Rubber Exp. Joint Gusset Plate Thickness.

[Anthony.A]

Please point me in the right direction if I’m posting on the wrong section.

I’m wrapping my head around the right size (thickness) of the gusset plates on a rubber expansion joint. I’m installing a few rubber exp. joints at the discharge side of pumps/chillers. The conditions are as follows:

Pipe Dia: max. 600mm
Bellow dia: (lets say +20% of pipe dia) 720mm
Operating pressure: 4.6 bar
Testing pressure: 13.7bar
Flow velocity: max. 3m/s
The formula used here is: Thrust = (pi/4)*D^(2)*P


For 4.6 bar case that gives us 187kN, 13.7 bar gives us 557kN. Now if I use 3x M24 grade 8.8 bolts (yield strength 560N/mm2), they could withstand the calculated force. But its so much easier for the gusset plate to bend, and lead to failure that looks like this.(attachment 1) But from most ready-made products/ existing installations, the gusset plate doesn’t exceed 20mm in thickness - so how do they not bend already?

I have also attached the calculation sheet for the M24 bolt and gusset plate as reference. (attachment 2)

Attach. 1:

https://files.engineering.com/getfi...06e-430a-b10d-42e292ce7e06&file=attach._1.jpg

Attach. 2:

https://files.engineering.com/getfi...file=Attach._2_Exp._Joint_Limiter_Details.pdf

 

[StressGuy]

I'm curious why you're trying to design these. They should have been designed and provided by the expansion joint manufacturer in their scope.

You really don't want to hydrotest that with the expansion joint in the system - especially at that high of a pressure. Why is the test pressure 4x operating anyway? The joint should have been tested in the shop already for the design pressure it was spec'd to and your pipe spool should be tested separately. Then you are joining two previously tested subassemblies.

An expansion joint is a compromise by it's nature. You might be able to test a metal piping system to "flange rating" even though it will never see such a thing. You really can't do that with an expansion joint - you are going to end up with something so stiff to handle the pressure that you are going to end up with high springs rates...defeating the purpose of getting a joint to begin with.

Edward L. Klein
Pipe Stress Engineer
Houston, Texas

"All the world is a Spring"

All opinions expressed here are my own and not my company's.