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Thrust Restraint - Buried Steel Pipe.

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ist49869

ist49869

Mechanical
Jun 1, 2011
6
Dear all,


I want to evaluate if we have to use thrust blocks for direction changes of a welded buried pipe installation. I found a Restrained Joints design procedure herein:


,but for ductile iron pipes. Does anyone have used a similar procedure for steel pipes? I did not find anything in AWWA M11.


Thanks in advance.
 
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Interesting thought - in Table 1 of the first publication, the auther is allowing 4000 psf for bearing on sand. That's OK for sands above the high water table, but not so for those beneath it, particularly for a quick condition induced during a seismic event. The water table needs to be factored in here.



Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
 
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Dear ione,

Thanks for the reference. If I am reading it correctly, if we have let’s say a large diameter buried conduit of 500 m between two anchor concrete blocks, we only have to guarantee a certain distance between fittings (where direction changes) without requiring the use of extra blocks. Do you agree?

Best regards.
 
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It may help if you clarify a little what you are doing. While thrust blocks are sometimes employed where it is desirable to very strictly control pipeline deformation in even some welded pipe systems, fully "welded" and buried e.g. many steel piping systems (and specially restrained joint ductile iron piping) are however often installed without any thrust blocks behind bends etc. (maybe why you didn't see much in AWWA M11).
Concrete thrust/reaction blocking on the other hand is traditionally employed if one is utilizing on the other hand UNRESTRAINED rubber-gasketed joining systems, e.g. simple "push-on" (with no restraint features) for steel or ductile iron pipes in pressure pipelines.
 
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If you have a fully welded pipeline why do you need any thrust blocks? The pressure results in a longitudinal stress in the pipe wall. it is only rubber ring jointed pipes such as DICl , PVC etc tht need such things.

Go look at any refinery and you will masses of steel pipelines all aboveground fully welded and not a thrust block in sight. Piping operating successfully for years at 100batg or more.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

 
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stanier,

I have worked for refineries for less than one year and I also have never used a thrust block. However this is different and I am sorry if I didn´t explained the context of my problem correctly.
I am a junior designer of pipelines for penstocks (small hydropower plants), which have relative high pressures and big diameters (more than DN700) which result in great unbalanced forces at bends that are restrained by thrust blocks in aerial pipelines. I think that for buried pipelines we won’t need thrust blocks, but since this is my first time I am designing one, I just wanted to show this.

rconner,

Thanks for your comment, like I say above I just want to show that we don’t need thrust blocks in buried pipelines.

Cheers
 
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Now knowing at least your general application, you may find ASCE Manual #79 regarding steel penstocks to be a helpful reference (I believe it has an extensive section of a dozen pages or so devoted to specifically “Anchor Blocks and Piers”). While I guess others can debate why it should or should not have been/be done, as I have seen even photographed in this manual it appears reinforced concrete anchorages and tie downs and/or encasements at vertical downbends are in fact at least sometimes employed in even substantially welded and exposed steel penstocks, and I believe some nuances/potential reasons why the designers do this are discussed in some detail in the referenced sections of this manual.
 
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Pressure elongation tension and end cap forces are one thing. Liquid mass flying around and changing velocities and directions produces quite another set of load vectors which may not all be directly resisted by pipe wellds.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus
 
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If the dynamic behaviour of liquid results in a pressure that cannot be resisted by the pipe welds then the pipe wall is compromised. Hoop stress is twice the longitudinal stress.

If the pipe is installed per AWWA standards in stable native soil then it isnt going to move. If the native soil is weak and you are worried about the pipe causing movement of the embedment then a thrust block will sink in the native soil.



"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

 
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Hoop stress is twice "end cap pressure" stress, but all stress, including longitudinal bending, and when combined against a max shear stress allowable, could be even more critical than that.



Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus
 
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I appreciate that however if a system is having dynamic stresses of the type that result in max shear stress then should not the engineer be looking at mitigating the transients that cause such conditions to arise rather than building thrust blocks? After all such events would cause damage to pumps and valves just as much as the pipeline.

I doubt if most water industry engineers ever apply Von Mises or similar in analysing a pipeline. Generally they pick a pipe from a catalogue.

I have yet to see thrust blocks on a fully welded steel pipeline in the oil & gas sector. Why is it th water industry keeps putting thrust blocks in.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

 
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Stresses should be reduced to minimum in any way economically possible. Normally its more economical to use thicker walled pipe for many pipelines as these forces are typically low and can usually be easily contained in such a manner. If you're in a nuclear plant, or have a high velocity heavy product and can only get thin walled pipe, you should check for other solutions.

Who knows why they continue with thrust blocks. Probably because most of their standards are derived from the non-welded joint era, but not ever having used non-welded/glued/mechanically coupled jointed pipe, I surely don't know.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus
 
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It just never ceases to amaze me how inexperienced the foreign engineers are that post here, and yet they are doing such complex work.

I mean really. Designing 6ft diameter piping for a hydropower dam is not something to mess around with and google up the answer too. God help them all.



Real world knowledge doesn't fall out of the sky on a parachute, but rather is gained in small increments during moments of panic or curiosity.
 
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That's what he's doing right now.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus
 
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Hi all,

I’m sorry for the late answer, I had other things to do, like enjoy the sun.
If you don’t mind I won’t comment your latest posts.
I would like to give you a Kisse – kind of stupid and [too] simplified example:

Pipe:
OD = 711 mm;
Thickness = 8,8 mm;
Bend:
Angle = 37º;
Pressure: 231 m of water;
=> Thrust force ~ 53 tons.
If we don’t grab this little fellow we’ll have, for a fixed beam with 16 m length:
Sigma=Mc/I= 1171 MPa = 170 ksi (I multiplied the force by cos(angle/2)).

I leave this for discussion if you’re still willing to put up with me. I honestly appreciate discussing with experienced engineers.

Cheers
 
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Could you please clarify if the pipe is a welded product with rubber ring joints or a pipe joint by welding? If the latter why do you think you need a thrust block and not allow the thrust to be contained by the pipe wall in developing a longitudinal stress?

What standard are you working to?

Have you done you calculations for hoop and longitudinaal stress? What results did you get?

Have you considered the dynamics of the system? ie had a waterhammer analysis carried out.

Answers to these questions may determine if we are willing to help any further as we too would like to get out in the sun!

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

 
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