Predicting Pipe Deflection 6

[aleland]

I am trying to determine the deflection of a pipe between two supports. Does anyone have an equation that I could use to predict the deflection. I want to take the temperature, psig and saddles into account. I'm not asking for the answer just the way to find it.
Thanks

 

[Frank1344]

Hello,

The following is for Horizontal Pipe.

It is similar to find a deflection for a simply supported beam with uniform load distribution and or local forces as a valve, saddle and ... on it.
As far as i know in this case, deflection is not affected by internal pressure or temperature of the pipe.

Hope it helps,
Frank
Calgary

 

[rconner]

This is dependent on the actual piping involved and other specifics of the installation. Complex piping arrangements can involve forces in every direction that effect displacement or "deflection". However, for flexibly joined basic "horizontal", metal piping at normal temperatures and with one support per pipe very near joints, a "simple beam" analysis based on the weight/loadings, the moment of inertia or section modulus of the (normally circular) pipe cross-section, and the modulus of the actual material is normally adequate to reasonably predict deflection. An example of such an analysis/procedure with ductile iron piping is available for download at

http://www.dipra.org/pdf/pipeOnSupports.pdf.

Predicting deflection of plastic or composite etc. pipes can be a much more complex procedure, as I believe the deflection of such pipe beams is much more time and temperature etc. dependent (and to do a good prediction job in such cases probably requires more intimate knowledge of the material, exposure conditions, and "long-term modulus(i)" etc. of the material(s). Incidentally, unlike e.g. metal pipes the long-term modulus of plastics might be only a very small percentage of a short-term or instantaneous modulus, sometimes requiring many supports/short support spacings for such materials.

 

[StressGuy]

The long out of print Kellogg Design of Piping Systems calls for this equation and it's the basis for most span tables that you'll find in B31.3 engineering.

Sag = 17.1 * (w * (L^4) / (E*I) (inches)

w = weight per foot of pipe+liquid+insulation (lbs/ft)
L = pipe span (ft)
E = modulus of elasticty (psi)
I = moment of inertia (in^4)

Most span tables are based on a maximum deflection of 5/8", which is intended to keep the natural frequency of the span above 4Hz and therefore less likely to resonate in the wind.

Oh and, temperature is a factor as it will affect your elastic modulus, especially at high temperatures.

Edward L. Klein
Pipe Stress Engineer
Houston, Texas

"All the world is a Spring"

All opinions expressed here are my own and not my company's.

 

[aleland]

Well with that said how do I account for the temperature. The elastic modulus was 29,000,000 lb/in^2 before the 200 degrees F in taken into consideration, I'm sure with such a low temperature it can't have much effect..

 

[rconner]

Youngs modulus for carbon steel decreases slightly with increasing temperature, but I doubt you will see it reflected in significant variations in deflection from your calculations at the temperature you describe (see for example data at

http://www.engineeringtoolbox.com/young-modulus-d_773.html).

 

[StressGuy]

Yes, I should have been clearer on that. For most low temperature cases, the modulus is not going to vary enough to cause problems. I just didn't want to leave the notion hanging out there that temperature does not have an impact on deflection as it is not always true.

Edward L. Klein
Pipe Stress Engineer
Houston, Texas

"All the world is a Spring"

All opinions expressed here are my own and not my company's.

 

[Frank1344]

please give an example of a piping between two supports which the temperature is affecting the deflection?

 

[JohnBreen]

Frank1344

Turn on your calculator and run the numbers. Ed gave you the equation above and you can go to B31.3, Appendix C, Table C-6, to look up the E (Young's modulus) for the material and temperature that you are using. Try different "E"'s in the Kellogg equation and see the difference. Not a great difference for some carbon steels but as an extreme example if you have aluminun ............

Reductions in Young's modulus (E) due to temperature can really cause a significant redistribution of loads over a pipe support (hanger) system especially when spring hangers are in use. That is why we always recommend that the hanger design case for high energy power plant piping systems be run with the hot Young's modulus (you must use the Young's modulus at 70 degrees to calculate the stresses though).

Regards, John.

 

[rconner]

I suspect temperature can be even more of a deflection factor in plastics -- I happened to see some 48" black, rather thick (2-1/2"-3"?) solid-walled polyethylene pipe strung out on a jobsite in the quite hot Mississippi summer sun many years ago, and I actually saw the piping obviously ovalled (ring) deflected, it appeared maybe even a few inches apparently under its own weight on top of the ground! In retrospect I don't know if this was just the pipe weight and the sun/temperature doing this, or it had been previously deflected due to the weight of a pipe or two over it in shipping, or what. However, I always thought the hot temperatures had something to do with this. I still have a picture of this pipe.

 

[Frank1344]

JohnBreen,

I turned on my computer and ran a simple model with Caesar II software and changed the temperature from ambient to 650 degree C and there was no change in the deflection at the middle of the pipe.

This is a very good point to discuss this with COADE guys who develop this software.

Thank you.

 

[JohnBreen]

OK Frank,

You are right, I am wrong, I am sorry. I regret if I offened you.

regards, John.

 

[Zapster]

Frank1344,

I ran a simple A-106B model with AutoPipe software and changed the temperature from 70 to 750 degF. Provided I changed the default settings to use the hot modulus, there was a change in deflection 20%. For the model, the deflection went from -0.375” using 70 degF modulus to 0.445” using the 750 degF hot modulus. I am not a Caesar II user. Is there a setting to change the default to use the hot modulus in Caesar II? According to their web site, Caesar II version 5 gives “The ability to perform an analysis using the “hot” material elastic modulus.”

Regards

 

[Frank1344]

Thanks Zapster.

Yes, you are right. The similar setting is also available in Caesar II, but as you said, by default, this is not taken into account unless you clicked it in the settings.

Thank you again.

 

[cpewhf]

To Stressguy,

Where and which book is the euquation stipulated?

Thank you very much!

 

[IRstuff]

Was there something besides

The long out of print Kellogg Design of Piping Systems calls for this equation

that you needed?

It's quite expensive to buy:

http://used.addall.com/SuperRare/su...rePowells=on&StoreStrandbooks=on&StoreZVAB=on

TTFN

 

[JohnBreen]

Hi guys,

The equation in the Kellogg book is just a standard beam deflection equation. Nothing special about it just because it is in Kellogg. The whole set of such equations is in Roark's Formulas and in any number of other introduction to mechanics books. These text books come and go and a large number of them can be had on the cheap from various used (or overstocked) book sellers.

Regards, John.

 

[stanier]

WHy not get as demo copy of Autopipe. I have one that has 20 bnodes. Plenty to model what you are doing.

Geoffrey D Stone FIMechE C.Eng;FIEust CP Eng

http://www.waterhammer.bigblog.com.au