Pipeline Expansion Loops.
Pipeline Expansion Loops.
(OP)
Hello Folks,
Why do we often use 45 degrees bends (3D, 5D, etc.) rather than a Standard 90 Degrees Elbow?
My understanding is that we will have less pressure drop and less stress.
Also it looks that bends can handle hammering, slug flow better that 90 degrees elbows.
I would really appreciate if I can have your comments on the subject.
Best Regards,
Frank
PS: Sorry if the subject has been discussed before.
Why do we often use 45 degrees bends (3D, 5D, etc.) rather than a Standard 90 Degrees Elbow?
My understanding is that we will have less pressure drop and less stress.
Also it looks that bends can handle hammering, slug flow better that 90 degrees elbows.
I would really appreciate if I can have your comments on the subject.
Best Regards,
Frank
PS: Sorry if the subject has been discussed before.





RE: Pipeline Expansion Loops.
Consider the vertical dimension difference available between north/south runs in a pipeway and the crossing east/west runs.
Two 90 degree elbows may exceed the space available. Whereas a 45 degree ell and a rolled 90 degree ell (with or with-out a short pup piece may work just fine.
Sometimes the simplest answer is the right answer.
RE: Pipeline Expansion Loops.
RE: Pipeline Expansion Loops.
The loop I am asking is in one plane, there is no vertical legs.
I think the one you are talking about is usually on the pipe rack inside the plant.
Mine is on the Pipe Way in off site and pipe is laid out very close to the ground.
Or I misunderstood your response.
Thanks again,
Frank
RE: Pipeline Expansion Loops.
There is not going to be any pig launch/receive station.
RE: Pipeline Expansion Loops.
RE: Pipeline Expansion Loops.
Plant piping on the other hand is either mostly up and down or horizontal anyway and, if you use 45s, you wind up blocking out a much bigger 3D block of space, which may inhibit adding more pipe to the piperacks and pipeways in the future, or interfering with the cable trays.
RE: Pipeline Expansion Loops.
RE: Pipeline Expansion Loops.
Hard solid content, dense slurries, and Roto-Router type cleanout requirements for some kinds of pipelines might also lead you to favor 45[deg] or X[deg] bends.
RE: Pipeline Expansion Loops.
If using bending machine the limitation could be the maximum allowable changes in the pipe wall thickness and if you use miters then it could be the welding and fitting.
Thrust forces are also important in water pipelines depending on pipe diameter, pressure and backfill height. Direct 90 degree bend could mean a big thrust block.
As you said, pressure loss can also be of concern all depending on your size and pipe flow velocity. In a biginch it could be not that important for bends less than 30 degree but you should do the calculations case by case. For a water project with 60" pipe and a normal velocity for a 90-degree break with 4 pieces each 22.5 degree (according to AWWA), the headloss is equivalent to 40 meter straight pipe while for a direct 90 degree bend it would be equal to 250-300 meter pipe.
Another concern is the more smooth flow streamlines when dividing the break (bend)to smaller pieces. Sudden 90-degree bend distracts the flow streamlines. This causes a rotating vortex right in downstream of the bend which blocks the pipe partially and makes the nearby passing flow to accelerate. This will bring localized high velocity which could be of concern and also stimulates flow-induced vibrations. Take a look to flow velocities in this CFD model (red and blue are high and low velocity magnitudes respectively) and observe the rotating vortices next to 90-degree bend.
BTW, pigging is not common in water pipelines.
RE: Pipeline Expansion Loops.
While all have made good points, and I suspect any sorts of bends might work for some expansion loop circumstances, see however a fundamental design rule expressed for these specific devices e.g. at http://bo
While I would not purport to be an expert in these specific devices, I suspect depending on the geometry and space constrictions you could be dealing with, all 45 or 90 cornered loops might not necessarily be equal in this regard (unless perhaps you put two very close together 45's at each otherwise ell location?) More information and sketch of what you are really trying to do might elicit more specialized help.
RE: Pipeline Expansion Loops.
Here are the line data:
24", Steam 354 C @ 17240 kPag,
Pipe Material Carbon Steel Grade 550 MPa Yield Strength
Wall Thickness 19.1 mm
Design Code :CSA-Z662, 2007
RE: Pipeline Expansion Loops.
RE: Pipeline Expansion Loops.