Dec 2014, AutoPIPE V8i 09.06.01.10
Could you please advise how to properly model a nozzle at the dished end. We have modeled using nozzle flexibility option (using "spherical" in flexibility method) Length = head thickness. Then we tried with the "user" input in the flexibility method using the stiffness values from the "spherical" option but the results are completely different between using "spherical" and "user" selection.
As you already know the nozzle flexibilities calculated for a Spherical head are Radial and Bending stiffness of the nozzle, where User flexibilities fields are Radial, Circumferential, and Longitudinal stiffness of the nozzle. In the background, AutoPIPE use flexible joint elements to model a nozzle.
The stiffness calculations for Nozzles connected to cylindrical vessels are more sensitive to diameter of the pipe.
The difference with your modeling is to be sure that the Radial direction is the same for both User and Spherical stiffness's, and that Spherical Bending values/directions matches the User's Cir & Long values/directions.
Radial = Resistance to Axial load on the nozzleCircum = Resistance to bending moments along the circumference of the vesselLong = Resistance to bending moments along the vessel axis
Consider the following, if you are using the flexibility method = User, what direction is Rad, Cir, Long given global X, Y, & Z. In AutoPIPE a vessel is modeled as a pipe component. So the program does not know which direction a nozzle element Rad, Cir, Long is until you specify the direction of the vessel.
Direction of vessel = Global Y, that means Long. is Global Y and Rad. & Cir are set to Global X or Z.
Direction of vessel = Global X or Z, That mean Rad. is set to Global Y, Long. is set to Global X or Z and Cir is set to Global axis left over.
Again, by defining the nozzle element vessel direction, you are setting direction for Long and consequently Rad and Cir.
Modeling Vessel / Nozzles