Good evening. I am up against a 42" slip joint in which I need to somehow account for what the manufacturer calls the "activation force", which is the force required to overcome the friction in the packing to get the joint to start slipping. I have also heard this called the breakaway force. This particular joint has an activation force of 20,000 lb.

I think the correct technique would be an expansion joint in conjunction with a bilinear restraint along the pipe axis, somehow. But I am struggling with how to set it up so it "works" properly. Have any of you done one of these in the past?

Thanks in advance for any suggestions!

Hi Pete!

Your right to use the bilinear restraint. There is a small example for this in the Application Guide, page 5-21. Usually the hard thing to find out is the value of the "activation force", because of the different factors involved like friction, pre-loading of the item and so on. But this seems already be available.

Greetings,
Veit Bockemühl
www.pipestressanalysis.com
www.surgeanalysis.com

Hi Pete,

Its about time that you showed up here - welcome.

Slip joints are interesting. The Pittsburgh area is powered by steam that is supplied like any other utility. There are miles of (district heating) steam tunnels. When you are in the tunnels during piping system "heat-up" or "cool-down" (the slip joints are telescoping either elongating them or shortening them) you can hear each of the joints reach their "breakaway" point. This is a violent dynamic event. As this multiple "banging" happens, the anchors (concrete and steel) get a multiple event dynamic pounding - sometimes they are not properly designed with a dynamic load factor and they fail. Sometimes the anchor designer does not realize that the piping goes through the same loading (opposite direction) on "cool-down" and the design of the guides and anchors ignores the need for accommodating multiple two-way loadings. It would be nice if the joints would reach their "breakaway" point in some sort of order but this is not likely. They usually "pop" at different times on each side of the anchors, further exacerbating the anchor loadings.

This (when the pipe is under compressive loading) is one of those rare times in pipe stress analysis when the axial F/A calculation is important. Of course it is also important to locate the guides as to preclude buckling due to axial loading and slenderness ratio.

Another interesting thing is that with cycling the gland seals wear and the friction at the packing that causes the "breakaway" issue decreases a bit (and the "breakaway force" lessens). Of course, the wear progressively results in leakage and eventually the gland seals must be tightened by turning the seal bolts (pushing the packing into the bell tighter). Now since it is hot and nasty in the tunnel, "Bubba" doesn't like to go down there to tighten the seals very often so he is motivated to over tighten them and during the next "start-up/heat-up" cycle the "breakaway" force is greater than the OEM predicted.

Regards, John

Guys thanks for the replies. I will proceed with the bilinear.

Hi John - thanks, it took me awhile to get here. I have been playing process engineer as of late, and am just now getting back to more mechanical and piping. Well, at least it all pays the same. :-) Thanks for the great insights on the slip joints; I passed them along to our structural guy for his use in design of the anchors.

OK I built the model in accordance with the Aplications Guide.

I note that the example in the Applications Guide does not account for pressure thrust. Since the axial stiffness of a slip joint is less than the stiffness of the adjacent pipe wall, equilibrium requires that an axial force due to pressure thrust will appear. How do I account for the pressure thrust?

I guess I could calculate it by hand and input it into the model as a point load in the Forces/Moments inout box at the end of the pipe near one of the anchors. Any other ideas?

What if I add an expansion joint into the model with zero axial stiffness in all directions? That would let CAESAR II calculate the thrust and add it in, while the X2 restraints handle the slip joint.

CAESAR II does not, by default, include any structural response due to pressure, except, when expansion joints are modeled, in which case the user can specify an effective ID for the joint and this is used to add a force (=P*Aeffective)to either side of the joint. There is a switch to include bourdon pressure effects - both axial extension and opening bends.

Yes, you could model the slip joint as a low axial stiffness/high other stiffnesses, untied expansion joint to get this thrust load in there and it might cause fewer questions than simply adding a thrust load on the up- and downstream thrust points (elbows, reducers, closed valves) around this joint. But I think the forces would be easiest. Be sure to include that force set in your load cases.