Large slip joints are usually difficult to install and difficult to accurately model.
Smaller diameter slip joints are telescoping, axial displacement devices that permit considerable axial displacement of the slip joint ends and moderately rigid resistance to pipe bending. They are usually categorized as having two annular packing glands that are separated axially along the joint by a dead air space or by a small bellows sleeve.
The following figure shows the crosssection of a typical large slip joint. The stiffnesses between nodes 15 and 25 are a function of the packing stiffness for transverse and rotational relative deformation and of packing stiffness and tightening for axial relative deformation.
Slip Joint

Typical delta dimensions are:

5  10. The distance from the closest guide or support to the end of the joint. The same values are also used for 25  30.

10  15. The effective length of the joint, if known, or the travel expected plus 4inches, or a 12inch estimate, if nothing else is known.


K1 is the spring stiffness for forces below the yield force, FY.

K2 is the spring stiffness (for joint compression) for forces greater than FY. The best estimate for this resistance is cumulative friction effects of guides and supports given by the vendor.
K2 =((100)N/(a) (Approximation)
Where (N) is the nominal pipe diameter in inches, and (a) is the thermal expansion at the operating temperature in inches per 100feet.

FY is the joint friction thrust from the vendor catalog. Typical values are given as 400 pounds multiplied by the nominal pipe size.