Steam Line Expansion Loop

Dear All,
We had long steam line header without any brnaches.
The steam line is designed with the expansion loops at certain distances.
When we analyze the system then a virtual anchor point is created between the two loops, which is as good as anchor point.

Do we need to introduce an anchor between the two loops to equally distribute the displacement.

Or there is no need of anchor point in between the two loops, virtual anchor from analysis is suffice.

Please advice the good engineering practice.


.....I am just a begineer

Yes you do need intermediate 'anchors'. Use the search facility to see the several previous posts on this subject.

Danish,
what exactly is this "virtual" anchor point?

just curious

cheers,

Hi Sanjay
Virtual anchor means acting anchor not a physical anchor.
You make long line with number of symmetric loops. Check thermal axial load & displacement at the mid point support between two loop it will show zero load & displacement. Axial load become zero because of equal & opposite forces. Putting a physical anchor at that location also will not effect the result of axial load & axial movement. This point is called virtual anchor.
However physical anchor is recommended to take care of discontinuity in piping tolerance, Seismic & wind load.

Hi Shr,

So, what kind of physical support should be provided at vertual anchor point? My understanding is that it should be a guide and rest support without an axial restraint...

__________________
Chandrasekhar

Hi chandras

Use that support location as anchor not virtual anchor.
You may not like/require welded/bolted ANCHOR support in rack, So you may use (Rest+guide+axial stop). At least use (rest+axial stop) on that support point.

The "virtual anchor" concept is often associated with buried pipe. I am sure there are several threads on that subject in this Forum.

In this case, the "virtual" anchor being described is just a point of zero movement along the axis of interest (in this case, along the axis of the header). Caesar will gladly give you a "perfect" calculation that says that point won't move.

In the real world, you need a directional anchor/line stop/bumper in order to insure that the real piping system behaves as the model predicts. Friction (which will also read zero at that point in Caesar) and uneven expansion from a progressive heating of the line means that your "virtual anchor" won't stay put. You can end up with one loop taking more expansion than it was designed for and the adjacent loop not taking enough.

So, definitely call for a stop in between the loops and give your structural engineer enough load to account for it having to resist a percentage of the expansion load when one side heats up before the other.

I have literally seen beams twisted over in the field at such "virtual anchor" points because they were put in without any worthwhile load being designed for.

Dear Sir,

I have doubt regarding this virtual point in Pipe line stress analysis.
To get this point how long distance we want to model from Pig receiver\launcher.
After some distance we will get some point with zero displacement and load..
What does it implies we should provide there anchor block or we don't want anchor block there..
Then up to how long induction bend radius we can take in this pipe line stress analysis...
Is there any limit for this..
Please clear on this...

Thanks...

The "virtual anchor point" is a "fake/imaginary" point that indicates the location where the pipe run is long enough such that the axial friction along the pipe is sufficient to provide a point of zero displacement. No you don't put an anchor there. Rather it indicates a point where you can stop your model.

Note also that the virtual anchor point is from a specific reference location. If you move the reference point, the virtual anchor moves.