Dear all,

I'm currently doing a CAESAR II analysis for inter stage piping for a 6 stage centrifugal compressor. For the same when the CAESAR II model is built how is the compressor to be modelled in CAESAR II. Should there be a common resolution point for the all the 6 stages of suction and discharge piping ? or A separate model is to be built for each of the stages ?

If each stage correctly ends with anchors or specified displacements/rotations and there is no common node between stages, they are independent of one another.
If each stage is independent of one another, then they can be evaluated separately.
But if any of these stages interact, they should be evaluated as a single system or, at least, share the same boundary conditions at their common point.
You may find it useful to display the 6 stages in a single model. You can use the "Include Piping Files" to bring the individual models together. You may have to adjust your node numbers between models (using the "node Increment" feature in the File Include) and you may have to Set Global Coordinates for the individual systems (stages) to position them correctly in space.

From a vendor and API 617 perspective, it is common practice to only resolve/combine nozzle loadings for the nozzles on a single CASING. If the compressor has three casings connected by shafts then you will have three API 617 evaluations with three distinct resolution points. There typically can only be ONE fixed point for axial thermal displacements for the entire machine near the motor/driver end.

Many large multistage compressors have two casings.

Dear Dave, Thank you for your valuable guideline on my query. However I'm unable to decide (From option 1 & 2 from your quote) as to what is to be considered while building the model. I've attached the image of the 3D model and the built C-II model and C-II file. Can you please take a look at them and provide me with a technical guide line as to is my C-II model built correctly. The piping is highlighted in blue color in the 3D model image.

Dear Bob, Thank you for your valuable guideline on my query. As per my understanding when we perform a API 617 or NEMA Sm-23 analysis for a piping system, a common resolution point is to be created. This point as per the guide line of CAESAR-II manual is

" DX Specifies the X-distance from the force/moment resolution point to the nozzle.

NEMA SM 23 is ambiguous about the point of resolution of the combined forces and moments. The resolution points are interpreted to be the following two points:

The face of the flange at the exhaust nozzle connection.
The intersection point of the exhaust nozzle centerline and the equipment shaft centerline.
In order to resolve the forces and moments at the current nozzle connection, enter the X-distance from the current nozzle to each connection. Distance from the exhaust to the exhaust nozzle is 0.0. In order to resolve the forces and moments at the intersection point of the exhaust nozzle and the shaft center lines, enter the X-distance from the intersection point to each connection."

Now when we have a single stage compressor this can be done. However if we are to create a common resolution point for a 6 stage compressor, what should that point be. Request you to please elaborate on this subject.

I cannot open your two illustrations.

Hello Dave,

Hope you can view the image files now. In case you are still unable to see the images of the 3D model and the CAESAR II model you can drop me a test e-mail to chandrakant.salunkhe@in.atlascopco.com. I'll reply to the same with the image file as well as the CAESAR II file of the model

My earlier response was focused on the possible interaction of the piping. In reading Bob's response and re-reading your question, I see I am not answering your question.

Oh I see. Well as a Director of training at Intergraph, you must have had many of your customers having a similar problem like mine. Would you please take some references from your previous experiences and provide me concrete guide line as how should I solve this query?

I would suggest that the compressor manufacturer would be in the best position to estimate movements and allowable loads for each of your flange faces, and how any combination of loadings on any individual casing may interact.

Chasndrakant, if I had such advice, I would give it. Sorry.

Dear Michael_Fletcher,

I guess you have not gone through my earlier posts or have some mis understanding about the query.

1.) I want to know weather we can create a common resolution point for a 6 stage compressor. If yes how can we do it and what should that point be?
2.) If not a common resolution point, then should it be modeled as a different piping system with no interaction between any of the stages? If this approach is used for modeling the piping, suction and discharge ends of the pipe connected to the nozzles of the compressors will have to modeled as an anchor point. This will make the system stiff as CAESAR II default value for the anchor is 1*10^12 N/mm. This will result in values of loads at anchor point to be very high and will not satisfy the allowable limits of NEMA sm-23 or API 617 analysis
3.) Please guide elaborately on point 1 and 2 with practically applicable data with supporting document / code / standard