Localized pipe stress near support on large (30") diameter lines

[ARohb]

Hello,
I was tasked on analyzing the localized pipe stress near the support for some 30" diameter runs with a current support span of 20 ft. i have already run a CAESAR run and have support loading per node. if we were using pipe shoes this would not be an issue but the support that war are using are just the bare pipe sitting on a 6 in Teflon pad. i am currently using two methods
1. Roark's ring model (table 9.2 Case 20) to determining the loading on the contact point. i have been using the 8th addition as well as

http://www.calcstress.com/local-stress-in-support

as reference.
2. line stress equations for bare pipe from "piping and Pipe Support systems" paul R. Smith and Thomas J. VAn Laan

1. for Raork's ring model i am getting a bending stress of about 235 ksi for a 7 kip load, which does not seem correct since they (as well as other ) have used the support in the past and have not had any issues. i got the moments from the Raork's model but i used the stress calc Sbc = M/Z m being the moment, and Z being the section modules calculated to be Z = Beff*Tnom^2/6. in the calcstress reference they made a mention that Beff can be estimated to beff= b+1.56*Squrt(r*tnom). this is from Troitsky use of ring girders and i believe used in the Zick model for pipe saddles mentioned in ASME section VIII Div 1. since this calc toke into account the with of the contact strip i assumed this could be used. is there any thing im missing or may have miscalculated

2. the other methods was more straight forward since the stress equation were give in the book. however they are around 70 ksi as well. this seams really high. im not sure if there is something i am not accounting for

Thank you in advance for any tips or tricks

 

[pennpiper]

Look here and consider just the plain Saddle, not a Shoe Saddle, but just a plain reinforcement Saddle.

https://www.google.com/search?q=pip...AgDEAE&biw=1024&bih=633#imgrc=U72GHAInXHaK8M:

Sometimes its possible to do all the right things and still get bad results

 

[XL83NL]

Have you considered the (excellent) book by Peng & Peng called ‘Pipe Stress Engineering’?

https://www.asme.org/products/books/pipe-stress-engineering

I believe they cover some methods on this topic.

 

[GD2]

ARohb,
I am not a stress engineer but with my piping knowledge I can say that supporting pipe directly on steel ( as your current situation) becomes a concern with increasing D/t ratio. The circumferential bending stress causes the pipe to collapse. Equation for the circumferential bending stress can be found in “Roark’s Formulas for Stress & Strain”. The allowable limit can be found in “Design of Piping Systems”. If the circumferential bending stress exceeds the limit, the pipe is required to be supported on a shoe or saddle. As a minimum, the shoe need to be a wide flange.

GDD
Canada

 

[KevinNZ]

When doing these calculations, you should check the stress at various pressures and contents loads. Like a balloon, the local stress will quite different if these is pressure in the pipe or not.

 

[saplanti]

It is a small span for that diameter. It should be OK unless you use very thin pipe and short line or point contact only. If you consider point contact you may find very large stresses with thin pipe, however if you consider the line contact (this should be maintained at all load cases) the stresses may be reduced a lot (Roark's has line contact case as well).
In case you cannot get away from point contact with thin pipe you should implement saddles.