axis, and .21 inches in y-axis. I am using ASME B31.3 to analyze the stresses within the system.

I am however having a problem determining the in-plane, out-of-plane, and torsional moments for the bending stresses. Any guidance on how to determine these values would be greatly appreciated. It seems fairly simple but am at a loss. Thanks

 

[C2it]

In a two leg system there will be no torsional moment, nor out of plane moment. The inplane moment is clearly a function of thermal expansion of the two legs, but also pipe and particularly bend stiffness. There are many solutions to this, look in Kellogg Design of Piping Systems, or Tube Turns, Grinnell or the Piping Handbook. The elastic centre method will give accurate results.

 

[JohnBreen]

Well said, C2it.

mxms80809, C2it is referring you to "the classics" of piping design and these are all worth finding and reading. You can also download (for free) this file which will give you the pertinent section of the olde "Grinnell" book mentioned by C2it:

http://www.anvilintl.net/db/catalogs/Pipe_Hanger_Design-07.pdf

Regards, John.

 

[mxms80809]

Thanks guys.

It took me a while to find the suggested reading material, but I did find the Design of Piping Systems (extremely old) and Piping Handbook. Unable to locate the others. The ones I did get are valuable resources, Lots of great information.

I have evaluated my system and have established a max Se= 34,000psi which is over the allowable Sa= 32,650psi. Looks like i need to add an expansion joint, and determine guides. Does anyone have any helpful hints when it comes to expansion joints?

 

[pennpiper]

Just my opinion.
You do not "NEED" an expansion Joint.
What you need is more flexibility in your current routing.

Example:
Your current design "runs about 35' along the x-axis and then 10' along the y-axis.
If you have the room I suggest that you try the following"

Run 30 feet in the x-axis turn in the opposite direction as before and run 6 feet then turn in the original direction and run 5 feet then turn and run 16 feet to your end point.
This will give you a more flexible configuration and may be below your stress allowable.
It will be cheaper than the expansion joint and it eliminates the possible of a weak point in the system in the future.

 

[mxms80809]

Thanks for the suggestion. We discussed such an expansion loop, however there is not enough room, and its not very cost effective as we are dealing with 8" Pipe. Sorry looks like i forgot to mention the pipe size earlier.

 

[BigInch]

Don't say cost effective without including maintenance and perhaps a number of replacements for your joint, as well as any associated loss of production during repair times.

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[Cuemaster64]

Gentlemen,

Would it be unwise to say that if you stepped this line, depending on support availability, that you would lessen the stress at more points and still have functionality. Proceeding 15' on x then 5' on y then 15' on x and finally 5' on y. I would only suggest this because it doesnt seem possible to put in a simple swing joint on the z axis.

Your comments would be appreciated since I too would like further information on this subject.

Regards,

 

[btrueblood]

Similarly, is there no room to step in z-direction?

 

[mxms80809]

There is just over 1 foot in the z direction from pipe centerline. I know most would advise against an expansion joint, but at this point I need suggestions/references to properly design one into the system.

Thanks

 

[Cuemaster64]

Further look into my memory banks shows that we have used many metallic bellows type expansion joints in lines that connect to equipment. Order of operations then would be:

y +10 connection, 90 to x, (attach flange on x at elbow), insert metallic bellows, (attach flange on x), x +35 connection.

With this type of connection you would get both sets of deflections at one point.

Hope this might help. As for locating the metallic bellows I can not help for I do not know the manufacture that my client uses.

Regards

 

[stanier]

The old adage used to be "change of direction change of elevation". This was designed to ensure that you did not sterilize an area on a pipe rack with a pipe cutting across future runs. If you were to employ two LREs at your change in direction it may prove to be that your stress levels are reduced.

 

[mxms80809]

Im trying to fully understand, I thought the expansion joint could either move axial or lateral not both. What is the advantage to placing the bellows at the elbow as opposed to the center of x? How do you determine the correct spring rate for the bellows?

Thanks

 

[rconner]

I was just curious, this may be stupid question, and while you didn't mention the actual size pipe, pressure, nor specific type of "air" service, would all this agonizing be going on if this were simply a more flexibly joined piping system (employing e.g. lengths of flexibly grooved end piping and fittings) as opposed to rigid welded? If flexibly joined pipes of stainless steel or other, could be suitably applicable (with actual service temperature, exposure etc. involved), what would be the relative economics compared to the schemes that are thus far developing on this thread (it would appear consisting of bellows, expansion joints, and or otherwise welded more convoluted piping arrangements)?