All ducting or piping systems, with or without expansion joints, can have pressure. With rigid pipe installed between two closed flanges, the internal pressure is restrained by the strength of the pipe. The longitudinal rigidity of the pipe prevents extension to a point.
When you replace the pipe with an unrestrained metal bellows without anchors or guides, the bellows will eventually extend due to the pressure thrust which can result in bellows squirm, or over-pressurization. This can lead to permanent deformation and/or immediate bellows failure.
Adding main anchors and pipe guides into the piping system will prevent the expansion joint from overextending. The main anchors must be able to resist the pressure thrust force and a small amount of force due to the deflection of the bellows.
Determining Pressure Thrust Force
All expansion joints exert a force that pushes the pipe ends apart, pressure thrust; which is proportional to the operating pressure and the diameter of the bellows. If the expansion joint in unrestrained, the piping system must withstand the force or restraints, such as hinges, gimbals and/or tie rods can be installed on the expansion joint to constrain the thrust force.
Example:
Pipe Size = 10” nominal
• System Pressure (P) = 200 psig
• Effective Area = (Aeff) of 10” part = 126 in2
• Pipe growth between anchors (?) = 0.5 in (compression)
• Axial Spring Rate of the 10” Bellow = (SR) = 5,255 Lbs/in
1. Find the force due to the pressure:
Force due to the pressure = Pressure thrust = (P) x (Aeff)
200 psig x 126 in2 = 25,200 Lbs
2. Find the force due to the bellows deflection:
Bellows Spring Force = (G) x (S)
0.5 in x 5255 Lbs/in = 2,628 Lbs
3. The main anchors must resist the sum of the above two forces:
Pressure Thrust + Bellows Spring Force
25,200 Lbs + 2,628 Lbs = 27,828 Lbs
Managing Pressure Thrust
As mentioned, installing restrained expansion joints to a system is one way to manage pressure thrust in a system. Intermediate anchors are required to withstand the spring force generated when the expansion joint is deflected, but the need for main anchors is eliminated.
Restrained expansion joints include tied single bellows, tied universal expansion joints, hinged expansion joints, gimbal expansion joints. and pressure balanced expansion joints.