If you need to restrain axial vibration, and after doing the flexibility analysis using bumper (Limit Stop), thermal expansion loads exceed the allowable values, you'll need to use some assembly that allows thermal displacement without thermal reaction, but acts against vibration.
Some support devices exist. In this case, I would prefer to use Sway Brace Assemblies (see CAESAR II, Applications Guide, Chapter 3).
Other type is the Spring Loaded Hold-Down Supports. The pipe is rigidly supported at the bottom, with guides, and with an assembly spring type with a pre-load in the vertical direction (at the top of the pipe) that results in a friction force opposing to an axial vibration movement.
The HD Support manufacturer must guarantee a friction force higher than the necessary force calculated by the Pulsation and Mechanical Studies.
If you need to use HD Supports (I agree with the opinion of Mr. Detlef Drews), consult client about considering friction forces resisting effectively to vibration.
I liked also some other comments. They are also addressing the problem in a good manner.
Anyway, how to model HD Supports? This is your question. I would do according the following manner:
Suppose the HD Support has to be installed at Node 20 and that X axis is axial, Y is vertical and Z is tranversal.
So at RESTRAINTS blocks you should consider, for example, for Node 20:
a) Y restraint; Type = Y, friction Mu = 0.2 or 0.3 (depends of the type of support material used);
b) Z restraint (or GUIDE); Type = Z or G; friction Mu = 0.2 or 0.3;
c) X restraint is modeled as a spring in the axial direction, with an initial force (Fy) that instantaneously opposes vibration (this is not the same force obtained by the Pulsation and Mechanical Study, lets call it Fpm that must be considered in the specification of the HD Support); the manufacturer will design and furnish the Fy friction force for its designed support; this friction force will be the input in the restraint box, and results also as the reaction force in the SUS case analysis (not in the thermal analysis). It will also act as double direction restraint in your dynamic analysis to oppose Fpm (Fy must be higher than Fpm).
Type = X2,
K2 = 0.5 Kgf/mm (not higher than 1.0),
K1 = blank (this means Rigid),
Fy = friction force furnished by the HD support manufacturer
The hold down supports like Technology & Products, Inc. are installed for the purpose of vibration isolation, So, the vibration is suppressed in horizontal and/or vertical directions as same guide/Y restraint with some stiffness, friction. Also the rod restraint is modeled In cases where pre-load is applied
Then, it is permissible to move thermally not to generate higher loads to rotating equipment nozzles. In addition, higher natural frequencies are achieved in piping system if applied.
P.S. An additional recommendation is that during Expansion Load Case, you must confirm that the upward vertical force shall be lower than the Pre-Load Force in the HD Mounting Assemblies, because if it is higher than the Pre-Load Force, the pipe will lift-off the support. If the support was adequately located during P & M Studies, this will not happen.