Externally pressurised expansion joints are the perfect solutions when the expansion joints must absorb very large axial movements under high pressure. In principle, there are no limits to the size of axial movement that can be absorbed by this type of expansion joint.
In externally pressurised expansion joints, the bellows element(s) is arranged so that the media ﬂow is on the external side of the bellows, while the inside part of the bellows is only subjected to atmospheric pressure with this side being in direct connection with the atmosphere. External pressurising of the bellows eliminates pressure instability as a design limitation and permits the absorption of large axial compression. Where an internally pressurised bellows will become unstable and buckle due to internal pressure, the pressure around an externally pressurised bellows will have a stabilising effect on the bellows. This makes it possible to achieve a construction of externally pressurised expansion joints with great ﬂexibility for large axial compensation. With only the outside of the bellows subjected to pressure and the convolutions packed under a cover, the perfect conditions for external insulation or underground installation can be achieved.
As the convolutions of externally pressurised expansion joints are well protected under a cover, transport damage to the convolutions is unlikely to occur but most importantly, the cover offers maximum protection against leaking bellows or bellows failure.