What are the definitions of FE/Pipe’s boundary conditions?
There are many types of boundary conditions in FE/Pipe. Below is a list of all boundary conditions.
- FIXITY
- DISPLACEMENT
- FORCE
- HEAD
- RIGID
- FLAT
- PFIX
- SYMFIX
- NONE
FIXITY – Fixity will restrain all 6 degrees of freedom, not translation or rotation will be allowed at the specified node selected by the user. There is some radial displacement allowed in this boundary condition. If the user does not want any radial displacement, then RIGID should be used, see definition below.
DISPLACEMENT – The user can specify which displacements are allowed. The figure below shows the only allowed displacement in the FY direction and rotation in the RX axis.
“Free” can be placed by the user saying that this degree of freedom is free to translate or rotate. Note that the Weight and Operating case were both entered with the same values, these two are different cases and will be computed as separate load cases, and therefore it is a good idea to place the same values on both, if the user does not have different values for Weight.
FORCE – The user can add forces in any direction.
HEAD – For vessels, sometimes the user needs to model the top of the vessel to have a head boundary condition. FE/Pipe will add a head to the model so that the top of the vessel is not free to rotate or displace in any direction. The type of head placed is defined in the Optional window (only for NOZSHELL template).
RIGID – This boundary condition will place a rigid element at the desired location, this boundary condition will add stiffness, that is prevent radial displacement also, to the model as well as restrict it from moving in all 6 degrees of freedom.
FLAT – This boundary condition is very much alike HEAD boundary condition, only this time a flat plate will be added to the top of the vessel or nozzle, to prevent some radial displacement and some rotation.
PFIX – This will allow displacement in the axial direction. Usually what is used for a vessel is FIXITY at the bottom and PFIX at the top. PFIX allows axial displacement so that accurate PD/4t stresses can be computed at the boundary condition due to pressure.
SYMFIX – Provides axial restraint and local in-plane bending restraint. This boundary condition is more commonly used when only half of a symmetric model is used.
NONE – No boundary condition will be applied.