A Pipe Support from the ground 1

[somacast]

Hi everybody, its my first post in the forum,

We are facing a problem with a vent at our plant, and it was agreed by all parties that it needs to be re-routed to a further point.

The vent pot is 12 meters high, small in size and weighs about 60 Kgs, and in order to maintain the vent and due to pressure drop its vent pipe shall be enlarged to 6 inch pipe.

so to make things clearer its a 6 inch pipe starting from 6" x 3" reducer, and running horizontally for 6.5 meters , and at the 6.5 meters we shall introduce a support from the ground made of 10" sch 40 carbon steel pipe, this support is 12 meters high , and the vent line will be anchored to it by U-bolts..

my questions are if someone can answer them please:

1- I did basic calculations considering a wind speed of 92 mph, and assumed worst case scenario that a resultant force of this wind load will act at the top of the 10" sch 40 pipe, and the bending moments were fine at its base taking an allowable value of 21000 psi (approx. 0.6 Fy), so my question is will the 10" pipe stand by its own from ground given that a proper foundation will be provided underground with good pedestal?

2- do I have to worry about the thermal expansion of the 6" pipe stressing the 3" end ?

I know that the second question might not be so relevant to the forum but I hope someone might find a way to do it.

Note: my company belongs to stone age, no softwares, the only software I have is a casio calculator , and Iam a general Mechanical Engineer and no one else to do this, so I have to give an answer , thnx

 

[SlideRuleEra]

somacast - There are other considerations:

Question #1: The 10" pipe's bending moment may be ok, but what about deflection (in any horizontal direction) at the top of the 10" pipe? Either the 10" pipe will deflect or the vent pipe will have to resist the force to keep it from deflecting. Will that cause a problem with the 6" vent pipe?

You mention a proper foundation, but the foundation needs to withstand overturning. I doubt the 1 meter square footing shown on the drawing is anywhere close to satisfactory for overturning.

Question #2: You do have to consider thermal expansion, but that is of secondary importance until other issues are addressed.

A question you did not ask concerns the 6.5 meter (21 feet) simple span for the 6" vent pipe. The 6" pipe needs to be considered as a beam. A pipe, depending on its wall thickness, can be a pretty good beam, but 21' is a long span (Span to Depth Ratio of approximately 40). Take a look at bending and deflection.

Also, considering the forces above, will the 6"x3" reducer be a satisfactory structural support by itself? Does additional structural support need to be furnished at that end of the vent pipe?

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[somacast]

thnx for the reply SlideRuleEra

regarding the deflection at the top can you please tell me more about it? and how do you suggest to increase the rigidity of the 10 inch pipe and reduce the deflection amount on top? will cement filling help? and regarding the foundation we can increase it we might follow some existing foundations from another case, i think ive seen some going 3.2 metets then 1 meter on top on which the pipe base fits.

regarding the unsupported span as per a table i found on google from 31.1 i think it can go upto 6.4 meters for gas applications, and here its 6.5 meters on lower temp than that mentioned in the table, so i wonder if that is good enough.

and regarding the 6x3 side, any hints here on how to go over here?

also if the wind force made that 6inch pipe start to rotate, it will fail from the 3 inch pipe side due to torsion, so we were going to restrict its lateral movement by u-bolt, but again that might prevent it from expanding thermaly leading all expansion to the weak 3inch side, what do i have to do?

thnx

 

[SlideRuleEra]

somacast - IMHO, the answer is not to increase the rigidity of the 10" pipe but to provide both structural support and some movement of the 6" vent pipe at/near the 6" x 3" reducer - maybe a flexible coupling to accommodate movement. Support is a separate issue.

The foundation has to be adequate. Copying an existing foundation may or may not work. The calculations have to be done for this project.

Same thing for the 6" pipe span, looking for an answer in a table may or may not work.

Lack of software is not the problem, the Casio calculator should do just fine... but you have to know how to make use of it. I understand you are required to provide an answer, but you may need outside help on this project.

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[somacast]

thnx again SlideRuleEra

as a matter of fact my boss wont out source this so its me and him and all I hope for is getting as much help as possible here, ill check on flexibility on the weak end, and would welcome any additional hints

 

[SlideRuleEra]

Ok. The load is light, and the vent elevation is not too high. Look into embedding a pipe or pole into ground instead of designing a footing. If soil conditions will allow it (no rock) embedment will provide cost effective, simpler overturning resistance than a footing.

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[somacast]

Thnx again, yes loads are not much but the 12 meters high support makes me worry as much as I do about the 3 inch side, as I don't really know how would a 10 inch standing pipe behave regardless to the loads, I mean under its own weight..., but I will consider your advises wherever possible.

 

[SlideRuleEra]

...makes me worry as much as I do...

That's why I'm stressing an understanding of the calculations. Your boss has given you an excellent problem for a plant engineer. Don't try to just "get an answer", take the opportunity of learn how to address the problem, any why. Otherwise you are doomed to a career full of worry.

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[somacast]

thanks again, I will sure do