The in-line pressure balanced expansion joint is designed to absorb externally imposed axial movement without imposing pressure load on the system. This is accomplished by using two bellows at line pressure, tied together and acting in opposite directions. It is used where the pressure loading on the piping or equipment is not acceptable.
While the joint balances out the pressure forces, the force required for moving the joint is not balanced. Both the flow bellows and balancing bellows must be compressed or elongated in operation and the combined axial spring force acts on the piping or equipment. Since the forces need to move the bellows of, the in-line pressure balanced expansion joints are of low magnitude, and are usually not objectionable.