Bending from thrust on above to below ground pipeline

[angus89]

thread378-482187
Hi All,

I have a question regarding a below to above ground piping section I am currently designing.
The pipeline is a DN750 Ductile Iron pressure pipeline.

I have attached a sketch for reference.

I am looking at confirming what force an in line thrust block should be designed to restrain the pipe from effectively lifting above ground? Refer to point A. As the bend is Flanged, I understand the force in the direction of the vertical to be taken up within the pipe with the resultant force being in the horizontal due to the unrestrained Flange- Socket connector (clouded on the sketch).

To be conservative, I have looked simply at B as a fixed point with the force being applied at the unrestrained A over the 10m lever arm and ignored the soil which would counteract against this force, however I know this is far too conservative.

The bends are 22.5deg bends and the test pressure is 900kPa. The bend is flanged, however I have used a Flange- spigot connector to connect back into the rubber ring joint pipe above ground.(as clouded)

I realise this is not the most conventional design, however there are numerous constraints on site leading to the design.

Any help would be much appreciated.

Thanks

 

[LittleInch]

Is A to B a flanged fully retrained pipe?

If the temperature is the same as the install then forces should be balanced in the pipe and hence there is no resultant trust force.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[angus89]

That is correct, it is flanged between A & B. However one thing I didn’t mention is the 22.5 deg bends are each rotated over one bolt hole. It is still symmetrical however, so I’m assuming the forces are still balanced?

I have attached the sketch in plan view.

Thanks

 

[LittleInch]

Does that "Inline thrust(piled)" not take out the end cap force from the sliding connection?

And create an equal and opposite force F in the thrust block?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[angus89]

That was the intention with the in line thrust block, I was just looking for advise on whether my understanding of that force (F) is correct.
Thanks for your help.

 

[GD2]

Why do you use flange joints below ground? Any advantage over push-on and other joints? How far are you putting the first inline thrust from the above ground bend?

GDD
Canada

 

[angus89]

Hi GDD,

There are issues on site with high ground water and inability to dewater (contamination). The only advantage with the flanges is to simplify the installation process and limit the dewatering.

Typically it would be socket fittings.

Thanks

 

[GD2]

Got it. Is there any buoyancy issue?

GDD
Canada

 

[angus89]

No buoyancy issues at that depth with the weight of the soil.

 

[alchemon]

You can estimate thrust at the bends as 2PAsin(theta/2))

P = absolute pressure
A= pipe cross sectional area
Theta = the bend angle measured from previous alignment